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Theoretical Questions

1.   Explain the principle of operation of the polyphase induction motor.
2.   Show that a rotating magnetic field can be produced by the use of 3-phase currents of equal magnitude.
3.   What is meant by slip in an induction motor? Develop an expression for the frequency of rotor currents in it.
4.   Derive an expression for the torque of an induction motor and obtain the condition for maximum torque.
5.   Show that in a 3-phase induction motor with negligible stator impedance maximum torque is developed R at slip s =  R2/Xwhere R2 and X2 are rotor

resistance and standstill reactance respectively.

6.   State the effects of increasing rotor resistance on starting current, starting torque, maximum torque and full-load slip of an induction motor.

7.   Show that in an induction motor the rotor input: power developed: rotor copper losses: 1:(1 – s) : s, where s is the fractional slip.

8.   Draw equivalent circuit of a 3-phase induction motor.

9.   Describe and explain how to perform a locked rotor test. What data can be obtained by the test?

10.                What are no-load and blocked rotor tests? What sort of losses can be measured by these tests?

11.                How would you determine circle diagram of a 3-phaseinduction motor experimentally?

12.                Describe the constructional details of a 3·phase squirrel-cage and phase wound induction motors. Also discuss the applications of various types of starters used for starting these motors.

13.                Why at all starters are necessary for starting the induction motors? What are the various types of starters used for squirrel-cage motors? Discuss them.