The armature current is given by the relation ;
When the motor is at rest, there is no back e.m.f. Eb in the armature. Consequently, if the motor is directly switched on to the mains, the armature will draw a heavy current (Ia = V/Ra) because of small armature resistance.
For instance , 10 H.P., 220 V shunt motor has a full-load current of 40 A and an armature resistance of about 0.2 Ω. If this motor is directly switched on to supply, it would take an armature current of 220/0.2 = 440 A which is 27.5 times the full-load current.
This high starting current may result in:
- burning of armature due to excessive heating effect,
- damaging the commutator and brushes due to heavy sparking,
- excessive voltage drop in the line to which the motor is connected. The result is that the operation of other appliances connected to the line may be impaired and in particular cases, they may refuse to work.
In order to avoid excessive current at starting, a variable resistance (known as starting resistance) is connected in series with the armature circuit. This resistance is gradually reduced as the speed of the motor increases (and hence Eb increases) and eventually it is cut out completely when the motor has attained full speed. The value of starting resistance is generally such that starting current is limited to 1.25 to 2.5 times the full-load current.