ETO ETO Exam papers 📅 Jun 2023

Exam Question

(a) What is back emf? Derive the relation for the back emf and the supplied voltage in terms of armature resistance (6)

(b) A three-phase induction motor is wound for four poles and is supplied from a 50 HZ system. Calculate:

(i) The synchronous speed

(ii) The speed of the rotor when the slip is 4 per cent

(iii) The rotor frequency when the speed of the rotor is 600r/min.

Reference Answer

(a) Back electromotive force (back EMF, Eb) is the voltage generated in the armature of a DC motor when it rotates and cuts the magnetic flux. By Fleming's Right-Hand Rule , this induced emf opposes the applied voltage V, as per Lenz's law. Back EMF acts as a self-regulating mechanism that limits the armature current when the motor is running.
Consider a shunt motor:
$$V\:=\:Applied\:voltage$$
$$I\:=\:Current\:flowing\:through\:the\:circuit$$

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