Chapter 33, Problem 1RQ

To determine

The reason why the contacts often connected in series when controlling small DC motors.

Explanation of Solution

To break the motor circuit or to stop the motor the arc formation may occur.

In that case only load contact is required to break the circuit and to stop the motor but in that case current does not fall to zero as like the alternating current. This shows that the arc interruption in case of the DC motor is difficult.

In case the direct current do not fall the zero periodically so the severity of the arc and the risk to the damage is high.

To aid the arc interruption the load contacts in the small DC motor is connected in the series connection. Also, it increases the total air gap between the contacts and in arc quenching.

Thus, the contacts are in series in controlling small DC motor to aid the arc interruption.