Transient operation of converter fed async. motor - operates during supply system interruptions by sudden inverter frequency reduction

摘要

The asynchronous motor (M), typically part of a variable speed drive, is supplied with power from the mains (N)via a rectifier/inverter unit with an intermediate d.c. link. During transient supply interruptions the motor changes to generator operation provided that sufficient load is available in the d.c. link. To ensure a rapid change to the generating mode the frequency of the inverter stage (INV) is reduced by a predetermined and sufficiently large step as soon as the supply (N) suffers an interruption. Considerable power is thereby generated giving rise to adequate current in the d.c. link to ensure continued commutation. The size of the loading capacitor (C) in the link can thus be kept small. Following the transient the unit can readily revert to normal operation, and the motor brought back to set speed.