Download AC Drive Basics

TORQUE PRODUCTION WITH
AC DRIVES & MOTORS:
Understanding the technology
Developed by,
Rockwell Automation Drives Business
Reliance Electric
Spring Update CD, May 2001
Presentation Abstract
After 25 years of AC Drive
acceptance, drive manufacturers
offer the industry many types of
control methods.
We’ll review some motor & drive
basics and then discuss the
technologies offered in AC Drives
along with the selection process.
Spring Update CD, May 2001
AC & DC Motor Basics
REVIEWING MOTOR
FUNDAMENTALS
Spring Update CD, May 2001
Motor Basics
Motor nameplate HP is achieved at Base RPM:
HP = Torque * Speed / 5252
Torque
100%
Constant Torque
Range
Constant Horsepower
Range
Nameplate HP is only
achieved at base
speed, NOT BEFORE!
Base Speed
Spring Update CD, May 2001
RPM
Motor Basics - AC Motor Construction
Motor Frame
Assembly
Stator Winding
Assembly
Rotor & Shaft
Assembly
3 phase stator winding circuit w/ connections T1, T2 & T3
Spring Update CD, May 2001
Motor Basics - AC Motor Operation
2 Pole Motor
Motor RPM is equal to:
120 * Frequency
# Motor Poles
Note that Frequency
is the only variable to
affect motor speed
Rotating Magnetic Field of a 2 Pole AC Induction Motor
Spring Update CD, May 2001
Motor Basics - DC Motor Construction
Commutator &
Brush Assembly
Armature
Assembly
Field Poles
Assemblies
NOTE: The Armature & Field
Circuits are mechanically
fixed at 90° at all times
Distinct Armature & Field Circuits are mechanically separated
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Motor Basic - DC Motor Operation
Simple Model
S
V
V
N
Motor RPM is equal to:
Voltage Arm - ( Voltage Drop )
Field Flux
Both Armature Terminal
Voltage & Field Strength
affect DC Motor speed
To create motor torque
at the shaft, we increase
Armature Current
Rotating Magnetic Field of a 2 Pole AC Induction Motor
Spring Update CD, May 2001
Motor Basics - AC & DC Summary
Key Points of Understanding
• AC Induction Motors have one circuit to connect
• Connection to T1, T2 & T3 for the stator
• DC Motors have 2 separate circuits to connect
• Connection to F1 & F2 for the Field
• Connection to A1 & A2 for the Armature
• To make AC Motors perform like DC Motors
• Treat the AC motor like a 2 circuit machine
Mechanical differences must be overcome mathematically
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AC Drive Basics
PWM AC DRIVE
FUNDAMENTALS
Spring Update CD, May 2001
Drive Basics - PWM AC Drive Construction
Motor
AC Line
Diode
Rectifier
DC Bus
Filter
IGBT
Inverter
• Diode rectifier converts AC line voltage to fixed voltage DC.
• DC voltage is filtered to reduce current ripple from rectification.
• Inverter changes fixed voltage DC to adjustable PWM AC voltage.
Spring Update CD, May 2001
AC Drive Basics - PWM AC Waveforms
VLL @ Drive
500 Volts / Div.
+ DC Bus
1
- DC Bus
3
Phase Current
10 Amps / Div.
M2.00s Ch1
1.18V
PWM waveform is a series of repetitive Voltage pulses
Spring Update CD, May 2001
AC Drive Basics - V/Hz Operation
At 100% of the motor’s base speed, the V/Hz ratio is
determined: HP = 100% of motor nameplate
Operation at Base Speed
Output
Voltage
460
Ratio @ 460VAC
= 7.67 V/Hz
230
115
0
15
30
60
Base Frequency
90 Output
Frequency
Motor speed is controlled by ramping Voltage & Frequency
Spring Update CD, May 2001
Hz
AC Drive Basics - V/Hz Operation
At 50% of the motor’s base speed, the V/Hz ratio is
maintained: HP = 50% of motor nameplate
Output
Voltage
Operation at 50% Base Speed
460
Ratio @ 460VAC
= 7.67 V/Hz
230
115
0
15
30
60
90 Output
Hz
Frequency
Base Frequency
At 50% of base speed, Voltage & Frequency decrease by 1/2
Spring Update CD, May 2001
AC Drive Basics - V/Hz Operation
At 25% of the motor’s base speed, the V/Hz ratio is
maintained: HP = 25% of motor nameplate
Output
Voltage
Operation at 25% Base Speed
460
Ratio @ 460VAC
= 7.67 V/Hz
230
115
0
15
30
60
90 Output
Hz
Frequency
Base Frequency
At 25% base speed, Voltage & Frequency decreases by 3/4’s
Spring Update CD, May 2001
AC Drive Basics - V/Hz Operation
To increase starting torque, V/Hz Drives use Voltage Boost
to over-flux the motor to increase starting torque
Output
Voltage
460
Ratio @ 460VAC
= 7.67 V/Hz +
248
% BOOST
138
Voltage
Boost
0
15
30
60
Base Frequency
90 Output
Frequency
Offsetting the voltage ratio increases motor starting torque
Spring Update CD, May 2001
Hz
AC Drive Basics - V/Hz Operation
Voltage Boost over prolonged operating
periods may result in overheating of the
motor’s insulation system and result in
damage or premature failure.
CAUTION: Motor Insulation Life is
decreased by 50% for every 10C above
the insulation’s temperature capacity
Unable to perform like DC, the industry looks to Vector Control
Spring Update CD, May 2001
AC Drive Basics - Vector Operation
If we can de-couple and Regulate Current, the
component that creates torque at the motor, we
can regulate motor torque, not just motor speed!
This is the premise for
Vector Control
Current Regulation allows Torque Control
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AC Drive Basics
AC VECTOR DRIVE
FUNDAMENTALS
Spring Update CD, May 2001
AC Drive Basics - Motor Modeling
AC Drive Parameters create a “Motor Model”
based on data entered in the drive parameters
• Motor Magnetizing Current
• Motor Full Load Amps
• Motor Voltage
• Motor Base Frequency
• Motor Base (Slip) RPM
• Motor Horsepower
Correct Motor Data is the most important factor for success
Spring Update CD, May 2001
AC Drive Basics - Motor Modeling
AC Drive Parameters: “Magnetizing Current”
Magnetizing Current is the current required to excite the
motor laminations and copper winding w/o doing work.
• Magnetizing Current is: NO LOAD AMP draw less
friction and windage
• Establishes the motor’s Flux
• (FLA - Mag. Amps) = 100% Torque Current
 Wrong data will reduce motor torque production
Magnetizing Current will range from 35% to 50% of FLA value
Spring Update CD, May 2001
AC Drive Basics - Vector Operation
Torque is produced, as well as regulated even at “0” RPM
Magnetizing Current = Motor No Load Amps
100%
“a fixed value from “0” RPM to Motor Base RPM”
Torque
Current
90
Magnetizing Current
Magnetizing Current is the equivalent of Field Current
Spring Update CD, May 2001
AC Drive Basics - Motor Modeling
AC Drive Parameters: “Full Load Amps”
The motor FLA value may set the scaling for:
• Motor Overload
• Drive Overload
• Torque Current Available
• (FLA * %OL) - Mag. Amps = Max. Available Torque Current
 Wrong data affects available torque current and may
allow damage to the motor.
Since every Vector algorithm is unique, check w/ manufacturer
Spring Update CD, May 2001
AC Drive Basics - Motor Modeling
AC Drive Parameters: “Voltage & Base Hz”
Voltage & Base Hz values will:
• Establish the motor V/Hz ratio for the drive output
 Wrong data will cause motor heating and possibly
reduce motor torque as well as shorten insulation life.
Needed to assure proper motor operation w/o over-heating
Spring Update CD, May 2001
AC Drive Basics - Motor Modeling
AC Drive Parameters: “Base HZ & RPM”
Base Hz & RPM values will set the scaling for:
• Calculation of motor slip
• Identifies expected motor RPM at Frequency
• Allows for speed error detection & correction
• Establishing the point of field weakening
 Wrong data here can cause excessive current draw
AC Drives regulate speed based upon motor slip
Spring Update CD, May 2001
AC Drive Basics - Motor Modeling
AC Drive Parameters: “Horsepower”
The Horsepower value may be used to:
• Estimate the expected motor impedance
• Estimate the expected motor inductance
• Calculate the torque loop gains
 Wrong data here can cause poor speed and torque
regulation
Horsepower information gets us “in the Ballpark”
Spring Update CD, May 2001
AC Drive Basics - Vector Operation
Flux Vector Drives act very much like DC Drives
Magnetizing Current is decreased above Motor Base RPM
100%
100%
Torque
Current
Torque
Current
90
90
Magnetizing Current
Magnetizing Current
Field Weakening occurs whenever we exceed Motor Base RPM
Spring Update CD, May 2001
AC Drive Basics - Vector Operation
Torque at the motor shaft based upon load
Torque Current = Motor Load at the Shaft
100%
“a variable value” during speed regulated operations
Torque
Current
Torque
Current
10%
90
90
Magnetizing Current
Magnetizing Current
Torque Current increases or decreases dependent upon load
Spring Update CD, May 2001
AC Drive Basics - Vector Operation
Torque at the motor shaft based upon “Torque Reference”
Torque Current = Reference setting
100%
“a fixed value” during torque regulated operations
Torque
Current
Torque
Current
10%
90
90
Magnetizing Current
Magnetizing Current
Torque Current can be commanded as a reference value
Spring Update CD, May 2001
AC Drive Basics - Vector Operation
Torque production suffers if 90° is not maintained
100%
Improper tuning, incorrect motor parameters, problems
with motor speed feedback or undersized drive
applications will result in poor load (torque) regulation.
Torque
Current Optimized
Torque
Production
Poor Torque
Production &
Regulation
Torque
Current
ie: Impact Load
90
Magnetizing Current
?
Magnetizing Current
Motor torque is optimized ONLY when 90 is maintained
Spring Update CD, May 2001
AC Drive Basics - Vector Operation
Load Type: Forward Speed & Reverse Torque
?
How a load becomes applied
to the drive system can be
critical to system success.
A load where there is Forward
Velocity & Reverse Torque is
the most difficult load to
handle.
If the Nip Rolls are engaged during
web travel, a condition with
forward velocity and reverse
torque can occur.
Use either V/Hz or a closed loop
system if inertia or speed is high.
Time to find motor rpm & position is limited by inertia & speed
Spring Update CD, May 2001
AC Drive Basics - Vector Operation
Motor Current is = Vector Sum of Torque & Magnetizing
This is where the term VECTOR DRIVE is derived
100%
Torque
Current
100%
Motor
Current
A² + B² = C²
Torque
Current
Motor
Current
90
Magnetizing Current
90
Magnetizing Current
Motor Current is what’s measured with a clamp-on meter
Spring Update CD, May 2001
AC Drive Basics - Flux Vector Operation
Flux Vector Drives regulate current & torque using rotor
speed & position to optimize torque at the motor shaft
along w/ current feedback from the motor.
Current
Feedback
L1
L2
L3
Motor
E
Micro P
Encoders provide rotor speed & position information
Spring Update CD, May 2001
AC Drive Basics - Rotor Temperature & Torque
As motor temperature reaches nominal operating values,
torque linearity and accuracy improves in FVC operation
600
400
HOT
Motor
% Torque
200
0
-200
-150
-100
-50
0
50
100
150
200
30 deg
80 deg
Ideal Value
-200
Torque
accuracy of
5% or better !
-400
-600
COLD
Motor
-800
Inch - Lbs
Spring Update CD, May 2001
AC Drive Basics - Field Oriented Control
Field Oriented Control uses the same basic technology as
Flux Vector Control, but adds Voltage Feedback to
optimize / adapt to changes in motor temperature.
Voltage
Feedback
L1
L2
L3
Motor
E
Micro P
The drive continuously adapts to motor temperature change
Spring Update CD, May 2001
AC Drive Basics - Summary
Key Points of Understanding
• Errors in Encoder Feedback affect the Micro-Processor
• Speed instability will occur
• Encoder Feedback Signals must be NOISE FREE
• Select an appropriate encoder for Vector Motor use
• Proper grounding is very important
• Motor Data programmed in the drive must be accurate
Motor information, measured or programmed is key to success
Spring Update CD, May 2001
AC Drive Basics - Sensorless Vector Operation
There are actually 2 types of drives advertised as
Sensorless Vector;
• Those with a V/Hz Core
• Those with a Vector Core
All Sensorless Vector Drives are NOT the same!
Spring Update CD, May 2001
AC Drive Basics - Sensorless Vector Operation
SVC with V/Hz Core Technology
• Use sophisticated “Current Limiting” algorithms to
improve constant torque & starting torque operation
• Typically needs less motor information for setup adding
some simplicity
• Can operate multiple motors from one drive
• ONLY regulates V/Hz output, clamps CURRENT
• Can only operate as a Speed Regulator, NOT TORQUE
V/Hz Core SVC Drives can operate multiple motors
Spring Update CD, May 2001
AC Drive Basics - Sensorless Vector Operation
SVC with Vector Core Technology
• De-couples Torque & Magnetizing Currents to maintain
90 alignment
• Typically needs more motor information for setup adding
some complexity
• Can operate only one motor per drive due to the
information required to regulate current
• Regulates SPEED and Regulates TORQUE
Vector Core SVC Drives can operate only one motor at a time
Spring Update CD, May 2001
AC Drive Basics - Sensorless Vector Operation
SVC Drives w/ a Vector Core estimates rotor speed &
position
Current
Sensors
L1
L2
L3
Motor
Micro P
( FVC + Speed Estimator )
A “Speed Estimator” calculates rotor speed & position
Spring Update CD, May 2001
AC Drive Basics - Control Loops
There are 3 Basic Control Loops in High Performance
Drives:
POSITION
SPEED
10 rad/sec
100 rad/sec
Position Reference
is optional in most
Vector Controls,
internal in some
Speed Reference is
typical of how we
control motor
operation
TORQUE
1,000 rad/sec
Torque Reference can made
directly, bypassing the speed
loop as a reference for
applications such as Winders &
Test Stands
Bandwidth ratio between loops ranges from 3:1 to 10:1
Spring Update CD, May 2001
MOTOR
AC Drive Basics - Regulator Diagram
Typical Regulator Control Diagram for FVC
Speed
Reference
+
-
Speed
Loop
Torque
Command
Flux
Command
Torque
Loop
Gate
Signals
AC Line
PWM
Inverter
Current
Feedback
Field
Controller
AC
Motor
Speed
Feedback
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Rotor Speed
& Position
E
AC Motor Basics - Inverter Duty
INVERTER
DUTY
MOTORS
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AC Motor Basics - Inverter Duty
Blowers may be added to
motors to allow operation at
low speed including “0” RPM
with 100% Torque continuous
Some motor frames are sized so
that just the surface area is suitable
to dissipate motor heat w/o the need
of a fan or blower
Spring Update CD, May 2001
AC Motor Basics - Inverter Duty
Types of AC Motors
Match Motor
type to meet
your needs!
T-Frame Construction Motors allow
commonality in footprint & shaft
height.
Definite purpose “laminated frame”
designs provide higher power
densities & improved torque to
inertia performance.
Spring Update CD, May 2001
AC Motor Basics - Inverter Duty
Rotor Designs Vary by motor type:
Rotor design
affects torque
production!
Standard Industrial AC Motor “double
squirrel cage” Rotor Design for
improved across the line starting
torque.
Definite purpose “single squirrel
cage” rotor design for Variable
Frequency Drive use
Spring Update CD, May 2001
AC Motor Basics - Equivalent Circuit Diagram
Equivalent Circuit Diagram of an AC Induction Motor
Resistance
Inductance
Inductance
Stator
Stator
Rotor
+
AC Input
Voltage
-
Spring Update CD, May 2001
Inductance
Magnetizing
Current
Working
Rotor heating
affects torque
production!
Resistance
Rotor
AC Motor Basics - Drive Operating Region
NEMA Design ‘B” Motor
Breakdown Torque
Full Load Torque
Rule of Thumb:
Approximately 80% of BDT
(ft-lbs) is usable for PEAK
Torque needs when current
is available.
Therefore, current
headroom from the drive
can improve recovery from
sudden load changes.
Peak Torque capacity is dependent upon the motor BDT %
Spring Update CD, May 2001
AC Motor Basics - Drive Operating Region
NEMA Design “B” Motors vary in Breakdown Torque capacity
Breakdown Torque identifies Peak Torque capabilities
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AC Motor Basics - Operating Range
Speed / Torque Curve of an AC Drive & Inverter Duty Motor
100
Torque
90
%
T
O
R
Q
U
E
80
Torque
70
60
50
40
Acceptable Region
for Continuous Operation
30
20
10
0
0
6
12
18
24
30
36
42
48
54
60
66
72
78
84
90
HZ
Inverter Duty Motors operate at 1/10th Base RPM
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AC Motor Basics - Operating Range
Speed / Torque Curve of an AC Drive & Inverter Duty Motor
100
Torque
90
%
T
O
R
Q
U
E
80
Torque
70
60
Torque above
base RPM =
50
40
100%
% Above Base RPM
30
20
10
0
0
6
12
18
24
30
36
42
48
54
60
66
72
78
84
90
HZ
CHp Operation above Base RPM is typically limited to 150%
Spring Update CD, May 2001
AC Motor Basics - Operating Range
Speed / Torque Curve of a Vector Drive & Vector Duty Motor
100
Torque
90
%
T
O
R
Q
U
E
80
Torque
70
60
50
40
Acceptable Region
for Continuous Operation
30
20
10
0
0
6
12
18
24
30
36
42
48
54
60
66
72
78
84
90
HZ
Vector Duty Motors operate at “0” RPM w/ 100% Torque Cont.
Spring Update CD, May 2001
AC Motor Basics - Operating Range
Speed / Torque Curve of a Vector Drive & Vector Duty Motor
100
Torque
90
%
T
O
R
Q
U
E
Special motor & drive
designs can allow
operation up to 8 *
Base RPM
80
Torque
70
60
50
40
Vector Duty Motors may have
CHP Ranges of
2 * Base Speed or more
depending on their design
30
20
10
0
0
6
12
18
24
30
36
42
48
54
60
66
72
78
84
90
96
102
108
114 120
HZ
Some Vector Duty Motors can provide CHp ( 2 * Base RPM )
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AC Drive Performance
COMPARING
AC DRIVE
PERFORMANCE
Spring Update CD, May 2001
Control Selection
Starting into
rotating loads
V/Hz
SVC
FVC
Better
Good
Best
• FVC operation is best since the position and velocity of the
rotor is known and restarting is immediate.
• V/Hz being a soft speed regulator is very forgiving for
restarting into loads with high inertia.
• SVC may be more difficult to implement due to limitations by
manufacturer. Processor & algorithm dependent.
Spring Update CD, May 2001
Control Selection
Multi-motor
Operation from
one drive
V/Hz
Best
SVC
FVC
Not
Not
Recommended Recommended
• V/Hz operation inheriently controls multiple motors.
• SVC or FVC operation with multiple motors is only possible
when motor shafts are mechanically locked together and
assumptions are made about “total” motor current values.
Spring Update CD, May 2001
Control Selection
Constant
Torque Range
V/Hz
SVC
FVC
Good
Better
Best
• V/Hz is typically good for up to 10:1 Constant Torque.
• SVC is typically good for up to 40:1 Constant Torque.
• FVC is typically good for up to 1,000:1 which includes
continuous operation at Zero Speed.
Spring Update CD, May 2001
Control Selection
Dynamic
Response
V/Hz
SVC
FVC
Good
Better
Best
No
Adjustable
Adjustable
tuning Gains for tuning Gains for tuning
• V/Hz has no quantifiable response time or bandwidth.
• Typical SVC specifications may state 100 Radians/second.
• Typical FVC specifications may state 1,000 Radian/second.
Spring Update CD, May 2001
Drive Selection
Feature
Flux Vector - benefits
DC Drive - limitations
Power Factor
92% to 96% at all speeds &
loads
88% to 33% dependent on
speed & load
Torque Production
1,000 radian/sec
300 radian/sec
Operation at Stall
Closed Loop Flux Vector at DC operation at Stall limited by
Stall continuous
brushes & commutator
Motor Cost
AC Motor cost is less
DC Motor cost is higher due to
expensive due to simplicity labor complexity & parts
High Speed
Applications
Lower rotor mass allows
high speed operation
Mechanically limited in speed
due to construction
Both AC & DC Drives have specific areas of merit to consider
Spring Update CD, May 2001
Drive Selection
Feature
Flux Vector - limitations
DC Drive - benefits
Line Regeneration
60% to 100% premium over
drive cost to do
5% to 25% premium over drive
cost to do
Motor Lead Length
Limitation of lead length – can No concerns of lead length
affect operation & reliability
other than voltage drop
Drive Only Cost
More expensive due to
controller complexity
Less expensive due to
controller simplicity
Shock Load
Applications
Less inertia at motor requires
more tuning and setup time
Armature inertia helps to
dampen shock loads
Both AC & DC Drives have specific areas of merit to consider
Spring Update CD, May 2001
Drive Selection - Speed Range
Performance DC Drive DC Drive
Features
w/ Encoder w/ Tach
DC Drive
w/o Fdbk
Flux
Vector
Sensorless
Vector
Operating
0 RPM to 90 RPM to 90 RPM to 0 RPM to 45 RPM to
Speed Range Base RPM Base RPM Base RPM Base RPM Base RPM
CT Speed
Regulation
1,000 : 1
70 : 1
20 : 1
1,000 : 1
40 : 1
w/o load
0.01%
1.0%
3.0%
0.01%
0.5%
change
CT Speed
Regulation
100 : 1
30 : 1
10 : 1
100 : 1
20 : 1
w/ 100% load
0.05%
3.0%
5.0%
0.05%
1.0%
change
Digital DC Drives & AC Vector Drives performance similarly
Spring Update CD, May 2001
Thank You!
Any Questions?
Spring Update CD, May 2001