Abstract: In order to minimize the initial in-rush produced on first energisation of a polyphase induction motor (10), careful control is exercised over the firing of controlled switching means (11) connected in series with each of the supply phases (A, B, C) or motor windings. This control includes effecting an initial selective firing of the switching means (11) such as to cause current to flow between two only (B,C) of the supply phases, the firing being timed to occur such that the voltage between these two phases (B,C) is decreasing in value at the moment of firing. Thereafter, the switching means (11) are sequentially fired in a manner resulting in the progressive increase in the angle of conduction of each switching means (11).

Abstract: Motor power-factor controllers (12) act, in the presence of varying motor loads, to maintain the phase lag angle (.phi..sub.M) between motor voltage and current at a preset demand or reference phase-lag value (.phi..sub.R) by regulating the firing angle of gate-controlled switching devices (11) connected in series with the motor supply (A, B, C). The optimum reference phase-lag value (.phi..sub.R) depends on the motor characteristics and will be as small as possible while providing for a stable, unambiguous control characteristic. A reference-setting unit (14) is provided for automatically setting an appropriate reference phase lag (.phi..sub.R). This unit (14) monitors the motor phase lag (.phi..sub.M) during start up and sets the reference phase lag (.phi..sub.R) to a value corresponding to the minimum monitored phase lag plus a predetermined amount (for example 8.degree.). The unit (14) can be arranged to seek to improve on the set reference (.phi..sub.