Page 722 - Basic Electrical Engineering

For a given torque, T and rotor-induced EMF at standstill, i.e., E ,
20

R ∝ S
2

If R is doubled, S will be doubled. The slip at doubled R will be 6 per cent.
2
2

8.19 REVIEW QUESTIONS

A. Short Answer Type Questions

1. Explain the principle of working of a three-phase induction motor.
2. Explain why a three-phase induction motor can not run at synchronous speed.
3. What is meant by slip of an induction motor? What is the value of slip at starting and at
synchronous speed?
4. How can you change the direction of rotation of a three-phase induction motor? What is meant
by phase sequence of power supply?
5. Explain the purpose of making two types of rotor construction for three-phase induction
motors.
6. Write an expression for torque developed by an induction motor and write the meaning of
each term.
7. Draw the torque-speed characteristic of a three-phase induction motor and explain its shape.
8. Show that stator magnetic field and the field produced by the rotor mmf are stationary with
respect to each other.
9. What is the condition for maximum torque developed at starting?
10. Show how a rotating magnetic field is produced when a three-phase supply is connected
across a three-phase winding.
11. What is the expression for maximum torque developed in a three-phase induction motor?
12. Draw the power-flow diagram in an induction motor.
13. What are the various losses in an induction motor? State the factors on which they depend.
14. Explain how efficiency of an induction motor can be determined by performing no-load test
and blocked-rotor test.
15. Explain why a starter is required to start a large three-phase induction motor.