Considerations In Choosing a Bypass
For Variable Frequency Controllers

Tom Jawor
Reliance

One of the unique advantages of A-C variable frequency controllers (VFC) over other types of variable speed devices is the ability to drive the load even if the controller is inoperable. This is not possible with mechanical, hydraulic or even D-C static controllers. A bypass, which electrically circumvents the VFC and runs the motor directly across the line, is possible because the A-C motor was designed to operate in this manner.

Why Use Bypass

Although variable frequency controllers have become reliable devices in the last several years, some applications still require a back up system. A-C controllers are sophisticated electronic devices which are more susceptible to their surroundings (temperature, line spikes, etc.) than are the motor starters they typically replace. 

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 drive 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 drive 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.

Bypass Approaches

The two basic approaches to electrically bypass an A-C controller are a manual switch, and magnetic contactors.

Transfer Switch Bypass Magnetic Contactor Bypass

As detailed in the figures, the difference between the two is in how the motor is switched from the VFC to the line, and that is either by means of two electrically or mechanically interlocked contactors (contactor bypass) or by means of a manual switch (transfer bypass).

There are inherent advantages and disadvantages of each which determine the use of one or the other.

A manual transfer switch is less expensive to supply than two magnetic contactors, and this is usually the reason this approach is selected. There are other cost considerations, however, that sometimes are overlooked in the initial evaluation. To examine these it is important to look at what is required from a bypass in a given application.

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

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

The VFC will protect the motor from overcurrent providing the controller output current is close enough to the full load current rating of the motor. The VFC cannot, however, protect the motor when the motor has been switched to the bypass mode. A motor overload relay or thermostats in the motor are a requirement for proper protection of the motor by the National Electrical Code.

A motor overload relay or thermostat is not designed to break the motor connections but Instead opens a set of contacts when an overcurrent or overtemperature condition is sensed. It cannot by itself protect the motor. Since the contacts must have something to turn on or off, contactor bypass lends itself well to this by nature of the contactors being electrically energized. Transfer switch bypass, however, is a mechanical switch arrangement which cannot be actuated electrically. Additional hardware becomes a necessity if the transfer type bypass is to interface properly with user interlocks. To interrupt power on this bypass requires the addition of either an input contactor or a circuit breaker equipped with a shunt trip coil. The shunt trip coil allows the breaker to be physically tripped by the electrically energized shunt trip mechanism. The additional hardware required for a transfer bypass is either an input contactor or circuit breaker equipped with a shunt trip. This hardware is required to interface not only with the motor overload but for user interlocks as well. The cost of this additional hardware may offset the savings of this type bypass over contactor bypass. 

Safety considerations may require the capability not only to trip the motor off but also restart it. This cannot be accomplished after a circuit breaker has been tripped by a shunt trip mechanism; it must be manually reset. 

It may be a requirement for a service person to troubleshoot or test the VFC when the motor is being operated in the bypass mode. This requires a disconnect or a contactor on the input of the VFC 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. Reliance bypasses have always been horsepower rated and do not require a motor starter be supplied by others, although they may be interfaced with an existing starter if that is desired.

Reliance has elected to offer a single bypass option incorporating hardware to satisfy all of the above considerations for the VTAC V and GP2000 product line. The result is one simple bypass option as detailed below which includes:

  • Output contactor for VFC (N)
  • HP rated contactor for the bypass mode (BC)
  • Motor overload relay for motor protection in both modes (OL)
  • VFC input disconnect (ID)
  • Bypass/off/inverter selector switch
  • Common cabinet enclosure
  • U.L. listing


VFC Cabinet

The only option that may be desired for this bypass is a main disconnect or circuit breaker. This is not required if an existing disconnect/breaker is available that meets the National Electrical Code.
This approach provides an ideal solution to many application requirements because it is:

  • Simple
  • Capable of meeting most of the customer's/users interlocking requirements
  • U.L. listed
  • Factory, not field, wired
  • Easy to service thru separation of drive & bypass components
  • Very competitively priced

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