A motor which is designed to be operated from two independent voltage sources, in such a way that variations in one voltage source will determine the direction and speed of the motor. In the alternating current type the voltages are usually applied to a split-phase stator, whereas in the direct current type, one voltage is applied to the stat or and the other to the armature. Low inertia and rapid reversibility are inherent characteristics of this type of motor. See also servomotor, automatic pilot.