| 摘要 |
PURPOSE:To protect semiconductor elements and the like restraining a secondary overvoltage generated at the recovery of service interruption by a method wherein when an AC power supply is interrupted, a resistor is automatically inserted in the secondary circuit of an induction motor. CONSTITUTION:When an induction motor is controlled in speed by forming a commutator-less motor with a synchronous motor 3 of which revolving shaft is connected with the induction motor 2 and a rectifying circuit 5, a frequency converter 7, the DC voltage and current in the rectifying circuit 5 are controlled to be zero if service interruption occurs in the power supply 1. When the turning speed of the motor drops to the lower limit of speed which is set, the dropped speed is detected with a speed detector 20, switches SW2-SW4 are opened and at the same time a switch SW1 for a secondary resistor 19 is closed and further, a breaking signal is sent out to a thyristor gate of the frequency converter 7. Accordingly, at the recovery of service interruption, the resistor 19 is in the state that it is connected to the secondary side. Therefore, the overvoltages are consumed by the secondary resistor 19 and the semiconductor elements etc. are protected. |
