1. Derive the expression for active and reactive power when the power polyphase circuit is measured for balanced load by two wattmeter method. Draw connection and phasor diagrams also.

2. Three inductive loads of 50 mH are connected in star to a 3-phase, 200 50 Hz system. Calculate the inductance of each load, which when connected in delta to the same supply will take the same line current.

3. Draw the phasor diagram of a single phase transformer when the

inductive load is connected,

pure resistive load is connected.

4. Derive an equivalent circuit of a transformer referred to primary. Also draw the approximate equivalent circuit referred to secondary.

5. Draw slip -torque characteristics of a 3-phase induction motor and explain why the maximum torque is independent of rotor resistance but exact location is dependent on it. Derive the condition for maximum torque.

6. Explain the various factors which are to be considered while selecting a motor. What are the different types of loads

With the help of a neat circuit diagram explain the working of an SCR.

Explain the different methods to turn on a thyristor.

8. Explain the working of a step down chopper. Give its advantages and limitations.

9. A 6-pole, 3-phase, 50 Hz induction motor develops a maximum torque of 30 N-m at 960 rpm. Determine the torque exerted, by the motor at slip. The rotor resistance per phase is 0·6 ohm.

10. Write short notes on any two of the following:

Starting methods of single-phase induction motor

Short circuit test of a transformer

Load equivalization