Induction Motor Interview Question Answer - 2

Important Questions of Three Phase Induction Motor

In this post, some of the important questions of three phase induction motor are given. It includes effect of supply voltage on starting torque, maximum torque in the induction motor, stable operation of induction motor, pull out torque, flux in air gap of induction motor, magnetizing current, plugging in induction motor, induction generator and methods of slip measurement.

Effect of supply voltage on starting torque

Describe the effect on starting torque if the supply voltage is ( a ) Increases by 5% ( b ) Decreases by 5%.

The starting torque in the induction motor is directly proportional square of the supply voltage therefore   

                       T_st α V² 

( a ) T_st2 / T_st1 = ( V₂ / V₁ )²

                                  = ( 1.05 / 1.0 )²

                                  = 1.1025

                T_st2 = 10.25% T_st1

If the supply voltage is increased by 5% then starting torque is increased by 10.25%.

( b ) T_st2 / T_st1 = ( V₂ / V₁ )²

                                   = ( 1 - 0.05 / 1.0 )²

                                   = 0 .9025

                T_st2 = 90.25% T_st1

If the supply voltage is decreased by 5% then starting torque is decreased by 9.75%.

Which parameters greatly affect the maximum torque in the three-phase induction motor?

Factor affecting maximum torque

• The value of the maximum torque in the induction motor varies inversely proportional to standstill reactance. 
• The maximum torque varies as square of the supply voltage.

At which value of slip, stable operation of the induction motor depends?

• The stable operation of the induction motor depends for the value of slip between   s = 0 and slip corresponding to maximum torque.

Define : Pull out torque

Pull out torque

• The maximum torque which is developed by the induction motor is known as pull out torque in the induction motor.

What should be rated supply voltage if the rated supply frequency is increased by 20% for stable operation of the induction motor?

• If the supply frequency is increased by 20%, the rated supply voltage will be 1.2 times the name plate rating for stable operation of the induction motor.

Which parameters greatly affect the flux in the air gap?

Flux in the air – gap

• The ratio of the voltage and frequency must be kept constant in order to keep constant flux in the air gap. 
• Therefore, the supply frequency and voltage greatly affect the flux in the air gap.

           B_max  α  ( V / f )

Magnetizing current

What is magnetizing current in the induction motor?

Magnetizing current

• It is minimum current required to create flux in the induction motor.

At which speed the value of magnetizing current becomes minimum?

Synchronous speed

Describe the effect on torque and rotor current if the supply frequency decreases in the three-phase induction motor.

Effect of supply frequency on torque and rotor current

• If the supply frequency is decreased, the torque increases and rotor current decreases in the induction motor. 
• Hence, we obtain large torque with reduced current at reduced frequency.

On which parameter the active power of the Induction generator above the synchronous speed depends?

• Slip

Which parameter greatly depends upon straight torque – speed curve of the three-phase induction motor?

• Rotor resistance

Plugging in Induction Motor

What is meaning of the plugging in the induction motor?

Plugging in the induction motor

• When any two stator leads are interchanged, the direction of the stator revolving flux reverses and hence the direction of the torque also reverses, therefore brake will apply on the induction motor. 
• This period is called as plugging in the induction motor.

Why the speed of the rotor decreases during plugging period in the induction motor?

Plugging

• The speed of the induction motor rotor is decreased during plugging period because the induction motor absorbs some kinetic energy from revolving mechanical load.

What is induction generator or asynchronous generator?

Induction generator

• When the induction motor runs faster than its synchronous speed, it is called as induction generator.

What is direction of torque and speed in the case of induction motor and induction generator?

Induction Generator

• The direction of the torque and speed in the case of induction motor is same whereas torque and speed are opposite directed in the induction generator.

What is operating speed range when the induction motor works as ( a ) Induction motor ( b ) Induction generator ( c ) Brake

The speed range for

• Induction motor : ( 0 to N_s )
• Induction generator :  ( N_s to 2N_s )
• Brake : ( 0 to - N_s )

Rotor frequency

How to measure rotor frequency by using milli – voltmeter in the  slip ring induction motor?

Measurement of rotor frequency

• The centre zero DC moving coil mill voltmeter is used to measure rotor frequency. 
• The leads of mill voltmeter are lightly pressed against adjacent slip – rings. 
• The pointer oscillates about its main zero position which indicates rotor oscillation. 
• The number of complete cycle per second is known as rotor frequency.   ( One cycle is equal to movement of pointer from zero to maximum left, maximum left to zero, zero to maximum right, maximum right to zero. )

How to measure rotor speed without using Tacho – meter in the slip – ring induction motor?

The speed of the rotor in the slip-ring induction motor is measured by following methods: ( a ) By comparing stator and rotor frequency and

( b ) Stroboscopic method.

Methods of measurement of slip

Describe the different methods to measure slip in the induction motor.

Slip measurement

• By  measuring the actual speed of the rotor by tachometer
• By comparing stator and rotor frequency
• Stroboscopic method

Why rotor copper loss in the induction motor is neglected under running condition?

Rotor copper loss

• The frequency of the rotor current is small under running condition therefore the rotor copper loss is neglected in the induction motor.

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