In apparatus for controlling the speed and torque of a multiphase a.c. motor in accordance with digital control signals, a first regulation circuit regulates the frequency of the a.c. electrical signal applied to the motor and another regulation circuit regulates the voltage of the a.c. signal. Both regulation circuits are controlled by digital control signals in a series of discrete steps over a range of control values and the control signals are supplied periodically to the regulation circuits. The speed and torque are controlled in accordance with an algorithm relating speed and torque to the frequency and voltage of the source of electric power which is applied to the motor, calculated by computing apparatus on a periodic basis. In one embodiment the position of the magnetic flux of the stator is controlled directly.