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Fundamentals of Motors
5/05
FOM
Fundamentals of Motors
Table of Contents
Electrical and Magnetic Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fluid Flow versus Electron Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electro−Magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Single and Three Phase Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Torque, Horsepower and Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Torque . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Horsepower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Relationship of Torque, Speed and Horsepower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
NEMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Speed versus Torque and Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Motor Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Frame Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
NEMA Motor Enclosures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Service Factor and Duty Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Types of Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Three Phase AC Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Explosion Proof . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Single Phase AC Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DC Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
FOM
1-2
1-2
1-2
1-4
1-5
1-5
1-5
1-5
1-6
1-6
1-6
1-8
1-8
1-8
1-10
1-10
1-11
1-11
1-11
1-12
1-15
Fundamentals of Motors 1-1
Section 1
General Information
Motor Fundamentals consists of four chapters:
− Electrical and Magnetic Basics
− Torque, Horsepower and Speed
− Motor Basics
− Type of Motors
Electrical and Magnetic Basics
Electrical motors use magnetism and induction to change electrical energy to mechanical energy.
It is therefore important to understand electrical and magnetic fundamentals.
Fluid Flow versus Electron Flow
Electricity flows through conductors (wire) much the same as liquid flows through tubing or pipes.
Electricity is measured in Amps, Volts and Watts and may be compared to liquid flow as shown in
Table 1−1. There are similarities in the relationships of pressure, flow and energy.
Table 1−1
Unit
Fluid FLow
Electrical Flow
Media
Water
Electrons
Pressure
PSI (Pounds per Square Inch) Volts
Flow
Gallons
Amps
Energy
Gallons per minute
Watts
In electrical circuits, the relationship between voltage (electrical pressure), current (electron flow)
and resistance (resistance to electron flow) is stated as ohms law:
I = Amperes (AMPS)
E = Volts
I + E
R
R = Resistance (Ohms)
Therefore, if the voltage across the motor changes, either amps or ohms has also changed.
Electro−Magnetism
Magnetism can be produced by applying electricity to a coil of wire. The wire is wrapped around
an iron core. When current flows through the wire, Magnetic lines of force (called magnetic flux,
from North pole to South pole) are produced, Figure 1-1.
Figure 1-1 Electro−Magnetism
Ohms Law
When a wire is moved through a magnetic field, electric current is produced in the wire. This is
known as induction and is the principle of operation for an AC or DC motor generator. The
relationship of magnetic flux and current flow in a wire (amps) is shown in Figure 1-2.
Figure 1-2 Motor Generator
Wire
Amps
North
South
Magnetic FLux
Wire Movement
1-2 Fundamentals of Motors
FOM
When the wire is moved up through the magnetic field, electric current flows in one direction in
the wire. When the wire is moved down through the magnetic field, electric current flows in the
other direction. This relationship is shown in Figure 1-3.
Figure 1-3 Electro−Magnetic Generator
Simplified Alternator: (AC Generator)
The magnetic field is being moved through the wires rather than the wires moving through the
magnetic field. A complete current path is established to the Light by the wires that are rotating
through the magnetic field. The current and voltage produced lights the Light bulb shown in
Figure 1-4.
Figure 1-4 Simple AC Generator
Stator
Rotor
Slip
Rings
Brushes
Sine AC Generator
As the magnetic field makes one revolution through the wires, a complete sine wave of voltage is
produced. This means that both positive and negative voltages are produced and the speed of
the wires moving through the filed determines the frequency of the AC voltage, Figure 1-5.
Figure 1-5 AC Sine Wave Generator
FOM
Fundamentals of Motors 1-3
Simplified DC Generator
The DC generator works very much the same as the AC alternator. Brushes are in contact with
the commutator and each full rotation produces two pulses.
Figure 1-6 DC Generator
Basic DC Generator Operation
FIgure 1-7 illustrate how the voltage signal is produced during one complete revolution of the DC
Generator. As the wires rotate through the magnetic field, the brushes that are in contact each
half of the commutator switch to the other half of the commutator at the 180 degree points (“B”
and “D”). This changes the polarity of the signal that is produced. In this case, two positive
pulses are produced for each 360 degree rotation of the rotor.
Figure 1-7 DC Generator Operation
Single and Three Phase Power
AC Frequency
Figure 1-8 shows the relationship of single phase AC Voltage and AC Current. Notice that the
current lags slightly behind the voltage. Frequency is the number of cycles per second, called
“hertz”. 60 Hz (hertz) = 60 cycles per second. The green waveform shown in Figure 1-8 is called
a sine wave.
Figure 1-8 AC Voltage and Current Relationship
Three Phase AC Power
Three phase AC power consists of three individual voltages that are synchronized with each
other. These are Phase A, Phase B and Phase C voltages shown in Figure 1-9.
Figure 1-9 Three Phase AC Power
+
Phase A Phase B Phase C
−
1-4 Fundamentals of Motors
FOM
Rotating Magnetic Field
A rotating magnetic field is produced in the stator (stationary coils of wire in the motor). The 3
phase power applied to the stator causes the rotor (rotating part of the motor) to move which
converts the electrical energy to mechanical energy.
Torque, Horsepower and Speed
Torque
Torque is the twisting force applied to the load as shown in Figure 1-10. Although we think of
horsepower when selecting a motor, the load actually “sees” “Torque” supplied by the motor.
Standard units of measure are:
lb−in (inch pounds), lb−ft (pound feet), oz−in (ounce inches) and nM (newton meters).
Torque is a measured force at a specified distance.
Torque = Force X Distance
Figure 1-10 Torque
Speed
The rotational speed of the magnetic field within the motor is called the Synchronous Speed.
It is approximately the no load motor speed. The synchronous speed varies by motor design but
generally depends upon the number of motor poles and the operating frequency of the AC
voltage, Figure 1-11.
Figure 1-11 Synchronous Speed
Horsepower
Horsepower is a measure of the rate at which a motor or drive can produce work.
A 1hp motor can produce 33,000 lb−ft of work in one minute of time.
Figure 1-12 Horsepower Illustration
16.3’
2000
33’
1000
FOM
Fundamentals of Motors 1-5
Relationship of Torque, Speed and Horsepower Putting it all together.
There is a relationship between a motor’s Horsepower (hp), Torque and Speed.
Figure 1-13 Torque, Speed & Horsepower Relationships
Torque (lb−ft) x Speed
hp +
5252
Torque (lb−in) x Speed
hp +
63025
OR
hp x 5252
Torque (lb−ft) +
Speed
Torque (lb−in) +
hp x 63025
Speed
Rules of thumb:
As speed decreases, torque increases!
As Torque increases so does the amount of required current !
NEMA
Full load torque for common motor speeds at 1 hp are:
1hp = 1.5 lb−ft of Torque @ 3450RPM
1hp = 3.0 lb−ft of Torque @ 1750RPM
1hp = 4.5 lb−ft of Torque @ 1200RPM
1hp = 6.0 lb−ft of Torque @ 850RPM
National Electrical Manufactures Association. NEMA was established to provide a common
standard for the electrical industry. Baldor and other motor manufacturers build their motors to
the MG 1 specifications. Specifications also include:
Enclosures, Insulation Classes, Frame sizes, Energy Efficiency and Nameplates
According to NEMA a nameplate must include the following as a minimum:
Mfg. Type and frame design.
HP output
This can be used as a guide to
Time Rating ( Duty Cycle )
obtain needed information about
Max. Ambient Temp
the application so the correct
Insulation system designation
motor may be selected.
Frequency − 50 or 60 HZ
Number of phases
Voltage
Full Load Amps
Code Letter for locked rotor KVA
Efficiency
Speed versus Torque and Current
This is a standard for Speed/Torque curve for a three phase AC induction motor.
1-6 Fundamentals of Motors
FOM
Locked Rotor Torque is the torque the motor produces with full voltage applied and the
load is stationary. It is torque available to break the load away from its rest position.
Also called starting torque and breakaway torque.
Pull Up Torque the lowest point on the Torque/Speed curve for a motor that is accelerating
a load to full speed. The lowest point may be the locked rotor point for some motors.
Also called pull in torque.
Breakdown Torque the maximum amount of torque that is available from the motor shaft.
When a motor operates at full voltage and maximum speed, the load is increased until
the maximum torque is reached. Also called pull out torque.
Full Load Torque the rated continuous torque the motor can provide without overheating
within it’s time rating. Also called rated torque.
NEMA Design A, B, C and D motor comparisons. Design B is the most common three phase
design used. NEMA provides standardized ratings for starting and torque characteristics and the
amount of slip of a motor. See Figure 1-14.
Figure 1-14 NEMA Design A, B, C and D Comparisons
NEMA Torque Design Summary
Something to Know: Slip & Heat
Slip is loss of potential energy that shows up as heat.
Slip is the difference between synchronous speed and full load speed.
Design D motors have more losses from the stator, so more heat is produced.
NEMA Design
B
C
D
Starting
Locked Rotor
Breakdown
Percent Slip
Current
Torque
Torque
Medium
Medium
High
Maximum 5%
Applications: Normal starting torque for fans, blowers, rotary pumps,
unloaded compressors, some conveyors, metal cutting machine tools,
miscellaneous machinery. Slight speed change with load.
Medium
High
Medium
Maximum 5%
Applications: High inertia starts, such as large centrifugal plowers, fly
wheels, and crusher drums. Loaded starts, such as piston pumps,
compressors and conveyors. Slight speed change with changing
load.
Medium
Extra High
Medium
5% or More
Applications: Very High inertia and loaded starts. Also, considerable
variation in load speed. Punch presses, shears and forming machine
tools. Cranes, hoists, elevators, and oil well pumping jacks.
Torque Types for Motors
For higher starting torque applications, Design C or D motors should be used.
FOM
Fundamentals of Motors 1-7
Motor Basics
Frame Size
NEMA standardized the motor frame size that allows motors to be interchangeable regardless of
the manufacturer.
Three Generations
1. Prior to 1952 − “Original” − 284
2. 1952 to 1965 − U frame − 284U
3. 1965 to present − T frame − 284T
NEMA frame size Designation.
Two digit 56
56 = shaft height in 1/16 inch increments, 56/16 = 3.5 inches (Base to shaft center)
IEC motors (metric) measure the shaft height in millimeters.
For example, an 80 frame would have an 80 millimeter shaft height.
Three digit 284T
28 = shaft height in 1/4 inch increments, 28/4 = 7 inches (Base to shaft center)
4 = Relative frame length (6 is a longer frame than 4)
Common Frame Size Variations (suffix − letter appears after frame size, ie. 56C)
C Face mount
D Flange mount
S Short Shaft with smaller shaft diameter than standard
T 1964 generation
U 1952−1964 era
Y Special mounting configuration
Z Special Shaft − longer, larger, holes, threads, etc.
NEMA Motor Enclosures
US Voltage Ratings
3 Phase Industrial Power Sources; 240 − 480 − 600 VAC
3 Phase Motors
Prior to 1965;
220 − 440 − 550 VAC
1965 to present; 230 − 460 − 575 VAC
1-8 Fundamentals of Motors
FOM
Voltage Drop
As current leaves it’s source and travels through wires to the load, the resistance of the wire and
connections causes voltage drops across the wire or connections. This reduces the voltage
available at the load. Because voltage drop affects motor performance, voltage and current
should be measured at the motor, not at the fuse/breaker box. Figure 1-15 illustrates voltage
drops at various parts of the circuit. Notice the reduced voltage at the lamp.
Figure 1-15
Insulation Class
NEMA also classifies the insulation used inside the motor.
Centigrade and Fahrenheit Comparison
FOM
Fundamentals of Motors 1-9
Service Factor and Duty Cycle
Service Factor − Is a measure of the overload capacity designed within the motor. This
service factor should only be used if other factors are within nameplate ratings (such as
ambient temperature and voltage). Service factor for a motor should not be used when
the altitude for the application exceeds 3300 ft (1000 meters).
A SF of 1.15 means a motor is capable of running at 115% of rated load with out
exceeding temperature rise limits. Other common SF are 1.0 and 1.25
Duty Cycle − Motors are rated for Continuous or Limited (timed) Duty. Most motors are rated
Cont. Duty
Efficiency
Efficiency is the percentage of the input power that is actually converted to work output (motor
shaft).
Output Power (kW)
Efficiency +
Input Power (kW)
1kW = 1.34 hp
Nameplate lists Nominal Efficiency
There is also a Minimum Efficiency
Standard versus Energy Efficient
Electric Consumption by Motors
In industry, electric motors consume more electricity than any other item.
Operating Cost versus Purchase Price
Rule of Thumb: (Assume 24 hour operation @ $0.05/kWh)
It costs about $1 per horsepower per day to operate a motor.
1-10 Fundamentals of Motors
FOM
Types of Motors
Three Phase AC Motors
Squirrel Cage Induction Motors − These motors meet many of the requirements found in industry.
Because of their simple construction, low maintenance cost, and versatility, these motors are
used most often. They have good speed characteristics and can be easily reversed.
Three Phase AC Motor Parts:
“Inverter Ready” Super−E® Motors
ISR® Inverter Spike Resistant Wire is used in all Baldor AC Motors.
EXXON/Mobil POLYREX®"EM Grease
Baldor Inverter Duty Motors Meet all requirements of NEMA MG−1 part 31
Speed Range 1000:1 (except explosion proof)
TENV or TEBC
Matched Performance with Baldor controls
Class H Insulation
Thermostats in windings
C−Face standard
1024 PPR Encoder (2 styles)
Vector, Inverter
Explosion Proof
These are specially designed AC squirrel cage induction motors.
Explosion Proof motors are designed for use in the presence of combustible gasses and vapors.
These are required to be constructed so that gas vapor−air explosions that occur within the
motor will not ignite surrounding gas vapor−air mixtures. Joints are machined with close
tolerance, metal−to−metal contact, with adequate flame paths to arrest any propagation of flame
or hot gas from inside the enclosure to the surrounding atmosphere.
Explosion−Proof Parts
FOM
Explosion−Proof Cutaway View
Fundamentals of Motors 1-11
Single Phase AC Motors
Types of AC Single Phase Motors
Shaded Pole
Split Phase
Capacitor Start−Induction Run
Capacitor Start − Capacitor Run
Permanent − Split Capacitor
Repulsion − Start Induction Run
Single Phase Voltages
Single phase motors larger than 5hp
are normally 230VAC only.
Single phase motors smaller than 5hp
are normally 115/230VAC.
Shaded−Pole Motor
The shaded pole motor will operate loads that are very easy to start. It has low starting torque
from 50 to 75 % of the normal full speed torque. Typical applications are found on small fans
used on home appliances and computer fans. Baldor does not build these type of motors.
Split Phase Motors
Split phase motors are wound to divide the incoming current into two parts during the starting
phase. The run or main winding consists of many turns of low resistance, large diameter wire.
The auxiliary winding (sometimes called the start winding) consists of fine wire with high
resistance. This causes a shift in the magnetic field that is needed to start the motor. A switch
disconnects the auxiliary start winding when the run RPM is reached.
1-12 Fundamentals of Motors
FOM
Capacitor Start − Induction Run
These motors are similar to split phase motors, except a capacitor is connected in series with the
start winding. The capacitor causes the current phase shift between the run and start windings.
A centrifugal switch mounted in the endbell of the motor disconnects the start winding when the
run RPM is reached. This is the most common single phase motor used in industry. The start
capacitor is an electrolytic type that has a time rating. All capacitors of this type cannot be
“Online” all the time.
Capacitor Start − Capacitor Run
Similar to the capacitor start−induction run motor except an electrolytic capacitor is used with the
start winding to start the motor and an oil filled capacitor is used with the run winding to run the
motor. The run capacitor reduces the line current required to run the motor. The capacitor
(unlike the oil electrolytic) is not time rated and may be “Online” all the time. This motor type may
be found in many farm duty applications.
Permanent Split − Capacitor
The permanent split−capacitor motor uses the same oil filled type capacitor for starting and
running (no switch). This motor type is used in fan applications.
FOM
Fundamentals of Motors 1-13
Repulsion Start
The repulsion start induction run motor has a start winding similar to the single phase motor
except it has a wound armature with a commutator similar to the DC motor. Starting torque
develops through the interaction of the rotor current and single phase stator. As the motor is
running, the commutator is removed from the circuit and the motor operates as a single phase
induction motor. This motor type is used in hard to start applications and is the most expensive
single phase design.
Comparison of Motor Parts
1-14 Fundamentals of Motors
FOM
DC Motors
DC Motor Parts
Types of DC Motors
Permanent Magnet
Wound Field
Series Wound
Shunt Wound
Compound Wound
Permanent Magnet DC Motors
Permanently magnetized materials create the magnetic field (instead of an electromagnetic
Stator winding). This type of motor has a straight line sped torque curve. This design is used in
adjustable speed applications normally less than 5hp. Armature voltages are commonly 90VDC
and 180VDC.
Series Wound DC Motors
Field coils are series connected to the armature. As the load is reduced, current decreases and
this results in a weaker magnetic field. This causes the motor speed to increase. The high
torque at low speeds makes these motors ideal for electric vehicle applications. Rated voltages
are commonly 12, 24, 48, 72 or 96 VDC (multiples of 12VDC).
Shunt Wound DC Motors
Field coils are connected in parallel with the armature. This motor type has the flattest
speed−torque curve of wound stator design and is ideal for conveying applications. However,
this motor type does not perform well under heavy overload conditions. This design is used in
adjustable speed applications. Can be wound for very special applications. These have both
armature and field rated voltages (ie. 90VARM/100VFIELD).
FOM
Fundamentals of Motors 1-15
Compound Wound DC Motors
A combination of series and parallel windings are used. One filed coil is in series with the
armature and the other is in parallel with the armature. This motor develops more starting torque
than a shunt wound motor and has less speed variation under load than the series wound type.
Compound wound motors are used in fans and pumps. Rated voltages are commonly 115VDC
or 230VDC.
Benefits of Permanent Magnet DC Motors
Stable Magnetic field
Higher stall torque
More efficient use of electricity
Reduced frame size for a given output.
Very durable
Only two wires required
Lowest (perceived) cost
Benefits of Wound DC Motors
S
Can be wound to meet extremely specific operating characteristics, especially for
speed control and over−speeding.
S
They are the only type of DC motor available in some torque ranges.
Overview of Motors Characteristics
DC Motors
S
Change in voltage produces a change in speed.
S
Change in current produces a change in torque.
AC Motors
S
Change in input frequency produces a change in speed.
S
Change in current produces a change in torque.
1-16 Fundamentals of Motors
FOM
BALDOR ELECTRIC COMPANY
P.O. BOX 2400
Fort Smith, AR 72902−2400
(479) 646−4711
Fax (479) 648−5792
www.baldor.com
© Baldor Electric Company
FOM
Printed in USA
5/05
Fundamentals of Motors
Name:
Branch Address:
Phone:
email Address:
1.
What is the largest user of electricity in industry?
- Light bulbs
- Computers
- Electric Motors
- Electric Pencil Sharpeners
2.
Since Fluid flow and Electrical flow can be compared, what is the pressure or PSI in Fluid
flow most like?
- Electrons
- Volts
- Amperes
- Watts
3.
When a wire is moved through a magnetic field nothing happens.
- True - False
4.
In an AC system frequency is known as voltage change.
- True - False
5.
A rotating magnetic field within the stator of an AC motor is called the _________________
speed of the motor?
6.
An AC motor has stationary and rotating parts.
The stationary part of an AC motor is called the _________________________.
The rotating part of the AC motor is called the _________________________.
Fundamentals of Motors Questions 1
7.
For every one horsepower, a 1750 RPM motor will develop approximately _______ lb−ft
of Torque at full load.
- 3
- 33
- 333
- 3333
8.
NEMA stands for ...
- National Engineering Managers Association
- National Electrical Manufacturers Association
- National Electrical Manufacturing Association
- National Electrical Motor Association
9.
Locked Rotor torque is also called Starting Torque.
- True - False
10. In a DC motor, a change in
- voltage changes motor speed.
- current changes motor speed.
- hertz changes motor speed.
- temperature changes motor speed.
11. The higher the insulation class letter the greater the amount of heat the insulation can
withstand before breaking down.
- True - False
12. Service Factor is a measure of the overload capacity of a motor. If a motor is used in
Denver, Colorado (5,200 ft) it is OK to use it’s service factor to determine overload capacity.
- True - False
2 Fundamentals of Motors Questions
13. Where is an explosion proof motor used?
14. What is the most common NEMA winding design of three phase motor used in industry?
15. What is the most popular motor for industry?
- Capacitor Start, Induction Run Motors.
- Split Capacitor Motors
- Wound Field DC Motors
- Shaded Pole Motors
16. What are the alternate names for the Conduit box on a motor. (Check all that apply).
- Terminal Box
- Breaker Box
- Junction Box
- End Bell Assembly
17. Split Phase AC Motors have only one set of windings.
- True - False
18. Wound DC motors can be wound to meet extremely specific operation characteristics.
- True - False
19. Why is avoiding a large voltage drop important in motor applications?
Fundamentals of Motors Questions 3
Section 1
General Information
20. A customer has a three phase conveyor application. What type of motor would you supply
and why?
21. What data would you need to know before selecting the motor for the application?
Return your completed form to:
Baldor Electric Company
Marketing Dept.
P.O. Box 2400
Fort Smith, AR 72902−2400
Attn: Director of Education
4 Fundamentals of Motors Questions
Fundamentals of Drives
5/05
FOD
Fundamentals of Drives
Table of Contents
Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AC Motor Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AC Motor Poles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Synchronous Motor Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Full Load Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Slip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Torque and Horsepower Formulas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Wire Control Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Wire Control Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Load Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Motor Starters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Full Voltage Starting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reduced Voltage Starting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Adjustable Speed AC Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Multi Speed AC Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Inverter Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dynamic Braking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Line Regenerative Braking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Output Ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Vector Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Encoderless Vector Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Feedback Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Encoder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Resolver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DC Motor Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DC SCR Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PWM DC Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DC Power Supply Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Feedback for DC Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
FOD
2-2
2-2
2-2
2-2
2-2
2-3
2-3
2-3
2-3
2-3
2-4
2-5
2-5
2-5
2-7
2-8
2-8
2-9
2-10
2-10
2-11
2-11
2-12
2-13
2-13
2-13
2-14
2-14
2-14
2-15
2-16
2-16
Fundamentals of Drives 2-1
Section 1
General Information
Basics
AC Motor Speed AC Motors are essentially constant speed motors. The speed of the magnetic field is
determined by the line frequency and the arrangement of the stator windings (number of
magnetic poles).
AC Motor Poles The number of pole pairs (north and south electromagnetic poles) are determined by how the
stator windings are constructed.
Figure 2-1 AC Induction Motor Poles
A
C
B
B
C
A
Frequency
Frequency of the AC Line is determined by the number of positive and negative cycles occur per
second. Motor designs are based on the voltage and frequency specified. Frequency is
normally 60 or 50Hertz.
Figure 2-2 Three Phase AC Voltage Waveform
+
Phase A Phase B Phase C
−
Synchronous Motor Speed
Synchronous Motor Speed is the speed at which the magnetic field within the motor (stator)
rotates. This is also approximately the no load rotor speed. Synchronous speed (see Table 2−1)
is determined by the number of poles and operating frequency. More poles provide more torque
but slower speed.
Table 2−1
60Hz Speed
50Hz Speed
Poles
(synchronous)
(synchronous)
2
3600 RPM
3000 RPM
4
1800 RPM
1500 RPM
6
1200 RPM
1000 RPM
8
900 RPM
750 RPM
The formula to calculate synchronous rotation speed is:
120
AC Frequency
RPM +
Number of Poles
2-2 Fundamentals of Drives
FOD
Full Load Speed The full load motor speed is approximately the motor speed when the motor is producing full
torque. It is also called the base speed or running speed of the AC motor.
Table 2−2
60Hz Speed
50Hz Speed
Poles
(synchronous)
(full load)
(synchronous)
(full load)
2
3600 RPM
3450 RPM
3000 RPM
2850 RPM
4
1800 RPM
1725 RPM
1500 RPM
1440 RPM
6
1200 RPM
1140 RPM
1000 RPM
960 RPM
8
900 RPM
860 RPM
750 RPM
720 RPM
Slip
Slip is the difference between a motor’s synchronous speed and full load speed. As the load
increases, motor speed is reduced slightly to move the load. Large motors tend to have less slip
than small motors. High efficiency motors tend to have low values of slip. 3−5% slip for motors
less than 10hp and 1% for motors greater than 75hp.
Table 2−3
Motor hp
Running RPM
Slip RPM
Percent Slip
1/2
1725 RPM
75 RPM
4.0%
100
1760 RPM
40 RPM
2.2%
200
1780 RPM
20 RPM
1.1%
Torque and Horsepower Formulas
Here is the relationship of Torque and Horsepower.
Where: HP = Horsepower, S = Base Speed
Torque + HP X constant
constant = 5252 (for torque in lb−ft)
S
2 Wire Control Circuits An example of a 2 Wire control circuit is shown in Figure 2-3.
Figure 2-3 2 Wire Control
When water reaches a preset level, the float switch closes and the
pump motor runs. When the water level decreases to a preset level,
the float switch opens and the pump motor stops. Another example
of a 2 wire control is a light switch. Close the switch and the light
goes On. Open the switch and the light goes Off.
2 Wire controls are inexpensive but have a major draw back. When
the switch is closed and the motor is running then power is lost, the
motor stops. When power is restored, the motor will immediately
start running (switch is still closed). This can be extremely
dangerous in many applications.
L2
L1
L1
L2
L3
M
Float
Switch
OL
M
OL
T1
T2
T3
Three Phase Motor
3 Wire Control Circuits An example of a 3 Wire control circuit is shown in Figure 2-4.
Figure 2-4 3 Wire Control
Momentary contacts are used so that if power is lost and
restored, the motor will not operate. Closing the “Start”
switch energizes the “M” contactor to start motor operation
and the “M” contact across the start switch is closed (sealing
contact of the “M” contactor relay. The motor is connected
across L1 and L2 and continues to run until power is lost
(sealing contact opens) or one of the three “Stop” switches
is pressed. When a Stop switch is pressed, the motor
connection to L1 is lost and the motor stops. 3 wire control
is much safer if power is lost and restored.
FOD
L2
L1
L1
L2
L3
M
Start
Stop
OL
M
M
OL
T1
T2
T3
Three Phase Motor
Fundamentals of Drives 2-3
Load Types
Loads are generally divided into three categories: Variable Torque, Constant Torque and
Constant Horsepower.
Figure 2-5 Variable Torque
100
Variable torque loads have
characteristics requiring low
torque at low speeds and
increased torque as speed is
increased.
Examples of variable torque loads
are fans, and centrifugal pumps.
% HP & Torque
80
60
Torque
40
HP
20
50
100
% Speed
Figure 2-6 Constant Torque
Constant torque loads have
characteristics where torque is
constant regardless of motor
speed.
Examples of constant torque
loads are conveyors, extruders,
mixing applications and positive
displacement pumps.
% HP & Torque
100
Torque
80
60
HP
40
20
% Speed
50
100
Figure 2-7 Constant Horsepower
Constant horsepower loads require
reduced torque as motor speed is
increased.
Examples of constant horsepower loads
are drill presses, lathes, milling
machines and winding equipment.
Note: A constant horse- power load is
easily identified when either the
diameter of the tool or the load is
changed.
% Torque
100
80
% HP
60
40
20
% Speed
100
2-4 Fundamentals of Drives
200
FOD
Motor Starters
A starter controls two important characteristics of a motor: The motor current (torque produced
by the motor) and the frequency of the voltage applied to the motor (motor speed ).
Motor starters can be Full Voltage Starters or Reduced Voltage Starters.
Full Voltage Starting
Full voltage starters are also called across the line starters. Full AC line voltage is applied directly to the
motor. Typically full voltage starting is used for squirrel cage induction motors. This results in starting
current of up to 600% of full load current and from 200−400% of full load torque. (See Table 2−4 that
compares inrush current and torque for different starter types). This can cause mechanical and electrical
stress on the motor.
Table 2−4 Inrush Current
Starter Type
Across the Line
Resistor
Part Winding
Wye/Delta
Auto−Transformer
Solid-State Soft Start
Inrush Amps %
400−700% FLA
65% FLA
50% FLA
60% FLA
50/60/80% FLA
10−100% FLA
Torque %
150−400% FLT
42% FLT
50% FLT
60% FLT
50/60/80% FLT
10−100% FLT
* Depends on Motor Design
Reduced Voltage Starting
Reduced voltage starting methods include:
Part Winding, Primary resistor, Wye−Delta, Auto−Transformer and Solid−State Soft Starter.
Figure 2-8 Part Winding Starter
Part winding starters use two motor windings. Power is applied to
only one winding to achieve reduced voltage starting. Normal
current is approximately 70% and torque is approximately 50% of
locked rotor full voltage condition. After a time delay, power is
applied to the second winding that brings the motor to full speed
and torque.
Limitations of Part Winding Starters are:
− Maintenance of electro−mechanical contacts.
− Initial starting torque is not adjustable to load condition.
− Special motor and starter are required.
− Transition from initial start to full winding operation is
stepped instantaneously causing mechanical and
electrical stress.
FOD
Fundamentals of Drives 2-5
Figure 2-9 Primary Resistance Starter
Primary resistance starters insert power resistors to reduce voltage
to the motor. After starting, the resistors are removed and full
voltage is applied to the motor.
Limitations of Primary Resistance Starters are:
− Maintenance of electro−mechanical contacts.
− Transition from initial start to full voltage operation
causes mechanical and electrical stress.
Stop
Start
M
O.L.
A
Thermal
Cut−Out
A
M
T.C.
M
M
O.L.
L1
A
M
O.L. T2
L2
A
T1
M
Motor
O.L. T3
L3
A
Resistors
Figure 2-10 Wye−Delta Starter
Wye−Delta motors are specifically wound for use with a Wye−Delta
starter. The motor is started in the Wye configuration to achieve
reduced voltage starting. Current and torque are approximately 2/3
of locked rotor values. After a time delay, the windings are
switched to delta configuration to bring the motor to full speed and
torque.
Limitations of Wye−Delta Starters are similar to other reduced
voltage starters.
Figure 2-11 Auto−Transformer Starter
Auto transformer starting uses a standard AC induction motor. The
starter may have up to three taps usually 50%, 65% and 85% of full
voltage. Reduced voltage starting is accomplished by selecting the
desired voltage tap. After a time delay, the starter will switch to full
voltage.
There are thermal limitations with the
auto transformer that limits the number
of starts per minute.
L1
S
L2
L3
S
R
S
R
R
T2
T1
2-6 Fundamentals of Drives
T3
AC Motor
FOD
Figure 2-12 Solid State Soft Starter
Solid state soft starter controls use solid state switching devices
(SCR or Silicon Controlled Rectifiers) to produce a linear voltage
increase to the motor. Solid state starting eliminates switching
transients that cause mechanical and electrical stress for the
motor.
There are no inherent limitations to the number of starts in a period
of time.
Adjustable Speed AC Controls
AC Motors are designed to run at one speed at rated voltage. To control the speed of an AC
motor application, a speed reduction gearbox, pulleys, sprockets or other devices must be used.
Figure 2-13 Gearbox or Pulley Adjustable Speed Devices
FOD
Fundamentals of Drives 2-7
Multi Speed AC Motors
A multi speed AC motor has one or more windings allowing for multi speed operation. Most
operate as two speed motors. Three and four speed motors are not common but available.
Multi speed motors are normally single voltage.
Single winding motors are designed to be connected for speeds based on 2:1 ratio. For
example, at 60Hz speeds of 3450/1750 RPM or 1750/850 RPM are available. 4:1 ratios are
available but not as common.
Two winding motors are designed so that available speeds are not necessarily on a 2:1 ratio.
For example, two winding motor full load speeds could be 1750/1140 RPM or 3450/850 RPM.
When ordering multi speed motors, be sure to specify the number of windings (one or two
windings) torque characteristic (variable torque, constant torque, constant hp) and voltage rating.
Adjustable Speed Control
To adjust the speed of an AC induction motor you must adjust the frequency and to adjust the
torque you must control the current to the motor.
Figure 2-14 Control Selection Chart
Must Control
the Motor’s
Speed and Torque
Must control motor
starting
− Soft Start
Feedback
Adjustable Speed
Control
− DC SCR
− Inverter
− Vector
Must control
position
− Servo
− Vector
Open Loop: There is no speed or shaft position feedback from the motor to the control in an
open loop system.
Closed Loop: A feedback device mounted on the motor provides speed and shaft position
information to the control. Most common feedback devices are tachometers, resolvers and
encoders.
2-8 Fundamentals of Drives
FOD
Inverter Control An inverter control adjusts the speed of an AC induction motor. The inverter adjusts the
frequency and voltage of the output power that is applied to the motor. It is an open loop control
and does not monitor motor speed or position.
Internally the control converts fixed voltage, fixed frequency AC line power to DC power that is
stored in capacitors. This is done by the Converter section of the control. The DC Bus voltage is
approximately 1.4 times the RMS AC line voltage (230VAC line voltage will produce 322 volt DC
bus voltage).
The DC bus voltage is then converted to a PWM (Pulse Width Modulation) 3 phase AC signal to
be applied to the motor. The way that the PWM signal is controlled adjusts the frequency and
voltage applied to the motor.
Figure 2-15 Inverter Control
Inverter
Converter
DC Bus
L1
L2
L3
+
T1
Logic
Circuits
Logic
Circuits
T2
T3
Three Phase Motor
Control
Circuits
SCR
PWM signal has the a series of positive and negative pulses for it’s output signal. The width of
each pulse changes with the amplitude of the voltage it represents. A very narrow pulse
represents a lower voltage than a wider pulse. Figure 2-16 illustrates the PWM signal and the
corresponding AC motor voltage waveforms. All pulses are the same amplitude but their width
changes to control motor voltage. The frequency of the motor voltage is controlled by the
frequency of the positive and negative transitions of the PWM signal.
Figure 2-16 PWM Signal
Capacitor
Transistor
Carrier Frequency
changes pulse rate
Motor
Narrow pulse widths
produce less power
to the motor
Carrier Frequency
changes pulse
rate
Output
PWM Voltage
Output
Current
Output Frequency
Output Frequency
PWM signal pulses occur at the rate of the Carrier frequency or the switching speed of the
Inverter section transistors. For inverters this is adjustable between 1kHz and 15kHz. A 2kHz
carrier frequency has more audible noise than an 8kHz carrier frequency but the 8kHz produces
more losses (heat).
The resulting motor current is a three phase signal whose frequency is determined by the
positive and negative transitions of the PWM signal. If the PWM signal changes from positive
going to negative going 40 times each second, the output frequency to the motor is 40Hz. The
frequency of the output to the motor is generally adjustable from 10 to 400Hz.
FOD
Fundamentals of Drives 2-9
Audible Noise
Low carrier frequencies typically produce audible noise from the motor windings. Increasing the
carrier frequency will reduce the audible noise. The trade off is reduced output current from the
inverter due to increased power losses within the power devices themselves, increased motor
heating and faster application of the current into the winding.
Figure 2-17 Volts per Hertz Ratio
For a motor to be able to output the desired torque
at each frequency (speed) it is necessary to apply
the optimal motor voltage at each frequency. The
ratio of this Motor Voltage to Motor Frequency is
called “Volts per Hertz ratio” .
The ratio for a 460VAC inverter with a 60Hz motor
is 7.67 Volts/Hz. If the output frequency is 40Hz,
the output voltage is then 40x7.67=306VAC.
VAC
460
306
40
60
Hz
Dynamic Braking When a motor is stopped faster than it would coast to a stop, the motor becomes a generator.
It produces power which must be sent back to the power line (Regenerative Mode) or it must be
dissipated as heat. To dissipate the extra energy, a resistor called a dumping resistor or
“Dynamic Brake” resistor is used. During a stop, the DC bus voltage increases and is
automatically switched to the dynamic brake resistor for dissipation so the bus voltage remains
relatively constant. A dynamic brake is simply to dissipate heat during load deceleration, it does
not hold the load at zero speed.
A mechanical fail safe brake is required if “Holding Torque” is required at zero speed. The
mechanical brake must be wired directly to the power line, not through the control. If the brake
were connected to the control, because of varying voltage the control may not provide enough
power to release the mechanical holding brake.
Figure 2-18 Dynamic Brake and Mechanical Brake
Dynamic Brake Resistors
Mechanical
Brake Assembly
Line Regenerative Braking
The excess power produced during braking periods can be put back on the incoming AC. The excess
DC power on the bus must be converted to AC power and synchronized and be in phase with the
60Hz utility power. This requires an additional converter section and is more expensive.
2-10 Fundamentals of Drives
FOD
Output Ratings Motor torque is directly related to the current applied to the motor. Load conditions may require
greater or reduced torque (motor current) as conditions change. The average current required by
the load is within the “Continuous Operating Area” if the control. Peak torque available (for starting
and other changing load conditions) is generally 150% to 200% of the continuous output rating.
Figure 2-19 Inverter Output Rating Chart
Vector Control Like the inverter, a Vector control adjusts the speed of an AC induction motor. The vector adjusts
the frequency and current of the output power that is applied to the motor. An encoder feedback
device is used with the vector control to monitor motor speed. This creates a closed loop control
system that continuously calculates motor speed and adjusts the motor signal to increase or
decrease speed and current to match changing load demands. The vector control provides more
precise control than an inverter. Full motor torque is available independent of speed (including
zero speed).
Internally the control converts fixed voltage, fixed frequency AC line power to DC power that is
stored in capacitors. This is done by the Converter section of the control. The DC Bus voltage is
approximately 1.4 times the RMS AC line voltage (230VAC line voltage will produce 322 volt DC
bus voltage).
The DC bus voltage is then converted to a PWM (Pulse Width Modulation) 3 phase AC signal to
be applied to the motor. The way that the PWM signal is controlled adjusts the frequency and
voltage applied to the motor. The vector control uses a complex algorithm to separately control
motor excitation current called “flux current” and torque producing “load current”.
Figure 2-20 Vector Control
Inverter
Converter
DC Bus
L1
L2
L3
+
T1
Logic
Circuits
Logic
Circuits
T2
T3
Three Phase
Motor
Encoder
Control
Circuits
FOD
Fundamentals of Drives 2-11
The vectors’ microprocessor calculates magnetizing current and torque producing current values
and vectorially adds them and this current value is applied to the motor. This value of motor
current produces optimal speed/torque control of the load. The algorithm maximizes motor
torque while minimizing motor heating.
Figure 2-21 Torque Components
ÉÉÉÉ
ÉÉÉÉ
ÉÉÉÉ
ÉÉÉÉ
ÉÉÉÉ
ÉÉÉÉ
ÉÉÉÉ
Magnetizing Current
fA
90°
Rotor
fC
Stator
Torque Current
fB
The vector ratings are illustrated in Figure 2-22, compare it’s ratings with those of the inverter
(Figure 2-19). A vector control has more precise control of the motor and allows full motor torque
to be available at zero speed. This provides torque control (Vector) rather than speed control
(Inverter) and a wider range of operating speeds.
Figure 2-22 Vector Output Rating Chart
Encoderless Vector Control
The encoderless vector control is also called sensorless or open loop vector control. This control
uses an adaptive vector algorithm to calculate motor currents and provide torque control. The
control monitors motor current and voltage and calculates rotor and stator flux signals, calculates
motor speed based on these signals and controls the motor. In other words, the motor itself
becomes the speed sensor.
Figure 2-23 Encoderless Vector Control
Current Signals
Id
Voltage Signals
Vd
Vq
Iq
Adaptive Motor Model
Motor Speed
LM L1 Slip Rs
Inductance, Resistance, and Slip Signal
2-12 Fundamentals of Drives
FOD
Feedback Devices A feedback device attached to the motor provides feedback information to the vector control.
The device provides angular position and speed information to the control.
Encoder
The encoder feedback device is a photo optical device that produces pulses as the motor shaft
rotates. The encoder is constructed to produce two channels of pulses (A and B) and a once per
revolution marker pulse (C) channel. The control receives this data and can precisely calculate
motor shaft position, speed and direction of rotation.
Figure 2-24 Encoder Operation
1024 Pulses Per Revolution
A
Code Disk
B
C
B
A
C
0
Time
01 23
Three Light Sensors
Quadrature
Quadrature
0
1
2
3
Light Beams
Three Light Sources
Resolver
A
0
1
1
0
B
0
0
1
1
Pattern Repeats
1024 Times per Revolution
A resolver looks like a motor, it has a shaft, windings and electrical connections. More properly, it
is a revolving transformer. A signal from the rotor winding is induced into it’s stator winding. This
output signal varies according to the angle of the rotor windings relative to the armature
windings.
A resolver and associated electronics, provides information about the speed, direction and
position of the motor shaft that the resolver is connected to. Resolvers do this without parts that
wear out (like DC tachometer brushes). A resolver is more rugged and may be operated at
higher temperatures than an encoder.
Figure 2-25 Resolver Operation
R2
S2
R1
S4
S3
S1
SIN+
SIN−
COS+
COS−
REF+
REF− (Common)
FOD
Fundamentals of Drives 2-13
Table 2−5 Features of Feedback Devices
Feature
What it measures
Location
Output
Precision
Ruggedness
Cost
Tachometer
Speed
On motor shaft
Continuous voltage proportional to
speed (analog)
±1% set speed, no position information
Good. Brush wear, ripple, change in
output with temperature
Low. (But no position information)
Encoder
Position, can derive speed
On motor shaft or lead screw
Digital signal (On/Off)
Resolver
Position, can derive speed
On motor shaft
Sinusoidal (sine and cosine) signal
Depends on number of encoder lines
Depends on sample rate of
microprocessor (control)
Best. No internal electronic parts,
withstands greater temperatures
Highest. Requires additional electronics
to process complex sinusoidal signals.
Fair. Susceptible to shock, vibration, dirt
and dust
Mid Range. Digital output works well
with controls.
DC Motor Controls DC motor controls respond to feedback signals to generate proper DC voltage and current to
control the DC motor and regulate motor performance. DC Voltage controls motor speed, DC
Current controls motor torque. The actual motor signal from the control may be DC or a PWM
signal like the inverter.
DC SCR Control The DC SCR motor control is the least expensive control type for motor control applications.
The speed demand determines the conduction of the SCR (silicon controlled rectifier) devices
that convert the AC line power to DC motor power. The more speed required, the more
conduction is required from the SCR devices. The SCR devices automatically turn off when the
AC waveform crosses through zero volts. By controlling when each SCR turns on, the amount of
DC power produced is controlled.
The polarity of the DC voltage determines motor direction. The SCR’s can be arranged for half
wave or full wave rectification. Full wave rectification provides the most power to the motor.
Figure 2-26 DC SCR Conduction Operation
1A
VAC
1A
2A
VDC
1B
VAC
2B
ÉÉÉÉ
A
3A
VDC
VAC
1B
VDC
Natural
Commutation
2A
VAC
B
VDC
2B
3B
ÉÉÉÉÉ
É
Three Phase − Full Wave
Single Phase − Full Wave
PWM DC Control A PWM control are the most sophisticated DC motor control. Like the Inverter, incoming AC
line power is rectified to DC power that is stored in capacitors. This is done by the Converter
section of the control. The DC Bus voltage is then converted to a PWM (Pulse Width
Modulation) signal. The PWM signal is filtered and the DC voltage is applied to the motor.
Dynamic braking or Regenerative braking considerations also apply to DC controls.
Figure 2-27 PWM DC Control Operation
PWM Output
Filtered DC
To Motor
2-14 Fundamentals of Drives
FOD
DC Power Supply Types
NEMA (National Electrical Manufactures Association) was established to provide a common
standard for the electrical industry. NEMA defines typical DC power supplies in it’s specification
MG1−12.62.
Baldor DC rated motors are designed to operate from type K power supplies for the 90 and 180
volt motors. 240 and 500 volt motors are designed for type C power supplies. Many designs
also operate from type D power supplies.
Figure 2-28 NEMA Power Supply Types
Power Supply A
DC Gen.
+
−
Power Supply A
Designates a DC Generator or Battery.
Power Supply K
Designates a single phase full wave power supply.
Power Supply C
Designates a Three Phase, Full Wave
Power Supply.
Power Supply K
A1
Power Supply C
A1
A2
Power Supply D
A2
Power Supply E
Power Supply D
Designates a Three Phase, Half Wave
Power Supply.
Power Supply E
Designates a Three Phase, One Way
Power Supply.
A1
A2
A1
A2
A1
A2
Figure 2-29 DC SCR Rated Motor
FOD
Fundamentals of Drives 2-15
Feedback for DC Control DC Controls can use Armature, Tachometer or Encoder Feedback to measure motor
speed. For encoder feedback, see Figure 2-24.
Figure 2-30 Armature Feedback
Armature feedback senses the armature
current and this feedback signal is used
to correct the motor speed.
Most SCR DC Controls have a speed
regulation of 1% of base speed with
armature feedback.
Feedback
Error
Signal
10VDC
Summing
Junction
Voltage
Controller
0V
A tachometer is a motor that operates in
a reverse manner. The rotating shaft of
the tachometer is attached to the motor
shaft and rotates with it. As it rotates, a
voltage is induced in the stator of the
tachometer. The voltage is proportional
to to speed and direction of the motor.
Tachometers are rated in terms of rated
voltage, ripple voltage and linearity.
Tachometers are rated as volts per
thousand RPM (ie 50volts per
1000RPM).
Figure 2-31 Tachometer Feedback
Tach Magnet
Tach Armature
Brush Assembly
Terminology
Speed/Torque Regulation
Base Speed
The speed at which a motor delivers rated voltage (for DC motors) or rated frequency (for AC
motors).
Set Speed
The desired operating speed set by the operator at the control.
Speed Regulation The percentage of base speed change between full load and no load speed given as:
No Load Speed − Full Load Speed
% Regulation +
X 100
No Load Speed
2-16 Fundamentals of Drives
FOD
BALDOR ELECTRIC COMPANY
P.O. BOX 2400
Fort Smith, AR 72902−2400
(479) 646−4711
Fax (479) 648−5792
www.baldor.com
© Baldor Electric Company
FOD
Printed in USA
5/05
Fundamentals of Drives
Name:
Branch Address:
Phone:
email Address:
1.
We know in an AC motor more poles mean more Torque.
What is the trade−off?
- Smaller size
- Larger shaft
- Slower speed
- More Heat
2.
AC Motors are essentially constant speed motors.
- True - False
3.
What are the two most important concepts to know when controlling AC motors?
(Select 2 answers).
- To control the torque, you must control the current.
- The motor rotates.
- You must overvoltage the motor.
- To vary the speed, you must vary the power line frequency.
4.
What type of load has the characteristic where the amount of torque required is the same
regardless of speed?
- Constant Horsepower
- Constant Torque
- Variable Torque
- Variable Horsepower
Fundamentals of Drives Questions 1
5.
Limitations of part winding starters are_____. Select all that apply.
- Can produce more than full load torque at start
- Maintenance of electo−mechanical contacts
- You must have a special motor and starter
- The transition from initial start to full winding operation is instantaneous
6.
Full voltage starting of AC squirrel cage induction motors can result in starting currents
up to ______% of full load current.
7.
Solid State Soft Starts can only produce 2 starts per minute.
- True - False
8.
An inverter adjusts _____________ to control the speed of an AC motor.
9.
Multi−speed motors, designed for use without a control have one or two windings.
- True - False
10. The rate at which the PWM pulses are produced is called the carrier frequency.
- True - False
11. What two types of current are added in a vector control?
(Select 2 answers).
- Torque Current
- Over Current
- Flux Current
- DC Current
12. _____________________ is the industry standard optical feedback device used with a
vector motor and control?
13. An Encoderless vector control uses the control as a speed sensor.
- True - False
2 Fundamentals of Drives Questions
14. Encoderless Vector controls offer better performance then a closed−loop vector control.
- True - False
15. Which is not a type of feedback used with DC motors and controls.
- Armature
- Servo
- Encoder
- Tachometer
16. Base speed is the speed you set on the control.
- True - False
17. With DC drives, a change in voltage changes speed.
- True - False
18. A DC control performs which of the following?
(Select all that apply).
- Rectify incoming AC power to DC power
- Control the amount of current to the motor
- Control the pay scale
- Respond to feedback signals
19. When we say that a control is “Line Regenerative”, what do we mean?
20. What is a vector control?
21. How is a vector control different than an inverter?
Fundamentals of Drives Questions 3
Return your completed form to:
Baldor Electric Company
Marketing Dept.
P.O. Box 2400
Fort Smith, AR 72902−2400
Attn: Director of Education
4 Fundamentals of Drives Questions
Fundamentals of Servos
7/05
MIFOS
Fundamentals of Servos
Table of Contents
Servo Specifics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Servo Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Servo Motor Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PMDC Servo Motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Brushless Servo Motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Feedback Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tachometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Encoder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Resolver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hall Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pulse Width Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
MIFOS
3-2
3-3
3-5
3-5
3-6
3-7
3-7
3-7
3-8
3-8
3-9
Fundamentals of Servos 3-1
Section 1
General Information
Servo Specifics
A Servo is a system of devices used to control the position, direction and/or the speed of a load.
These devices are normally a Control, Motor and Feedback device. Within this guide, we will call
the control a Servo Control, the motor a Servo Motor and the feedback device can be a resolver
or an encoder. A Servo drive is a control and motor. Servo’s are used in applications where
there’s a lot of starting and stopping. Where motor direction is frequently reversed. The changes
in speed or position are rapid − almost instantaneous. In a potential servo application, it’s the
need for movement that will first attract your attention.
Open and Closed Loop Systems
An open loop system, the control provides voltage and current to the motor and hopes that the
motor is going in the correct direction and the correct speed. There is no feedback signal to
inform the control that the proper motion has taken place. This system is not self−correcting.
A closed loop system the control provides voltage and current to the motor so it will go in in the
correct direction and the correct speed. A feedback signal from the motor or load are returned to
the control. The feedback signals are compared to the desired setpoints and the signal to the
motor is changed to correct any system errors.
The feedback signal (return signal) provides the means to monitor the motor or process.
Feedback allows a closed loop system to perform more complex tasks with greater precision
than an open loop system.
Servo applications require tight control over position and velocity, or both. Tight control that
customers are not able to get from other drive types (Inverter, Vector or DC controls). Those
loads are typical large horsepower motors and heavy loads that create large inertia that cannot
be quickly accelerated, stopped or reversed.
Some examples of these applications are:
S
Packaging, where the load must be positioned for form/fill/seal and wrapping
applications.
S
Textiles, where a plotter−cutter must be accurately positioned according to the pattern
and cutting table.
S
Robotics, where the robot arm must be positioned in X−Y−Z coordinates.
Each X, Y and Z motion axis requires a separate Servo Control and Motor.
Figure 3-1 Open Loop and Closed Loop System
Closed Loop System
Open Loop System
Feedback
signal
Motor
Power
Input
Power
Servo
Control
3-2 Fundamentals of Servos
Motor
Power
Input
Power
Servo
Control
MIFOS
Tight Velocity control
Servos provide tight control over speed, ± 0.1 % of set speed, for example. A servo responds to
the feedback signal and change speeds within a matter of milliseconds. Velocity control is
important in applications such as:
S
Semiconductor industry, where silicon wafers are “spun out” to evenly distribute the
coating.
S
Laboratory applications like centrifuges and blood analyzers where products must be
spun at exact speeds for the desired separations.
Both Velocity and Position control
The majority of applications require some degree of control over both velocity and position.
S
Automatic fastening application such as tightening caps on tubes of toothpaste or
tightening nuts on automobiles, are primarily concerned with velocity and torque.
But there is a positioning component to these operations, as well.
S
Your customers may want their positioning applications performed within a controlled
time frame for maximum productivity. So the packaging, textile, and medical
applications previously discussed could also have a velocity component.
Servo Terminology
Load
Servo Motor
DC Servo Motor
Brushless Motor
Servo Control
Feedback
Power Supply
MIFOS
The work being moved by the servo drive.
The part of the servo drive that provides the motion. Servo motors are built to have the low
inertia and fast response. There are brush−type and brushless servo motors.
A DC PM motor has a stationary stator field that is produced by permanent magnets. A
second field, the rotor field, is set up by passing current through stationary brushes and a
commutator on the rotating rotor assembly. The rotor field rotates in an effort to align itself
with the stator field, but at the appropriate time (due to the brushes and commutator) the
rotor field is switched. The feedback device may be either a tachometer, encoder, or
resolver.
A brushless servo motor could be DC or AC but has no brushes for commutation. The
feedback device effectively determines whether it is considered DC or AC, since this dictates
the control scheme. The feedback device may be either Hall sensors, encoders, or resolver.
The part of the servo drive that monitors or controls the amount of power applied to the
motor. This will control how fast the motor rotates. For example: with more voltage supplied,
the motor will rotate fast; with less voltage supplied, the motor will rotate slowly.
A servo control commands the motor to move to a position at a specific speed. The control
continuously monitors the feedback signal. If the feedback signal indicates the motor position
or speed is not correct, the control changes the signals to the motor to correct the motor’s
speed or position.
A feedback device (located within the motor) generates a signal that’s sent back to the
control. Tachometers, Hall effect, encoders, and resolvers are examples of feedback devices
for servo systems.
The power supply converts AC power to DC power.
Fundamentals of Servos 3-3
The servo drive controls speed or velocity only. To command the machine to go to a specific
position, a means of monitoring position must be added. The positioner monitors the machine
position and is typically called a “Motion Controller” which is a programmable device.
Figure 3-2 Control System Components
Servo Axis
Programmable Motion Controller represents the “brains” of this package. It monitors position.
The user writes a program to accomplish a task i.e. run at a certain speed to a specific position.
This program is stored in the memory of the Programmable Motion Controller. The user interface
can be a PC (that is used to develop the software program) or an HMI panel to display results.
The motion controller monitors the position and velocity of each servo axis. To control the
machine position, it sends corrections to each axis so the load is moved to the correct position at
the correct speed. Acceleration and deceleration of each load must also be precisely controlled.
A Network (CAN or Ethernet etc.) is normally used to allow system elements to provide additional
inputs and outputs (I/O) and to provide communication or system monitoring.
3-4 Fundamentals of Servos
MIFOS
Servo Motor Benefits
An AC motor can be connected across the line to run at a fixed speed to operate a compressor, a
grinder or other machine. You expect and you get a certain performance. Likewise, take any
PMDC motor and connect it to a servo control and you get a certain performance, but not the
same performance you would get with a servo motor. What makes a servo motor ideal for many
applications? A servo motor must be used with a control and feedback device. The feedback
device provides position information to the control so that the load can be accurately controlled.
The following are benefits of using servos:
S
Smaller size − for a given amount of output torque, a servomotor has a smaller rotor
diameter, thus a smaller housing diameter. This makes it easy to fit servomotors into
tight confining locations. But the prime reason for smaller size is for lower rotor inertia.
S
Weighs less − a servomotor is smaller, thus it weighs less. This is important in
applications that move a load that includes the servomotor − the smaller the
servomotor, the smaller, less power required to move it.
S
More performance −servomotor have a higher speed capability and a higher peak or
acceleration capability, thus offering wider performance range. This improves the
machine’s performance empowering it to produce more parts.
S
Faster − lower rotor inertia means faster acceleration, faster positioning, thus improving
a machine’s throughput. By making the machine faster, it can increase the number of
parts produced, and increasing production helps to reduce part costs.
S
Highly accurate − accurate parts means better quality and less part rejects. This saves
money, improving the bottom line of the facility.
S
High efficient − servomotors are by design to be highly efficient, to have a high efficient
power conversion factor, making the best use of power/energy.
S
Controllability − motors used in closed loop applications offer very high controllability,
reducing reject rate, improving the quality of the parts being produced.
S
Rapid positioning − servos have the capability to accelerate faster, run at higher
speeds, and get into position faster. This improves the machines productivity.
S
Uses power only when ”on” − servos use power only when commanded to move to a
position. Then power to the servo motor is usually turned off.
PMDC Servo Motor (Permanent Magnet DC Motor)
This is a motor designed for incrementing (start−stop) applications. This motor has a stator field
that is produced by permanent magnets. This is a stationary field (as opposed to the AC stator
field which is rotating). The second field, the rotor field, is set up by passing current through a
commutator and into the rotor assembly. The rotor field rotates in an effort to align itself with the
stator field, but at the appropriate time (due to the commutator) the rotor field is switched. In this
method then, the rotor field never catches up to the stator field. Rotational speed (i.e. how fast
the rotor turns) is dependent on the strength of the rotor field. In other words, the more voltage
on the motor, the faster the rotor will turn.
Since the stator field is generated by permanent magnets, no power is used for field generation.
The magnets provide constant field flux at all speeds. Therefore, linear speed torque curves
result.
This motor type provides relatively high starting, or acceleration torque, is linear and predictable,
and has a smaller frame and lighter weight compared to other motor types and provides rapid
positioning.
Figure 3-3 DC Servo Motor Components
Termination
Tachometer
Commutator
Encoder
Tach Brushes
Tach
Magnets
Motor
Brushes
MIFOS
Rotor
Fundamentals of Servos 3-5
Brushless Servo Motors
Servo drives are closed loop systems. The feedback device is normally built into the servo motor
as shown in Figure 3-4.
Figure 3-4 Brushless AC Servo Motor Components
Terminal Box
Permanent Magnet
Feedback
Rotor
Bearing
Mounting Face
Feedback Stator
Stator Laminations
Windings
A brushless AC servo motor includes a resolver or encoder feedback as standard.
Power is applied through the terminal box to the stator windings. As power flows through the
stator, it sets up a magnetic field which interacts with the permanent magnets on the rotor. The
interaction of these two result in rotation of the motor shaft.
The rotor races to catch up and align with the magnetic poles of the stator windings. However the
resolver informs the control about the location of the rotor assembly. Just before the rotor
catches up, the control will switch power to another winding. Now the rotor must race to catch up
with another winding. This is called commutation. By electronic commutation, power is switched
from winding to winding, and shaft rotation continues. The resolver must be aligned (similar to
timing on your car) to the rotor’s back−emf.
Brushless servos are available as either DC brushless or AC brushless. The feedback device
effectively determines whether it is considered DC or AC, since this dictates the control scheme.
The feedback device may be either Hall sensors, encoders, or resolver.
With Hall sensor feedback, the three−phase brushless motor is powered by energizing two of the
three motor windings at a time. There are six different commutation sections for one mechanical
revolution, and with in each commutation section, a DC level of power is applied. The amount of DC
applied is directly proportional to the desired operating motor speed. Thus the term “DC brushless”.
With resolver or encoder feedback, a sinusoidal waveform is applied on the motor windings.
Thus the term “AC brushless”. The advantage of this technology is that, for the same torque
(compared to ‘DC brushless), the AC brushless will require less current. Therefore a smaller
control may often be used in the application. This becomes possible since the motor has a three
phase sinusoidal winding being powered by a three phase sinusoidal current waveform.
Advantages of brushless technology include higher speed capability, higher torques in a smaller
package, much lower inertia (thus faster acceleration capability), and of course, long reliable
maintenance free life in the application. There are different motor configurations. Some may be
motor and resolver, others may be motor and encoder or motor and Hall Effect Device. Brakes
may also be incorporated internally within the motor housing.
3-6 Fundamentals of Servos
MIFOS
Feedback Devices A feedback device attached to the motor provides feedback information to the servo control. The
device provides speed and/or angular position information to the control.
Tachometer
A tachometer is shown in Figure 3-3. Tachometers are used to indicate speed. Their output is a
DC voltage proportional to speed. Tachometers resemble a miniature motor, the faster the shaft
is turned, the larger the DC voltage that is developed. The output voltage will be positive or
negative depending on direction of rotation.
Voltage Constant − Also may be referred to as voltage gradient, or sensitivity. This represents
the output voltage generated from a tachometer when operated at 1000 RPM, i.e. V/KRPM.
Sometimes converted, and expressed in volts per radian per second, i.e. V/rad/sec.
Ripple − Also may be called voltage ripple, or tachometer ripple. Since tachs are not really an
ideal device, and design and manufacturing tolerances enter into the product, we should be
concerned with deviations from the norm. In an analog tach, when the shaft is rotated, a DC
signal is produced, as well as an amount of an AC signal, this is noise and can be measured as
RMS or Peak to Peak ripple expressed as a percent of the average DC level.
Linearity − The ideal tachometer would have a perfect straight line for voltage vs speed curve.
Linearity is a measure of how far away from perfect this is. Linearity is measured by driving the
tach at various speeds, while measuring both speed and output voltage. The reversibility error is
the difference between CW and CCW voltage constants expressed as a percentage of the
nominal.
Encoder
The encoder feedback device is a photo optical device that produces pulses as the motor shaft
rotates. The encoder is constructed to produce two channels of pulses (A and B) and a once per
revolution marker pulse (C) channel. The control receives this data and can calculate motor
shaft position, speed and direction of rotation. Some encoders have Hall signals to provide
commutation for a brushless servo motor.
Figure 3-5 Encoder Operation
1024 Pulses Per Revolution
A
Code Disk
B
C
B
A
C
0
Time
01 23
Three Light Sensors
Light Beams
Three Light Sources
MIFOS
Quadrature
0
1
2
3
A
0
1
1
0
B
0
0
1
1
Pattern Repeats
1024 Times per Revolution
Fundamentals of Servos 3-7
Resolver
A resolver looks like a revolving transformer. An AC reference voltage is applied onto windings
R1 and R2. This AC reference voltage is then coupled to the secondary windings by transformer
action (S1 and S3 and S2 and S4).
The amplitude of the signal picked up by the secondary winding depends on the angle between
the primary and secondary. In other words, this output signal varies according to the angle of the
rotor windings relative to the armature windings.
A resolver and associated electronics, provides information about the speed, direction and
position of the motor shaft that the resolver is connected to. Resolvers do this without parts that
wear out (like DC tachometer brushes). A resolver is more rugged and may be operated at
higher temperatures than an encoder.
Figure 3-6 Resolver Operation
R2
S2
R1
S4
S3
SIN+
SIN−
COS+
S1
COS−
REF+
REF− (Common)
Hall Sensors
Hall Sensors provide commutation information to the control. For electronic commutation to work,
the Halls must be “timed” properly to the motor’s back−emf. The output of the Hall sensor is a
DC level. Nominally +12 VDC is supplied. As a permanent magnet is passed by the Hall sensor,
and the magnetic field changes the Hall switches from “Off” to “On”.
As an example, the Hall would be “On” for 90 mechanical degrees, then “Off” for 90 degrees.
This repeats for the remainder of the 360 degrees rotation.
Figure 3-7 Hall Sensor Operation
Hall Sensors
Magnet
PC Board
Mounting Ring
Mechanical Degrees
Electrical Degrees
0
0
30
60
60
120
90 120
180 240
150
300
180 210
360 420
240
480
270
540
300
600
330
660
360
720
Hall Leads
Clamp
Hall
Signals
Table 2−1 Comparison of Feedback Devices
Feature
What it measures
Output
Ruggedness
Hall Sensor
Speed
Digital Signal (On/Off)
Good. No parts to wear.
Cost
Lowest
3-8 Fundamentals of Servos
Encoder
Position, can derive speed
Digital signal (On/Off)
Fair. Susceptible to shock, vibration, dirt
and dust
Medium. Digital output works well with
controls.
Resolver
Position, can derive speed
Sinusoidal (sine and cosine) signal
Best. No internal electronic parts,
withstands greater temperatures
Low. Requires additional electronics to
process sinusoidal signals.
MIFOS
Pulse Width Modulation
A Servo control adjusts the speed and direction of a servo motor. It is a closed loop control and
continuously monitors motor speed, direction and position. Internally the control converts fixed
voltage, fixed frequency AC line power to DC power that is stored in capacitors. This is done by
the Converter section of the control. The DC Bus voltage is approximately 1.4 times the RMS
AC line voltage (230VAC line voltage will produce 322 volt DC bus voltage).
The DC bus voltage is then converted to a PWM (Pulse Width Modulation) 3 phase AC signal to
be applied to the motor. The way that the PWM signal is controlled adjusts the frequency and
voltage applied to the motor.
Figure 3-8 PWM Control
OUTPUT
Converter
DC Bus
L1
L2
L3
+
T1
Logic
Circuits
Logic
Circuits
T2
T3
Three Phase Motor
Control
Circuits
PWM signal has the a series of positive and negative pulses for it’s output signal. The width of
each pulse changes with the amplitude of the voltage it represents. A very narrow pulse
represents a lower voltage than a wider pulse. Figure 3-9 illustrates the PWM signal and the
corresponding AC motor voltage waveforms. All pulses are the same amplitude but their width
changes to control motor voltage. The frequency of the motor voltage is controlled by the
frequency of the positive and negative transitions of the PWM signal. The rate at which the PWM
pulses are produced is called the carrier frequency.
SCR
Capacitor
Transistor
Figure 3-9 PWM Signal
Carrier Frequency
changes pulse rate
Motor
Narrow pulse widths
produce less power
to the motor
Carrier Frequency
changes pulse
rate
Output
PWM Voltage
Output
Current
Output Frequency
Output Frequency
MIFOS
Fundamentals of Servos 3-9
If the PWM signal changes from positive going to negative going 40 times each second, the
output frequency to the motor is 40Hz. The frequency of the output to the motor is generally
adjustable from 0 to 500Hz.
Figure 3-10 PWM Stepper Operation
PWM Output
Motor Current
3-10 Fundamentals of Servos
MIFOS
BALDOR ELECTRIC COMPANY
P.O. BOX 2400
Fort Smith, AR 72902−2400
(479) 646−4711
Fax (479) 648−5792
www.baldor.com
© Baldor Electric Company
FOS
Printed in USA
7/05
Fundamentals of Servos
Name:
Branch Address:
Phone:
email Address:
1.
What type of application would servos be used in?
- Positioning
- Constant Speed
- Constant horsepower
- Constant torque
2.
What is a closed loop system?
- A system with no feedback device
- A system which shares no information
- A system that uses a local area network
- A system with a feedback device to monitor the process
3.
What is electronic commutation?
- Mechanical switching resulting from the use of a copper commutator
- Switching of power from winding to winding through electronic switches
- Aligning the resolver to the motor’s EMF
4.
Linear speed − torque characteristics are the result of what?
- Superior engineering design
- High power permanent magnets
- Use of feedback devices
5.
A resolver provides sine and cosine output waveforms.
- True - False
Fundamentals of Servos Questions 1
6.
An encoder provides a train of output pulses; counting these pulses can provide position
information.
- True - False
7.
The main advantage of a servo system over other AC motor controls is (select all that apply)
- Precise control of motor speed
- Precise control of motor position
- Precise control of motor torque
- Precise control of line frequency
8.
A servo system always consists of a control, motor and feedback device.
- True - False
9.
The rate at which the PWM pulses are produced is called the carrier frequency.
- True - False
10. A tachometer is a good feedback device because it provides speed and direction feedback
and can be used for commutation for brushless motors.
- True - False
11. A tachometer is like a DC generator. The faster it spins, the more DC voltage it produces.
- True - False
12. The encoder can provide commutation for a brushless servo motor if the encoder has Hall
signals.
- True - False
13. Name 3 benefits of servos.
1.
2.
3.
2 Fundamentals of Servos Questions
14. Brushless servo motors are available as AC or DC brushless types.
- True - False
15. A resolver is the least expensive feedback type because it is an analog device.
- True - False
16. Hall sensors rely on a ________________ field to operate the Hall switches.
17. What part of a control system is programmed by the user?
- Operator Interface
- Network Input/Output
- Motion Controller
- Servo Control
18. What is the torque of a 3hp, 1750RPM motor? (Hint: Remember Fundamentals of Motors)
- 36 lb−in
- 72 lb−in
- 108 lb−in
- 144 lb−in
19. What applications may require a velocity control? (name three)
1.
2.
3.
20. A servo system is an open loop system.
- True - False
21. When selecting a motor for an application, what factors should you consider to decide
whether or not to use a servo system?
Fundamentals of Servos Questions 3
Return your completed form to:
Baldor Electric Company
Marketing Dept.
P.O. Box 2400
Fort Smith, AR 72902−2400
Attn: Director of Education
4 Fundamentals of Servos Questions
Fundamentals of
Speed Reduction
5/05
FOG
Fundamentals of Speed Reduction
Table of Contents
Speed Reducer Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Gear Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Gear Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Torque . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Horsepower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I/O Shafts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Comparison of Gear Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction to Speed Reducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Speed Reducer Mounting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Parallel Shaft and Right Angle Reducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Speed Reducer Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Input Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Gearing Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Helical Speed Reducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Worm Gear Speed Reducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Worm−Helical Speed Reducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Planetary Speed Reducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Speed Reducer Ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Speed Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Load Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Thermal Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Torque Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Power Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Service Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
OHL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
FOG
1-2
1-2
1-3
1-3
1-4
1-4
1-4
1-6
1-7
1-7
1-8
1-9
1-10
1-11
1-11
1-11
1-11
1-11
1-12
1-13
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1-13
1-13
1-13
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1-14
Fundamentals of Speed Reduction 1-1
Section 1
General Information
Gears and gear drives have been used for over 3000 years and are among the earliest forms of
power transmission. They are used in pairs to transmit power between two or more shafts
without slipping. Gears can be used to change both speed and direction of rotation. They are
used in a wide verity of applications from the tiny gears used in watches & toys to giant 100−foot
diameter gears used in radar antennas.
There are two classifications of Gears: Open or enclosed gearing.
Open gearing is supplied loose (unassembled), the machine builder incorporate a set or
multiple sets into the design.
Enclosed gearing is assembled and supplied as a gearbox, speed reducer or a gearmotor.
Introduction
Figure 1-1 Speed Reducers
Enclosed Gearing
Enclosed Gearing is assembled and
supplied as a gearbox, speed reducer
or a gearmotor. These devices come
in a variety of sizes, shapes and drive
configurations.
Figure 1-2 Gear Types
Gears come in many sizes and shapes. They
all share common characteristics, however,
they also have significant differences to allow
their use in a wide range of applications.
Gears are grouped into five basic design
categories: Spur, Helical, Bevel, Hypoid
and Worm. They are also grouped according
to the orientation of the shaft they are
mounted, either parallel or at right angle.
Generally, shaft orientation, efficiency and
speed determine which type should be used.
1-2 Fundamentals of Speed Reduction
FOG
Gear Basics
Gear Teeth Mesh
20
10
Gear Ratio
10 9
Various gear types are shown in Figure 1-2. Consider the first type, the spur gear. This is
perhaps the most widely used because it is simple and inexpensive. In Figure 1-3 we see the
basic characteristics of operation or application.
Figure 1-3 Gear Ratio
The teeth are designed to mesh together, or to be in contact with each other. For the teeth to
perfectly mesh, they must be the same size and have the same geometric shape. If the gears
have the same number of teeth, they are the same diameter.
Number of teeth of a gear determines the speed increase or decrease of the driven gear. If a
gear has fewer teeth it must then be smaller in diameter. If a gear has a greater number of teeth
it must be larger in diameter.
The drive gear and the driven gear have a relationship. This relationship is called a ratio which is
the ratio of the number of teeth of each gear. A drive gear with 10 teeth makes one revolution
and the driven gear with 20 teeth only makes one half revolution in the opposite direction. The
diameter of each gear determines the gear ratio of the drive gear to the driven gear.
Input RPM
Ratio +
+ 2 + 2:1
1
Output RPM
Drive Gear
If the Drive gear has 10 teeth and the driven gear has 20 teeth, the gear ratio
Rotates 8
8
is 2:1. This means the speed of the driven gear half the speed of the drive (input)
Teeth
7
gear. If the larger gear becomes the drive gear, the ratio is then 1:2 so the
6
5
smaller driven gear rotates twice as fast as the larger drive gear.
4
Reversing drive power is not possible with all gearing types.
3
Driven Gear
Rotates 8
Teeth
2
1
12
3
4
5
6
Rotation direction is opposite in this example. If the drive gear rotates
Clockwise, the driven gear rotates counterclockwise. The direction of the driven
gear can be reversed by adding an idler gear that is in contact with both gears.
7
8
9
10
Drive Gear
Drive Gear
Idler Gear
Driven Gear
Opposite Rotation
Gear Ratio
FOG
Driven Gear
Same Rotation
When two gears mesh together one is the drive gear. The drive gear is driven by a motor, a
gasoline engine or even a simple hand crank. The power from the drive source is transferred to
the driven gear. There are many forces acting upon each gear and bearings etc. so the power
transfer is not 100% efficient. Heat is a normal by product of the work being done by the energy
transfer. The amount of power that a gear system can handle is measured in terms of
horsepower and torque.
Fundamentals of Speed Reduction 1-3
Torque is the rotational or twisting force applied to the load as shown in Figure 1-4. Although we
think of horsepower when selecting a motor, the load actually “sees” “Torque” supplied by the
motor. Standard units of measure are:
lb−in (inch pounds), lb−ft (pound feet), oz−in (ounce inches) and Nm (newton meters).
Torque
Torque is a measured force at a specified distance. Torque = Force X Distance
Figure 1-4 Torque
Horsepower
Horsepower is a measure of the rate at which a motor or drive can produce work.
A 1hp motor can produce 33,000 lb−ft of work in one minute of time. This is equal to lifting one
ton (2,000 lbs.) 16.5 feet, or 1,000 lbs., 33 feet in one minute.
Figure 1-5 Horsepower Illustration
16.3’
2000
33’
1000
Typically, a gear has a shaft mechanical attached to it. gear configurations have their shafts
parallel with each other or at 90° to each other. Figure 1-6 shows these relationships.
Figure 1-6 Input/Output Shaft Relationships
Parallel shafts
Right Angle shafts
Some examples are:
Some examples are:
Spur Gears
Bevel Gears
Helical Gears
Hypoid Gears
Planetary Gears
Worm Gears
I/O Shafts
Spur Gearing
The spur gear set is the simplest to design
Gear
and manufacture. The spur gear and
Pinion
pinion both have teeth cut straight across
the width of the gear face. The gear set
transmits rotary motion and torque from
one parallel shaft to another. It is widely
used where rotational velocities and
torques are relatively low, and cost is an
important consideration.
Normally, the gear and pinion (smallest gear is always considered the pinion) rotate in opposite
directions. If the parallel shafts must rotate in the same direction an idler gear is used between
gear and pinion. The rack and pinion is a variation of the spur gear in which the gear (rack) is flat
rather than round. The rack and its round pinion convert rotation to linear motion or vice versa.
1-4 Fundamentals of Speed Reduction
FOG
Helical Gearing
Straight Helical Herringbone
Helical gear teeth are cut at helixes (teeth are oriented at an
angle) rather than straight lines across the gear face. This
creates smoother and quieter operation. Meshing teeth of two
gears come into gradual contact. They are able to carry more
load than spur gears. Helical gears operate at higher speeds as
compared with spur gearing. Unlike spur gears, which produce
only radial loads, helical gears also produce thrust load.
The greater the helix angle, the greater the thrust load. Bearings that are able to hand thrust must
be used. Double helical or herringbone design with opposing helix angles cancel thrust loads.
Worm Gearing
Worm gears look different and operate differently than other gear
types. These gears are called “a worm and a wheel”, although terms
like worm−gear and gear are often used to describe the two
components. Worm gear sets work similar to a screw. When you turn
a screw into a tapped hole, the screw moves up or down depending
on the direction the screw is turned. Worm gears use this principal.
The worm pushes the wheel into rotation. Worm gears use a sliding action. Due to the sliding
action, worm gear sets are not as efficient as spur, helical or bevel gear sets. The efficiency
range is typically 40% − 90%. This means more power is needed to do the same job as a more
efficient spur, helical or bevel gear set. However, one revolution of the input shaft only moves the
driven gear one tooth. So if the driven gear has 40 teeth, a 40:1 ratio results.
Straight Bevel Gearing
Bevel gears look different and operate differently than other gear
types. They transmit power between shafts that intersect at 90
degrees, but are sometimes used on shafts at other angles. The
teeth are cut into surfaces that are angular or conical. There are four
basic types, straight, zerol, spiral and skew tooth. The teeth are
tapered with the widest part on the outer edge.
Since the teeth are straight, as the two gears rotate together their teeth are in complete contact
with each other. This means a straight bevel gear is generally noisy and vibrates as compared to
some gear types.
Spiral Bevel Gearing (& Hypoid)
Spiral bevel gears have teeth that are spirals. The spiral angle of the teeth provides the same
benefit as helical gears. The teeth do not come in complete contact at one time and more teeth
contact each other at the same time. They also produce more thrust load than straight bevel
gears. Spiral bevel gears must be positioned accurately or they will not work properly and
produce noise and premature gear wear will result. Hypoid gears are similar to Spiral Bevel
Gears except that the axes of the shafts do not intersect (allowing both shafts to be supported at
both ends).
Zerol Bevel Gearing
Zerol bevel gears are similar to spiral bevel gears. The difference is the zerol bevel gear angle is
set to zero degrees. They operate similar to straight bevel gears. However they are less noisy
since their teeth do not slap together. They do not have as much load capacity as spiral bevel
gears.
Skew Tooth Bevel Gearing
Skew tooth bevel gears are typically used when large gears are required. Generally, they are 30
inch in diameter or larger. They have most of the same characteristics as spiral bevel gears.
Spiral bevel gears will operate more smoothly than a similar skew tooth bevel gear.
FOG
Fundamentals of Speed Reduction 1-5
Comparison of Gear Types
Gear Type
Spur Gearing
Helical Gearing
Bevel Gearing
Spiral Gearing
Worm Gearing
Advantages
Inexpensive to manufacture
Efficient
Does not produce thrust loads
Less noise and vibration (than spur gear)
Greater torque capacity than spur gear
Can be used on right angle shafts
See Worm Gearing
See Worm Gearing
Quiet operation
Handles shock load extremely well
High ratios available in a single reduction
Inexpensive to manufacture
1-6 Fundamentals of Speed Reduction
Disadvantages
Noisy, produces vibration
Can only be mounted on parallel shafts
More costly than spur gear
Does produce thrust loads
See Worm Gearing
See Worm Gearing
Produces heat
Low efficiency
FOG
Introduction to Speed Reducers
A speed reducer converts the rotational speed from a power source (electric motor) to a useable
torque and RPM for a load. A speed reducer is an enclosed gearing box that uses gear types
that were described in the beginning of this document along with an exclosure, bearings, seals
and lubricants to form a complete assembly. The primary tasks of a Speed Reducer is to:
S
Increase or Reduce Speed.
S
Increase or Reduce Torque.
S
Change Rotation Direction.
Figure 1-7 Inside Look at a Speed Reducer Housing
A speed reducer is an arrangement of
gears in an enclosed housing. The
reducer housing also contains shafts,
bearings, lubrication and other small
parts. The gears are mounted on shafts
that are support by bearings. Reducers
come in a wide range of shapes and
sizes to accommodate a board range
applications. Speed reducers are called
by names such as reducer, gearbox,
gear reducer, gear drive or enclosed
gear drive.
Worm Gearing shown
The name speed reducer comes from the most common use of the gearbox, to reduce the speed
at the input shaft to a usable speed at the output shaft. Another use is to change the torque. A
speed reducer will increase the input torque if the speed is reduced, it will decrease the torque if
the speed is increased. Another common use is to change the rotation direction of the shafts.
Clockwise/Counter−Clockwise or change direction to an angle as in a right angle gearbox.
A speed reducer can also be used to change the direction of rotation. Odd number of stages
(spur and helical sets) such as 1, 3 & 5 ... the output rotation is opposite input rotation. Even
stages 2, 4, 6 ... input and output rotation are equal. Speed reducers using worm, bevel or hypoid
gears other factors may determine if the input and output shafts rotate in the same direction.
Speed Reducer Mounting
Speed reducers have advantages over other means of reducing speed, one advantage is variety
of mounting. A speed reducer is mounted in one of three ways, foot mounted, flange mounted or
shaft mounted. Mounted means how the gearbox is attached. The speed reducer must be
attached so it does not move while it is transmitting torque.
Figure 1-8 Foot Mounted
Foot−mounted or base−mounted reducers have either a hole that a bolt will pass through
or have tapped holes where the reducer is mounted to the machine and the bolt passes
through the frame into the reducer. This is the most common type of reducer.
FOG
Fundamentals of Speed Reduction 1-7
Figure 1-9 Flange Mounted
Flange−Mounted or face−mounted reducers as the name implies, have a flange on the
output of the reducer. Most often, this type of reducer will support the weight of the reducer
and the motor and is usually bolted directly to the application. The mounting holes are
usually centered around the output shaft. The flanges purpose is to support the reducer
and hold it in place. A flange also prevents the gearbox from turning when the motor and
reducer are energized.
Figure 1-10 Shaft Mounted or Hollow−Bore
Shaft−mounted or hollow−bore reducers are mounted directly to the driven shaft.
The driven shaft passes through the output shaft of the reducer that is hollow. This
type of reducer is ideal as it eliminates the need for pulleys, belts, chains and
sprockets. Safety guards are also eliminated with type of reducer and take up the
lease amount of space. A possible disadvantage to using a shaft−mounted reducer
is that it may become difficult to remove the reducer from the application. The two
shaft surfaces become stuck through a process know as fretting corrosion. This
is cause by small movements between the two surfaces.
The shafts become bonded to each other. The more movement there is between
the shafts the worse the condition becomes. Applications that cycle on and off and
have shock loads are more susceptible to fretting corrosion.
With shaft−mounted reducers a torque arm or reaction arm must be used. Since
the speed reducer and motor are directly connected to the driven shaft, there is
nothing to prevent the speed reducer and motor from rotating once they are
energized.
Torque Arm
The most import factor about the mounting position is lubrication. The level needs to be high
enough to allow all components to get sufficiently lubricated. Some reducers have vent plugs or
breathers. Vents allow the pressure inside and outside the reducer to equalize. If too much
pressure builds inside the reducer, the seals can fail prematurely. This will allow oil to leak from
the reducer. The vent plug is usually mounted in the highest position as determined by the
mounting position.
Mounting position also affects the speed reducer’s efficiency. Some mounting positions require
the reducer to almost complete filled with oil. As the gears turn they must plow through the oil.
The more the gears are submerged in oil, the harder they have to work. This type of lost is
known as churning losses. Since the gears are churning the oil creating heat in the speed
reducer, energy is wasted.
Oil seal location is also another consideration concerning mounting position. In vertical
application with the motor below the reducer, the weight of the oil presses down on the seal,
which will increase the seal wear. Whenever possible, it is most advantageous to select a speed
reducer mounting position that requires the least amount of lubrication and one that keeps the
seals above the reducer’s oil level.
1-8 Fundamentals of Speed Reduction
FOG
Speed Reducer Nomenclature
AGMA
The American Gear Manufacturers Association is composed of member companies who
manufacture speed reducers, enclosed gear drives, open gearing, and gear type shaft couplings,
and an equally large number of technical members (domestic and foreign) who use these
products or supply tools, machines, or other products to the gear industry. Together, they
establish standards to help standardize the design and application of gear products.
Axial Movement
Endwise movement of input or output shafts, sometimes called endplay, is usually expressed in
thousandths of an inch (1/1000” or 0.001”).
Back−Driving
This is the converse of self−locking. It is difficult to predict the back−driving capability of a speed
reducer. Worm gear reducers are not intended to be used as speed increasers.
Backlash
Rotational movement of the output shaft when holding the input shaft stationary and rotating the
output shaft alternately clockwise and counter−clockwise. Backlash may be expressed in
thousandths of an inch (1/1000” or 0.001”) measured at a specific radius at the output shaft.
Center Distance
On a single reduction reducer, this is the distance between the center lines of the input and
output shafts. Shaft center lines may be parallel or at right angles to one another. The center
distance of multiple stage reducers usually refers to the lowest speed stage (last reduction).
Greater center distances have greater torque capabilities.
Efficiency
The amount of output power of the reducer as compared to the amount of input power. It is
usually stated as a percentage.
Fretting Corrosion
Input Horsepower The amount of power applied to the input shaft of a reducer by the prime mover. It is often used
as a selection basis for power transmission components, and it appears in the rating tables of
drive manufacturers published data. Input horsepower ratings represent the maximum amount of
power that the reducer can safely handle.
Output Horsepower The amount of power available at the output shaft of a reducer. Due to losses caused by
inefficiency, output horsepower is always less than input horsepower.
Mechanical Rating The maximum power or torque that a speed reducer can transmit based on the strength and
durability of its components. Reducers typically have a safety margin of two to three times their
mechanical rating. This is so the reducer can withstand momentary overloads during start−up or
other brief overload conditions.
Mounting Position The relationship of the input and output shafts relative to the floor line.
Overhung Load
A force applied at right angles to the input or output shaft of the reducer, beyond its outermost
bearing. The overhung load creates loads that the bearings must be able to support without
damage.
Ratio
The ratio of a drive is based on the relationship of its input shaft speed to its output shaft speed.
It is commonly expressed as a proportion.
Self−Locking
Baldor worm reducers, under no conditions, should be considered to hold a load when at rest.
Service Factors
The service factor is a multiplier applied to the known load, which redefines the load in terms of
operating conditions. It can also reduce ratings, thus redefining the rating in accordance with
drive conditions. The service factor is usually applied to the speed reducer, but it can also be
applied to the nameplate rating of the prime mover.
Thermal Rating
The maximum power or torques that a speed reducer can transmit continuously, based on its
ability to dissipate heat generated by friction, is its thermal rating.
Thrust Load
Forces imposed on a shaft parallel to the shaft axis is called thrust load. It is often encountered
on shafts driving mixers, fans, blowers, and similar applications. When thrust load acts on a
speed reducer, be sure that the thrust load rating of the reducer is high enough that its shafts and
bearings can absorb the load.
Torque
A twisting effort exerted around an axis is called torque. It is the product of a force and its
distance from the axis around which the force acts.
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Fundamentals of Speed Reduction 1-9
Parallel Shaft and Right Angle Reducers
Generally speaking there are two groups of speed reducers, Parallel Shaft and Right Angle.
These descriptions refer to the relationship between the input and output shafts of the reducer,
there is really no such thing as a “Left Angle” speed reducer. Usually, a speed reducer’s output
shaft will be at a 90° angle (right angle) or parallel to the input shaft.
Parallel shaft
Parallel shaft, in−line, offset parallel and concentric speed reducers as they are called, share the
same common traits. All have output shaft that run parallel to the input. An offset parallel speed
reducer’s output shaft is not in−line with the input shaft. It can be offset left, right, up or down. As their
names imply, in−line and concentric speed reducers input and output shaft are in the same line.
Right Angle
Right Angle speed reducers are usually available with right, left or double output shafts. They are
also available with hollow bores for shaft mounting.
Input Connections
Speed reducers need an input shaft rotation to have output shaft rotation. An electric motor,
internal combustion engine, hydraulic motors, air motors or even a hand cranks can drive a
reducer. The most common drive is the electric motor. For an electric motor to power a speed
reducer it must be connected to the reducer. There are three common connection methods.
1. Motors output shaft is connected to a solid input shaft of the reducer.
2. Motors C−face is connected to a flange on the speed reducer.
3. The motor is part of the speed reducer (Gearmotor)
Solid Input Shaft Connection
Solid Input shaft connection are made by either connecting the motors shaft to the reducers input
shaft with a shaft coupling or by using a belts and pulleys. The reducer and motor are usually
base mounted. Another option for this type of connection is called Scoop−mounting. The motor is
mounted to a base that is permanently mounted to the reducer. The scoop acts as a base for the
motor.
Flanged Input Connections
Flanged input connections are made through the use of an input flange that accepts the motors
input. NEMA C−face flanges are common in the US. There are generally two types.
S
Quill type − Quill type reducer has a hollow input shaft. This shaft is designed to
accommodate the motors shaft diameter. It is usually an integral part of the reducers
first stage gearing (pinion or worm depending on the type of reducer). The term Quill
comes from the period in which feather pens were dipped into ink for writing. The quill
(feather) was dipped into an inkwell to get ink. The motor is in effect is dipped into the
reducers hollow input shaft. As with the shaft−mounted reducer, a disadvantage to
using this type of reducer is fretting corrosion. The motor and gearbox can become
stuck together making it difficult to replace one or the other.
S
Coupling type − Coupling type reducer is similar to the solid input shaft design. A flange
is used to connect to the motors c−face and a jaw type coupling is used between the
motor and reducer. The flange usually adds considerable length to the reducer/ motor
arrangement. As an advantage this arrangement handles shock loading much better
than the quill reducer. The jaws absorb the shock and can be replaced. This makes this
reducer ideal for reversing application and frequent starts and stops.
Gearmotors
Gearmotors generally do not have input shafts, quills or
couplings. The motor and speed reducer are design to be
integral (connected to each other). The gearmotor will have
an input pinion or worm mounted directly to the motors output
shaft. The gearmotor has fewer components and is generally
less expensive and takes up less space as compared to a
speed reducer/motor combination. With a gearmotor you
know what your output torque and speed are going to be.
1-10 Fundamentals of Speed Reduction
FOG
Speed reducers and gearmotors are also put into groups based on the type of gears used.
Different gears will have advantages and disadvantages. Some gear types are better suited for
certain applications than other gear types.
Helical Speed Reducers
Helical speed reducers use helical gearing as the name implies. They usually have shafts that
are parallel or concentric to the motors shaft. Helical speed reducers are very efficient and can
have multiple gear stages. Each gear stage will have an efficiency range of 97% to 99%. This
means a speed reducer having up to three stages can be as high as 97% (99% x 99% x 99%)
efficient. Properly lubricated and applied, a helical speed reducer can have infinite life. Helical
gears do not wear when properly applied. It is most important to ensure that the load does not
exceed the reducer’s rating. Shock loads should also be considered when selecting helical speed
reducers, using proper service factoring will help eliminate early gear failure.
Worm Gear Speed Reducers
Worm gear speed reducers use worm gears and have an output shaft that is at a right angle to
the input shaft. They are the most common type of speed reducer used in low power ratings
(generally below 10 hp). Worm gear speed reducers have the advantage of being produced in
large gear ratios, up to 100:1 can be obtained in a single reduction. The down side is they are not
very efficient. An efficiency range from 40% − 90% is very common, this is a result of the sliding
action of the worm gear set. The higher the ratio, the lower the efficiency. A worm gear set having
a ratio of 5:1 can be 90% efficient, where as, a 100:1 might be 40% efficient. RPM will also affect
the efficiency, the higher the input speed the greater the efficiency. Worm gear speed reducer
can also be built in multiple stages for increased ratios. 3600:1 is possible with a double worm
gear set; the efficiency is extremely poor however. A typical 60:1 gear set may have an efficiency
of 69%, multiply that by 2 (69% x 69%) and you have an efficiency of 47%. This means you
would need a 2 hp motor to produce the power needed for 1 hp application.
Their center distance usually categorizes worm gear speed reducers. The center distance of a
worm gear speed reducer is the distance between the input shaft and output shaft. Worm gear
speed reducers having greater center distances will have greater load (torque) carrying capacity.
In general, speed reducers having the same center distance, will have similar load carrying
capacity. Worm gears handle shock loads better than helical.
Gearing Types
Worm−Helical Speed Reducers
Worm−Helical speed reducers offer some advantages of both helical and worm gears. They are
more efficient than standard worm gear speed reducers, however, they are not as efficient as an
all−helical reducer. They are efficient because they generally use smaller (lower) more efficient
worm ratios on the input or high speed and a highly efficient helical ratio the final output stage.
Planetary Speed Reducers
Planetary speed reducers always have concentric output shafts. They usually have 5 gears to
produce a ratio. Planetary speed reducers tend to be very strong and handle shock loads
because they have more teeth in contact at the same moment.
Figure 1-11 Planetary Speed Reducers
The pinion gear (often called the sun gear) is in
contact with three to four planet gears at one time.
The load on the sun gear is shared by three or four
meshes instead of one as in a standard pair of
Ring Gear gears. Planetary speed reducers must be built to
precise standards. If not, the loads may not be
Planet shared equally and failure will result. The precision
Carrier Plate required in machining and assembly increases the
overall cost as compared to other types of gearing.
3 Planet
Gears
Sun Gear or
Pinion Gear
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Fundamentals of Speed Reduction 1-11
Speed Reducer Ratings
Ratings for speed reducers depend on several factors. The mechanical and thermal capacities of
the individual gears are the main factors. The housing, bearings, keys and bolts are also
contributing factors. Selecting a speed reducer involves considering a number of different ratings,
capacities and limits. Speed reducers are rated in terms such as power, speed, torque, service
factor and overhung load (OHL) to name a few. Figure 1-12 shows how ratings are listed in a
catalog specification sheet.
Figure 1-12 Speed Reducer Selection Table
The ratio of speed of the input shaft compared to the output shaft.
Normal rated operating RPM to drive the input shaft.
Normal rated operating RPM of the output shaft (when input shaft is at normal RPM).
Maximum rated hp of drive power.
Maximum rated load hp at rated input hp.
Maximum rated torque provided at the output shaft.
Overhung load capacity of the output shaft (statement of bearing capability).
Described in detail later in this section.
Output Shaft Thrust Load A force imposed on the output shaft parallel to the axis or the shaft axis is called
thrust load.
Maximum Input Speed
Maximum rated operating RPM to drive the input shaft.
Ratio
Input RPM
Output RPM
Input HP
Output HP
Output Torque
Overhung Load
1-12 Fundamentals of Speed Reduction
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Speed Reducer Ratings Continued
Speed Limits All speed reducers have rotational speed limits or maximum speeds at which they will operate.
There are usually two factors that limit the speed; one is lubrication. Rapidly spinning gears will
sling off the oil as it spins. In high−speed applications it may be necessary to spray lubrication
into the gear mesh. Most speed reducers are partially or completely filled with oil. As the gears
(or mating gear) rotate they dip into the oil and carry the oil into the gear mesh. As the speed is
increased the gears plow (churn) through the oil at a faster rate. This plowing generates heat.
The faster the gears are turned the more heat is generated. The energy lost is typically called
churning losses. The heat generated by the churning losses can limit the high−speed operation
of the speed reducer. The other factor limiting speed is balancing. If the gear and shaft are not
balanced properly for a particular speed, they will vibrate and cause problems. Think of your
automobile tires, if they are not balanced properly, the car may be fine around town, however, at
highway speeds it can become unsafe. Speed reducers are limited by the speed of the
high−speed (input shaft) shaft. Most speed reducers can handle rotation speed between 2500
and 4000 RPMs. If this speed is going to be exceeded, it is likely a special speed reducer
designed for high−speed input will be needed. This may require special gears, lubrication, etc. to
handle the higher input speed.
Load Capacity It is important to understand how much load a speed reducer can safely handle. Load ratings are
expressed as maximum torque and maximum power. They are usually expressed at the output or
input shaft and sometimes both. These rating are usually mechanical or thermal ratings.
Mechanically limited speed reducers have their capacity limited by a mechanical component
such as a gear, bearing, bolt, etc. If operated above the reducer’s mechanical capacity, some
component will eventually fail.
The speed reducer will be limited by the strength of a component perhaps a gear tooth, bearing
or other internal component. If the mechanical rating is exceeded the speed reducer will wear to
quickly. At some point above the full load rating, the reducer will fail instantaneously.
Thermal LimitsThermally limited speed reducers are limited by the internal operating temperature. If the thermal
rating is exceeded, part of the speed reducer, usually the lubrication, will get too hot to operate
properly. As the reducer is overloaded beyond it’s thermal limit, the internal temperature climbs.
This heat is cause by friction form the internal component rubbing against each other. This
overloading pushes the lubrication beyond its ability to protect the gears and bearings, the speed
reducer fails.
It’s important to understand what these ratings mean. As an example, depending on the overload
condition, the speed reducer may not break instantly. If this rating is exceeded of an extended
period, the speed reducer may not last as long as it should. Speed reducer ratings are based on
statistics. If a speed reducer is operated at full load, it is expected to last a certain amount of
time. Of course this doesn’t mean it will last exactly that amount of time. If you operated 50
identical speed reducers, under the same exact load conditions, each one would fail at a different
time. Some will fail sooner than expected and other will last long after the expected life. The
ratings are based on the average amount of time the reducer will last. A speed reducer is
designed to last 25000 hours; this is 2.8 years at 24 hours per day, 365 days a year. Some
reducers may fail within the first year of operation while others may last 8, 10 or more years.
However, the average will be 25000 hours.
Efficiency
Efficiency in a speed reducer is measured by how much power is available at the reducer’s
output shaft as compared to the power at the input shaft. If a speed reducer were 100% efficient
it would generate no heat. As an example: a 95% efficient helical speed reducer rated 10hp
(input) will output 9.5hp, the power loss to heat is 0.5hp. A 70% efficient worm gear speed
reducer rated 10hp will generate a power loss of 3.0hp. A speed reducer’s efficiency will typically
change with input speed. However, the efficiency of a helical speed reducer will be approximately
the same regardless of input speed. In worm gearing the slower the input speed, the less
efficient it becomes. This is because the lubrication becomes less effective.
Torque Rating Speed reducers are often rated in terms of torque capacity. This torque rating can be limited
either mechanically or thermally. These ratings are normally at the speed reducer’s output shaft.
All manufacturers publish ratings tables. These tables usually list the maximum torque a speed
reducer can transmit.
Power Rating Power ratings are also published. These ratings are usually stated as maximum input
horsepower at the input shaft. This information is necessary, so the proper horsepower motor
can be selected that will not exceed the speed reducer’s power rating. The horsepower capacity
is always higher on the input side than the rating on the output. The lower the efficiency of the
reducer the lower the output power capacity.
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Fundamentals of Speed Reduction 1-13
Service Factor Service Factors must be considered when selecting a speed reducer. A service factor is the
amount of extra capacity in a speed reducer based on the actual torque required. Factors that
need to be considered for applying a service factor are hours of operation, frequent starts and
stops and shock loading. The purpose of applying more reducer for the application is to increase
the operating lifetime of the speed reducer. If a speed reducer is operated 8 hours per day it may
last 3 years, if that same reducer was operated 24 hours per day, it may only last 1 year. To
make that speed reducer last 3 years, a larger service factor than the one used for 8 hours
should be applied. Another reason to apply a service factor is to cope with difficult applications. A
rock crusher may require the same torque as a radar antenna; however, the rock crusher will be
much harder on the speed reducer. The rock crusher will send a shock through the speed
reducer. A shock load is a very sudden increase load. To accommodate this the shock load, the
speed reducer should have a large service factor. AGMA has set up a table (from 6034--B92,
Practice for Enclosed Cylindrical Wormgear Speed Reducers and Gearmotors) of recommended
service factors based on application. This table was established based on the experience of
many gear manufacturers.
OHL
Overhung Load is also import in sizing and selecting a speed reducer. Overhung load is a force
that either pushes or pulls against the input or output shaft of the speed reducer. This force in
applied in the direction of the shaft radius line. This can be applied from anywhere on the shaft,
12, 9, 6, 3 o’clock. Overhung load is produced when you transfer torque from the speed reducer
output shaft to another shaft. Chain & sprockets, gears, v--belts, flat--belts and timing belts all can
create overhung loads. The speed reducer must be able to handle the applications overhung
load. If the application exceeds the reducer overhung load capacity, one of several failures is
possible, the shaft can break, the bearings can fail or the housing can break. When the output
RPM is high, the bearings are usually the limiting factor, whereas at lower speeds, the other
components are likely to fail first. It is also possible to create overhung load simply by over
tensioning of belts, chain & sprockets, etc. The standard calculation for OHL is Torque/Radius.
For loads that will be tensioned, an overhung load factor (Table 1--1) should be added to the
equation.
Table 1--1 OHL Factor
Factor
Drive type
Single or multiple chain
1.00
Timing belt
1.00
Cut pinion run with cut gear
1.25
Single or multiple v--belt
1.50
Flat belt
2.50
Variable pitch pulley
3.50
Other types
Consult speed reducer manufacturer
The equation then becomes Torque/Radius x Factor.
As an example, a speed reducer producing 2000 lb--in of torque through a 10” diameter pulley
has an overhung load:
Torque
= (2000) x 1.0 = 400lb
OHL =
5
Radius
If that same reducer is driving a single v--belt, the load is:
Torque
OHL =
= (2000) x 1.5 = 600lb
5
Radius
The speed reducer must be able to handle the OHL.
1-14 Fundamentals of Speed Reduction
FOG
BALDOR ELECTRIC COMPANY
P.O. BOX 2400
Fort Smith, AR 72902−2400
(479) 646−4711
Fax (479) 648−5792
www.baldor.com
© Baldor Electric Company
FOG
Printed in USA
5/05
Fundamentals of Speed Reduction
Name:
Branch Address:
Phone:
email Address:
1.
Straight bevel gears are similar to spur gears.
- True
2.
- False
What are the 5 basic gear design categories?
1.
2.
3.
4.
5.
3.
Give two examples of gears types that are mounted on parallel shafts.
1.
2.
4.
What are the three methods for mounting a speed reducer?
1.
2.
3.
5.
What is another term used to describe a flange−mounted reducer?
Fundamentals of Speed Reduction Questions 1
6.
What is a torque arm used for?
7.
What are the two classifications of gears?
1.
2.
8.
Name three types of parallel shaft reducers.
1.
2.
3.
9.
What output shafts are available for right angle speed reducers? Hint; there are 3.
1.
2.
3.
10. What are two common methods for connecting a motor to a reducer? Hint; there are 3.
1.
2.
11. If a speed reducer has a hollow input, it is called a __________________________ type.
2 Fundamentals of Speed Reduction Questions
12. In a gearmotor, what is mounted directly to the motor’s output shaft?
13. In general, what is the most important factor to consider when selecting a mounting position?
14. What effect can oil/lubrication have on the efficiency of the reducer?
15. What is the efficiency of a typical 3 stage helical gear speed reducer?
16. Name two advantages of worm gear speed reducers?
1.
2.
17. What is center distance as it relates to a worm gear speed reducer?
18. What advantage does a helical−worm speed reducer offer over a worm gear speed reducer?
19. Not counting any losses, if the input speed used with a 5:1 gearbox is 3480 RPM, what is the
output speed?
20. The output torque of a gear box is increased by the same ratio that the output speed is
decreased. In problem 19 above, if the input torque is 1.5 lb−ft, what is the output torque?
Fundamentals of Speed Reduction Questions 3
Return your completed form to:
Baldor Electric Company
Marketing Dept.
P.O. Box 2400
Fort Smith, AR 72902−2400
Attn: Director of Education
4 Fundamentals of Speed Reduction Questions