| 摘要 |
An electrical switching circuit for a multi-phase a.c. supply uses semiconductor rectifier switches SCR1 to 5 to reversibly switch power to a multi-phase electric motor so that the motor can be run in a forward or a reverse direction. The SCRs are provided in two sets, one for switching power to forward run the motor, and the other set for reverse running of the motor. Inductor elements DELTA L are connected in series with the SCRs. In certain situations gross spurious over voltages may occur at the input phases, when subjected to lightning strikes, for example, which may result in spurious firing of the SCRs. It has been found by the provision of varistors between the input phases and ground, the input voltage can be clamped a maximum preset value, so preventing spurious firing of the SCRs. Also disclosed is the provision of further varistors in order to reduce the effects of voltage standing waves which may be produced due to the internal capacitances of the first mentioned set of varistors. |
