Series Machine, Direct-Current

a commutator type direct-current machine in which the winding of the main poles (the field winding) and the armature winding are connected in series.

Motors are the most widely used class of direct-current series machines. In DC series motors, an increase in load entails an increase in the torque on the shaft and a decrease in the rotational speed. When such a motor is operated without a load, the speed can become dangerously high. Series DC motors are used where high starting torques are required—for example, in traction drives in electric locomotives, electric trains, diesel locomotives, trolleybuses, and streetcars. They are not used where operation without load is possible.