Specifying VFD driven low voltage motors for safety and operational life

AC induction motors operating on a variable frequency drive (VFD) have different operating characteristics from those operating on sine wave power and may benefit from specific design considerations. However, many applications will operate successfully using a readily available standard motor, which may offer the best overall value to the end-user. Today, many sine wave motor specifications include a generic addition that is simply labeled “suitable for VFD operation.” The rest of the specification can contain many contradictions and lack application details. Or the specification may outline many additional features for use on VFD power that add value only in a small number of applications. This paper will offer insights into how to specify VFD driven motors based on the real operational and safety needs of the customer or application. Items such as motor size, duty cycle, driven load characteristics, hazardous location rating, and equipment life will be addressed. Considerations such as insulation types, bearing current protection, special cabling, and other options will undoubtedly be beneficial to the user. Motor design aspects, such as insights into what makes a motor better suited for VFD application versus when a general purpose motor will be just as reliable, will be discussed. This paper will provide some examples of verbiage that might be included in the specification to enhance communication between the specifier and the motor manufacturer. It will not, however, attempt to delve deeply into many of these issues but will serve to provide guidance only.