I have an AC system that runs on single phase 120v AC. The "motor" is a hermetically sealed compressor, fractional horsepower. It has a single capacitor about the size of pill bottle. Based on that, I think the motor is a capacitor start type single phase motor.

What I want to do is use a VFD, or something similar to run the motor up to perhaps 120% of it's rated speed. This is for an experiment, so I only need the motor to work for a few hours, and if I accidentally destroy it that's an acceptable risk.

I've done a bit of research and it appears that this is not a trivial task. However, it seems like if I were able to create a voltage of 120V+, up to around 72 hz, then that should work. Would it not?

If so what's the simplest way to go about this? I'm considering buying a single phase 240v output vfd, and stepping the output down using a 2:1 transformer. Is there a better way?

  • \$\begingroup\$ Look for a VFD with 120Vac input then the output is created with V/f to change RPM to 120% V/f but starting current needs to overcome compression torque to avoid stall. You get to choose the V/f ratio. \$\endgroup\$ Mar 13, 2018 at 19:09

1 Answer 1


It appears that you have found one of the very few VFDs on the market that are designed for single-phase motors. It is likely that that the V/f ratio has sufficient adjustment range to do what you want without a transformer. Transformers are difficult to deal with on the output of the VFD because the V/f ratio at start and very low frequencies may need to be significantly higher than the rated V/f. Try to download the manual and check the adjustment range of the V/f.

The VFD that you selected may be manufactured by someone other than the seller. If the seller will not let you download the manual before buying, try to determine if it is available from the manufacturer.

  • 1
    \$\begingroup\$ The VFD that you selected IS in fact not manufactured by the seller. The seller is a 2 person firm that is just an import office in Chicago for unnamed Chinese products from various sources. I would be leery of using them. There are other more reputable sources for VFDs that run single phase PSC motors. Invertek from the UK is one of them, and they sell through two companies in the US, Bardac and Anacon. invertekdrives.com/variable-speed-drives/… \$\endgroup\$
    – JRaef
    Mar 14, 2018 at 23:29
  • \$\begingroup\$ The literature on the sales site is not good, but it appears that the VFD is designed to energize the windings without the capacitors to operate as a two-phase motor. It may be quite a cleverly designed product. However there is a lot more than clever design required for a good product. \$\endgroup\$
    – user80875
    Mar 14, 2018 at 23:41

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