Induction Motor Interview Question Answer - 4

Important Questions of Three Phase Induction Motor

In this post, some of the important questions of three phase induction motor are given. It includes tapping on auto transformer, definition of speed regulation, methods of speed control of induction motor, crawling, cogging, harmonic torque, phenomenon of crawling, fundamental torque and harmonic torque, rotor efficiency of induction motor, construction and working of double cage induction motor.

Tapping on Auto-transformer

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.

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 k² × Direct on line starting. ( Where k = Voltage transformation ratio )

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

• Slip ring induction motor

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%.

Speed control of Induction motor

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

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.

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.

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.

Crawling

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.

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.

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.

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   ( 120⁰ × 5 = 600⁰ - 720⁰ = - 120⁰ ) 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  ( 120⁰ × 7 = 840⁰ - 720⁰  = + 120⁰ ) 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.

Cogging

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.

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.

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.

Fundamental torque & Harmonic torque

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

• The magnitude of the harmonic torque is 1 / n² of the fundamental torque. ( Where n is order of the harmonic torque )

Rotor Input, Copper Loss & Rotor Gross Output

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)

Rotor efficiency of Induction Motor

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

Construction & Working of Double Cage Induction Motor

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.

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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