Purpose of a DC Drive. Alternating Voltage. Direct Voltage. . . A DC drive converts AC power to DC voltagein order to vary the speed and torque of the motor. DC Motor Review. Speed is primarily determined by Armature VoltageTorque is determined by Armature Current. DC Motor Review. Rotation is the result of two independent electromagnetic fields.Armature windingsField windingsExtended motor speeds above base speed are achieved by reducing the field voltage (current)Field Weakening .