PHASE SWITCHING SYSTEM OF THREE-PHASE MOTOR

PROBLEM TO BE SOLVED: To provide a compact, inexpensive, and safe phase switching system for detecting the negative-phase connection and open phase of a three-phase AC power supply for switching to a positive phase in reverse connection. SOLUTION: The phase switching system of a three-phase motor comprises: a main relay 5 for opening and closing a first phase of the three-phase power supply 1; a phase switch relay 4 for exchanging the second phase and the third phase of the three-phase AC power supply to change a phase order; a selection relay 6 for selecting one of a relay 41 for the positive phase of the phase switch relay 4 and or a relay 42 for a negative phase connected in series to the load side of the main relay 5; a phase detection circuit 7 for detecting each phase signal of three-phase ACs at the load side of the main relay 5 and the phase switch relay 4; and a control section 8. When it is determined that the signal of the phase detection circuit 7 is not in a positive phase after closing the main relay 5, the main relay 5 is opened for changing the selection relay 6. After that, the main relay 5 is closed. When output from the phase detection circuit is detected while the main relay is being opened, closing the main relay 5 is prohibited. COPYRIGHT: (C)2008,JPO&INPIT