Chapter 6 Induction Motor Construction

7/30/2019 Chapter 6 Induction Motor Construction

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5/9/2013 360 Chapter 7 Induction Motor 1

7.2 Construction


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Induction Motors

For industrialapplications, thethree-phase

induction motoris used to drivemachines

Figure 7.2 Large

three-phaseinduction motor.(CourtesySiemens).

Housing

Motor


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Induction Motors

Figure 7.3

Induction motor

components.


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Induction Motors

The motor housing consists of three parts:

The cylindrical middle piece that holds the stator ironcore,

The two bell-shaped end covers holding the ball bearings.

This motor housing is made of cast aluminum or cast iron.Long screws hold the three parts together.

The legs at the middle section permit the attachment of

the motor to a base.

A cooling fan is attached to the shaft at the left-hand side.This fan blows air over the ribbed stator frame.


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Induction Motors

Figure 7.4 Stator

of a large

inductionmotor.

(Courtesy

Siemens).


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Induction Motors

The iron core has cylindricalshape and is laminated with slots

The iron core on the figure haspaper liner insulation placed insome of the slots.

In a three-phase motor, the threephase windings are placed in theslots

A single-phase motor has two

windings: the main and thestarting windings.

Typically, thin enamel insulatedwires are used

Figure 7.5 Stator iron core without windings


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Induction Motors

A single-phase motor has

two windings: the main and

the starting windings

The elements of the

laminated iron core are

punched from a silicon iron

sheet.

The sheet has 36 slots and 4holes for the assembly of

the iron core.Figure 7.6 Single-phase stator with main windings.


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Induction Motors

Figure 7.7 Stator iron core sheet.

The elements of the

laminated iron core

are punched from a

silicon iron sheet.

The sheet has 36 slots

and 4 holes for theassembly of the iron

core


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Induction Motors

Figure 7.8 Stator and rotor

magnetic circuit


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Induction Motors

Squirrel cage rotor. This rotor has a laminated iron core with slots,

and is mounted on a shaft.

Aluminum bars are molded in the slots and thebars are short circuited with two end rings.

The bars are slanted on a small rotor to reduceaudible noise.

Fins are placed on the ring that shorts thebars. These fins work as a fan and improvecooling.


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Induction Motors

Rotor bars (slightly skewed)

End ring

Figure 7.11 Squirrel cage rotor concept.


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Induction Motors

Figure 7.10 Squirrel cage rotor.


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Induction Motors

Wound rotor.

Most motors use the squirrel-cage rotor because of therobust and maintenance-free construction.

However, large, older motors use a wound rotor withthree phase windings placed in the rotor slots.

The windings are connected in a three-wire wye.

The ends of the windings are connected to three slip rings.

Resistors or power supplies are connected to the slip ringsthrough brushes for reduction of starting current and speedcontrol


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Induction Motors

Figure 7.9 Rotor of a large induction motor. (Courtesy

Siemens).


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7.3 Three-Phase Induction Motor

7.3.1 Operating principle


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Induction Motors

This two-pole motor has threestator phase windings, connectedin three-wire wye.

Each phase has 2 3 = 6 slots.

The phases are shifted by 120

The squirrel cage rotor has short-circuited bars.

The motor is supplied bybalanced three-phase voltage atthe terminals.

The stator three-phase windingscan also be connected in a deltaconfiguration.

A+

A-

B+

B-

C+

C-

A BC

Figure 7.12 Connection diagram of a

two-pole induction motor with

squirrel cage rotor.


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Induction Motors

Operation Principle

The three-phase stator is supplied by balanced three-

phase voltage that drives an ac magnetizing currentthrough each phase winding.

The magnetizing current in each phase generates a

pulsating ac flux.

The flux amplitude varies sinusoidally and thedirection of the flux is perpendicular to the phase

winding.


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Induction Motors

Operation Principle

The three fluxes generated by the phase

windings are separated by 120 in space andin time for a two-pole motor

The total flux in the machine is the sum of thethree fluxes.

The summation of the three ac fluxes resultsin a rotating flux, which turns with constantspeed and has constant amplitude.


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Induction Motors

Operation Principle

The rotating flux induces a voltage in the short-circuited bars of the rotor. This voltage drives

current through the bars. The induced voltage is proportional with the

difference of motor and synchronous speed.Consequently the motor speed is less than thesynchronous speed

The interaction of the rotating flux and the rotorcurrent generates a force that drives the motor.

The force is proportional with the flux density andthe rotor bar current


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Induction Motors

The figure shows the threecomponents of the magnetic field at a

phase angle of60.

Each phase generates a magnetic fieldvector.

The vector sum of the componentvectorsFa, Fb, Fc gives the resultingrotating field vectorrot,

The amplitude is 1.5 times theindividual phase vector amplitudes,and rot rotates with constant speed.

A+

A-

B+

B-

C+

C-

Fb

Fa

Frot

Fc

Fb

Fc

Figure 7.13 Three-phase winding-

generated rotating magnetic field.


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Induced Voltage Generation


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Induction Motors

Faradays law

Voltage is induced in a

conductor that movesperpendicular to a

magnetic field,

The induced voltage is:

Magnetic field B into page

Conductor

moving

upward with

speed v

Induced voltage V

Conductor length L

v v

Figure 7.14 Voltage induced in

a conductor moving through a

magnetic field.


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Induction Motors

The three-phase winding onthe stator generates a rotatingfield.

The rotor bar cuts the

magnetic field lines as thefield rotates.

The rotating field induces avoltage in the short-circuitedrotor bars

The induced voltage isproportional to the speeddifference between therotating field and the spinningrotor

A+

A-

B+

B-

C+

C-

Fb

Fa

Frot

Fc

Fb

Fc

V = B L (vsynv m)


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Induction Motors

The speed of flux cutting isthe difference between themagnetic field speed and therotor speed.

The two speeds can becalculated by using the radiusat the rotor bar location and

the rotational speed.

A+

A-

B+

B-

C+

C-

Fb

Fa

Frot

Fc

Fb

Fc

mrotmot

synrotsyn

nrv

nrv

2

2

msynrotrotbarnnBrV 2


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Induction Motors

The voltage and current generation in the rotor bar require aspeed difference between the rotating field and the rotor.

Consequently, the rotor speed is always less than the magneticfield speed.

The relative speed difference is the slip, which is calculatedusing

sy

msy

sy

msy

n

nns

2p

fnsy The synchronous speed is


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Motor Force Generation


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Induction Motors

The interactionbetween the magneticfield B and the

current generates aforce

F = B L I+

B B B B

F

B

Figure 7.15 Force direction on a current-carrying conductor placed in a magneticfield (B) (current into the page).


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Induction Motors

Force generation in a motor

The three-phase windinggenerates a rotating field;

The rotating field induces acurrent in the rotor bars;

The current generation requiresa speed difference between therotor and the magnetic field;

The interaction between thefield and the current producesthe driving force.

Brotating

Force

Ir

Rotor Bar

Ring

Figure 7.16 Rotating

magnetic field generated

driving force.


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7.3.2 Equivalent circuit


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Induction Motors

An induction motor has two magnetically coupled circuits:the stator and the rotor. The latter is short-circuited.

This is similar to a transformer, whose secondary is

rotating and short-circuited.

The motor has balanced three-phase circuits; consequently,the single-phase representation is sufficient.

Both the stator and rotor have windings, which haveresistance and leakage inductance.

The stator and rotor winding are represented by aresistance and leakage reactance connected in series


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Induction Motors

A transformer represents the magnetic couplingbetween the two circuits.

The stator produces a rotating magnetic field that

induces voltage in both windings.

A magnetizing reactance (Xm) and a resistance connected inparallel represent the magnetic field generation.

The resistance (Rc) represents the eddy current and hysteresis

losses in the iron core

The induced voltage is depend on the slip and the turnratio


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Induction Motors

Stator Rotor

Xrot_m

= rot

Lrot

Rrot

Irot

Vrot= s V

rot_s

Rsta

Irot_t

Vsta

Vsup

Ista Xm

Rc

Xsta

= sy

Lsta

Figure 7.17 Single-phase equivalent circuit of a

three-phase induction motor.


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Induction Motors

In this circuit, the magnetizing reactance generates a flux thatlinks with both the stator and the rotor and induces a voltage inboth circuits.

The magnetic flux rotates with constant amplitude andsynchronous speed.

This flux cuts the stationary conductors of the stator with thesynchronous speed and induces a 60 Hz voltage in the statorwindings.

The rms value of the voltage induced in the stator is:

2

max systa

sta

NV

F


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Induction Motors

The flux rotates with the synchronous speed and the rotorwith the motor speed.

Consequently, the flux cuts the rotor conductors with the

speed difference between the rotating flux and the rotor.

The speed difference is calculated using the slip equation:

The induced voltage is:

ssymsy )(

22

)( m axm ax sNNV

syrotmsyrot

rot

F

F


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Induction Motors

The division of the rotor and stator induced voltageresults in:

This speed difference determines the frequency of the rotorcurrent

sVsVN

N

V srotstasta

rot

rot _

sy

symsyrotrot fs

sf

222


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Induction Motors

The division of the rotor and stator induced voltageresults in:

This speed difference determines the frequency of the rotorcurrent

The rotor circuit leakage reactance is:

sVsVN

NV srotsta

sta

rotrot _

sy

symsyrotrot fs

sf

222

sXsLLX rotsyrotrotrotmrot _


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Induction Motors

The relation between rotor current and the rotor-induced voltage is calculated by the loop voltageequation:

The division of this equation with the slip yields

The implementation of this equation simplifies theequivalent circuit

)( sXjRs rotrot rotrot_srot IVV

rot

rot Xjs

R

rotrot_s

IV


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Induction Motors

Stator Rotor

Xsta

Xrot

Rrot

/s

IrotV

rot_s

Rsta

Irot_t

Vsta

Vsup

Ista

Xm

Rc

Figure 7.18 Modified equivalent circuit of a three-phase

induction motor.

The rotor impedance is transferred to the stator side. This

eliminates the transformer


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Induction Motors

Figure 7.19 Simplified equivalent circuit of a three-phase

induction motor.

Stator Rotor

Xsta

Xrot_t

Rrot_t

/s

Irot_t

Vsta

Rsta

VsupIsta

XmRc

Air gap


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Induction Motors

Stator Rotor

Xsta

Xrot_t

Rrot_t

Irot_t

Vsta

Rsta

Vsup

Ista

XmRc

Air gap

Rrot_t(1-s)/s

Figure 7.20 Final single-phase equivalent circuit of a three-phase

induction motor.

Documents

Chapter 6 Induction Motor Construction