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7/23/2019 Basics of Electric Motor.pdf
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11 March 2013 1Mano j Barsaiyan
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Motor Enclosures
Energy Efficiency Opportunities
Specifications of Motor
Construction Details of Motor
Motor Fundamentals
CONTENTS
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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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How the Electric motors work
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B SIC WORKING PRINCIPLE
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How Does an Electric Motor Work?
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DC Motor example
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Dc motor working
http://localhost/var/www/apps/conversion/tmp/scratch_3/Direct%20Current%20Electric%20Motor.flvhttp://localhost/var/www/apps/conversion/tmp/scratch_3/Direct%20Current%20Electric%20Motor.flv7/23/2019 Basics of Electric Motor.pdf
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How Does an Electric Motor Work?
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How Does an Electric Motor Work?
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How Does an Electric Motor Work?
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Type of Electric Motors
Electric Motors
Alternating Current (AC)
Motors
Direct Current (DC)
Motors
Synchronous
Induction
Three-Phase
Single-Phase
Self Excited
SeparatelyExcited
Series
Shunt
Compound
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Stator
Rotor
Terminal Box
Enclosure
Insulation
Structure of Motors
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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 motorfor 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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Commutaor
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Commutaor
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DC Motor
Sparking or arcing near the brushes or on thecommutator 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 theireffectiveness.
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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AC MACHINESNEMA MG 1-2003 has the following definitions:
An induction m chine 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 m chine is an alternating-currentmachine 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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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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Synchronous Motor
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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, untilthe 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 atsynchronous speed, it is possible to alter the power
factor by varying the excitation supplied to the motor
field.
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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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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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Torque Angle
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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 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 bre kdown torque.
Its value varies from 1.5 to 3.5 times the full load
torque.
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Synchronous motor power factor
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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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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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Synchronous motor power factor
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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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Synchronous motor power factor
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INDUCTION MOTOR
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Advantages: Induction motorThree-phase induction motors are the most common andfrequently 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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Disadvantages: Induction motor
Speed is fixed for a fixed voltage and frequency
Low power factor at start and no load condition
High starting current for a cage induction motor
Always operates at lagging power factor.
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Squirrel Cage 3 phase winding in stator
Copper bars in rotor
Wound Rotor 3 phase winding in stator3 phase winding in rotor
(Shorted internally)
Wound Rotor 3 phase winding in statorwith Slip Ring 3 phase winding in rotor
(Terminated to slip rings)
Types of Induction Motors
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The induction motor derives its name from the factthat 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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Rotating Magnetic Field
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TheStatorin an AC motor is a wire coil, called a statorwinding, when this coil is energized by AC power, arotating magnetic fieldis produced
This rotating field is produced by the contributions ofspace-displaced phase windings carrying appropriate
time displaced currents by120 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 thestator winding induces a current in the rotor
This induced rotor current produces a secondmagneticfield necessary for the rotor to turn
Induction Motor
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The rotating magnetic field generated in the stator induces a
magnetic field in the rotor.
The two fields interactand 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 fieldthat induces it, here the changing field is the
motionof the resultant stator field
A force is exerted on the rotor by the induced emf and theresultant magnetic field
This force tends to cancel the relative motionbetween the rotor
and the stator field and the rotor, as a result, moves in the same
directionas the rotating stator field
Induction Motor
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The rotor reacts to the magnetic field, but does nottravel at the same speed
Also the rotor speedactually lags behindthe speed of
the magnetic field and rotor runs at the speed Nrwhich is close to the speed of the stator field, Nsat noload, but the rotor speed decreases as the load is
increased
The termslipquantifies the slower speed of the rotorin comparison with the rotating speed of the statormagnetic field and is expressed mathematically as:
S=(Ns-Nr)/Ns
SLIP
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The rotor is not locked into any position and therefore will
continue to slipthroughout 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 increaseonly if the magnetic field
cuts through the rotor at a faster rate
To increase the relative speedbetween 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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Typical torque-speed characteristics of induction motor
Torque Speed Characteristic
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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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On start-up the slip is s=1and the starting torque (also known
as abreakaway torque) is sufficiently large to accelerate the
rotor (the rotor has previously been 'locked' - stationary)
As the rotor runs up to its full-loadspeedthe torqueincreases
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 littlewhen the load is raised from 0 to its
full value- this is a normal operating region
Analysis of Operation
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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 onthe internal circumference of the stator house thestator winding.
A rotor also composed of punched laminations, withrotor slots for the rotor winding.
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Stator
consisting of a steel
frame that supports ahollow, cylindrical core
core, constructed from
stacked laminations,
having a number ofevenly spaced slots,
providing the space for
the stator winding
Induction Motor - Construction
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Stator Frame
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Wound Stator Core
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Stator coils
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VPI of Stator Winding
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Stator core insertion
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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 insulatedwire, which produce a wound-rotor induction motor
(special characteristics)
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Induction Motor: Squirrel cage rotor
Squirrel cage rotor consists of copper bars, slightly longerthan the rotor, which are pushed into the slots.
The ends are welded to copper end rings, so that all thebars are short circuited.
In small motors, the bars and end-rings are diecast inaluminium to form an integral block.
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Squirrel Cage Rotor
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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 ringswhich turn with the rotor. The slip rings/brushes allowexternal resistors to be connected in series with the winding.
The external resistors are mainly used during start-up under normal running conditions the windings shortcircuited externally.
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Rotor before end ring brazing
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Rotor with end ring
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Squirrel cage rotor
Woundrotor
Noticethe slip
rings
Rotor
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Cutaway ina typical
wound-rotor IM.Notice the
brushes andthe slip
rings
Brushes
Slip
rings
Induction Motor
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Induction Motor
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Making of an induction motor
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NEMA class D
small bars near
surface
NEMA class A
large bars near
the surface
NEMA class B
large, deep rotor
bars
NEMA class C
double-cage
rotor design
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Typical torque-speed curves for different rotor designs
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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 thesame speed of the rotating magnetic field, then the
rotor will appear stationary to the rotating magneticfield and the rotating magnetic field will not cut therotor. So, no induced current will flow in the rotor andno rotor magnetic flux will be produced so no torqueis 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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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 CoolingFrame Mfg. details
Efficiency Ambient Temperature
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f = P N / 120
(Where f is frequency in Hz, P is no. of pole and N is speedin rpm)
1 H.P. = 746 Watts = 0.75 KW (approx.)
P D2L n
(Where P is output, D is diameter, L is length and n isspeed)
slip s = (ns- nr) / ns(Where ns is synchronous speed in rpm and nr is rotorspeed in rpm )
Motor Fundamentals
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The nameplate details of a motor are given as
Power, P = 15 kW, Efficiency, = 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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The nameplate details of a motor are given as
Power, P = 15 kW, Efficiency, = 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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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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M L
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Motor Losses Core Losses: A combination of eddy-current and hysteresis
losses within the stator core. Accounts for 15 to 25 percent ofthe 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 I2
R 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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Motor Losses
P F t
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kVA
kWCosFactorPower
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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Energy Efficiency Opportunities
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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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Power Loss Area Eff ic iency 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 & reduceslosses 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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Range of losses in Induction motors
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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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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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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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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 IPfollowed bytwo 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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T f E l
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Types of Enclosures
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T f E l
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Types of Enclosures
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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 terminalbox for space heaters & RTDs shall be provided
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TETV VENTILLATION CIRCUIT
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VENTILATION ARRANGEMENT for CACA MOTOR
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