Download Technology Training that works

FUNDAMENTALS OF
MOTOR TECHNOLOGY
Technology Training that works
Electromechanical Energy
Conversion
• Electromechanical energy conversion device is a
link between electrical and mechanical systems
• When mechanical system delivers energy through
the device to electrical system, device is called
generator
• When electrical system delivers energy through
the device to mechanical system, device is called
a motor
Technology Training that works
Basic principles of rotating
electric machines
• Electric machines utilize the concepts of
electromagnetism
• A magnetic field interacts with either electric
field or mechanical force to produce the other
• A current carrying conductor produces a
magnetic field in its vicinity
Technology Training that works
Right-hand Rule
• Hold the conductor in right hand with fingers
closed around conductor and thumb pointing
towards the direction of the current
• The fingers will point towards the direction of the
magnetic lines of the flux produced around the
conductor
Technology Training that works
Cork Screw Rule
• Direction and travel in which it has to be rotated
are related to each other the same way as
direction of current in a conductor and direction of
the field produced due to current
• Magnetic field exists in the plane perpendicular to
the conductor
Technology Training that works
Magnetic field lines around a current
carrying conductor
Technology Training that works
Flux produced by Current
Carrying Coil
• Flux can be produced by causing the current to flow
through a coil instead of a conductor
• The direction of the magnetic flux in the coil is given
by the Right-hand rule
Technology Training that works
Force on a Conductor in a
Magnetic Field
If a conductor carries current in a magnetic field, then
a mechanical force is exerted on it. The force exerted
on the conductor is given as :
F = BLI
Technology Training that works
Generator
S
N
Motor
Technology Training that works
Motoring Action
• As voltage is applied to stationary conductors, magnetic
field is produced
• This magnetic field in turn induces voltage in the rotor
conductors, in case of some motors (induction motors) or
voltage is externally applied to the rotor conductors
• This voltage also produces magnetic field
• Magnetic field of stator and rotor, together, put the rotor
in running condition
Technology Training that works
Depiction of a motoring action
Technology Training that works
Electric machine concepts
• Electrical machines (either a motor or a generator) can
be broadly classified as DC Machines and AC Machines
– depending on the nature of supply given to them
• An electrical machine can be interchangeably associated
with either a motor or a generator
• Fundamentals for a particular category (like DC or
Synchronous) are identical for both the generator and
motor
Technology Training that works
Revolving magnetic field
• 3-phase AC power supply is connected to the stator
terminals of an induction motor
• 3-phase alternating currents flow in the stator
windings
• These currents set up a changing magnetic field
(flux pattern) and rotates around the inside of the
stator
• The speed of rotation is in synchronism with the
electric power frequency and is called the
synchronous speed
Technology Training that works
Principles of operation
• 3-phase AC Voltage connected to the Stator windings
– Currents establish magnetic field (flux pattern)
– Rotates around the inside of the stator
– Rotation Speed in synchronism with the power
frequency
Technology Training that works
Principles of operation
• Flux distribution in a 4 pole motor at any one moment
– Shows the 2-North and 2-South poles
Technology Training that works
Principles of operation
• In its simplest form ...
– 3-phase Stator windings connected to power supply
– flux completes one rotation for every cycle of mains
– On 50Hz, the stator flux rotates at 50 revs per second
– Rotor turns at 50 x 60 = 3,000 revs per minute.
– Called a 2 pole motor (2 poles 1-North, 1-South)
• The design of the Stator windings can be changed to be
suitable for 4-pole operation ...
– Therefore rotates at half the speed ... 1,500 rev/min
– Called a 4 pole motor (4 poles 2-North, 2-South)
Technology Training that works
Motor Armature
• The rotational or speed e.m.f is produced in
opposition to the applied voltage. This is known as
counter or back e.m.f.
• Mechanical torque is produced as required by the
load driven by the motor. For more torque and
mechanical power output, there must be more input
to the motor from the mains.
• The motor draws current according to the
requirement of the load.
Technology Training that works
Idealized Machine
• All rotating electrical machines have common
features
• There is a stationary member called a stator
and a rotating member called a rotor
Technology Training that works
Types of Electrical Machines
• An Induction Machine
• A Synchronous Machine
• A D.C. Machine
Technology Training that works
3-Phase AC induction motor
• AC Induction Motors are one of the most successful inventions ……
consume > 50% of all electrical energy generated
• They are very popular for Industrial Applications
– Simplicity …
easy to manufacture
– Reliability …
very little maintenance
– Relatively low cost … more kW per Rupee
• Work well even in a bad environment
– Dust-proof
– Water-proof
• Can be used for Variable Speed Control
– Speed proportional to frequency
Technology Training that works
Simple depiction of a motor
Technology Training that works
Basic construction
• Two types of Rotor Construction
– Wound Rotor type, which comprises 3 sets of windings
with connections to 3 sliprings on the shaft
– Squirrel Cage Rotor type, which comprises a set of
copper or aluminium bars installed into the slots, which
are connected to an end-ring at each end
Technology Training that works
Squirrel Cage Induction Motor
• Most common type of AC motor
• Known as workhorse of Modern Industry
• Most cost effective motor
• Can be designed for any kind of environment
• Construction of rotor gives this name
• Rotor consists of a series of conducting bars laid into
slots carved in the face of the rotor and shorted at either
end by large shorting rings
Technology Training that works
Stator
Core
Rotor
Core
Frame
Stator
Winding
Rotor
Winding
Shaft
Bearing
Technology Training that works
Assembly details of a typical AC
induction motor
Technology Training that works
Assembly details of a typical AC
induction motor
• A Motor (whether DC
electromagnetic parts:
or
AC)
comprises
of
2
• Stationary part called the Stator consists of the
frame, which provides the physical support.
• Rotating part called the Rotor, supported at each
end on bearings.
Technology Training that works
Assembly details of a typical AC
induction motor
• The other parts, which are required to complete a motor
are:
• Two end-flanges to support the two bearings
• Two Bearings to support the rotating shaft
• Steel shaft for transmitting the torque to the load
• Cooling fan located at the NDE to provide forced
cooling for the stator and rotor
• Terminal box on top or either side to receive the
external electrical connections
Technology Training that works
Basic construction
• Basic Design unchanged in over 50 years … but now have
smaller physical size and lower cost per kW due to
– Modern insulation materials
– Computer based design optimisation techniques
– Automated manufacturing methods
– International standardisation ... physical dimensions
• Both Stator and the Rotor are made up of :
– Magnetic circuit …
– Electric circuit …
laminated grain oriented steel
insulated copper or aluminium
Technology Training that works
Stator & Rotor
Technology Training that works
Stator and rotor laminations
• The magnetic path of a motor comprises a set of
slotted steel laminations
• These silicon steel laminations are pressed into the
cylindrical space inside the outer frame
• The magnetic path is laminated to reduce eddy
currents.... lower losses and lower heating
• A set of insulated electrical windings, which are placed
inside the slots of the laminated magnetic path
• Rotor laminations are varied based on the type of
torque characteristic to be realized
Technology Training that works
Simple view of squirrel cage
induction rotor
Technology Training that works
Squirrel Cage Rotor
Technology Training that works
Stator core lamination
The basic stator structure, called as core, is composed of
steel laminations (or stampings)
Silicon steel is used for making the laminations
These are shaped in such a fashion as to form poles around
which are wound the copper wire coils
Set of coils put together and grouped into various patterns
form a winding
These primary windings connect to, and are energized by,
the voltage source to produce a rotating magnetic field
Technology Training that works
Stator core lamination
Technology Training that works
Typical Stator of a Motor
Technology Training that works
Winding coils at the time of
insertion into Stator
Technology Training that works
Single phase induction motor
• This motor is used mostly in small sizes, where
polyphase current is not available
• Characteristics are not as good as the polyphase motor
and for size larger that 10 HP, the line disturbance is
likely to be objectionable
• These motors are commonly used for light starting and
for running loads up to 1/3 HP Capacitor and repulsion
types provide greater torque and are built in sizes up to
10 HP.
• Mostly finds application in domestic sector
Technology Training that works
Slip Ring Induction Motor
• The motor starts with a full resistance bank
• As speed of the motor increases, the resistances are
shorted, one by one
• As the motor reaches full speed, the whole bank of
resistance is shorted out and the motor now runs alike
a squirrel cage induction motor
• Very much useful to develop high starting torque
• Reduced starting current puts less burden on the
power system
Technology Training that works
Slip Ring Motor
• Wound Rotor type, comprises 3 sets of insulated
windings
• Connections brought out to 3 slip rings mounted on the
shaft
• The external connections to the rotating part are made
via brushes onto the slip rings
• Consequently, this type of motor is often referred to as a
slip ring motor
Technology Training that works
Simple view of slip ring induction
rotor
Technology Training that works
Simple view of a Slip Ring Rotor
Technology Training that works
Rotor core laminations
Technology Training that works
Synchronous motor
• Synchronous motor is a constant speed motor
• Can be used to correct the power factor of the 3phase system.
• Like the Induction motor in terms of the stator, the
synchronous machine has either a permanent
magnet arrangement or an electromagnet (with
current supplied via slip rings) rotor.
• In simple terms, the rotor will keep locking with the
rotating magnetic field in the stator.
• In many synchronous machines, a squirrel cage is in
incorporated into the rotor for starting.
Technology Training that works
Constructional features of a
Synchronous Motor
Technology Training that works
Speed of AC induction motor
• AC Induction motors can be designed and manufactured
with the number of stator windings to suit speed
requirements
–
–
–
–
2 pole motors .... stator flux rotates at 3,000 rev/min
4 pole motors .... stator flux rotates at 1,500 rev/min
6 pole motors .... stator flux rotates at 1,000 rev/min
8 pole motors .... stator flux rotates at 750 rev/min
f x 60
f x 60
=
=
rev/ min
n
o
etc
pole - pairs
p/2
• Speed of Stator Flux is called Synchronous Speed
no =
f x 120
rev/ min
p
Technology Training that works
Actual rotor speed
• When Rotor Speed approaches synchronous speed .....
– Magnitude and frequency of rotor voltage becomes small
– If rotor reached synchronous speed, the rotor windings would be
moving at the same speed as the rotating flux
– Induced voltage (and current) in the rotor would be zero
– Without rotor current .... no rotor field and no Torque
• To produce Torque .....
– Rotor must rotate at a slower (or faster) speed
– So, the rotor settles at a speed less than rotating flux called the
Slip Speed
– The difference in actual speed to synchronous speed is called the
Slip
Technology Training that works
Rotor slip
• The slip will vary according to Load Torque
– As load torque increases, the slip increases
– More flux lines cut the rotor windings
– Increases rotor current and magnetic field
– Consequently increases rotor torque
– Typical slip between 1% (no-load) to 6% (full-load)
• Slip (in per unit) is given by :
Slip = s =
( no - n)
per - unit
no
• Actual rotational speed is
n = no (1 - s) rev/ min
Technology Training that works
Equivalent circuit of AC motor
• Electrical circuit can be represented by an equivalent circuit
• Sketch shows ... motor does not have separate field
windings
• Stator current therefore serves a double purpose ....
– Carries Magnetising current for rotating magnetic field ...
IM
– Carries Rotor current that provides shaft torque .………..
IR
Technology Training that works
SIMPLIFIED EQUIVALENT CIRCUIT
• Equivalent circuit simplified by taking out 'transformer'
– adjusting XR and RR values by the turns ratio N =
NS/NR
ie. 'transferring' them to the stator side
– So, must also adjust for frequency ... which depends
on slip
Technology Training that works
More simplified circuit
• Rotor Resistance is Variable ....
– Rotor current IR …. depends primarily on the slip (s)
• Magnetising Inductance is roughly Constant ....
– Magnetising Current IM ...... depends on voltage (V)
Technology Training that works
Current vectors
• Stator current IS represents the vector sum of :
– Magnetising current IM ... generates rotating magnetic
field
– Rotor current IR ... which produces the rotor Torque
Technology Training that works
Motor performance
• Torque and Speed are the most important
considerations
– Power is derived from the Torque
• Torque is the Rotating Force developed
by the motor
Technology Training that works
Motor performance
• Torque-Speed Curve is the basis of all
motor applications
– Curve derived from the equivalent circuit
• Fundamental equation for a 3-phase AC
induction motors,
– Refer to any standard textbook
– Represents the equivalent circuit
Technology Training that works
Speed-Torque curves of Motor Vs
Load
• Machine and load are two components of
electro-mechanical energy conversion system
• Machine characteristics play a very important
role in the operating behavior of the entire
system
• Steady operating point is the point where speed
torque characteristics of load and motor intersect
each other
• Drive system will settle to a speed
corresponding to point of intersection of these
two curves
Technology Training that works
Speed-Torque curves of Motor Vs
Load
Technology Training that works
Torque – Speed Characteristics
(Squirrel Cage Induction motor )
A: Breakaway Starting Torque
B: Pull-up Torque
C: Pull-out Torque or Breakdown Torque or Maximum Torque
D: Synchronous Speed (Zero Torque)
Technology Training that works
Torque- Speed curve
Mechanical Load Torque is shown as a dashed line
• At starting .....
– motor will not pull away unless A > Load Torque
• Acceleration .....
– Motor Torque always exceeds the Load Torque
• Final Speed and Torque
– Final Speed and Slip stabilise at the point D where
– Load Torque exactly equals the Motor Output
Torque
• Range C-D is the stable operating range for the motor
Technology Training that works
Drive systems engineering
• At the time of engineering the drive systems, following
shall be considered for both the Motor as well as the
Load:
– the speed / torque characteristics
– the speed / power characteristics
• Load characteristics can be classified as
– Constant Torque
– Constant Horsepower
– Squared-Exponential Loads - torque varies directly as
the speed, and power as the square of speed
– Cubed-Exponential Loads - torque varies as the
square of speed, and power as the cube of speed
Technology Training that works
Simple Loads - Constant Torque &
Linear Torque requirement
• Constant Torque - About 90% of all general industrial
machines, other than pumps, are constant torque
systems. The machine's torque requirement is
independent of its speed. If the machine speed is
doubled, its power requirement gets doubled.
• Constant Horsepower - For machines with constant
horsepower loads, the power demand is independent of
speed, and torque varies inversely with speed. This
type is most often found in the machine tool industry
and with center driven winders / coilers in steel rolling
mills & paper mills.
Technology Training that works
Simple Loads - Constant Torque &
Linear Torque requirement
Technology Training that works
Special Loads - Constant Power &
Exponential Torque requirement
• Squared-Exponential Loads - With machines of this type,
torque varies directly as the speed, and power as the
square of speed. Such relationships are frequently found in
positive-displacement pumps and mixer applications
• Cubed-Exponential Loads - It is a characteristic of these
machines that torque varies as the square of speed, and
power as the cube of speed. This type of load is imposed
on centrifugal pump drives and most fan or blower drives.
In some uses, fan or blower horsepower varies as the fifth
power of speed
Technology Training that works
Special Loads - Constant Power &
Exponential Torque requirement
Technology Training that works
Design Classes of Squirrel Cage
Induction Motor
Technology Training that works
Design Classes Comparison
NEMA Design
classification
Starting
Current
Starting
Torque
Breakdown
Torque
A
600%
to
700%
70%
to
275%
175%
to
300%
B
C
D
600%
to
700%
600%
to
700%
600% to
700%
70%
to
275%
200%
to
250%
> 275%
175%
to
300%
190%
to
225%
275%
Full load
Slip
0.5%
to
5%
0.5%
to
5%
1%
to
5%
15%
to
25%
Comparative
Efficiency and
power factor
Typical
Applications
Medium to High
Injection
molding
machines,
Machine tools
Medium to High
Fans,
centrifugal
pumps,
compressors,
and blowers
Medium
Crushers,
agitators,
reciprocating
pumps,
compressors,
conveyors
Medium
Punch
presses,
elevators,
hoists winches
Technology Training that works
NEMA Design Class A
Starting torque
motors);
rated torque (large
200 % rated torque
(small
motors)
Starting current
500 – 800 %
Full load slip
low (< 5 %)
Pullout / full-load torque
200 –300 %
Slip at pullout torque
< 20 %
Typical applications
Fans, blowers,
pumps
Technology Training that works
NEMA Design Class B
Starting torque
Starting current
same
Full load slip
Pullout / full-load torque
Slip at pullout torque
Typical applications
rated torque (large motors);
200 % rated torque (small
motors)
25 % that that of “A” for the
torque
low (< 5 %)
200 –300 %
< 20 %
Same as “A”
Technology Training that works
NEMA Design Class C
Starting torque
Starting current
Full load slip
Pullout / full-load torque
Slip at pullout torque
Typical applications
250 % rated torque
low
low (< 5 %)
200 –300 % (slightly less
than that of “A”)
< 20 %
High starting torque loads
like loaded pumps,
compressors & conveyors
Technology Training that works
NEMA Design Class D
Starting torque
Full load slip
275 % rated torque
high (7 to 11 %)
Typical applications requiring the acceleration of
extremely high inertia loads like fly wheels.
Technology Training that works
Difference in the rotor laminations
of various designs
Technology Training that works
Motor duty cycles
• Other Duty Rated output of Motor in manufacturer's
catalogues is based on assumptions about proposed
application and duty cycle
• If used for this duty ..... temperature within limits
• "Standard" motor .... continuous running duty cycle S1
• Cycles specified in IEC 34.1 and AS 1359.30
– Ranging from S1 to S10
Technology Training that works
Duty Cycles
S1:
S2:
S3:
S4:
S5:
S6:
S7:
S8:
Continuous running duty
Short-time duty
Intermittent periodic duty not affected by the starting
process
Intermittent periodic duty affected by the starting
process
Intermittent periodic duty affected by the starting
process and also by electric braking
Continuous operation, periodic duty with intermittent
load
Uninterrupted periodic duty, affected by the starting
process and also by electric braking
Uninterrupted periodic duty with recurring speed and
load changes
Technology Training that works
AC Motors for Oil and Gas
Applications
• Motors for oil and gas applications can be
further categorized as
– Motors for upstream operations
– Motors for midstream operations
– Motors for downstream operations
Technology Training that works
AC Motors for Oil and Gas
Applications
• Upstream comprises all activities relevant to the exploration &
production sectors within the oil and gas industry.
• Midstream refers to those industry activities that apply to processing,
storage and transportation of crude oil & natural gas - pipeline
pumps, blowers and compressors for the transportation of oil and
gas.
• Downstream operations include oil refining, marketing,
and natural gas transmission and distribution.
• This sector represents the most diverse range of applications &
requires products with the best technologies
Technology Training that works
AC Motors for Oil and Gas
Applications
•
Squirrel cage induction motors are
the workhorses of the industry due
to their versatility, reliability and
simplicity.
•
In the power range up to 12 MW a
squirrel cage induction motor is
usually the first choice.
•
The motors may be horizontally or
vertically mounted.
•
A squirrel cage motor having output
ratings up to 50,000kW (70,000HP)
and speeds between 3600 and 300
rpm is shown.
Technology Training that works
AC Motors for Oil and Gas
Applications
• Centrifugal and
reciprocating compressor
applications may use both
induction and synchronous
motors
• A 7,000 HP (5,200 kW), 4
pole,13.2 kV Induction Motor
for a natural gas Liquid
project manufactured by GE
is shown
Technology Training that works
AC Motors for Oil and Gas
Applications
Induction motor
Synchronous motor
Technology Training that works
AC Motors for Oil and Gas
Applications
• Again, both induction &
synchronous motors may
be used for most pump
applications, including
high pressure pumps.
• A 2,250 HP 400 RPM
vertical motor for a sea
water lift pump is shown.
Technology Training that works
3-Phase Induction Motors
• In oil and gas
applications, these
motors allow easy
maintenance and
enable high
performance.
• They can be applied
on a wide variety of
applications with
different operational
requirements.
Technology Training that works
3-Phase Induction Motors
• An induction motor with output
ratings between 100 & 3,150kW
(135 to 4000HP) & speeds
between 3600 – 600 rpm is
shown.
• As seen, the frame construction
consisting of a high resistance
solid block is fitted with external
cooling fins.
• This provides an homogeneous
blow of cooling air inside the
frame, for optimum performance.
Technology Training that works
Efficiency of motors
• Overall Efficiency of a machine .... is a measure of how
well it converts electrical energy into mechanical output
energy
• Efficiency roughly depends on .....
– Constant losses ...
independent of load
– Load dependent losses … mainly copper losses
Technology Training that works
Insulating materials for electrical
machines
Class A: The limiting hot-spot temperature is 1050C
Cotton, silk, paper, and similar organic materials impregnated or
immersed in oil, and enamel applied on enamelled wires.
Class E Insulation: An intermediate class of insulating materials
between Class A and Class B insulation material
Class B Insulation: The limiting hot-spot temperature is 1300C
Mica, asbestos, glass fiber, and similar inorganic materials in built-up
form with organic binding substances.
Class F Insulation: The limiting hot-spot temperature is 1550C
Includes insulation having mica, asbestos or glass fiber base with a
silicone or a similar high temperature resistant binding material.
Class H Insulation: The limiting hot-spot temperature is 1800C
Includes insulation having mica, asbestos, or glass fiber base with a
silicone or a similar high temperature resistant binding material.
Technology Training that works
Thermal rating of motors
• Motor Life depends on the integrity of Insulation
– Mechanical Loads must be within thermal rating
– Duty cycle of the Load .... continuous or cyclical
• Temperature in motor windings should not rise to a level
which exceeds the Critical Temperature.
• Classified by standards such as IEC 34.1 and AS
1359.32 based on an Ambient Temperature of 40OC
Insulation Class
Max Temperature
Rated Temp Rise
E
0
120 C
0
70 C
B
0
130 C
0
80 C
F
0
155 C
0
100 C
H
0
180 C
0
125 C
Technology Training that works
Thermal rating of motors
• Motors are designed with a Thermal Reserve
– Operating continuously at maximum rated temperature
– The life expectancy of the insulation is about 10 years
– Class-B rating .... use Class-F insulating materials at
higher ambient temperatures
Technology Training that works
Thermal derating of motors
• When motors are operated in abnormal conditions .....
– need to apply a de-rating factor
• Typical de-rating tables as follows :
Ambient
Temp
o
30 C
o
40 C
o
45 C
o
50 C
o
55 C
o
60 C
o
70 C
Output
% of Rated
Altitude
above Sea
Output
% of Rated
107 %
100 %
96 %
92 %
87 %
82 %
65 %
1000m
1500m
2000m
2500m
3000m
3500m
4000m
100 %
96 %
92 %
88 %
84 %
80 %
76 %
Technology Training that works
Motor control
•
•
•
•
•
•
•
Power circuit
Control circuit
Full online voltage starting
Reduced voltage starting
Braking
Speed control
Reversing
Technology Training that works
Motor control
• Motor control is to realize the full potential of a
Drive System
• This may include
– reliable starting at the moment of requirement
(vertical startup),
– controlling their start / stop through some safety
/ operational interlocks
– control of speed either smoothly or in steps
– Reversing the direction or braking the motor
speed
• The back bone of all these controls is the power
circuit
Technology Training that works
Power Circuit
• The power circuit of a motor typically includes
– isolators / disconnects
– fuses
– contactors / circuit breakers
– overloads
– cables / conductors and
– lug connections
• All the conductors and connections that exist from
the point at which the voltage is tapped from the
MCC (Motor Control Center) bus through all
intermediate points up to the connections at the
motor.
Technology Training that works
Control circuit
• Similar to the nervous system of a human body
• Various stimuli are received, processed and necessary
commands are issued
• Separate power source for the control circuit
• All the logics are fulfilled at a lesser voltage for safety reasons
– Lesser voltage is economical also
• Normally at 110 V AC given by a step down transformer
– Typically 415 V / 110 V for LT motors
– Called as a control transformer
• It can be a DC supply
– For critical applications
Technology Training that works
Methods of starting
• DOL starting is most common ... but Current is High
– between 3 to 8 times of FLC ... depends on motor design
– mechanical shock to the system
– starting torque low ... only 1.0 to 2.5 times FLT
• Other methods of Starting are ...
–
–
–
–
–
–
–
Star-Delta
Series Inductance
Auto-transformer
Series Resistance (liquid)
Solid State Soft-Starting
Rotor Resistance Starting
VVVF Converter
reduced stator voltage
reduced stator voltage
reduced stator voltage
reduced stator voltage
reduced stator voltage
requires a slipring motor
also gives speed control
Technology Training that works
Full Online voltage starting
• Also known as Direct-On-Line (DOL) starting
• For starting smaller squirrel cage induction motors
– The motor must accelerate to normal speed as
quickly as possible
– Otherwise the motor winding may get overheated
beyond tolerable limits
• Hence the load on the motor must be very light, to
facilitate acceleration of the motor, at the time of
starting
• Starting of higher rating motors – a serious concern
for the power system.
Technology Training that works
DOL Starting
Direct-On-Line (DOL) starting is the simplest and most
economical method.
Disadvantages:
• The starting current is very high .... between 3 to 8 times
the full load current. Depending on the size of the motor,
this can result in voltage sags in the power system
• The full torque is applied instantly at starting and the
mechanical shock can eventually damage the drive
system, particularly with materials handling equipment,
such as conveyors
Hence various types of reduced voltage starting
methods are used.
Technology Training that works
Typical DOL Starter
Technology Training that works
Legend for the components of
DOL Starter
Technology Training that works
Motor acceleration
• During Starting ..... Current is High
– usually about 6 times rated current . (discuss why ?)
• Manufacturers specify a Maximum Starting Time
– Avoid overheating of the motor windings
• Acceleration time depends on
– Motor torque (TM) characteristic
– Load torque (TL) characteristic
– Total Moment of Inertia (JTot) of rotating parts
T A = ( T M - T L ) Nm
• Acceleration torque is the difference between TM & TL
Technology Training that works
Reduced Voltage starting
• The starting current can be reduced considerably
• Starting torque reduces in direct proportion to the
square of the voltage
• Starting current inrush decreases as the square of
the reduction in supply voltage
• Can be carried out on no-load or very light load
• Not suitable for applications that require a high breakaway torque
Technology Training that works
Typical connections of a reduced
voltage starter
Technology Training that works
Star Delta starting method
Technology Training that works
Typical connections of a Star
Delta Starter
Technology Training that works
Star/ Delta Starting
• Cheapest method of starting
• Commonly employed for both small and medium
sized motors
• Can be utilized only when
– the reduction in voltage per phase and
– the resultant reduction in starting torque to onethird of its full-voltage value
• will be able to accelerate the drive in a reasonable
time
Technology Training that works
Auto-transformer starting method
Technology Training that works
Typical connections of an
Autotransformer start
Technology Training that works
Auto transformer starting
• If the voltage is reduced by a fraction x,
– the current drawn from the system becomes x2
– Also the starting torque reduces by the same factor
• This method of starting is much superior to that of
stator impedance starting.
• Also a smooth starting and high acceleration can be
achieved by smoothly varying the voltage to the full
line value.
Technology Training that works
Soft start
• Voltage to frequency is maintained constant so that the torque
will remain constant.
• Reduced voltage (reduced torque) soft starting has the
following main advantages:
– Reduces mechanical shock on the driven machinery, hence the
name soft starting
– Reduces the starting current surge in the electrical power supply
system
– Reduces water hammer during starting and stopping in pumping
systems
• Usually this method of starting is applied for very large,
frequently started machinery.
• It is highly expensive but has a promising future because of the
ever improving electronics and their economic feasibility.
Technology Training that works
Reversing a Three -Phase Motor
For war d
Rever s e
A
B
C
A
B
C
M otor
A
B
C
A
B
C
L1
L2
L3
St ar te r
L1
L2
L3
Technology Training that works
TROUBLESHOOTING,
MAINTENANCE AND PROTECTION
OF AC ELECTRICAL MOTORS
Technology Training that works
Main causes for Motor Damage
Technology Training that works
Protective Functions Needed
Technology Training that works
Electric motor protection
• Useful life of a motor is dependent on
–
–
–
–
–
–
Mechanical Overloading
Frequent Starting, Jogging, Plugging and Reversing
Single-phasing or Unbalanced Power Supply
Locked Rotor or Stalling
High Ambient Temperature
Loss of Cooling
• Most common cause is Thermal Overloading ...
–
–
–
–
Load Current exceeds the Maximum Rated Value
Temperature in insulation exceeds Critical Value
Insulation Fails
Motor Short-circuit or Earth Fault
Technology Training that works
Life of electric motor
• Long Insulation Life is important for Electric Motors
– Operate in Strategic Locations in industry
– Continuously Changing Load Conditions
– Constantly applied Temperature Rise of 10oC above
the maximum rated temp can reduce useful life to
50%
Technology Training that works
Motor protection
• To protect a motor from insulation damage
– Potentially damaging condition should be detected
– Motor disconnected from the power supply before
insulation damage can occur.
• Current Sensing devices ... sense Current Such as
Thermal Overload Protection Relays
– Continuously Monitor Stator Current flowing into
motor
– Motor Model ... heating and cooling time-constants
• Direct
Temperature
Sensing
devices...sense
Temperature
– Thermostats, Thermistors, Thermocouples, RTDs
– Continuously monitor Actual Temperature in Windings
Technology Training that works
Currrent sensing
• Small motors ...... Bimetallic type of TOL relay
–
–
–
–
Normally mounted with the Motor Contactor
In-adequate protection for repeated starting, jogging, etc
Heating and Cooling Time-constant are different to Motor
Typical cage motor has cooling time constant approx. twice its
heating time-constant ... cooling fan stops
• Larger motors ..... Electronic Motor Protection Relay
– Better Modelling of Motor Thermal Characteristic
– Also ... provides protection against short circuits, earth faults,
stalling, single phasing, multiple starts, etc
– Matched to the Type, Size and Application of the Motor
Technology Training that works
Best maintenance Practices for AC
Motors
• Regular Upkeep:
– Cleaning of motor surfaces and ventilation openings periodically.
– Proper lubrication of moving parts.
– Ensuring proper alignment of motor couplings.
– Proper alignment & tensioning of belts & pulleys when installed.
– Keeping the bearings clean, lubricated, and loaded within
tolerances.
– Checking for proper supply voltages.
Technology Training that works
Best maintenance Practices for AC
Motors
• Record - keeping practices:
 Maintaining an up-to-date inventory.
 Keeping maintenance logs.
 Considering the possibility of using a computerized
maintenance program incorporating inventories and
logs.
Technology Training that works
Sample Maintenance Schedule
Technology Training that works
Sample Troubleshooting Charts for
Induction Motors used in Oil & Gas
Technology Training that works
Sample Troubleshooting Chart for
Induction Motor used in Oil & Gas
Technology Training that works
Sample Troubleshooting Chart for
Induction Motor used in Oil & Gas
Technology Training that works
Power Circuit – possible
problems
• Corrosion of terminals
– due to oxidation resulting out of moisture or
corrosive environments, can be avoided by
proper sealing of the motor terminal box.
• Looseness of cables
– due to vibrations can be avoided by
ensuring proper tightness at regular intervals
or by providing suitable locking mechanisms.
• Looseness in bus bars shall be kept under
check by carrying out regular maintenance.
Technology Training that works
Motor Failures
Technology Training that works
Motor failure analysis
•
•
•
•
•
•
•
•
•
•
Frequent starts
High inertia
Inadequate cooling
Congestion on fan cover
Improper spacing at end of motor
Incorrect belt alignment
Solid belt guards
Excessive loading causing bearing clearance problems
Insulation failures
Bearing current problems
Technology Training that works
Motor failure analysis
• Motor failures can be broadly classified as
– Insulation failure
– Rotor bar failure
– Mechanical failure
– Auxiliaries failure
• Insulation failures can manifest in various forms like winding
shorts, insulation to ground faults etc.
• Rotor bar failure is an important failure mode of especially large
motors.
• Most common mechanical failure is bearings related.
• Auxiliaries failures are related to the power supply, electrical circuits &
cable termination.
Technology Training that works
Typical heating and cooling curves
of a motor
Technology Training that works
Heating and cooling curves of a
motor
• A motor’s temperature (mainly of the winding as it is the main
concern) typically rises exponentially in response to the time taken for
the motor to start.
• Even in operation this temperature continues to increase but with a
declining rate of temperature rise.
• Based on the heat dissipating efficiency of the cooling circuit –
comprising the cooling fans assembly, finned structure of the yoke
etc. – the temperature of the motor drops exponentially with respect
to time, once the motor is de-energized and allowed to coast down.
• During starts the cooling circuits will be almost ineffective.
Technology Training that works
Frequent Starts
•
•
•
•
•
•
The rotor will be running at much lesser speeds during starting.
Hence the induced currents are also high.
Because of this excessive heat is generated.
This results in thermal uneven expansion.
Rotor bars expand unevenly with respect to the rotor.
It causes the rotor bars to crack (at the joints where the bars are
welded to the shorting ring).
Technology Training that works
Frequent Starts
•
•
•
•
•
•
Due to the cracks the electrical resistance of the bars increases.
Hence heating of the rotor bars also increases.
The current is diverted through other rotor bars
Hence they get overheated.
All these result in a localized overheating of the rotor bars.
These high temperatures of the rotor may cause bowing effect
thereby reducing air gap / bearing clearances.
• This can result in mechanical damage to the rotor.
Technology Training that works
High Inertia
• As a motor draws very high current during acceleration phase, the
windings get overheated.
• Typical high-inertia loads are certain fans, blowers, pumps, and
some kinds of machine tools.
• As a thumb rule, if the load’s moment of inertia is more than twice
that of the motor, the acceleration gets prolonged and it can be
considered as a high inertia load-motor combination.
• A high inertia load usually demands high torque and hence lesser
acceleration torque – difference of motor torque and the load torque is available for driving the load.
Technology Training that works
High Inertia
Methods of protecting the motor:
• Internal temperature protection for the motor – instead of relying on
indirect temperature measurement methods.
• Solid-state protection using modern day sophisticated relays.
• If efficiency is not a bar, NEMA Design D motor can be used.
• Close / regular monitoring.
Technology Training that works
Incorrect belt alignment
• Wherever possible, make lower side of the belt the driving side
• Pulley ratio must not exceed 8:1. If not, the manufacturer must
approve it
• The drive sheave on the motor should be centered on the shaft
extension
• The overhung load (OHL, a bending moment that results from drive
tension) shall be kept to a minimum. An increase in OHL can gave
doubling effect in the reduction of the L10 bearing life. Reducing the
shaft extension can minimize this
• When using a belt drive on a horizontal application, any belt sag
should be on the top. Optimum drive/pulley contact occurs when the
tight segment of the belt is on the bottom
Technology Training that works
Incorrect belt alignment
• Vertical belt drives cause more problems than horizontal belt drives
• If an application requires a vertical belt drive, make arrangements to
mount the drive pulley at the top
• Optimum belt / drive pulley contact is achieved when the tight part of
the belt is pulling upward
• A belt drive must be not be too tight as to overload the motor or put
unwanted extra force on the motor bearings
• At the same time it should be tight enough to avoid it from slipping
• Adjust the tension by changing the distance between the motor and
driven load. The tension must be just enough to prevent excessive
bow on the slack side
Technology Training that works
Typical Belt drive
Technology Training that works
Misalignment
• Misalignments in belt drives can be categorized as angular & parallel:
• Angular Misalignment normally results from improper mounting of
motor / reducer
• A skewed bushing or a bent shaft can also contribute to angular
misalignment
• It is measured as angle between shafts or in Mils per Inch of
coupling diameter
• Normally an angular misalignment of less than 0.002 in for each
inch diameter of pulley is considered to be fine
• Parallel Misalignment is the misalignment arising out of mounting of
motor / reducer on different planes as shaft centerlines don’t coincide
• It is normally measured in Total Indicated Run out, TIR in Mils
(0.001”)
Technology Training that works
Misalignment
• Bearing misalignments also can play havoc and can be
classified as static or dynamic.
• Static misalignment arises due to a non varying static load
(like deflection) and is due to axes being not co-linear or
the supports being not in the same plane.
• Dynamic misalignment normally arises due to a bent
shaft, which results in a balance problem as well as
clearance problems in the bearings resulting in undue
fatigue.
Technology Training that works
Insulation system
• The insulation system of an AC induction motor mainly consists of
the
 Ground wall insulation - the slot liner paper that protects the
insulated copper to ground
 Phase-to-phase insulation - a sheet of insulation paper that is laid
between the phases
• ·Turn-to-turn insulation - often the weakest link in the insulation
system, the enamel on the copper of a random wound motor or the
tape found on form coils. This insulation’s purpose is to protect from
copper to copper failures.
Technology Training that works
Insulation system – degradation
process
• Thermal Stresses are imposed on the insulation by the operating
temperature.
• Overloads will also cause the motor temperature to rise steeply, for
very short periods may be, causing the degradation of the mica / resin
bond.
• Electrical Stress is a result of the working voltage of the machine and
increases as the voltage is raised above normal values, even for brief
time intervals.
Technology Training that works
Insulation system – degradation
process
•Mechanical stresses are due to the operating philosophy.
• For example, direct on-line starting of motors will exert severe
forces on the end winding structure.
•Environmental stresses are a result of oxidation of the organic
material, contamination (from water, oil, dust, carbon, salinity,
sand, corrosion etc), deposition etc.
• As a result of these, the insulation can age and crack. Surface
deposition and ingress of moisture make the stator windings to
suffer a lot.
Technology Training that works
Bearing current problems
• Bearing currents are produced in different forms and
almost all rotating machines, either large or small in size,
have a bearing current problem whether it is DC or AC.
• Even though bearing current is caused by an electrical
phenomenon, it results in mechanical damages.
• Electric current flow in bearings can be seen
simultaneously on both the races and the rolling
element.
• The bottom of the depression will be dark in color and is
known as fluting.
Technology Training that works
Bearing current problems
• The various sources of shaft voltage can be broadly
categorized as –
• Electromagnetic induction
• Electrostatic coupled from internal sources and
• Electrostatic coupled from external sources.
Technology Training that works
Fluting of a bearing
Technology Training that works
Bearing current problems
• IBGT technology has resurrected bearing problems due to electrical
discharge, creating a new challenge to manufacturers of electric
motors
• New problems arose because PWM inverters equipped with IGBT
inverters distort the sinusoidal supply generating high frequency
harmonics and high (dv/dt)s
• Inverter switching mechanism also creates what is called commonmode voltage
• Due to the high switching frequencies of IGBT inverters, parasitic
capacitances between stator winding and stator, and between rotor
and stator winding become relevant. Lead to a common mode current
flowing through the motor bearings.
Technology Training that works
Bearing current problems - remedies
• Insulated bearings serve as a barrier – to break the path for circulating
currents.
• Shaft brush reduces stray current through the motor bearings by half,
as a result of short-circuiting the path between rotor and stator.
• ABB has recently patented a motor winding designed to eliminate
circulating bearing currents.
Technology Training that works
Testing
•
•
•
•
•
•
•
•
•
•
•
Insulation life and resistance
Polarization index
Dc hipot
Dc ramp test
Ac hipot
Capacitance test
Dissipation factor
Partial discharge
Surge test
Mechanical testing
Online testing
Technology Training that works
Why the Tests are required?
• A motor has many rotating parts
– Deterioration due to wear and tear becomes
inevitable.
• Stator winding insulation operates at high temperatures
– Insulation material degrades over a period of usage.
• To assess the condition of these components and
materials
– To check the suitability for charging
– To re-condition in a predictive manner
• These tests taken up in a pro-active manner provide
– very cost effective
– condition-based solutions
for the asset-manager
Technology Training that works
Insulation resistance
•
•
•
•
Easiest field test
Monitors the health of the winding’s insulation
Popular in most of the countries as “megger," test
This test
– Applies DC voltage, usually 500 or 1000 Volts
– Measures the resistance of the insulation
– Low current leakage is measured
– Converted to a measurement of Meg, Gig or TerraOhms
Technology Training that works
Insulation resistance trending
• A megger reading of a motor, alone, conveys very little
information
– about incipient faults, if any
• Trending of the insulation resistance
– a curve recording resistance, with the motor cold and
hot, and date
– indicates the rate of deterioration.
• This curve provides the information needed
– to decide if the motor can be safely left in service until
the next scheduled inspection time
Technology Training that works
Trending of Insulation Resistance
Technology Training that works
Temperature correction
• Insulation resistance is highly sensitive to
temperature
• Actually measured values when plotted may
show wild variations
– Even when the insulation is healthy
• Temperature correction of insulation test results
for trending is a must
• Also the temperature of the object must stabilize
– can be 30 minutes or more after deenergization
Technology Training that works
Temperature correction
•
IEEE Standards – regarding IR values – stipulate that
– All measurements have to be related to a temperature of 40 deg C.
R40 = k R
R – insulation resistance measured at the
specific temperature.
R40 – insulation resistance corrected to a
temperature of 40 0C.
k - the temperature correction factor (to be
taken from the reference curve)
Technology Training that works
Reference curve for temperature
correction of IR value
Technology Training that works
Polarization Index
• A gradual increase in the reading of the insulation resistance shows
the Polarization phenomenon. This is because of
• the charging of the insulation system, much like a capacitor
• results in the charging of the capacitor like dielectric medium and
• hence a reduction in the absorption current.
• The ten minute reading divided by the one minute reading gives the
Polarization Index, the PI value of the insulation system
• It serves as an indication of the average polarization of the material.
• IEEE recommends a value of 2.0 or higher as acceptable.
Technology Training that works
Polarization Index
• According to the IEEE 43-2000, insulation values over
5,000 Meg Ohms need not be evaluated using PI.
• Even though this test gives an insight into the
healthiness of the insulation system,
• it also looks at only the ground insulation and
• will not see the problems neither in the turn-to-turn
insulation nor the weaknesses in the insulation system.
Technology Training that works
DC Hi Pot Test
• This test can uncover insulation weaknesses.
• In addition to measuring overall insulation resistance to
ground, it provides information on insulation dielectric
strength.
• It can detect insulation weaknesses that are likely to fault
to ground if subjected to the high transient voltage
surges that commonly occur on industrial power
systems.
• However a weak insulation may permanently breakdown
• As it can cause violent polarization of the insulation
system
• Hence due care is to be taken while carrying out the test
• A voltage up to twice the rated voltage plus 1,000 Volts
multiplied by 1.732 is applied in order to stress the
insulation system
Technology Training that works
DC Ramp Test
• It requires only one person to perform the test.
• Also it provides the testing engineer with better control
and sufficient foresight of impending failure to avoid
damage to the insulation.
• The elimination of the human factor in controlling the
voltage and current parameters yields overall test results
that are much more accurate and repeatable.
• The slow and continuous increase in applied voltage
(normally 10 – 20 volts per second) is less likely to
damage insulation than the step-method voltage
increments (approx. 1 kV per second).
• Typical ramped-voltage test response curves are a
composite overlapping of the capacitive charging
current, absorption current and leakage currents plotted
against time.
Technology Training that works
AC Hi Pot Test
• AC high potential tests impress a high voltage sine wave
• A voltage of twice the motor rated voltage plus 1,000 volts
is applied across the insulation system.
• Hence only a good insulation system without any
contamination or degradation can pass the test.
• In case of even moisture being present in moderate
quantities can damage the insulation system.
• Unlike the DC high pot test the defect point gets ionized
very easily as the AC voltage has the potential for
penetration through the dielectric.
• Hence this test is much more vicious than other tests.
Technology Training that works
Equivalent circuit of a dielectric
• An insulation system can be represented, on a
macroscopic level, as a parallel circuit
• containing resistance and capacitance in each branch
and
• can be represented as shown in the earlier slide.
• The total current drawn by the insulation under testing,
the specimen, can be considered to be having two
components:
• capacitive and resistive.
• Depending on the capacitive current the capacitance
value can be determined.
Technology Training that works
Equivalent circuit of a dielectric
Technology Training that works
Dissipation Factor
• The currents drawn by the specimen can be vectorially
represented as shown in the next slide.
• The ratio between the resistive component and the total
current drawn gives the power factor.
• The dissipation factor of the insulation system can be
defined as the ratio of the capacitive current to the
resistive component.
• The Dissipation Factor is usually known as loss tangent
or Tan d of the insulation system
Technology Training that works
Vector components of test voltage
and current
Technology Training that works
Partial Discharge measurements
• This is an on-line test
• Monitors the healthiness of the machine in energized
condition
• Depends on the principle that an insulation as it ages
or due to some voids in it will allow leakage currents
to pass through as sudden discharges.
• Once Tan d results are found alarming, but the
equipment is out in service, this measurement helps
in regular monitoring and can be relied upon to avoid
an untoward incident.
• Premature failures of insulation systems have been
attributed to the action of partial discharges. Hence
this test.
Technology Training that works
A typical PD measurement setup
Technology Training that works
Partial Discharges
• Partial discharges occur when minor defects are present
in electrical insulation systems.
• When partial discharges occur repetitively in solid
insulation material, the destructive energy released
deteriorates the insulation material at that site.
• Over a period of time, this deterioration spreads to
others in its vicinity and may lead to failure of insulation.
Technology Training that works
Stress - PD activity versus time
Technology Training that works
Surge comparison
• Surge comparison testing is used to detect winding faults and
defective insulation in coils, motors, transformers & generators.
Using surge comparison, following faults can easily be detected:
– Turn-To-Turn faults
– Coil-To-Coil faults
– Phase-To-Phase faults
•
The faults that can be detected are:
– Short Circuits
– Open Circuits
– Grounding
– Misconnections
– Wrong turns count
Technology Training that works
Surge comparison testing
• Insulation is tested by applying a series of brief, high
voltage pulses to a pair of windings.
• If the two windings are identical, their patterns will get
superimposed and match perfectly. Hence only a single,
stable waveform can be observed.
• Otherwise the pattern will be unstable, as the insulation
breaks down.
• If a double line appears there must be a fault in the
winding.
• If the pattern is unstable and flickering, the insulation is
at the verge of its breakdown.
• Various types of faults that can be detected are as
shown in the following slides
Technology Training that works
Turn-to-turn short & coil-to-coil short
detection in a Surge comparison test
Technology Training that works
An open coil connection / complete
grounding detection - Surge
comparison test
Technology Training that works
Group-to-group / phase-to-phase
shorts detection in a Surge
comparison test
Technology Training that works
Under-voltage / Over-voltage
• As per NEMA MG1 standards,
– AC induction motors shall operate satisfactorily at rated
load, with
– the voltage varying within + / - 10 % of rated value
– at rated frequency.
•
With the voltage decrease in this range, power factor of
the AC induction motor increases.
• Same way an increase in voltage results in a decrease of
the power factor.
• The torque developed by the motor, whether of locked
rotor or of breakdown will be proportional to the square of
the voltage applied.
Technology Training that works
Under-voltage / Over-voltage
• Average accelerating torque is given as
2
voltage available at motor bus
rated motor voltage
•
* Rated torque
Load torque
Due to the reduced accelerating torque,
motor will have problems in starting and reaching full
speeds. Also a running motor may lose speed and
draw heavy currents.
Hence under voltage protection is invariably provided
for induction motors
Technology Training that works
Under frequency
• AC motors shall operate successfully under running conditions
at rated load and at rated voltage with a variation in the
frequency
– up to 5 percent above or below the rated frequency.
• At a frequency lower than the rated frequency,
– the speed is decreased.
• Since the magnetic flux in the machine, which is proportional to
the inverse of frequency at a particular voltage, increases
– locked-rotor torque also increases and power factor
decreases.
• Also this may result in over magnetization of the core of the
motor that in turn may result in
– overheating of the stator due to increased iron losses.
Technology Training that works
Pole slip / Out of step
• These aspects are purely applicable to synchronous machines only.
• During a pole-slip condition,
– negative currents can be induced into the field which is opposite
of the normal positive current flow produced by the excitation
system.
• The out-of-step conditions (loss of synchronism) of a synchronous
machine may occur as a result of pole slipping and hence
– pole slipping protection also detects loss of synchronism, but with
the excitation intact.
• Synchronous motors can develop torque only in synchronism.
• Overloading, beyond motor’s capability, may result in slowing down of
the rotor.
• Once synchronism is lost, motor will not be able to develop any
torque. This is called motor going out of step.
Technology Training that works
Loss of Excitation
• Synchronous motors can be protected against loss of excitation
– by a low-set undercurrent relay connected to the field.
– This relay should have a time delay drop out.
• On large synchronous motors
– an impedance relay is frequently applied that operates on
excessive VAR flow into the machine, indicating abnormally low
field excitation.
• If an under voltage unit is part of the relay, its function should be
shorted out because loss of motor field may produce little or no
voltage drop.
• Operation of synchronous motors drawing reactive power from the
system can result in overheating in parts of the rotor that do not
normally carry current. Some loss-of-field relays (device 40) can
detect this phenomenon.
Technology Training that works
Inadvertent energization
• Used for synchronous motors
– especially to avoid any accidental closing of the breaker
when the supply to the motor fails and the motor is coasting
down.
• Due to the stored energy in the drive, especially from the driven
side,
– motor starts acting like a Generator.
• Under such circumstances, the supply being restored will be out
of phase with motor generated voltage and there can be a
resultant flashover.
Technology Training that works
Over fluxing
• It is a phenomenon of over magnetization and results due to low
frequency at the same voltage level.
• This in turn may result in overheating of the stator, due to
increased iron losses.
• If left unchecked, further fall in frequency will result in saturation
of the magnetic core thereby impairing its torque delivering
capability.
• This kind of protection must invariably be provided in
applications where the frequency of the supply is varied in order
to obtain variable speeds.
• All modern day variable frequency drives have this protection
built into the logics and hence they are called as variable voltage
variable frequency drives, VVVF drives in short.
Technology Training that works
The Positive, Negative and Zero
Components
Technology Training that works
Unbalanced Supply Voltages
•
It can happen due to
– Unbalanced loading of the power system
– single phase loads, blown fuses, accidental opening of one phase lead
•
•
•
Higher voltage unbalances will result in reduced efficiency, overheating of the
motor calling for de-rating the power rating of the motor.
NEMA Standard MG 1–14.35, recommends the derating of the motor where the
voltage unbalance is between 1% and 5% beyond which operation shall not
continue.
Unbalanced voltages will give rise to a pulsating flux in the rotor bars.
– This will result in uneven heating of the rotor bars and hence localized overheating
will be taking place.
– Uneven expansion due to the localized heat of the rotor bars can be detrimental to
the rotor's integrity.
– This can result in the development of cracks ending up finally as rotor bar failures.
Technology Training that works
Negative sequence currents
• Larger rating motors are more prone to dangers arising out of
negative sequence currents flowing.
• The presence of negative sequence can be expressed as a
percentage with respect to the positive sequence currents.
• To detect whether the unbalance can have a deleterious effect or
not,
– it is required to analyze the three phase voltages both by means
of the phase angle as well as magnitude difference
– not just magnitude alone - as is the case with unbalanced
voltage protection
Technology Training that works
Derating Factors
• The factors that need to be considered in de-rating a motor’s
performance are:
– Supply Voltage
– Supply Frequency
– Ambient Temperature
– Altitude of the location of installation
• When voltage is 10% below the rated voltage of the motor, the
motor has 20% less starting torque.
• An increase in frequency of 5 % results in a 10 % decrease in the
motor starting torque.
• Standard motors are designed to operate below 3300 feet (1000
m). Roughly for every 1600 feet rise in altitude, the derating factor
reduces by 0.04.
Technology Training that works
Earth fault Protection
• Faults that occur within the motor windings are mainly earth
faults
– caused by a breakdown in the winding insulation.
– This type of fault can be very easily detected by means of an
instantaneous relay,
– usually with a setting of approximately 20% of the motor full
load current,
– connected in the residual circuit of three current transformers.
• Unbalanced load currents do not cause nuisance earth-fault
trips. If there is no leakage to earth, unbalanced load currents
add to zero and do not cause an output from a core-balance
CT.
Technology Training that works
New technologies and developments
•
•
•
•
•
•
•
•
Energy efficient designs – the modern trend
Digital protection
Intelligent controllers
Focus on Lubrication techniques
Bearing Isolators
Improvised condition monitoring
Reliable on-line diagnostics
Improved Testing methods
Technology Training that works
Energy efficient motors
•Re-designed shaft mounted cooling fans
• reduced windage losses
•Thin gage of sheet steel used in the stator
• Reduces eddy current losses
• Careful, close machining of the rotor
•reduced stray load losses
• Motor manufacturers are adopting different, better
designs and specific techniques to minimize the losses
Technology Training that works
Digital protection
•
•
•
•
Lot of protections integrated into one assembly
Versatility of the algorithms
Universal relays helping interchangeability
Very compact in size
• as a result of surface mounted technology of the
PCB components
• Require very little power
• little stress on the station power backup facility
• the heat dissipation will also be lower
Technology Training that works
Intelligent controllers
• Highly sophisticated, reliable controllers that can take
care of the mind-boggling logics
• Application of squirrel cage induction motors is
made possible for a majority of applications
• Easy integration with the higher end control systems
without any hassles
• Enhanced connectivity
• typically a PROFIBUS or a MODBUS
• Reduced efforts in physical monitoring
• due to availability of HMI (Human Machine
Interface)
Technology Training that works
Focus on Lubrication techniques
• Synthetic greases
• Long lasting
• Temperature tolerant
• Little maintenance, longer re-greasing intervals
• Grease guns with the quantity calibrated
Technology Training that works
Bearing Isolator
• Makes the bearing long lasting
• Keeps a check on the contamination of the
grease
• Prevents water contamination
• Stops the ingress of humid air
• Serves as even Emergency sleeve bearings
• for short periods
Technology Training that works
Improved condition monitoring
• FFT (Fast Fourier Transform) analyzers
• To pin point the source of vibration
• Very sophisticated software
• to collect, retrieve & analyze the data about
various vibration parameters
• Infra red thermography
• useful for the detection of hot spots
• Modern balancing machines
• Facilitate easy balancing without a trial & error
method
Technology Training that works
Reliable on-line diagnostics
• Motor Current Signature Analysis (MCSA)
• to identify the health of stator winding, rotor,
air gap eccentricities etc.
•FFT Analyzers
• for analyzing or trending dynamic, energized
systems
• Electrical signature analysis
• problems related to supply, control, motor,
coupling, load and process
•On-line partial discharge (PD) measurements
• effective, reliable method of assessing stator
insulation condition
Technology Training that works
Improved Testing methods
• Sophisticated equipment for
• Insulation Resistance measurement
• Ramp Testers etc.
• Hi Pot Test equipment with built-in safety features
• Surge Comparison testers
• Motor Circuit Analysis (MCA)
• Involving low voltage tests
• Promising future for MCA techniques – as the tests utilize
resistance, inductance, phase angle, current / frequency response
etc. to detect various defects like winding defects.
Technology Training that works
Any questions ?
Technology Training that works