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My residence is supplied with single phase power. The previous owner installed a single phase to 3 phase converter to run a 12 ton compressor (Phase Perfect PT355). I am getting rid of the 12 ton compressor and replacing it with 3 5-ton compressors. I can buy these in single phase or 3 phase.

Correct me if I'm wrong but if I'm taking single phase power, converting it to 3 phase power, and then running compressor, I'm not gaining any efficiency. Any increased efficiency of 3 phase compressor is going to be offset by the losses in converting from single to 3 phase.

The sticker on the converter reads: Input 240VAC, 50/60Hz, 1 phase 98A; Output 240VAC, 50/60Hz, 3 phase 55A. Also, the single phase compressors have a 10 year warranty, while the 3 phase units have a 5 year warranty.

  • Are the new 3 phase compressors inverter rated? – ThreePhaseEel Jan 15 '17 at 17:42
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That's a crazy-large unit for a house. That's why he was stuck with 3-phase.

Stand-by power consumption on that inverter is 240 watts. That will cost you between $240 and $500 per year in electricity simply to have it idle depending on your local electricity prices.

Generally there's no earthly reason to go from single-phase to 3-phase (variable frequency drive for tools notwithstanding).

Single phase motors aren't significantly less efficient; if they were, they'd need enormous cooling fins. Because inefficiency turns into heat.

I think you're better off with 3 units instead of one. Not least, if one fails, it's not such a big deal.

  • I'm wondering if his best bet is to get inverter rated compressors and run them off VFDs rated to take 1ph 240VAC input? That way, it'd be possible to have modulating control instead of the crude 4-step stuff that he has now... – ThreePhaseEel Jan 16 '17 at 5:35
  • 3 phase is more efficient than a single phase by a factor of 1.732 but if you are having to convert single phase to 3 phase the efficiency is lost in the conversion or most is depending on how it is converted. 12 ton is crazy large unless you have a 10,000 sq foot home in a hot climate. A smaller high efficiency single phase system may be a better choice. – Ed Beal Jan 16 '17 at 14:51
  • You're talking about wire efficiency, and you're right. A single phase drive needs 1.732x the conductor. I'm talking about motor efficiency, i.e. the efficiency of turning electric-bill dollars into torque x RPM. – Harper Jan 16 '17 at 16:20

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