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3ϕ Induction Motor
 Construction:
 Stator:
Stationary part of motor part of motor made up of silicon steel stampings
...


 Rotor:
There are 2 types of Induction Motor depending on construction of rotor
...
Squirrel cage rotor :
- This is simplest & most rugged construction
...
Rotor conductors
are thick copper bars that are placed in these slots & welded to the end rings
...
Therefore it is not possible to
any external resistance in rotor circuit
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2
...

Rotor is made up of laminations with slots on the outer periphery in which a 3ϕ rotor
winding s placed
...

Slip rings are made up of copper or phosphor bronze & 3 brushes resting on them
...

Under normal conditions to slip-rings are short circuited by metal collar which is
pushed along the shaft & brushes are lifted from slip-rings to reduce frictional losses
...

The magnetic flux cuts the rotor conductors which are stationary, due to relative speed
between rotating flux & stationary conductors an emf is induced in the conductors
...

Since conductor forms closed circuit rotor currents are produced & according to
Lenz’s law it will try to oppose the relative speed
...


Ns = speed of rotating magnetic field
...

Ns-N = relative speed
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Relative speed is main cause of induced emf in rotor
...
e
...
Eventually the motor stops
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But due to inertia of rotor this does not happen in practice & rotor continues to rotate
with a speed slightly less than synchronous speed of rotating magnetic field
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N < Ns
So it can be said that rotor slip behind the rotating magnetic field produced by stator
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Slip speed = Ns – N

-

The slip speed is generally expressed as the % of synchronous speed of the rotating
magnetic field
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S=

–

%S=

–

---------------------- (absolute slip)
* 100----------------- (% slip)

∴ S=1

At start, M=0

This is maximum slip and minimum slip is zero for which N=Ns
...

Ns =
`

=

&
–

=S

Ns – N =

`


-

Torque of Induction Motor:
The average torque produced at any slip‘s’ depends on the flux, rotor current at slip‘s’
i
...
cosϕ2
...

K = constant
∴ T α E2I2cos ϕ2

Denoting rotor emf at standstill by we have that E2 α ϕ
or

T = K1E2I2COS ϕ2

 Starting torque:
The torque developed by motor the instant of starting is called starting torque
...

Let,

E2 = rotor emf per phase at standstill
R2 = Rotor resistance per phase
X = rotor reactance per phase at standstill
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Then,

I2 = E2/Z2 = (E2)/(√ (R22+X22))
Cosϕ2 = R2/Z2 = R2/√ (R22+X22)

Standstill or starting torque
Tst = K1E2I2cosϕ2
Or
√

TST = K1E2*

∴ Tst = K2*R2/ (R22+x22) = K2 *R22/Z22

√

*

=

If supply voltage ‘V’ is constant, then flux ϕ & hence E2 both are constant
...



Condition for maximum starting torque:
[ R2 = SX2 ]
Tmax = (3/2πNs)*(E22/2X2) N-m

 Torque – slip curve:
-

The maximum torque is independent of rotor resistance but speed or slip at which
maximum torque occurs depends on the rotor resistance so by changing rotor
resistance ‘R2’ maximum torque can be made to occur at any desired slip
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(b) At speed near to synchronous speed slip ‘S’ is very small & SX2 is very small in
comparison with R2
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∴ T α S/R2 also R2 is constant
∴ TαS
Thus variation is straight line at the beginning when load increases, the slip &
therefore torque increases & becomes maximum (Tm) at particular slip to satisfy R2 =
SX2
...

(c) Further increase of load then SX2 also increases & R2 becomes negligible with
respect to SX2
...
The motor slows down &eventually stops
or fuses blow out
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(d) As for maximum torque R2 = SX2, for greater R2, the slip is more &the point of
maximum torque shifts toward left A to B to C for 2R, 4R etc
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- In short circuited test of x’mer reduced voltage is applied to circulate the full load
current
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c
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- Similarly in case of 3ϕ induction motor it’s stator is connected to 3ϕ rated voltage
the motor draws a heavy current which might damage the motor
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A) Starters for cage motors:
These starters reduces the voltage per phase applies to starter
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I
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- When motor attains rated speed, the resistances are removed form circuits
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- The stator has no-volt coil & over load relay based on thermal contact releasing
mechanism
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Star-delta starter:
- It is simple mechanical switch which connects the stator to supply in star on ‘start’
position
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Hence stator connections are changed to delta
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In run position the rated line voltage is restored per
phase
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- This starter is suggested for the motors up to 30 HP only
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III
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The taps on autotransformer can be changed but are identical on all three
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-

The handle can’t be taken on ‘run’ position at start, because of the locking
arrangement
...

- This starters is generally suggested for higher capacity motors [above 25HP]
- The cost is very high & it reduces the starting torque
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Rotor resistance starter:
- Three equal resistance in the form of circular arrangement are connected in the
rotor circuit
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- The motor resistance is maximum at start & progressively reduced till run
position
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- This prevents starting of 3ϕ induction motor on the ‘run’ position
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O
...
):
- For small capacity motors having less than 5HP, the motors can withstand high
starting currents
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- Motors are directly switched ON supply lines hence the starter is called as direct
on-line starter
...

- At start ‘NO’ is pushed for fraction of second due to which coil gets energised &
attracts the contactor so starter directly gets supply
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- When NC is pressed circuit gets open due to which coil gets de-energised & motor
gets switched OFF from supply
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The thermal
relay gets opened due to high temperature protection the motor from overload
condition
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1) Stator copper loss:
3I12R1
2) Rotor copper loss:
3I22R2
b) Iron losses:
Due to hysteresis and eddy currents there is loss of power
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2) Rotor iron loss : It is very small because the rotor frequency is very
small and hence the rotor iron loss is negligible
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The induction motor takes an electrical input power
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Total rotor Cu loss = 3I22R2 ,
Let ,
Tg = Gross torque in Nm
N = Actual speed of rotor in rps
Rotor Gross oputput = 2πNTg
Tg = rotor gross output / 2πN

-

-

If there were no Cu loss in rotor then the rotor gross output = rotor input and rotor
would run at synchronous speed (Ns)

Tg = Rotor input / 2πNs
Rotor input = 2πNsTg
Rotor Cu loss = rotor i/p – rotor o/p
= 2πNsTg - 2πNTg
= 2πTg (Ns – N)

-

–

∴ rotor cu loss = s*rotor input
Rotor gross output = rotor input – rotor cu loss
=

= slip (s)

= rotor input – (s*rotor input)
= rotor input (1 - s )
= (1 – s)
= [1-(

–

)]

=
∴ rotor ‘η’ =
Motor’ η ’ =

= actual speed/ synchronous speed
=

=

∗
1−

∗



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