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.