1. Write short notes on any two of the following: Heat run test of a transformer EMF generation in dc machines Speed-torque characteristic of dc shunt motor

2. What is armature reaction in a dc machine? Explain the different methods used to reduce the effect of armature reaction in detail.

3.(a) Discuss the voltage build-up process of a self-excited shunt generator. A 2-pole dc shunt generator charges a 100 V battery of negligible internal resistance. The armature of the machine is made up of 1,000 conductors, each of 2 milli-ohm resistance. The charging currents are found to be 10 A and 20 A. generator speeds of 1055 and 1105 rpm, respectively. Determine the shunt field current of the generator.

4.(a) Using schematic circuit diagram and speed,torque characteristics, discuss the method of shunt motor speed control by varying the armature circuit resistance. A 250 V dc shunt motor has an armature resistance of 0·5 ohm and a field resistance of 250 ohm. When driving a constant torque load at 600 rpm, the motor draws 21 A. What will be the new speed of the motor, if an additional 250 ohm resistance is inserted in the field circuit?

5.(a) Explain the components of various transformer losses.
The ohmic, hysteresis and eddy current losses in a transformer at 50 Hz are 0·9% and respectively. For Steinmetz's coefficient of 1.6, determine the hysteresis and eddy current losses for same system voltage and current at 60 Hz of supply frequency.

6.(a) With the help of a transformer phasor diagram for leading p.f. load, explain its working. Restrict your discussion to leading p.f. condition.

(b) A 2200/220V, 50 Hz, 1-phase transformer has exciting current of 0·6 A and a core loss of 361 watts, when its h.v. side is energized at rated voltage. Determine the core loss and magnetising component of exciting current.

7.(a) Compare an autotransformer with a two-winding transformer.

(b) State the conditions essential for the successful parallel operation of three-phase transformers.