Electrical Engineering ⇒ Topic : Need of starter for d.c motors
Need for Starters
A motor at rest has no back or counter e.m.f. At staring therefore, the armature current is limited only by the resistance of the armature circuit. The armature resistance is very low,however, and if full voltage were impressed upon the motor terminals at stand still, the resulting armature current would be many times full-load value usually sufficient to damage the machine. For this reason, additional resistance is introduced into the armature circuit at starting.As the motor gains speed, its back e.m.f. builds up and the starting resistance is cut out.
Note. Very small D.C. motors, either shunt, series or compound wound, have sufficient armature resistance so that they may be started directly from the line without the use of a starting resistance and without injury to the motor.
Fig.(1) , shows the connections of a starting resistance in three types of D.C. motors
(a) A series motor;
(b) A shunt motor; and
(c) A compound motor.
Fig. (1). Circuits incorporating starting resistance
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