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Service Call: Motor won’t run in bypass

Recently a customer requested a VFD with a bypass option to replace a thirty year old stand-alone VFD for his HVAC application. A bypass option permits the motor to be started across the line in the event the VFD fails.

This is a great idea for applications where down-time is critical. In this particular instance however, the customer didn’t realize that his supply transformer didn’t have the capacity to provide the roughly 600 percent inrush current that a line started motor requires.

As a result, when we attempted to line start the motor, the overloaded transformer forced a significant voltage drop, causing among other things, the output contactors to drop out.

This application ran fine on the VFD, because in general, the starting current on centrifugal type loads can be kept below 115 percent of the motor full load amps. In fact, it can usually be even lower by adjusting the acceleration time.

By understanding the problem, we were able to recommend a higher

KVA transformer, which resolved the issue. These are simple solutions, and at EMA, we look for simple solutions. It’s why customers call us, and it’s why our customer satisfaction rating remains the highest in our industry.


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