PWM motor drive with torque determination

Claim 1

In a pulse-width-modulated (PWM) motor drive including an induction motor; an inverter for supplying AC current to said motor; a voltage-source including first rectifier means supplied with an AC power supply; a DC link between said voltage-source and said inverter for applying a direct current I DC to said inverter; and pulse-width-modulation means for controlling said inverter in accordance with pulse-width-modulated time waves of selected frequency and selected modulation index M; the combination of: current transformer means coupled with said AC power supply for deriving a vectorial representation of the sum of the AC currents supplied to said first rectifier means; second rectifier means responsive to said current transformer means for generating a voltage signal representative of said AC currents sum vectorial representation; said voltage signal being representative of said direct current I DC ; operational amplifier means responsive to said voltage signal current representative signal; and switch means operative on the gain of said operational amplifier means for repeatedly reducing the gain thereof to a lower value determined by the ON and OFF states of said switch means, the duty cycle thereof being in relation to the modulation index M of said pulse-width-modulation means, whereby said operational amplifier means derives an output signal representing I DC /M, said output signal being applied to said motor drive as a torque representative signal.