Page 723 - Basic Electrical Engineering

16. Draw a push-button-operated direct-on-line starter for an induction motor.
17. Draw the connection diagram for a manual star–delta starter.
18. Show the effect of variation of rotor-circuit resistance on the torque-speed characteristic of an
induction motor.
19. What is the limit of increasing the rotor-circuit resistance for achieving a high starting torque?
Explain your answer.
20. If the applied voltage is reduced to half, what will be the reduction in torque developed?
21. Prove that in an induction motor the rotor copper loss is slip times the rotor input.
22. Show how the maximum torque developed in an induction motor is independent of rotor-
circuit resistance.
23. How can you determine experimentally, the full-load copper loss of an induction motor?
24. Establish the relation rotor frequency, f = S × f.
r
25. Mention various applications of three-phase induction motors.
26. What are the advantages and disadvantages of squirrel-cage induction motors over slip-ring
induction motors?
27. Explain why the rotor-circuit reactance of an induction motor varies with speed?
28. Establish the relation, X = S X where S is the slip, X is the rotor-circuit reactance under
20
2
2
running condition, and X is the rotor-circuit reactance when the rotor is at rest.
20
29. Establish the similarity between a transformer and an induction motor.
30. At what slip will torque developed by an induction motor be maximum when rotor resistance
equals half the rotor reactance at standstill.
31. In an induction motor, slip is always positive, why?
32. Explain why an induction motor draws heavy current at starting when started on full voltage.

B. Numerical Problems

33. A four-pole, three-phase, 50 Hz induction motor rotates at a speed of 1440 rpm. Calculate its slip in
percentage. Also calculate the frequency of the induced EMF in the rotor circuit.

[Ans 4 per cent; 2 Hz]

34. A six-pole, three-phase, 400 V, 50 Hz induction motor is running at a speed of 940 rpm. Calculate
its slip.

[Ans 6 per cent]

35. A four-pole, three-phase, 400 V, 50 Hz induction motor develops an induced EMF in the rotor of 2
Hz. What is the speed of the motor?

[Ans 1440 rpm]