A motor has an armature resistance \( 4\,\Omega \). It is operating on 220 V. At its full speed, it develops a back emf of 200 V. The current in the armature when the motor is switched on will be:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 55 A
The back emf of a motor is produced by its rotation. At the instant of switching on the armature is still at rest, so the back emf is zero and the whole supply voltage drives the current through the armature resistance.
\( \displaystyle I = \frac{V - E_b}{R_a} = \frac{220\,\mathrm{V} - 0}{4\,\Omega} = 55\,\mathrm{A} \)