Q5
(a) What is back emf? Derive the relation for the back emf and the supplied voltage in terms of armature resistance
(6)
(b) The earth-lamps on a main switchboard comprise two 240V 60W lamps connected in the usual manner. The potential difference at the busbar is 220V. Damage by sea water occurs to a distribution cable so that the insulation resistance to the earth is reduced to 16 ohms and 6 ohms for +ve and -ve cables respectively. Find by calculation:
(10)
(i) Which of the two lamps burns the brighter and
(ii) The additional load on the generators occasioned by the fault. The resistance of the cables and the ship's structure may be neglected and that of the lamps taken as constant at the value corresponding to the 60W rating.
Reference Answer
### Part (a): Back EMF and its Relation to Supplied Voltage #### Concept of Back EMF (E_b) When the armature of a DC motor rotates within the magnetic field produced by the field windings, the armature conductors cut the magnetic flux lines. In accordance with Faraday's Laws of Electromagnetic Induction, an electromotive force (EMF) is induced in these conductors. According to Lenz's Law, the direction of this induced EMF is such that it opposes the very cause producing it. The cause is the supply voltage (V) that drives the armature current (I_a) and causes the rotation. Therefore, this induced EMF is termed **Back EMF** or **Counter EMF (E_b)**. It acts in direct opposition to the applied supply voltage.
Full answer on MeoMock — surveyor-grade reference answers for 7,000+ written exam questions.