Why use Bypass in Variable Frequency Drive system?

Although VFDs have become reliable devices in the last several years, some applications still require a back up system.

Some applications where bypass is required are those where downtime for any reason is unacceptable such as the ventilation of an operating room, computer room or clean room. Some industrial applications also require bypass to enable operation in the event of a VFD failure, and typically are requested on centrifuges, induced and forced draft furnace fans and pumps.

In commercial buildings, fire and safety requirements often call for bypass as part of a VFD package to insure the evacuation of smoke from a building after a fire. In such cases, bypass must allow for the remote control of the motor.

The bypass must switch the power supply to the motor between the variable frequency drive and the power line, but it also must be able to:

• Protect the motor in both variable frequency drive and bypass modes.
• Interface with safety controls.
• Isolate the variable frequency drive for service while maintaining motor operation.
• Meet code requirements as a motor starter.

It may be a requirement for a service person to troubleshoot or test the variable frequency drive when the motor is being operated in the bypass mode. This requires a disconnect or a contactor on the input of the variable frequency drive in addition to a main disconnect/ breaker.

Many bypasses are sized to handle only the full load current of a motor and not the locked rotor current. These are less expensive contactors or switches but cannot act as motor starters. A bypass which is not horsepower rated must only be switched when power is off and therefore requires a motor starter upstream.