2. Motor Enclosures
Energy Efficient Motors
Classification of Motors
Construction Details of Motor
Motor Fundamentals
CONTENTS
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3. ELECTRIC MOTOR
• An electric motor is an electromechanical device
that converts electrical energy to mechanical
energy.
• The mechanical energy can be used to perform
work such as rotating a pump impeller, fan, blower,
driving a compressor, lifting materials etc.
Input = Electrical Power
Output = Mechanical Power
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10. How Does an Electric Motor Work?
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11. Type of Electric Motors
Electric Motors
Alternating Current (AC)
Motors
Direct Current (DC)
Motors
Synchronous Induction
Three-Phase
Single-Phase
Self Excited
Separately
Excited
Series Shunt
Compound
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13. DC Motor
Although AC motors are the most common type of motor
used in industry, direct current (DC) motors are also
used.
One common use for a DC motor is as a backup motor
for a critical process.
DC motors can run on the direct current supplied by a
battery when there is a failure in the alternating current
supplied to an AC motor.
For example, a DC motor can used with a backup pump
that supplies oil to the bearings in a large piece of
equipment.
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14. DC Motor Parts
DC motor stator consists of permanent magnets.
The rotating part, which is shown as a loop of wire, is
called the armature.
The armature is connected to a source of DC power.
Two components called brushes are connected to a DC
power source.
A conducting ring, known as a commutator, is mounted
on the end of the armature. The commutator is not a
solid ring. It consists of conducting segments that are
separated from each other.
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15. DC Motor
During operation, the commutator makes sliding contact
with the brushes.
Current flows from the power source to armature, through
the brush and commutator.
The current flow through the armature creates a magnetic
field with a north pole and a south pole.
The poles are perpendicular to the armature. When
current is supplied to the armature, motor action is
produced.
The interaction between the stator's magnetic field and
the armature's magnetic field causes the armature to
rotate.
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17. DC Motor
However, as the armature turns, the commutator
physically changes the direction in which the current flows
through the armature. This change in direction changes
the polarity of the magnetic field created by the armature.
The brushes and the commutator in a DC motor enable
the armature to change its magnetic field. As a result, the
armature turns continuously.
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19. DC Motor
The major parts of the example DC motor include the
frame used to house the stator and armature; the stator,
which may also be referred to as the field; the armature;
the commutator; and end bells.
The stator is made of coils of wire that are wrapped
around iron cores. The coils are electrically connected to a
DC power source.
The armature contains several loops of wire that are
wound back and forth. All of the loops make an electrical
connection to the motor's commutator segments. The
electrical connections to the commutator segments are
protected and held in place by a wrapping of varnish-
coated fibers.
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20. DC Motor
Current flows from one commutator segment, through the
armature, and back through another commutator
segment.
The commutator makes sliding contact with a set of
brushes. The brushes, which fit in holders, are held
against the commutator by springs to maintain contact
and to provide a path for current flow from the power
source to the commutator.
Brushes are a frequent maintenance item for DC motors.
Because they rub against the commutator, they wear
down, so they must be replaced periodically.
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22. Types of D.C. Motors
Shunt-wound motor
Field winding is connected in parallel with the armature
The current through the shunt field winding is not the
same as the armature current. Shunt field having
relatively large number of turns of wire having high
resistance.
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23. Types of D.C. Motors
Series-wound motor
Field winding is connected in series with the armature.
Series field winding carries the armature current
Series field winding has a relatively small number of
turns of thick wire and with a low resistance.
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24. Types of D.C. Motors
Compound-wound motor
It two field windings; one connected in parallel with the
armature and the other in series with it. The compound
machines always designed so that the flux produced by
shunt field winding is considerably larger than the flux
produced by the series field winding.
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25. DC Motor
Sparking or arcing near the brushes or on the
commutator can mean that the brushes need to be
replaced or that they are not making good contact with
the commutator.
In addition, brushes can chip, which impairs their
effectiveness.
The commutator should also be checked periodically. Any
scoring or grooving on the face of the commutator may
indicate a problem.
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26. AC MACHINES
NEMA MG 1-2003 has the following definitions:
An induction machine is an asynchronous machine that
has a magnetic circuit interlinked with two electric
circuits, or sets of circuits, rotating with respect to each
other. Power is transferred from one circuit to another
by electromagnetic induction.
A synchronous machine is an alternating-current
machine in which the average speed of normal
operation is exactly proportional to the frequency of the
system to which it is connected.
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27. SYNCHRONOUS MOTOR
A synchronous motor is a synchronous machine used
for a motor. A synchronous motor cannot start without
being driven. They need a separate starting means.
There are several types of synchronous motors.
Direct current excited synchronous motor (field poles are
excited by direct current)
Permanent magnet synchronous motor (field excitation is
provided by permanent magnets)
Reluctance synchronous motor (starts as an induction
motor, is normally provided with a squirrel cage winding,
but operates at synchronous speed).
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28. Synchronous motors have fixed stator windings
electrically connected to the AC supply.
Three-phase stator is similar to that of an induction
motor.
A separate source of excitation connected to a field
winding on the rotating shaft.
The rotating field has the same number of poles as the
stator, and is supplied by an external source of DC.
Magnetic flux links the rotor and stator windings
causing the motor to operate at synchronous speed.
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Synchronous Motor
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30. Synchronous Motor
Synchronous motors can be classified as brush
excitation or brushless excitation.
Brush excitation consists of cast-brass brush holders
mounted on insulated steel rods and supported from
the bearing pedestal. The number of brushes for a
particular size and rating depends on the field current.
Sufficient brushes are supplied to limit the current
density to a low value. The output of a separate DC
exciter is applied to the slip rings of the rotor.
A brushless excitation system utilizes an integral exciter
and rotating rectifier assembly that eliminates the need
for brushes and slip rings.
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31. Synchronous Motor
An important drawback of a synchronous motor is that
it is not self-starting and auxiliary means have to be
used for starting it.
A synchronous motor starts as an induction motor, until
the rotor speed is near synchronous speed where it is
locked in step with the stator by application of a field
excitation.
When the synchronous motor is operating at
synchronous speed, it is possible to alter the power
factor by varying the excitation supplied to the motor
field.
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32. A synchronous motor runs at synchronous speed or not
at all. Its speed is constant (synchronous speed) at all
loads.
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Synchronous Motor Speed
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33. In d.c. motors and induction motors, an addition of load
causes the motor speed to decrease. The decrease in
speed reduces the counter e.m.f. enough so that
additional current is drawn from the source to carry the
increased load at a reduced speed.
This action cannot take place in a synchronous motor
because it runs at a constant speed (i.e., synchronous
speed) at all loads.
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Synchronous Motor On Load
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Synchronous Motor On Load
What happens when we apply mechanical load to a
synchronous motor?
The rotor poles fall slightly behind the stator poles while
continuing to run at synchronous speed. The angular
displacement between stator and rotor poles (called
torque angle a) causes the phase of back e.m.f. Eb to
change w.r.t. supply voltage V. This increases the net
e.m.f. Er in the stator winding. Consequently, stator
current Ia ( = Er/Zs) increases to carry the load.
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36. Pull-Out Torque
There is a limit to the mechanical load that can be
applied to a synchronous motor. As the load increases,
the torque angle α also increases so that a stage is
reached when the rotor is pulled out of synchronism
and the motor comes to a standstill.
This load torque at which the motor pulls out of
synchronism is called pull—out or breakdown torque.
Its value varies from 1.5 to 3.5 times the full load
torque.
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37. Synchronous motor power factor
37
One of the most important features of a synchronous
motor is that by changing the field excitation, it can be
made to operate from lagging to leading power factor.
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38. Synchronous motor power factor
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Under excitation: When the rotor is underexcited, i.e.
the induced e.m.f. E is less than V, the stator current
has a lagging component to make up for the shortfall in
excitation needed to yield the resultant Weld that must
be present as determined by the terminal voltage, V.
Normal excitation: With more field current , however,
the rotor excitation alone is sufficient and no lagging
current is drawn by the stator.
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39. Synchronous motor power factor
39
Over excitation: And in the overexcited case, there is so
much rotor excitation that there is effectively some
reactive power to spare and the leading power factor
represents the export of lagging reactive power that
could be used to provide excitation for induction motors
elsewhere on the same system.
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42. 42
Introduction: Induction motor
Three-phase induction motors are the most common and
frequently encountered machines in industry
Simple design
Rugged
Inexpensive
High power to weight ratio
Easy to maintain
Direct connection to AC power source
Easy maintenance
Wide range of power ratings: fractional horsepower to MW
Run essentially as constant speed from zero to full load
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43. Squirrel Cage 3 phase winding in stator
Copper bars in rotor
Wound Rotor 3 phase winding in stator
3 phase winding in rotor
(Shorted internally)
Wound Rotor 3 phase winding in stator
with Slip Ring 3 phase winding in rotor
(Terminated to slip rings)
Types of Induction Motors
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44. The induction motor derives its name from the fact
that AC voltages are induced in the rotor circuit by the
rotating magnetic field of the stator
An Induction motor operates on the principle of
induction.
The rotor receives power from the stator due to
Induction The rotor is not connected to an external
source of voltage (Singly excited m/c).
The induction motor is the most commonly used type
of AC motor as It is simple, rugged in construction and
low in cost
Induction Motor
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46. The Stator in an AC motor is a wire coil, called a stator
winding, when this coil is energized by AC power, a
rotating magnetic field is produced
This rotating field is produced by the contributions of
space-displaced phase windings carrying appropriate
time displaced currents by 120 electrical degrees
When a magnetic field comes close to a wire, it
produces an electric voltage in that wire
This is called induction – (as Faraday's law)
In induction motors, the induced magnetic field of the
stator winding induces a current in the rotor
This induced rotor current produces a second
magnetic field necessary for the rotor to turn
Induction Motor
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47. The rotating magnetic field generated in the stator induces a
magnetic field in the rotor.
The two fields interact and cause the rotor to turn
To obtain maximum interaction between the fields, the air gap
between the rotor and stator should be very small
As you know from Lenz's law, any induced emf tries to oppose
the changing field that induces it, here the changing field is the
motion of the resultant stator field
A force is exerted on the rotor by the induced emf and the
resultant magnetic field
This force tends to cancel the relative motion between the rotor
and the stator field and the rotor, as a result, moves in the same
direction as the rotating stator field
Induction Motor
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48. It is, however, impossible for the rotor of an induction motor to
turn at the same speed as the rotating magnetic field
If the speeds were the same, there would be no relative motion
between the stator and rotor fields; without relative motion
there would be no induced voltage in the rotor
In order for relative motion to exist between the two, the rotor
must rotate at a speed slower than that of the rotating magnetic
field
The difference between the speed of the rotating stator field
and the rotor speed is called slip
The smaller the slip, the closer the rotor speed approaches the
stator field speed
Induction Motor
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49. The speed of the rotating magnetic field of the stator
can be calculated with the formula below (we shall
not go into details of how it is derived, but it is
simple, and follows from the equations for poly-phase
machines):
Ns=120fs / P
where P is the number of poles and fs is the
frequency of the stator applied voltage
When the stator is supplied by a balanced three-
phase source, it will produce a magnetic field that
rotates at synchronous speed determined by the
above relation
Induction Motor
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50. The rotor reacts to the magnetic field, but does not
travel at the same speed
Also the rotor speed actually lags behind the speed of
the magnetic field and rotor runs at the speed Nr
which is close to the speed of the stator field, Ns at no
load, but the rotor speed decreases as the load is
increased
The term slip quantifies the slower speed of the rotor
in comparison with the rotating speed of the stator
magnetic field and is expressed mathematically as:
S=(Ns-Nr)/Ns
SLIP
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51. The rotor is not locked into any position and therefore will
continue to slip throughout the motion
The speed of the rotor depends upon the torque requirements
of the load, higher the load, stronger the turning force needed
to rotate the rotor
The turning force can increase only if the rotor-induced e.m.f.
increases and this e.m.f. can increase only if the magnetic field
cuts through the rotor at a faster rate
To increase the relative speed between the field and the rotor,
the rotor must slow down
Therefore, for heavier loads the induction motor turns slower
than for lighter loads and the amount of slip increases
proportionally with increase in load
SLIP
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52. Actually only a slight change in speed is necessary to produce
the current changes required to accommodate the changes in
load (this is because the rotor windings have a low resistance)
As a result, induction motors are called constant-speed motors
(similarly to DC shunt motor)
SLIP
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54. 54
Locked rotor torque – the minimum torque that the motor
develops at rest for all angular positions of the rotor at rated
voltage and frequency.
Locked rotor current – the steady state current from the line at
rated voltage and frequency with the rotor locked.
Breakdown torque – the maximum torque that the motor
develops at rated voltage and frequency without an abrupt
drop in speed.
Pull up torque – the minimum torque developed during the
period of acceleration from rest to the speed that breakdown
torque occurs.
Common terms
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55. On start-up the slip is s=1 and the starting torque (also known
as a breakaway torque) is sufficiently large to accelerate the
rotor (the rotor has previously been 'locked' - stationary)
As the rotor runs up to its full-load speed the torque increases
in essentially inverse proportion to the slip
After the torque reached its maximum, it rapidly falls to zero, at
the synchronous speed, Ns
Looking backwards: as rotor speed falls below Ns the torque
increases almost linearly to a maximum dictated by the full
load (plus rotor losses)
the speed only falls a little when the load is raised from 0 to its
full value - this is a normal operating region
Analysis of Operation
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56. Other key features:
The maximum speed is a synchronous speed, Ns,
independent of the applied voltage
Torque is proportional to the V2 at an arbitrary speed
When operating at 90-95% Ns heat losses are at
minimum
Analysis of Operation
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57. Components Of Induction Motor
A 3-phase induction motor has two main parts:
• A stator – consisting of a steel frame that supports a
hollow, cylindrical core of stacked laminations. Slots on
the internal circumference of the stator house the
stator winding.
• A rotor – also composed of punched laminations, with
rotor slots for the rotor winding.
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58. Stator
consisting of a steel
frame that supports a
hollow, cylindrical core
core, constructed from
stacked laminations,
having a number of
evenly spaced slots,
providing the space for
the stator winding
Induction Motor - Construction
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61. COMPONENTS OF INDUCTION MOTOR
There are two-types of rotor windings:
• Squirrel-cage windings, which produce a squirrel-cage
induction motor (most common)
• Conventional 3-phase windings made of insulated
wire, which produce a wound-rotor induction motor
(special characteristics)
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62. Induction Motor: Squirrel cage rotor
Squirrel cage rotor consists of copper bars, slightly longer
than the rotor, which are pushed into the slots.
The ends are welded to copper end rings, so that all the
bars are short circuited.
In small motors, the bars and end-rings are diecast in
aluminium to form an integral block.
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64. Induction Motor: Wound Rotor
A wound rotor has a 3-phase winding, similar to the stator
winding.
The rotor winding terminals are connected to three slip rings
which turn with the rotor. The slip rings/brushes allow
external resistors to be connected in series with the winding.
The external resistors are mainly used during start-up –
under normal running conditions the windings short
circuited externally.
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67. 67
Induction motor speed
At what speed will the IM run?
Can the IM run at the synchronous speed, why?
If rotor runs at the synchronous speed, which is the
same speed of the rotating magnetic field, then the
rotor will appear stationary to the rotating magnetic
field and the rotating magnetic field will not cut the
rotor. So, no induced current will flow in the rotor and
no rotor magnetic flux will be produced so no torque
is generated and the rotor speed will fall below the
synchronous speed
When the speed falls, the rotating magnetic field will
cut the rotor windings and a torque is produced
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68. Specifications
Following basic parameters are embossed on motor name plate
Voltage Bearing
Frequency Insulation class
Current Degree of protection
Kilo Watt Duty
Phase RPM
Serial number Cooling
Frame Mfg. details
Efficiency Ambient Temperature
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69. 70
Temperature :
Insulating materials are divided into seven classes in terms of
withstanding maximum temperature
Y up to 90°C Un impregnated Paper, Cotton, Silk ….
A up to 105°C Paper, Cotton, Silk impregnated with oil
E up to 120°C Phenol formaldehyde mouldings
B up to 130°C Inorganic fibrous and flexible materials (mica
glass etc) bonded with suitable organic resins such as
shellac bitumen, alkyd, epoxy etc.
F up to 155°C As Class B – but with resins such as alkyd,
epoxy, silicone alkyd etc.
H up to 180°C As Class B – but with silicone resins
C above 180°C – mica, asbestos, ceramics, glass (alone or
with inorganic binders or silicone resins), polyamides or
polytetrafluoroethylene - PTFE
Electrical Insulation
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71. f = P N / 120
(Where f is frequency in Hz, P is no. of pole and N is speed
in rpm)
1 H.P. = 746 Watts = 0.75 KW (approx.)
P α D2 L n
(Where P is output, D is diameter, L is length and n is
speed)
slip s = (ns - nr) / ns
(Where ns is synchronous speed in rpm and nr is rotor
speed in rpm )
Motor Fundamentals
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72. The nameplate details of a motor are given as
Power, P = 15 kW, Efficiency, η = 0.9
Using a power meter the actual three phase
power drawn is found to be 8 kW
Find out the loading of the motor
Example
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73. The nameplate details of a motor are given as
Power, P = 15 kW, Efficiency, η = 0.9
Using a power meter the actual three phase
power drawn is found to be 8 kW
Input power at full-rated power in kW, Pir = 15 / 0.9
= 16.7 kW
Percentage loading = 8 / 16.7
= 48 %
Example
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74. Input Power Measurements
First measure input power Pi with a hand held or in-line power meter, Pi
= Three-phase power in kW
Note the name plate rated kW and Efficiency
The figures of kW mentioned in the name plate is for output conditions
So corresponding input power at full-rated load
ηfl = Efficiency at full-rated load
Pir = Input power at full-rated power in kW
The percentage loading can now be calculated as follows
Loading of Motor
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75. Performance Terms and Definitions
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Efficiency :
The efficiency of the motor is given by
in
loss
in
out
P
P
1
P
P
Where Pout – Output power of the motor
Pin – Input power of the motor
PLoss – Losses occurring in motor
Motor Loading :
Motor Loading % =
Actual operating load of the motor
Rated capacity of the motor
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76. Motor Efficiency
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Electric motors are electromagnetic energy converters
whose function is based on the force exerted between
electrical currents and magnetic fields – which are
usually electrically excited as well. A typical value for an
11 kW standard motor is around 90 per cent and, for
100 kW, up to 94 per cent.
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77. Efficiency of Electric Motors
Motors loose energy when serving a load
• Fixed loss
• Rotor loss
• Stator loss
• Friction and Windage
• Stray load loss
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78. Motor Losses
Core Losses: A combination of eddy-current and hysteresis
losses within the stator core. Accounts for 15 to 25 percent of
the overall losses.
Friction and Windage Losses: Mechanical losses which occur
due to air movement and bearings. Accounts for 5 to 15
percent of the overall losses.
Stator Losses: The I2R (resistance) losses within the stator
windings. Accounts for 25 to 40 percent of the overall losses.
Rotor Losses: The I2R losses within the rotor windings.
Accounts for 15 to 25 percent of the overall losses.
Stray Load Losses: All other losses not accounted for, such as
leakage. Accounts for 10 to 20 percent of the overall losses.
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80. kVA
kW
Cos
Factor
Power
As the load on the motor reduced, the magnitude of active current reduces. However,
there is not a corresponding reduction in the magnetizing current, with the result motor
power factor reduces, or gets worse, with a reduction in applied load.
Power Factor
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81. Energy Efficiency Opportunities
82
Use energy efficient motors
Reduce under-loading (and avoid over-sized
motors)
Size to variable load
Improve power quality
Rewinding
Power factor correction by capacitors
Improve maintenance
Speed control of induction motor
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82. 83
Reduce intrinsic motor losses
Efficiency 3-7% higher
Wide range of ratings
More expensive but
rapid payback
Best to replace when
existing motors fail
Use Energy Efficient Motors
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83. 84
Reduce Under-loading
• Reasons for under-loading
• Large safety factor when selecting motor
• Under-utilization of equipment
• Maintain outputs at desired level even at low
input voltages
• High starting torque is required
• Consequences of under-loading
• Increased motor losses
• Reduced motor efficiency
• Reduced power factor
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84. 85
Sizing to Variable Load
• Motor selection based on
• Highest anticipated load: expensive and risk
of under-loading
• Slightly lower than highest load: occasional
overloading for short periods
• But avoid risk of overheating due to
• Extreme load changes
• Frequent / long periods of overloading
• Inability of motor to cool down
X
Motors have
‘service factor’
of 15% above
rated load
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85. 86
Improve Power Quality
Motor performance affected by
• Poor power quality: too much fluctuations in
voltage and frequency
• Voltage unbalance: unequal voltages to three
phases of motor
• Keep voltage unbalance within 1%
• Balance single phase loads equally among
three phases
• Segregate single phase loads and feed them
into separate line/transformer
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86. 87
Rewinding
• sometimes rewinding reduces motor
efficiency considerably
• Can reduce motor efficiency
• Maintain efficiency after rewinding by
• Using qualified/certified firm
• Maintain original motor design
• Replace 40HP, >15 year old motors instead of
rewinding
• Buy new motor if cost of rewinding is more
than 50 cost of new motor.
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87. 88
Improve Power Factor (PF)
• Use capacitors for induction motors
• Benefits of improved PF
• Reduced kVA
• Reduced losses
• Improved voltage regulation
• Increased efficiency of plant electrical system
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88. Power Loss Area Efficiency Improvement
1. Fixed loss (iron) Use of thinner gauge, lower loss core steel reduces eddy current
losses. Longer core adds more steel to the design, which reduces
losses due to lower operating flux densities.
2. Stator I2R Use of more copper & larger conductors increases cross sectional area
of stator windings. This lower resistance (R) of the windings & reduces
losses due to current flow (I)
3 Rotor I2R Use of larger rotor conductor bars increases size of cross section,
lowering conductor resistance (R) & losses due to current flow (I)
4 Friction & Windage Use of low loss fan design reduces losses due to air movement
5. Stray Load Loss Use of optimized design & strict quality control procedures minimizes
stray load losses
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Use Energy Efficient Motors
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90. PMI Revision 01 91
Motor Efficiency
Electric motors are electromagnetic energy converters
whose function is based on the force exerted between
electrical currents and magnetic fields – which are usually
electrically excited as well. A typical value for an 11 kW
standard motor is around 90 per cent and, for 100 kW, up
to 94 per cent.
91. PMI Revision 01 92
Motor losses
The % losses indicated are for 3000 rpm motors, and
1500 rpm motors in brackets.
Core Loss : approx 18% (22%) of total loss at full load
Stator and Rotor Resistance (I2R) Loss: approx 42% (56%) of total loss
at full Load
Friction and Windage Loss approx 30% (11%) of total loss at full load
Stray Load Loss : approx 10%(11%) of total loss at full load
92. Range of losses in Induction motors
93
Range
Energy Loss at
Full Load (%)
1 - 10 HP 14.0 - 35
10 - 50 HP 9.0 - 15
50 - 200 HP 6.0 - 12
200 - 1500 HP 4.0 - 07
1500 - HP & ABOVE 2.3 - 04
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93. Type of Enclosures (IP55, IP23 etc.)
Provides protection to person against contact with live wire
and moving parts and to machine against ingress of solid
foreign bodies and harmful ingress of water
Ingress protection code consists of the letter ‘IP’ followed by
two numbers, first numeral designates the extent of
protection to person and protection to machine against solid
foreign bodies, while the second designates the extent of
protection to machine against water
General suffix letter for protection IP XY
Types of Enclosures
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96. S1: Continuous operation at rated load
S2: Short time operation
S3: Intermittent periodic operation
S4: As for S3 but with starting
S5: As for S3 but with electric braking
S6: Continuous cyclic operation
S7: As for S6 but with electric braking
S8: As for S6 but with related load/speed characteristic
Duty Cycles
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97. Air cooled motors
70 deg. C by resistance method for both class B&F
insulation.
Water cooled Motors
80 deg. C over inlet cooling water temperature
mentioned elsewhere, by resistance method for
both class B&F insulation
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Temperature Rise
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98. Cooling
All motors shall be either Totally enclosed fan cooled
(TEFC), Totally enclosed tube ventilated (TETV), or Closed
air circuit air cooled (CACA) type. However, motors rated
3000kW or above can be Closed air circuit water cooled
(CACW)
Suitable single phase space heaters shall be provided on
motors rated 30KW and above to maintain windings in
dry condition when motor is standstill. Separate terminal
box for space heaters & RTDs shall be provided
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