Basics of Electric Motor.pdf

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
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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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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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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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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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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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Basics of Electric Motor.pdf