Importance

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Abstract

An AC induction motor for driving a varying mechanical load is powered via a Triac switch electrically inserted in series with the motor and its AC supply. A low ohmic value resistor, also in series with the motor, continuously samples the load current. When the peak value of the sampled load current is below a predetermined value indicative of a near full load condition, the Triac is switched to a conducting state at a predetermined electrical angle from each immediately preceding trailing edge of the sampled load current pulses or of sampled line voltage pulses applied to the motor. Such a phase triggering switching scheme reduces the power applied to the motor by reducing the period of time that each half-cycle of the supply voltage is applied to the motor, which, in turn, advantageously reduces the losses caused by the reactive current in the less than fully loaded induction motor. At or near a full load condition, the Triac is held or latched in a conducting state to continuously apply full supply voltage to the motor.

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First Claim