| 摘要 |
<p>A three-phase electric motor (12) is coupled to each phase line (A, B, C) of an electric source by pairs of SCR's (21, 24; 22, 25; 23, 26) coupled in inverse parallel relationship. One of the SCR's (21, 22, 23) in each pair is triggered by a signal from a conventional motor control circuit (20) which regulates the phase angle at which the SCR turns on. The other SCR (24, 25, 26) in each pair is an optically triggered device which is activated by light emitted from an LED (41, 42, 43) in one of the legs of a three-phase rectifier bridge (31 to 36) in the control circuit power supply (30). When the LED with a leg of the power supply is forward biased so as to conduct current, it emits light which triggers a correspondingly biased opto-SCR (24, 25, 26) to supply current to the motor. A separate transistor (56) is connected in parallel across each of the LED's to provide a current shunt path when the motor (12) is to be turned off so that the LED does not emit light and triggers the SCR.</p> |
