DC motors are used in many applications, some of which require the speed to remain constant as the load applied to the motor shaft changes. In others it is necessary to vary the speed within a certain range. It is important to know the speed curve as a function of torque. DC motors are rated according to the field winding connections.

In the motor with independent excitation the field winding is connected to an independent voltage source and the armature winding is connected to another source, having no connection between the two windings. In the branch motor the armature winding and the field winding are placed in parallel and connected to a continuous power supply. The engine in series exhibits a great variation of speed as the torque developed by the engine increases and should not be used without load.

The composite excitation motor is a combination of series type and bypass type motors. A winding composed of winding with many turns of fine wire, connected in parallel with the armature winding, and another with few turns of coarse wire, connected in series with the armature winding.