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VFD bypass contactor

A bypass contactor is intended for use in case of a VFD failure for short-time emergency service. A typical diagram of the power circuit connection of a variable frequency drive bypass contactor is shown in the figure.
Variable frequency drive bypass systemThe isolation contactor electrically isolates the VFD during bypass operation & is mechanically & electrically interlocked with the bypass contactor to ensure that both cannot be closed at the same time. Upon a sensed malfunction of the variable frequency drive, the control circuit automatically opens the VFD isolation contactor & closes the bypass contactor to keep the motor connected to the source.

When automatic transfer to bypass operation occurs, the motor continues to operate at full speed. The VFD isolation contactor must be opened during closing of the bypass contactor so that AC power is not fed into the output of the variable frequency drive, causing damage. The automatic switch to bypass ensures no downtime & no interruption of service to critical loads. E.g, in HVAC applications, this allows heating or cooling to be maintained at all times.

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