Class 4 (4th Engineer) MET 📅 Oct 2022

Exam Question

(a) Explain and describe why and how space heaters are fitted to motors. (6)

(b) Give a clear explanation of the following effects in a three phase induction motor,

(i) the production of rotating field,

(ii) the presence of an induced rotor current,

(iii) the development of torque.

A 4-pole, 250V motor has its armature removed in order to test the continuity of the field windings which are connected in series and consist of 2000 turns each. What is the average e.m.f. induced when the current is switched off, if the flux falls from 0.026Wb to 0.001 Wb in 0.2s? (10)

Reference Answer

### Part (a): Motor Space Heaters
#### Why Space Heaters are Fitted
Space heaters, also known as anti-condensation heaters, are fitted to electric motors, especially those operating in humid environments or subject to intermittent duty cycles, to prevent the ingress of moisture. The primary reasons are:
1. **Preventing Condensation**: When a motor is stopped, its internal components cool down. If the component temperature drops below the dew point of the surrounding air, moisture will condense on the surfaces, particularly the winding insulation.

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