Inverter bypass switch

Not all inverters are intended for continuous operation; in some instances, they are simply a starter for a rotating machine. In these cases, one frequency inverter 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 frequency inverter, having a bypass capability will allow operation when the frequency inverter 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 an inverter 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 frequency inverter 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 switches and an overload) where one switch is connected to the output of the inverter and one switch is connected to the mains. Both power paths have protection. This is often called a Two Switch Bypass. A more complete system places another switch in front of the frequency inverter so as to completely isolate the frequency inverter for service. Surprise! That is called a Three Switch Bypass. Using a reversing starter prevents closing both switches by a mechanical interlock (but you knew that!!).

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

I have seen bypassed frequency inverters run for years that way. I actually sold a project to a hospital where the specification required a bypass on each inverter. When I started up the frequency inverters 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!