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AC Tech – 3 contactor

There has been a movement among some VFD manufacturers to reduce the cost of the drive bypass, required by most engineering specifications, by eliminating the drive input isolation contactor, the 3^rd contactor. While this certainly does reduce the cost of the bypass it has also been a disservice to the HVAC industry. When bypasses were first conceived it was quickly learned that a method of isolating the VFD from the bypass was needed. The first failed effort was to advise the customer to remove the VFD input fuses. This was quickly rejected as difficult and awkward and could prove to be unsafe in certain situations. Also, not all VFD inputs are or were fused. It was then that the third or isolation contactor was introduced to provide a safe and convenient method of VFD isolation. Over many years and tens of thousands of installed bypass equipped VFD’s the 3-contactor bypass has proven itself to be a safe and reliable method of VFD isolation.

The 3^rd contactor allows the drive to be powered up and down for testing and set up in a safe and reliable manner while not interrupting the motor operating in bypass mode. Also, if you where to operate for long periods of time in bypass, the 3^rd contactor allows power to be easily and safely removed from the VFD to reduce energy consumption and just as easily and safely reapplied when normal operation is resumed. You might do this if your application required 100% flow for extended periods.

Today we are finding many installations with 2 contactor bypasses where the specifications clearly require 3 contactor bypasses. It is important to advise the design engineering community that they may be approving and having installed bypass equipped VFD’s that do not comply with their specifications in this very important detail.

AC Tech also continues the use of rugged steel enclosures, while in an effort to further reduce costs of their VFD’s some of our competitors use plastic. Obviously plastic does not have the strength nor the structural integrity of steel. We have heard reports of customers having difficulty getting the plastic covers back in place when they have been removed for installation or service. The MCH is a high value product that gives your customers’ full 3 contactor bypasses with rugged steel enclosures, easing the installation concerns of the contractor and specifying engineer.