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Three Phase RTU

....a three phase Lennox rooftop unit yesterday. The power at this building is only single phase, so they installed a "phase converter" (I believe that's what it's called, anyway). Why they didn't just put in a single phase unit, I have no idea.

Has anyone had any experience with these, and what effect on the overall amp draw they may have? It seems to me that you would need a larger than normal circuit feeding the phase convertor, since you're taking single phase power and making it do the work of three phase???? WHen I amped the compressors, I expected to see fairly similar amp draws on each leg, but actually saw much higer draw on one leg. I have next to no experience with three phase electricity, so I'm flying by the seat of my pants here.



  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Three phase-converters

    Typically, the single phase circuit requirement is three times the rating of the three phase amperage draw when using a converter. So, if your three phase load has an amperage draw of 15 amps, the single phase power supply should be able to handle a minimum of 45 amps.

    As far as the high voltage reading goes, it depends on how the three phase windings are wound. If wound as a star or "wye", all three voltage readings to ground will be more or less the same. If wound as a delta, you will have one high-leg, which will yield a voltage reading that is approximately 75% higher (square root of 3) than the single phase line voltage.

    In a nutshell, here is the concept of a single-phase to three-phase converter:

    Single phase power (two hot leads) are connected to two of the three power terminals on a three-phase motor (referred to as an idler motor). A third lead is connected between the third power terminal on the motor and a capacitor. The other side of the capacitor is connected back to one of the hot, single phase power leads.

    Your three-phase power supply can then be tapped into from the three power terminals on the idler motor.

    A slightly more complicated method of generating three phase power involves the use of two more cpacitors.

    I hope this helps a little
  • Paul Fredricks_3
    Paul Fredricks_3 Member Posts: 1,549

    As far as why didn't they just put in a single phase unit, my guess would be that was all that was available and they had to get the building running. Or maybe they ordered the wrong one, couldn't send it back, and put in the converter to get the job done.

    Just a couple of guesses.
  • Techman
    Techman Member Posts: 2,144
    3 phase

    Evening!What is the tons of the RTU?Anything larger than 5 ton would be 3 phase!
  • Jeff Lawrence_25
    Jeff Lawrence_25 Member Posts: 746

    A local establishment had a kitchen remodel done 8-10 years ago. The contractor looked at the job and gave them a price. He ordered the new dishwasher that was 3 phase and the place had single phase, so he got a phase converter. Most people that go out there refuse to work on that dishwasher. The GM told me he wishes it would die so they could get the correct one in, but it won't.
  • J Mac
    J Mac Member Posts: 1
    How much BTU do I really need

    How much BTU do I really need to heat a 1500 sq ft raised rancher with full basement. Home is 9 years old and well insulated.
  • John Starcher_4
    John Starcher_4 Member Posts: 794
    Thanks, Prof!

    I will have to get the specific information off of the unit next time I'm there.

This discussion has been closed.


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