Self calibration of the thyristor firing angel of a motor controller using a current window to determine a final value of a reference current lag phase angle

摘要

A reference current lag for a motor controller is selected so that the reference current lag approximates the motor current lag when the motor is fully loaded. A motor controller has a plurality of thyristors for controlling electric current flow to the motor. A firing circuit fires the thyristors at a selected firing angle. A current lag, as a phase angle between a power line voltage applied to the motor and electric current flow to the motor, is measured. The selected firing angle is adjusted so that the measured current lag is substantially equal to a reference current lag. A temporary value of the reference current lag is selected. The motor current is monitored, and in the event that the motor current becomes between an upper predetermined motor current and a lower predetermined motor current, the reference current lag is given a final value. The final value may be equal to the current lag measured when the motor current is between the upper predetermined motor current and the lower predetermined motor current. The temporary value of the reference current lag may be taken as the measured current lag during a predetermined line cycle after initiation of current flow to the motor. The predetermined line cycle may be the third line cycle after initiation of current flow to the motor. The temporary value of the reference current lag may be a minimum value of current lag measured as the motor starts from a stopped condition to a full speed condition. The upper predetermined motor current may be 100% of a motor full load current, and the lower predetermined motor current may be 85% of a motor full load current, thereby defining a motor current window.