VFD Single Phase to Three Phase

A VFD (variable frequency drive) is an electric motor drive and although it will convert single phase to threephase it should never be used as a power source for control circuits, instrumentationcircuits, or to operate other electronic components. The solution should be to isolate and feed the machine control circuit from a separate single phase source and feed the motor from the VFD output. Controlcircuits are almost always single phase even when the machine input is threephase. VFD’s that are rated for three phase input must beproperly rated when single phase inputis used.Mains (input) current will be much higher when single phase power isapplied to three phase rated equipment. Components within a three phase VFD of the same horsepower ratingwill most often not be rated for enough current and will fail. A mathematicalcalculation based upon the motor full load current will determine which VFD touse for the application. When applying single phase to a three phase rateddrive, the horsepower of the drive will always be a higher rating than the motor horsepower rating.

Installation and Commissioning

A contactor or transition switch should never be used on the output of a VFD.Wiring from the VFD output terminals should go directly to the motor. A contactor may beused on the input but never on the output. Properly rated circuit breakers or fuses should always be usedon the input of a VFD.Motor rated circuit breakers and fuses will allow too much currentto pass for too long. Electronic components are less tolerant of short term overcurrent conditions than motors. This information is found in the VFD User Manual.Installation of a VFD and other related equipment should only beperformed by qualified personnel.VFD programming is typically easy for most applications. Programming for anapplication can usually be successfully performed once user manual has been read.