Since as early as the 1990s, there have been questions, discussions, and application notes on the topic of using a 2-contactor bypass or 3-contactor bypass with variable frequency drive (VFD) configurations.
This article will offer information on topics of “safety” and “reliability” issues when using a 2-contactor or 3-contactor bypass. In order to better understand these issues and configurations, we will first review the purpose of a bypass. A VFD bypass package addresses the need to run the motor directly across the line. Using contactors, the bypass package redirects the 3-phase AC line voltage around the VFD and directly to the motor. This removes the VFD from the circuit. A common purpose for this type of package would be to allow the motor to run when there is a problem with the VFD.