Proper phasing of a VFD with bypass

A local college contacted Delta Automation with a request for on-site service to repair a VFD on the hot water pump for their heating system. This heating system was for the gym where a basketball game was scheduled for later in the day.
As they described the problem, words like “popping sounds” and “crackling sounds” were used to indicate what the VFD was doing. These are usually not good things to hear coming from a VFD! Then they also said that the motor was now running backwards as well!
Delta’s field service engineer arrived on site within a short time. What he found was that the VFD had expired quite violently. It was several years old and was a bit undersized. This particular manufacturer has a history of under sizing VFDs on some applications. This is usually done to “beat” the competition in pricing, who properly sizes their VFDs to the application. Even in a slightly under sized application, most VFDs will survive their warranty period.
This VFD was beyond economical repair, so a replacement of the correct size was to be installed.
But why had the motor begun to run in reverse?
This system had an automatic bypass installed. These systems work in a manner that should the VFD fault (or fail violently) the system automatically switches the motor to a bypass position. This puts the motor directly across the line voltage to run full speed. During the original installation, the phasing was not checked while in bypass. So, although the bypass function did perform the switch, the result was still a pump/heating system that could not perform. Total system failure.
Once again, an installation by personnel inexperienced with VFDs and bypass applications, caused this easily corrected issue.

click here to download the VFD bypass diagram

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About deltaautomation

President of Delta Automation, Inc. Working in industrial automation in excess of 30 years
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