a variable speed direct current shunt motor control system is disclosed for regulating the strength of the field to achieve and maintain a desired motor speed . the control system comprises a comparator circuit operable to compare a control voltage that is dependent on the armature voltage with a reference voltage that varies with torque demand . when the control voltage differs from the reference voltage , the comparator circuit develops a field demand or command signal that provides for the strengthening of the field to return the motor to the desired speed . a sensing circuit is incorporated into the control system to sense motor plugging and provide for field weakening .