Ask an Engineer: VFD Phase Conversion

Ask an Engineer: VFD Phase Conversion

How do you convert variable frequency drives from single-phase to three-phase power?

Phase converters convert a single-phase supply to three-phase power. If you’ll be using the phase converter device to create a three-phase output from single-phase input, you should know that variable frequency drives (VFDs) typically require a more balanced voltage than off-the-shelf phase converters can provide. VFDs may also create harmonics, and this could potentially create problems with the operation of the phase converter.

To use a VFD with single-phase power the VFD should be oversized by a factor of roughly double the drive’s horsepower (hp) capacity. If you need to run a 10 hp, 230 volt three-phase motor from single-phase power, purchase a 20 hp VFD to do the job. This is because only half of the input rectifiers are available to energize the direct current (DC) bus.

VFDs provide single-phase to three-phase conversion as a part of the speed control process. To use single-phase power, simply connect the voltage source wires to the L1 and L2 terminals on the VFD. To use three-phase power, connect the input voltage wires to L1, L2 and L3 terminals on the VFD. The output (load) voltage is always three-phase regardless of the input voltage source.

There are specially designed phase converters which can serve sensitive loads, such as CNC machines and VFDs. An isolation transformer is included to change the typical output, three-phase delta high leg to a balanced wye configuration. A delta high leg taps one of the three-phase legs to supply single-phase voltage, resulting in an unbalanced transformer. A VFD requires a more balanced three-phase supply voltage.

There are also phase converters with automatic adjusting circuitry that help keep the voltage in balance so three-phase VFDs can be supplied equal voltage on all three input phases. In most cases, these units must also be derated, but this depends on the type of load being served. Because VFDs can create harmonics, line reactors are sometimes used to avoid this problem.

Phase converters that serve mostly motor loads will need to be sized larger than those that serve non-motor loads. If you have an existing phase converter, contact the manufacturer to see if that unit can serve a VFD.