61

What percentage of tapping is provided in the auto
transformer starter?


The auto transformer starter is provided with 50%, 65% or 80% of line
voltage tapping.

62

Describe the relation between line current during
auto transformer starter and direct on line starter.


The line current in the auto transformer starter is equal to k^{2}
× Direct on line starting. ( Where k = Voltage transformation ratio )

63

In which type of induction motor starting current is
limited by rotor circuit?


Slip ring induction motor

64

Explain the term : Speed regulation


Speed regulation
 The speed regulation indicates difference between no load to full load
speed.
 The speed of the induction motor slightly drops at full load therefore
its speed regulation is less than 5%.

65

Describe the difference method of speed control of
induction motor.


Method of speed control
Stator side
 Varying the supply voltage
 Varying the supply frequency
 Changing number of poles
Rotor side
 Rotor rheostat control
 Injecting emf in the rotor circuit
 Operates two motors in cascade

66

Write any two disadvantages of rotor rheostat speed
control of the slip ring induction motor.


Disadvantages of rotor rheostat speed control
 As the rotor resistance increases, the rotor copper loss increases and
consequently the motor efficiency decreases.
 The speed control not only depends upon rotor resistance but also load
as well.

67

Describe the effect of external rotor resistance on
the speed of the slip ring induction motor.


As the external rotor resistance increases, the speed of the slip ring
induction motor decreases and vice versa.

68

Describe the effect of injecting rotor emf in the
slip ring induction motor.


Injecting emf speed control
 If the injecting emf is in the phase opposition to the rotor induced
emf, the rotor resistance increases therefore the speed decreases.
 Similarly if the injecting emf is in phase with rotor induced emf, the
rotor resistance decreases therefore the speed increases.

69

Explain the term : Crawling


Crawling
 Sometime the induction motor runs at one seventh speed of the
synchronous speed.
 This tendency of the induction motor is called as
crawling.

70

Explain the term : Cogging


Cogging
 Sometime the squirrel cage induction motor refuses to start
particularly at low voltage due to locking tendency of stator and rotor
teeth.
 It is called as cogging.

71

Give reason : The number of rotor slots prime to the
number stator slots in the induction motor.


Number of rotor and stator slots
 When the number of rotor slots is same as that of stator slots, the
induction motor refuses to start particularly at low voltage.
 This is due to minimum reluctance between stator and rotor teeth and
magnetic locking tendency of the stator and rotor teeth.
 Therefore the number of rotor slots is not equal to the number of
stator slots in the squirrel cage induction motor.

72

Describe the relation between magnitude of
fundamental torque and harmonic torque.


The magnitude of the harmonic torque is 1 / n^{2} of the
fundamental torque. ( Where n is order of the harmonic torque )

73

Explain the phenomenon of crawling in the induction
motor?


Crawling
 The torque develop by the induction motor is sum of fundamental torque
and seventh harmonic torque if all the other harmonic torque is neglected.
 The speed of the seventh harmonic is N_{s} / 7. The seventh harmonic
torque reaches its maximum value just before one seventh synchronous speed.
 If the torque developed by the induction motor falls below constant load
torque, the induction motor will not accelerate up to normal speed but run at
1/7^{th} synchronous speed.

74

Describe the relation between rotor input, rotor
copper loss and gross mechanical output in the induction motor.


Relation between rotor input, rotor copper loss and gross rotor output
 Rotor input : Rotor copper losses : Gross rotor output
= 1 : s : 1 – s
 Rotor copper loss = s × Rotor
input and
 Gross mechanical output = ( 1 – s ) × Rotor input
 Gross mechanical output = [( 1 – s ) / s ] × Rotor copper losses
Where s = slip

75

Write the equation of the rotor efficiency in the
three phase induction motor.


Rotor efficiency = ( 1 – s )
= ( N / N_{s })
Where s =
Slip
N =
Rotor speed
N_{s}
= Synchronous speed

76

What is speed of the harmonics in the three phase
supply waveform?


Speed of the Harmonics
 The speed of the harmonics is 1 / nth speed that of fundamental
waveform resulting if the third harmonics present in the supply waveform it
will rotate at Ns / 3 speed , 5th harmonic rotates at Ns / 5 speed and 7th
harmonic rotates at Ns / 7 speed.

77

Which harmonics is responsible for effect of
crawling in the three phase induction motor? Why?


Crawling – Harmonics
 The third harmonic current are absent in the three phase supply system
resulting it does not produce any rotating field.
 The fifth harmonic current produces rotation of ( 120^{0
}× 5 = 600^{0}  720^{0 }=  120^{0 }) flux wave
in the reverse direction with a speed of Ns / 5 resulting it produces
negative torque or braking action in the induction motor.
 The seventh harmonic current produces rotation of ( 120^{0
}× 7 = 840^{0}  720^{0 }= + 120^{0 }) flux in the
forward direction with a speed of Ns / 7 resulting it produces a positive
torque in the induction motor.
 Therefore the seventh harmonic is responsible for the crawling effect
in the induction motor.

78

Why the phenomenon of cogging occurs in the three
phase induction motor?


Phenomenon of cogging
 When the number of stator teeth is equal to the number of rotor teeth
in the induction motor, there is possibility of magnetic locking between
stator and rotor due to minimum reluctance between them.
 The induction motor refuses to start particularly at low voltage due
to this magnetic locking.

79

Which type of rotor construction the double cage
induction motor consists of?


Construction of Double cage induction motor
 The stator of the double cage induction motor three phase winding
whereas the rotor consists of outer cage and inner cage rotor.
 The outer cage consists of high resistance whereas low ratio of
reactance to resistance whereas the inner cage consists of low resistance and
high ratio of reactance to resistance.

80

Explain the operation of the Double cage induction
motor.


Operation of the Double cage induction motor
 The frequency of the rotor current at starting is same as that of the
supply frequency therefore the reactance of the inner cage becomes the high.
 Due to very high impedance, most of the current flows through the inner
winding and motor develop high starting torque.
 The frequency of the rotor current decreases during running condition therefore
the reactance as well as impedance of the inner cage decreases and most of
the current flows through the outer cage.

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