| 摘要 |
A drive circuit for a three-phase induction motor comprises an inverter bridge consisting of six bridge switches, and a logic speed controller which controls switching of the inverter bridge in dependence on the motor current as sensed by a total current sense resistor. The speed controller is arranged to switch the bridge switches so as to supply alternating current to each phase of the motor at a fundamental frequency which may be varied in accordacne with the required motor speed. However, additionally, at least during low speed operation, the speed controller switches at least three of the bridge switches at high frequency during each cycle of the fundamental frequency switching. In this case the switching points are not predetermined but rather occur when the current as sensed by the resistor reaches a preset limit for switch turn-off and when the current falls to a specified point for turn-on. Such a circuit can be realized simply and enables the motor to be driven at a range of speeds.
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