Class 4 (4th Engineer) MET 📅 Nov 2024

Exam Question

(a) Explain how excitation of the rotor is produced and supplied. (6)

(b) A shunt motor has an armature resistance of 0.2 ohms and with an armature current of 120 amperes runs at 750 r.p.m. off a 400-volt supply. Calculate the speed and armature current of the motor if the flux per pole is reduced to 75 per cent of its initial value, the total torque remaining unaltered. (10)

Reference Answer

### (a) Production and Supply of Rotor Excitation (6 Marks)
**1. Principle of Excitation**
Excitation is the process of generating a magnetic field (flux) in an electrical machine by passing a direct current (DC) through a set of field windings. This magnetic field is essential for the operation of the motor, as the interaction between this field and the current-carrying conductors of the armature produces the torque that causes rotation.
While the question refers to the "rotor," in a conventional DC machine, the main excitation field windings are located on the stationary part (stator or yoke), and the armature winding is on the rotating part (rotor). The term is therefore interpreted as the method of producing the main magnetic flux for the machine.

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