| 摘要 |
<p>The operating power factor of an AC induction motor (34) is controlled by selectively controlling the firing angle delay of a gate signal to a triac (36) during the cycle of the power source (32). Source voltage zero crossing is sensed and a plurality of consecutively sequenced discrete windows are initiated. If the current zero crossing occurs in the first window then a sudden load on the motor (34) is indicated, and any delay in the firing angle of the triac (36) is substantially eliminated. This enables conduction of the triac (36) during substantially the entire cycle of the source voltage thus providing instantaneous full power to the motor (34). If the current zero crossing occurs in the second window then the power factor is too great, and more power is needed at the motor (34). The gate firing angle is reduced, whereby the triac (36) will conduct for a longer cycle portion and hence deliver more power to the motor (34). If the current zero crossing occurs in the third window then an optimum power factor is indicated and the gate firing angle is not affected. If current zero crossing does not occur in any of the first three windows, then it will occur somewhere in the fourth window and too much power is getting to the motor (34). Upon termination of the third window without zero crossing of the current, the gate firing angle delay is increased so that the triac (36) will be ON for a shorter portion of the cycle, and hence less power is delivered to the motor (34). <IMAGE></p> |
