Motor power control circuitry

摘要

To control the power supplied to a three-phase induction motor as a function of the motor load, the primary winding of a current transformer T1, T2, T3 is included in each phase input line to sense the phase currents, and the three line- to-line voltages are sensed by voltage transformers. Square waves in phase with the sensed currents and voltages are summed at a point S4 in a circuit producing firing signals FIRE A, FIRE B and FIRE C for controlling triacs connected respectively in series with the current transformer primary windings. The signal at S4 is integrated by an operational amplifier A4 and the integration signal supplied to three differential amplifiers A5, A6, A7. The voltage square waves VsA, VsB and VsC are also used to produce sawtooth signals RAMP A, RAMP B and RAMP C with a repetition rate of twice mains frequency which are respectively summed with the integration signal at inputs of the differential amplifiers A5, A6, A7. The FIRE A, FIRE B and FIRE C signals are applied to blocking oscillators for controlling the triacs. The conduction angles of the triacs are thus controlled in dependence upon motor load.