Topic3Three Phase Induction Motors

8/18/2019 Topic3Three Phase Induction Motors

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THREE PHASE INDUCTION MOTOR

INTRODUCTION:

Three-phase Induction motor is an AC motor. Of all the AC motors available, it is

extensively used because of the followin advantaes.!. Its construction is simple, rued and almost unbrea"able.

#. Its cost is low and is hihly reliable.

$. Its efficiency is hih.

%. Its wor"s with reasonably ood power factor at rated load.

&. Its maintenance is less.

'. Induction motors are self startin. (ence motors of small ratins do not re)uire a

starter. The startin arranements for lare motors are simple.

*rawbac"s:

The startin tor)ue is inferior to that of a *C motor.

CONSTRUCTION:

A three-phase induction motor mainly consists of two parts

a+ tator

+ otor

The rotor which is the rotatin part is separated by the stator, which is the stationarypart by a small air ap.

STATOR:

It is the stationary part of an Induction motor. It consists of stator frame, stator coreand windins.

Stator Frame: /ncloses a hollow, cylindrical core. It provides only a mechanicalsupport and is not desined to carry the stator flux.

Stator Core: tator core is a stac" of cylindrical steel laminations which are slottedalon their inner periphery for housin the three-phase windins. The core islaminated to reduce eddy current loss. The stator core fits closely in the cast- ironstator frame.

Stator Windings: tator conductors are placed in these plots, which are insulatedfrom one another and also from the slots. These conductors are connected as abalanced three-phase star windin or *elta windin.


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ROTOR:

The rotor is the rotatin part of the induction motor and is mounted on the shaft of the motor to which any mechanical load can be connected.

There are # types of rotors.

!. )uirrel cae rotor.

#. lip rin rotor or wound rotor.

Accordin to the type of rotor used, Induction motors are classified as s)uirrel caeinduction motor and slip rin or wound rotor induction motor.

SQUIRREL CAGE ROTOR:


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- 0ormally 123 of the induction motors are of s)uirrel cae type.

- otor is simple and rued in construction.

- It consists of a cylindrical laminated core with parallel slots for carryin rotor

conductors.

- The rotor conductors are heavy bars of copper or aluminium. One bar is placed in

each slot. All the bars are bra4ed or welded at both ends to two copper end rins,

thus short circuitin them at both ends.

- As the rotor bars are short circuited on themselves, it is not possible to add any

external resistance in series with the rotor circuit durin startin.

- The rotor slots are slihtly s"ewed which helps in # ways.

a+ It reduces the noise due to manetic hum and ma"es the rotor to run )uietly.

b+ It reduces the loc"in tendency between the rotor and the stator that is the

tendency of the rotor teeth to remain under the stator teeth due to direct manetic

attraction between the two.

The only disadvantae of this type of rotor is that, it has a low startin tor)ue.

5ithout the rotor core, the rotor bars and end-rins loo" li"e the cae of a s)uirrel

hence the name s)uirrel cae rotor.

PHASE WOUND ROTOR OR SLIP RING INDUCTION ROTOR

The rotor is laminated, cylindrical core havin uniform slots on its outer periphery. A

three-phase windin which is star connected is placed in these slots. The open ends

of the star windins are brouht out and connected to $ insulated slip rins, mounted

on the shaft of the motor, with carbon brushes restin on them.

The $ brushes are externally connected to a three-phase star connected rheostat,

which is used as a starter durin the startin period.

5hen runnin under normal conditions, the slip rins are automatically shortcircuited by means of a metal collar, which is pushed alon the shaft and connects

all the rins toether. 0ext the brushes are automatically lifted from the slip rins, to

reduce the frictional losses, wear and tear.

Adantages o! S"i# Ring Ind$%tion Motor :

!. The facility of havin external resistances in the rotor circuit has the followin

advantaes:

a+ They can be used for startin the motor, especially with load, with hiher startin

tor)ue, at lower startin current.


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b+ They can be used to control the speed.

c+ They can be used to start the motor with hih startin tor)ue and

d+ They can be used to improve the p.f

#. The motor is smooth runnin.

$. The motor can be specifically built for very hih capacities as several !22s or

!222s of horse powers and

%. The motor, under normal wor"in conditions, wor" at hih efficiency.

Disadantages&

!. lip rin motor re)uires more space than the s)uirrel cae motor of the same

capacity.

#. This motor is more expensive, since the construction is complicated.

$. 6aintenance and repair costs are )uite hih.

ROTATING MAGNETIC FIELD

5hen a three-phase supply is iven to the three-phase windin of the stator, a

rotatin manetic field of constant manitude and rotatin with synchronous speed is

produced. This fact can be proved as follows.

7iure below shows the three-phase windin of the stator of an induction motor

which is connected to the three-phase 8$-9+ supply. The startin point of the windins

A , , C are connected to the $-9 lines , and . The other $ ends 7 A , 7 and

7C are connected to the neutral ;0


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=et 9 A > 9m sin wt, 9 > 9m sin 8wt ? !#22+ , 9C > 9m sin 8wt ? #%2

2+ 'Re!er t(e #o)er

#oint *+d #i%t$res ,


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In!eren%e&

!. The resultant flux is of constant value > !.& 9m

#. The resultant flux rotates round the stator at synchronous speed iven by 0 s >

PRINCIPLE OF WOR-ING

5hen the $-9 stator windins are fed by a $-9 supply, manetic flux of constant

manitude, but rotatin at synchronous speed, is set up. The flux passes throuh the

air ap, sweeps past the rotor surface and so cuts the rotor conductors, which as yet

are stationary.

*ue to the relative speed between the rotatin flux and the stationary rotor

conductor, an emf is induced in the stationary rotor conductor in accordance with

7araday


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SLIP:

In practice, the rotor never succeeds in ;catchin up< with the stator field. If it really

did so, then there would be no relative speed between the two, hence no rotor emf,

no rotor current and so no tor)ue to maintain the rotation. That is why, rotor runs at a

speed which is always less than the speed of the stator field. The difference in speed

depends upon the load on the motor.

3 lip >

Definition: The difference between the synchronous speed N S and the actual speed

‘N’ of the rotor is known as slip.

0 ? 0 > lip speed

6otor 8rotor+ speed >haft sped>0 > 08 !- +

0ote :

!. The term slip is descriptive of the way in which the rotor slips bac".

#. It may be "ept in mind that revolvin flux is rotatin synchronously, relative to the

stator but at slip relative to the rotor.

FREQUENC. OF THE ROTOR CURRENT&

5hen the rotor is stationary, the fre)uency of the rotor induced emf is the same as

that of supply fre)uency.


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0 > or 80 ? 2+ > --------------------@ !

f > upply fre)uency in (4

but when the rotor starts rotatin at ;0< rpm,, then the fre)uency depends upon the

relative speed 80 ? 0+.

=et at any slip speed, the fre)uency of rotor current be f !

80 ? 0+ > -------------------------@ #

/)uation # /)uation !

f ! > sf > slip fre)uency

SPEED OF ROTOR FIELD 'OR MMF,

-The rotatin field set up by stator currents rotates at synchronous speed ;0< relative

to the stator surface.

- the currents havin a fre)uency f ! > 8sf+ when flowin throuh the rotor windin set

up the rotor manetic field which rotates at a speed of ;s0< rpm relative to the rotor

surface, in the direction of rotation of the rotor.

> s0

- (owever, the rotor itself is runnin at a speed of ;0< rpm w.r.t. stator surface.

- o the speed of the rotor manetic field w,r,t stator surface or space is e)ual to thesum of

> 0 B s0 > 0 8! ? s+ B s0 > 0

Thus the rotor manetic field also rotates, in space, at the same speed and in the

same direction as that of the stator field.

This concludes that, both the stator and rotor fields rotate synchronously, which

means that they are stationary w.r.t each other at all possible rotor speeds.


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These two synchronously rotatin manetic fields, super impose on each other and

ive rise to the actually existin rotatin field, which corresponds to the manetisin

current of the stator windin.

ince the two fields set up by stator 8primary+ and rotor 8secondary+ currents are

stationary relative to each other, so the polyphase I.6 can be considered as bein

e)uivalent to a transformer.

Pro/"ems

0. In case of an pole I.6, the supply fre)uency is &2 (4 and the shaft speed is D$&

rpm. Calculate,

a+ ynchronous speed

b+ lip speed

c+ Eer unit slip

d+ 3 slip

1. A ' pole, &2 (4, s)uirrel cae I.6 runs on load at a shaft speed of 1D2 rpm.

Calculate

a+ 3 slip

b+ 7re)uency of the induced current in the rotor.

*. A pole alternator runs at D&2 rpm and supplies power to a ' pole I.6 which has

at a full load a slip of $3. 7ind the full load speed of the I.6 and the fre)uency of its

rotor emf.

2. A % pole $ ? 9 I.6 operates from a supply whose fre)uency > &2 (4. Calculate

a+ The speed at which the manetic field of the stator is rotatin.

b+ The speed of the rotor when the slip is 2.2%.

c+ The fre)uency of the rotor currents when the slip is 2.2$.

d+ The fre)uency of the rotor currents at standstill.

34 A $ ? 9 I.6 runs at almost !222 rpm at no load and 1&2 rpm at full load when

supplied with power from a &2 (4, $ ? 9 line.

a+ (ow many poles has the motorF

b+ 5hat is the percentae slip at full loadF

c+ 5hat is the correspondin fre)uency of rotor voltaeF


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d+ 5hat is the correspondin speed of the rotor fieldF

e+ 5hat is the correspondin speed of the rotor w.r.t the statorF

f+ 5hat is the correspondin speed of the rotor field w.r.t stator fieldF

+ what is the rotor fre)uency at the slip of !23F

ROTOR EMF AND REACTANCE UNDER RUNNING CONDITIONS

=et /# be the standstill rotor induced emf per phase.

G# be the standstill rotor reactance per phase >

where, ;f< is the supply fre)uency.

5hen the rotor is stationary 8s>!+, the fre)uency of the rotor induced emf is same asthat of the supply fre)uency. The value of the rotor induced emf at standstill is

maximum because the relative speed is maximum.

5hen the rotor starts rotatin at ;0< rpm, the relative speed between the stator

revolvin flux and the rotor is reduced. (ence, the rotor induced emf which is

proportional to the relative speed is also decreased.

The fre)uency of the rotor induced emf under runnin condition>

*ue to decrease in the rotor fre)uency, the rotor reactance under runnin condition

will also decrease.

otor windin impedance per phase under runnin condition>

5here, # is the rotor windin resistance per phase.


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otor current per phase >

otor power factor> >

EQUI5ALENT CIRCUIT

The induction motor e)uivalent circuit is similar to transformer e)uivalent circuit. The

only difference is on account of the fact that the secondary windin 8otor windin+

of an Induction motor rotates and therefore, involves the development of mechanical

power. The derivation of e)uivalent circuit proceeds in the same manner as in the

case of transformer. All the e)uivalent circuit parameters have per phase values.

STATOR EQUI5ALENT CIRCUIT&

=et


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B

ROTOR EQUI5ALENT CIRCUIT&

=et


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E6$ia"ent %ir%$it re!erred to Stator side&


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POWER STAGES

otor Core loss is neliible and hence nelected.

E7PRESSION FOR TORQUE

Electrical

Power Input

to Stator

OR

STATOR

INPUT

Pin

Power

transferred

across the

air-gap to

rotor

Rotor

Input

Or

Air-gap

Power

Stator

Coppe

r Loss

&

Mechanica

l Power

Deelope!

"# the

rotor

PMech

Rotor

Coppe

r Loss

Sha$tPower

Output

Or

Output

Power

Or

Output

%riction&

in!ag

e loss

or

Mechani


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INFERENCE&

!. 6aximum tor)ue is independent of rotor circuit resistance.

#. The slip at which the maximum tor)ue occurs depends upon the rotor resistance

and therefore by varyin the rotor circuit resistance, the maximum tor)ue can bemade to occur at any desired slip or motor speed.

$. 6aximum tor)ue varies directly as the s)uare of supply voltae.

%. It is clear that maximum tor)ue varies inversely as standstill reactance of the rotor,

hence to have maximum tor)ue, standstill rotor reactance should be "ept as small as

possible. This is achieved by placin the rotor conductors very close to the surface of

the rotor and reducin the airap between stator and rotor to the smallest possible

value.

TORQUE SLIP AND TORQUE SPEED CUR5ES&

T > ---------------------@ !

T >

!. 5hen s > 2, T > 2, 8i.e when the speed is synchronous+

T > 2

#. 5hen the speed is very near to synchronous speed, 80 is close to 0+ i.e when

the slip is ;< very low, is very lare compared to , Gs, and Gr !

7rom e)uation !

T ∝


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∝ K

T ∝ if the rotor resistance is constant.

At speeds near to synchronous speeds, the tor)ue-speed or tor)ue-slip curves are

approximately straiht line.

$. As slip increases ie as the speed drops with the increase in the load, tor)ue

increases, reaches its maximum value when . The maximum tor)ue is also

"nown as brea"down or pullout tor)ue. The slip correspondin to brea"down tor)ue

is called brea"down slip b or mt.

%. 5ith further increase in slip or drop in speed due to increase in load beyond the

point of maximum tor)ue, the tor)ue beins to decrease. The result is that motor

slows down and eventually stops. The motor operates for the value of slip between

4ero and that correspondin to brea"down or pullout tor)ue.

T >

5ith hiher slip, r ! H H 8 Gr

!+

T∝

if Gr !

is constant.

i.e speed tor)ue or slip tor)ue curves are rectanular hyberbola with the speed or

slip beyond that correspondin to maximum tor)ue.


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Topic3Three Phase Induction Motors

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