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VFD bypass

Not all variable frequency drives are intended for continuous operation; in some instances, they are simply a "starter" for a rotating machine. In these cases, one VFD may be used to sequentially start several machines - switching each to "bypass" mode once operating voltage and frequency are established, and leaving the machine connected to the supply line for loading/running.

This is similar to using an auto-transformer approach, albeit with smoother transitioning. The whole idea is to limit inrush current ... which can be significant on larger equipment.

For equipment that does normally operate full-time on the drive, having a bypass capability will allow operation when the drive must come out of service. It may (if the machine is also capable of a direct-across-line start) allow starting in the event of a drive failure so that high-profile processes can continue.

In a commercial building the HVAC system is a critical system as it supplies fresh air and comfort to the people who pay rent. The VFD with bypass was used to insure there was never an interruption in air flow.

If you have not seen a wiring diagram, a bypass is most commonly a reversing starter (two contactors and an overload) where one contactor is connected to the output of the VFD and one contactor is connected to the mains. Both power paths have protection. This is often called a Two Contactor Bypass. A more complete system places another contactor in front of the VFD so as to completely isolate the VFD for service. Surprise! That is called a Three Contactor Bypass. Using a reversing starter prevents closing both contactors by a mechanical interlock (but you knew that!!).

Normal operation has the VFD supplying power to the motor through its contactor with variable frequency. If the VFD dies (or at least goes into cardiac arrest) you would open the variable frequency drive contactor(s) and close the mains contactor putting the motor at full speed.

I have seen bypassed VFDs run for years that way. I actually sold a project to a hospital where the specification required a bypass on each drive. When I started up the VFDs I wanted to test the bypass to check for rotation. I was told to just bump the motors because the HVAC duct system could not handle the volume supplied by a motor running at full speed and could suffer rupture if I did so!

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