| 摘要 |
A control circuit for d-c motors, which are supplied by an a-c supply through phase angle control, and which obtains information regarding the actual current value from a current transformer secondary voltage, and which is utilized additionally also for detecting the point of current zero-axis crossing, which, for example, is required before a motor current direction reversal. For recognition of the current zero-axis crossings the fact is utilized, that the ends of the current pulses occuring with low motor current (interrupted current) have voltage peaks opposite to the pulse current direction, which is caused by the reduction of the residual magnetic energy of the current transformer after each current pulse. The pulses of alternating direction generated by the current transformer are converted by controlled polarity reversal into pulses of the same polarity, the current zero-axis crossing of which at the pulse end is detected with a comparator circuit at the correct time.
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