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AC motor coupling with VFD

Can your VFD do "catch-on-the-fly"? The easiest thing to do is to couple an identically rated motor to the motor connected to your VFD. Start the second motor with a normal motor starter, ie soft starter, and then start your VFD in "catch-on-the-fly" mode at full rated speed. Slowly turn UP the VFD frequency reference ABOVE line frequency, monitoring the current draw, until you reach full-rated current. The motor being run from the starter will be regenerating power to the utility by operating above synchronous speed. If you try to run the VFD motor BELOW line frequency, it will regenerate power into the VFD and most probably cause it to trip due to high DC bus voltage. Also, make sure that both motors spin the same direction! The reason some people recommend DC drives and motors is that some DC drives can absorb regenerative power and allow you to load your VFD at lower frequency, but you can do it as I have described above with an AC motor.

Generally using a DC motor as a load is done due to costs involved with regenerative drives required to dump the energy. AC can and is being used when the driven motor and load motors drives share a common DC bus as the energy created by the load motor is used by the driven motor with cost to run only being losses.

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AC Motor Control