Induction Machine Viva Questions

Define Slip in Induction Machine?

Answer: Slip is the difference between the speed of the stator rotating magnetic field and the speed of the rotor of the induction motor expressed as a fraction of synchronous speed is known as slip of the induction motor.

Why Induction Machine always runs below synchronous speed

Answer: Induction motor works on the principle of induction. When both rotating magnetic field of stator and rotor of induction motor rotates with same speed no emf is induced in the rotor conductors resulting no torque is produced in the rotor of induction machine and motor will stop. Therefore in order to have torque for rotating the rotor induced emf is required. Hence Induction motor rotates with speeds less than synchronous speed.

How direction of 3 phase Induction Motor can be changed

Answer: Direction of 3 phase induction motor can be changed by changing any two supply phases of the induction motor. Changing any two phases will result in change in rotating magentic field to opposite direction. Therefore rotor direction is also changed.

What are the different starting methods of 3 phase induction motor?

Answer: Different starting methods employed while starting 3 phase induction motors are:

• Direct on line starting method
• Primary resistance starting method
• Auto transformer starting
• Star-Delta Starting method

When we can get Maximum Torque in Induction Motor while starting?

Answer: Starting torque for 3 phase induction motor is low. In order to increase the starting torque an external resistance should be inserted while starting of the 3 phase induction motor. It is to be remembered that starting torque of slip ring induction motor is high compared to squirrel cage rotor machine due to the provision of adding external resistance while starting the machine.

What are the different losses in 3 phase induction motor?

Answer: Different losses in the 3 phase induction motor are stator copper loss, stator iron loss, rotor copper loss, friction and windage loss and stray load loss.

What is the relation between torque and supply voltage in 3 phase induction motor?

Answer:  Torque T = sV²