Start=up circuit for variable speed asynchronous motor - ensures initial rotation of motor by overriding triac control pulses on switch=on

摘要

The control circuit for a variable speed asynchronous motor consists of a triac (2) in series with the motor (1), connected between the conductors of an A.C. supply. The gate of the triac is connected to a decoupling resistor (3) and a power adjusting circuit (4) which varies the conduction angle. This may be in the form of a diac (9) with a variable resistor (5) connected to a capacitor (6), and a second resistor (7) connected to a second capacitor (8) to form a phase shift circuit supplying interruption pulses to the gate of the triac at a selected angle of conduction. An auxiliary circuit ensures initial start-up of the motor by maintaining conduction of the triac for a predetermined time after power is first applied to the circuit. It includes a capacitor (21) which is charged by the first negative-going impulse and applies a control signal to the triac for a period determined by the value of an R.C. circuit (23,24) controlling a switching transistor (27) connected to its output.