VFD Single Phase to Three Phase

A VFD (variable frequency drive) is an electric motor drive and although it will convert single phase to three phase it should never be used as a power source for control circuits, instrumentation circuits, 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. Control circuits are almost always single phase even when the machine input is three phase. VFD’s that are rated for three phase input must be properly rated when single phase input is used.  Mains (input) current will be much higher when single phase power is applied to three phase rated equipment. Components within a three phase VFD of the same horsepower rating will most often not be rated for enough current and will fail. A mathematical calculation based upon the motor full load current will determine which VFD to use for the application. When applying single phase to a three phase rated drive, 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 be used on the input but never on the output. Properly rated circuit breakers or fuses should always be used on the input of a VFD. Motor rated circuit breakers and fuses will allow too much current to pass for too long. Electronic components are less tolerant of short term over current conditions than motors. This information is found in the VFD User Manual. Installation of a VFD and other related equipment should only be performed by qualified personnel. VFD programming is typically easy for most applications. Programming for an application can usually be successfully performed once user manual has been read.