| 摘要 |
The phases of a three-phase AC power supply are applied to a three-phase load, such as a motor, by respective ones of three triacs. Three alternating voltages, phase-displaced by 120 DEG , appear across the triacs. When each alternating voltage is in its half cycles of one polarity, for example when the main terminal T2 is negative relative to the main terminal T1 of the triac to which the voltage is applied, the triac conducts continuously. During the half cycles of the opposite polarity, however, the conduction angle or time of each triac is varied in response to a control effect. In this way, the magnitude of the phase voltages applied to the load are varied to control the operation of the load, such as to vary the speed of the motor. No neutral point exists for the load currents so a triac, during the negative excursion of its main terminal T2, cannot conduct unless the terminals T2 of the other two triacs are in a positive excursion and have been triggered into conduction. A feedback arrangement insures that for a given control effect any undesired variations of the load voltages will be balanced out or compensated. |
