METHOD AND APPARATUS FOR AUTOMATICALLY SETTING THE DEMAND PHASE LAG INPUT TO AN INDUCTION-MOTOR POWER FACTOR CONTROLLER

摘要

Motor power-factor controllers (12) act, in the presence of varying motor loads, to maintain the phase lag angle (O &cir& /M) between motor voltage and current at a preset demand or reference phase-lag value (O &cir& /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 (O &cir& /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 (O &cir& /R). This unit (14) monitors the motor phase lag (O &cir& /M) during start up and sets the reference phase lag (O &cir& /R) to a value corresponding to the minium monitored phase lag plus a predetermined amount (for example 8). The unit (14) can be arranged to seek to improve on the set reference (O &cir& /R) during motor running by repeatedly comparing the actual phase lag (O &cir& /M) plus said predetermined amount, with the set reference (O &cir& /R) and resetting the latter to the former if this has a lesser value.