Transient peak currents in permanent magnet synchronous motors for symmetrical short circuits

To enable constant-power areas with permanent magnet synchronous motors, flux weakening has to be applied. In that mode, the inverter protection measure, e.g. in case of overcurrent, is a crucial point, since the open-circuit induced terminal voltage may exceed the allowed inverter limit. That is why a symmetrical short circuit (SSC) is currently favoured as alternative protection measure. Though the steady state short circuit current may be acceptable, this contribution shows that the transient current may seriously exceed the allowed limit. The worst case SSC will not occur during flux weakening but in regenerative operation at rated speed. Such high peak currents include the risk of permanent demagnetization of the magnets. As a result, SSC should applied only during flux weakening as protection measure, while for the lower speed range the conventional converter shut down is proposed