Soft start for d.c. motors

Thee starting current of heavy-duty d.c. motors can be 3-8 times as high as the normal operating current, which puts a heavy load on the on / off switch. Also, when the motor is switched off, a breaking spark occurs which causes erosion and oxidization of the contacts. This obviously shortens the life of the switch appreciably. Moreover, the starting torque is much greater than the operating torque and this does neither the motor nor the load to which it may be coupled any good. The usual remedy for this deficiency is a series resistor in the motor leads which llmits the starting current and is short-circuited shortly after the motor has started by a relay. It does not nullify the breaking spark, however.