Motor speed control circuit providing armature current sensing and feedback action for each half cycle and control circuit reducing energy consumption of induction motors when running under no load and light loading

摘要

A motor speed control circuit in which a source of alternating current (AC), a universal-type motor, a triac and a large capacitor are connected in series, with the armature current flowing through the large capacitor and with the control providing armature-current sensing and feedback action for each half cycle of motor operation for stabilizing motor speed at any speed setting in spite of changes in the torque load being imposed on the motor. A triggering circuit for the triac includes a diac connected on one side to the control electrode of the triac and on the other side to an RC network across the portion of the series circuit containing the large capacitor and the triac. The RC network includes a variable resistor for adjusting the speed of the motor by setting the phase of the signal to the triggering diac. Power is supplied to the motor during both positive and negative half cycles of the AC supply voltage. In the operation of the circuit embodying this invention, as the torque load imposed on the motor is increased for any given speed setting, the power supplied thereto is also increased to maintain the motor speed constant at that given setting over a wide operating range. This advantageous feedback action utilizes changes in the charging voltage across the large capacitor for changing the firing angle of the triac, thereby immediately changing the power being supplied to the motor and accordingly maintaining the speed constant under varying load conditions as they occur. Feedback occurs from the large capacitor during each and every half cycle for stabilizing the motor speed under variable conditions of loading for any particular setting of the motor speed.