| 摘要 |
PROBLEM TO BE SOLVED: To stably detect the open phase of a three-phase AC power supply by reducing the number of voltage detection circuits. SOLUTION: In a voltage sensing circuit 34, a voltage which is generated due to the potential difference between a terminal P and a terminal N1 is used as an input. When the value of the input becomes a prescribed value or more, a DC voltage sensing signal 52 which is electrically insulated from the input is turned on. When the value of the input is smaller than the prescribed value, the sensing signal 52 is turned off. An open-phase discrimination circuit 35 turns on an open-phase sensing signal 53 as the sensing of an open phase when the detection signal 52 becomes square waves which repeat an ON-OFF operation. In the sensing circuit 50 for the open phase of the three-phase power supply, the voltage sensing circuit 34 can be reduced to one. When the open phase is not generated in the three-phase power supply 5, a voltage which is input to the voltage sensing circuit 34 becomes a nearly definite voltage, a change with the passage of time in a sensing level is not generated, and the open phase of any one phase from among respective phases of the power supply 5 can be sensed stably.
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