For driving a direct-current motor in tune with the frequency of an external synchronizing signal regardless of any abrupt change in the load or in the synchronizing signal frequency, a rotor position signal is produced in the form of a succession of pulses in phased relationship to the rotation of the motor. A first control circuit is provided to initiate the supply of electrical energy from a DC power supply to the motor in response to each pulse of the synchronizing signal and to suspend the supply of electrical energy in response to each pulse of the rotor position signal. This first control circuit is combined with second and third control circuits adapted to modify the operation of the first control circuit in event the motor has fallen out of synchronism. Thus, when the actual speed of the motor becomes higher than the required synchronous speed, the second control circuit operates to permit the motor to be fed continuously from the DC power supply, and when the actual motor speed becomes lower than the synchronous speed, the third control circuit operates to prevent the motor from being fed from the DC power supply, until the motor is pulled back into synchronous operation. Several other embodiments are disclosed.