Application of multi-function motor protection relays to variable frequency drive connected motors

This paper reviews the application of multifunction motor protection relays to motors that are connected by variable frequency drives (VFD). Typical VFD internal motor protection functions are reviewed to understand the required roles of external motor protection relays. Applications discussed include protection requirements when the relay is located on the source side of the VFD and the motor side of the VFD. The impact of using a bypass switch around the VFD on protection requirements is also discussed.