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3 phase vs. single phase power

  1. Sep 19, 2008 #1
    I have an opportunity to purchase a surplus but new 3 ton central AC (heat pump) system for my home at an attractive price but it has a three phase compressor motor and I have single phase 220V service into my home. As I understand my options, I could replace the motor with a single phase unit or install a phase converter to convert my single phase service to three phase power. I assume that there are advantages to using three phase power but are they lost if I have to go through a converter? If I decide to buy a phase converter, how do I size it? The plate on the unit does not show a HP rating for the compressor motor but it has an RLA of 10.4 and an LRA of 65.5 at 208V. If I decide to go with a conversion to a single phase motor, what size do I need? If I decide to go with a phase converter, should I go for a rotary or digital converter (or are there other options)?
    Last edited: Sep 19, 2008
  2. jcsd
  3. Sep 19, 2008 #2
    3 ton?? How big a house have you got? I would have thought buying a new single phase pump was a cheaper option. I don't know what LRA is ?? Current?
  4. Sep 19, 2008 #3


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    I would check if your residential service can even give you the current you need at 230volt single phase. RLA is the running load amperage and LRA is the locked rotor amperage.

    Maybe you can get your power company to bring in three phase for you. It will cost however.
  5. Sep 19, 2008 #4
    That would be 2.4 kW then ignoring power factor and about 3HP. Big pump?

    Do you have a castle?
  6. Sep 19, 2008 #5


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    That's 3.7 kW (compressor only) or 19 Amps (total system), and no, that's not really all that big. I'm not sure where you guys are getting the idea it is. It depends a little on where he lives, but that covers about 1500 sq ft. It may even be a townhouse - I live in a 1500 sq ft townhouse with a 3ton AC unit. Anyway, houses of that size often have 100A panels.

    For the initial question, if you can get the AC unti and the phase converter for considerably less than the price of a single phase AC unit (a few hundred dollars, at least), I'd consider it. However, be wary of rotary phase converters - they are essentially motors driving generators and they have a pretty decent efficiency loss involved. Solid state phase converters are better.
  7. Sep 19, 2008 #6


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    The rule of thumb is 1 ton for every 500 square feet, so that's not really that big.

  8. Sep 19, 2008 #7


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    It shouldn't affect the efficiency as far as the compressor is concerned. I'd go with the digital converter. Contact the manufacturer for the proper size.

    In the long run it may just be cheaper to buy a standard single phase unit once you add everything up. I replaced one of my condensing units recently for about $1950 installed (4 ton unit).

    What size is your service entrance?

  9. Sep 21, 2008 #8

    I’d contact my supplier and get the cost of having a star/delta 3 phase transformer installed, you never know, it may be a cheaper option. In general the main advantage of having 3 phases available is that there is one less component to go wrong, the phase converter. Another little side benefit is that if you lose a phase, you will know about it straight away as the motor will probably sound like a tumble dryer full of ball bearings, so you wont have to argue with your supplier over who is at fault. Just on a practical note, phase converters can generate a bit of heat, it depends on the construction, so if its for a AC unit I’m assuming its going to be hot outside and placing a potentially heat generating device outside can cause its own problems on really hot days.

    Oh, and having 3 phase to the house can come in handy if you want to start your own business as you could setup a saw mill in your back garden for example with very little additional wiring :-)

  10. Sep 21, 2008 #9
    I didn't read the original post well enough. I thought you meant the normal pump used to pump water around the radiators around a house which is usually pretty small..
  11. Sep 29, 2008 #10
    I am no expert, but i had a similar situation where I had to power a 3 phase 480 volt 5 HP motor with with single phase 220.
    I contacted a trusted friend and electrical engineer of mine and he suggested a step up transformer and an AC drive/inverter ( used in cnc machines to regulate the power requirements).

    They are fairly cheap and i paid $250 on eBay for mine and i still have it and am very satisfied with it.
    I am not too sure how exactly it works but it seems to be a kept confidential even as my friend didn't want to tell me too much about them. The AC drives are more efficient then rotary converters and have a lot of extra features witch wouldn't be of any benefit in your situation (unless you would like to supercharge your AC and adjust the settings) If anyone has any explanation on how exactly they work i would be interested in hearing them.
  12. Oct 2, 2008 #11
    LRA = Locked Rotor Amps...
  13. Aug 17, 2009 #12
  14. Aug 23, 2009 #13
    I hope you haven't purchased the unit - there is no winning scenario. Even if the utility gave you the extra phases for love, the extra wiring, electrician work + panel would kill any money you could gain on this deal.

    Changing out the compressor would also be a bit scary. There would be all the issues of breaking into the system, paying for a new compressor, and then rewiring the thing with something that UL never reviewed (note 1).

    Finally, putting in a phase converter sounds just awful. Most of these things are fairly good sized with heat pouring out, and they're not made to sit outside. Any 1-2 seer points you might have gotten from this machine are gonna be lost to the converter.

    note 1 - When I worked at Trane, it was common for R&D technicians to purchase over-sized units and swap compressors for a smaller size. It wasn't something to try with a heat pump, and it definitely wasn't a project for the faint of heart. If performed properly, the end result was less pressure in the outdoor coils, and a significant boost in efficiency. Now, due to the improved efficiency mandates, your off-the-shelf machines probably rely on similar tactics.
  15. Aug 24, 2009 #14


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    This post is about a year old so he's probably already decided on a purchase! Just FYI...

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