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I am needing to convert either 240 V or 120 V single-phase into 120/208 V three-phase 4-wire. The load needed is 30 A. My panel can supply up to 100 amps, so the amperage isn't an issue.

I've looked into static inverters, but then I'm going to need a step-down transformer which just adds to cost.

I'm considering a VFD rated for 120 V single-phase input, 208 V three-phase output. Yes, I know VFDs are used almost exclusively for motors. I will not be running a motor, there is already a VFD in my unit for a motor.

What is the best way to accomplish this? Can a VFD supply three-phase AC for something other than a motor?

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  • \$\begingroup\$ You haven't explained what the load is and that will be crucial. Also, should "My panel can pull up to 100 amps" be "My panel can supply up to 100 amps"? \$\endgroup\$
    – Transistor
    Apr 1, 2020 at 17:44
  • \$\begingroup\$ Thanks for the corrections! I'm not really sure how to calculate the load. I know how to calculate general load, but I'm needing to supply three-phase to a very complicated "computer" of sorts that runs 4 x 115v single phase motors, a 300v 9.2A DC motor, and multiple other devices such as control panels and read-out panels. There are already VFDs in the computer to control the motors, so I really just need to supply safe 3-phase 4 wire 120v/208v power... \$\endgroup\$
    – DMNYC
    Apr 1, 2020 at 17:53
  • \$\begingroup\$ You probably need an inverter. The VFD will output PWM at √2 times the mains voltage. That's fine for a three phase motor but a lot of other stuff may not like it. \$\endgroup\$
    – Transistor
    Apr 1, 2020 at 18:09
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    \$\begingroup\$ If your load is not a three phase motor, what is your load then? If you have a three phase heater operated in wye or star with 120 V across each heater, it may be rewired for single phase 120 V. No VFD neccessary. Of course you may connect 2 heaters to one 120 V leg and the remaining heater to the other leg. \$\endgroup\$
    – Uwe
    Apr 1, 2020 at 18:11
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    \$\begingroup\$ VFDs are three-wire devices. If you really need a four-wire connection, you can always add a delta-star transformer. If you do not have rotating plant, you do not need a three-phase supply. 100 A at 240 V is about 23 kVA. \$\endgroup\$
    – skvery
    Apr 1, 2020 at 18:48

2 Answers 2

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What is the best way to accomplish this?

Make a list of all the individual loads. Determine what voltage, current and number of phases is actually supplied to each. If you have VFDs and a DC motor controller that requires 3-phase input, you can get 240 V, single-phase alternatives for those. It seems like most of the loads may be 120 V, single-phase.

It is not a good idea to use a VFD to power something like that. There are likely loads that will not tolerate the VFD output waveform very well. There are also likely loads that the VFD will not tolerate very well.

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Using a VFD as a 3 phase source is a strict no-no.

An ideal application of a VFD is where the motor size matches the VFD size.

A smaller motor may be managed with some motor hardware and VFD programme alterations.

A lightly loaded VFD would be rich in harmonics and cause damage to electrical equipment when used as a 3 phase source.

With 3 phase mains supply connection, 3 phase generator and 3 phase static invertor being ruled out, a single phase to 3 phase rotary converter would be the best option.

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