Induction Motor Interview Question Answer - 5

Important Questions of Three Phase Induction Motor

In this post, some of the important questions of three phase induction motor are given. It includes features of double cage induction motor, cascaded operation of three phase induction motor, Kramer speed control of induction motor, locus of circuit diagram, no load of induction motor, block rotor test of induction motor, effect of supply voltage on torque and current and effect of speed on rotor emf.

Features of double cage induction motor

Describe any two features of the Double cage induction motor.

Features of Double cage induction motor

• High starting torque
• Reasonable starting current
• High efficiency
• Better speed regulation

Cascade operation of induction motor

Describe the conditions to be necessary for the cascade operation of the three-phase induction motor.

Cascade operation of three phase induction motor

• Both motors must be connected on the same shaft resulting both run at same speed.
• The main supply is given to the wound rotor induction motor. The turn ratio of the stator to rotor winding of the wound rotor induction motor should be 1:1.

Which type of cascade connection of the induction motor should provide very low or zero torque?

• Differential connection

What should be the frequency of the rotor induced emf in the injected emf speed control method of induction motor?

• The frequency of the injected emf should be same as the slip frequency ( rotor frequency ).

Kramer Speed Control

What are the advantages of the Kramer speed control of the induction motor?

Advantages of Kramer speed control

• If the rotary – converter is over excited, it will take a leading current which compensates lagging current drawn by the main wound rotor induction motor and improves power factor. Any speed control can be possible within working range

Circle Diagram

Which parameter the locus of the circle diagram represents?

• Rotor current as refer to stator side

Describe the method to find out no load parameter G₀ and B₀ of the three-phase induction motor.

No load parameters     

• The no load parameters of the three-phase induction can find out by running the induction motor at synchronous speed resulting slip becomes zero. 
• However, it is impossible to run the induction motor at synchronous speed due to friction and windage loss, it is run by another machine which supplies friction and windage loss. 
• The rotor circuit becomes open hence motor draws only no-load current and from no load current and no-load power we easily calculate

           I₀ = VY₀

           And W₀ = 3G₀V²

           G₀ = W₀ / 3V²

           And B₀ = √ (Y₀² – G₀²)

Give the reason : The one wattmeter shows negative reading in the no load test of the three-phase induction motor.

Induction motor at no load

• As the induction motor run at no load, the power factor is very low i.e., less than 0.5 therefore one wattmeter shows negative reading. 

           W1 α Cos ( 30 + Ф )

           W2 α Cos ( 30 – Ф )

           Where Ф is the load power factor

• As Cos Ф < 0.5, the power factor angle Ф > 60⁰ therefore wattmeter W1 indicates negative reading.

Which parameters can find out by performing blocked rotor test in the three-phase induction motor?

Short circuit current at normal voltage

• Power factor Cos Ф_sc at short circuit
• Total winding impedance Z₀₁, reactance X₀₁ and resistance R₀₁ as referred to the stator side

Effect of voltage on torque and current

Describe the effect on torque and starting current if the supply voltage is reduced by 50% in the three-phase induction motor?

Effect of supply voltage

• If the supply voltage is reduced by 50%, the starting current is also reduced by that percentage but at the same time the starting torque ( T_st α V² ) is reduced by 75 % of the normal voltage torque.

Is there any effect on power factor by using primary resistor starter in the three-phase induction motor?

• The power factor improves slightly due to resistors connected to the line.

Effect of speed on rotor emf

Describe the effect of speed on the rotor induced emf.

Effect of speed on rotor induced emf

• The rotor induced emf is directly proportional to the slip speed ( N_S – N ). The rotor speed at standstill is zero therefore the rotor induced emf is same as that of supply frequency. As the rotor speed increases, the slip speed decreases therefore rotor induced emf decreases.

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