Define and explain the terms 'energy' and 'co-energy'. Also show that energy and co-energy are numerically equal for a linear system.

Discuss briefly the general analysis of electromechanical system, and derive an expression for the mechanical force developed in a singly excited system.

Describe the working of a four point starter for a d.c. shunt motor, with the help of a neat diagram.

Explain what would happen if the d.c. motor is directly switched on to the supply, without any starter.

A d.c. shunt motor draws 80 A at 220 V on full-load. The armature and field resistances are 0·2 ohm and 110 ohms respectively. If the stray losses amount to 800 determine the following:

Output power

Electrical efficiency

Mechanical efficiency

Overall efficiency

Explain the torque-current characteristics of a d.c. series motor.

Explain with circuit diagrams, the open-circuit and short-circuit test to be carried out for the determination of the parameters of a single phase transformer.

Derive the condition for maximum efficiency of a single phase transformer.

Draw and explain complete phasor diagram of a single phase transformer for capacitive load.

Explain the construction and working of an auto-transformer.

What are the conditions for parallel operation of two 3-phase transformers? Also write the advantages of a 3-phase transformer over a single-phase transformer.

Draw and explain the open delta connections of a transformer. Also write the applications and disadvantages of it.

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

Hopkinson's Test

Interpoles and Compensating Windings

Harmonics in three-phase transformer