An apparatus for controlling an AC power supply for an electric motor, said AC power supply being derived from a DC voltage. The apparatus including a comparer configured to provide a comparison of a modulation index of a motor control signal with a reference value. This current data provider is configured to provide current data based on a torque demand signal; a speed signal indicating the speed of rotation of the AC motor; and an indication of the DC voltage modified on the comparison for control of the motor control signal which is based on the motor current data.