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Phase Converters

  1. Apr 5, 2006 #1


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    We have a residential client who is building a 7,000 sq ft vacation home in Cape May, NJ (right across the street from mine :cool: ) that we are going to air condition with fan coil units. That means an 18 ton chiller, and 18 ton chillers don't come in single phase. There are smaller chillers that are single phase that we could gang together, but we'd reaaaaaaally prefer not to.

    I spoke to the electric company and the street they are on is all 120/240 single phase, and to get three phase from 900 feet away would cost on the order of $70,000. My boss is of the opinion that if you can afford a 7,000 sq foot house on the beach at Cape May, you can afford to run a thousand feet of electric cable down the street. Sure, maybe, but I'm not sure the client will see it that way.

    Anyway, the electrical engineer that we use knows a tiny little bit about phase converters, but not enough to explain to me how they work, much less make educated decisions on them. I've done some research and found a few....
    ...and it appears to me that there are two basic types: rotary and rectfier/inverter converter. Let me explain how they work and you EEs can tell me if I understand it:

    The rotary ones are essentially just 3 phase motors that when run by one phase induce current in the other phases, outputting not-well-balanced 3 phase power of the same voltage that you put in.

    The rectifier/inverter ones (do they have a concise name?) rectify the power to regular DC, then invert it back to A/C, three phase, similar to how a VFD works. According to the marketteers, that yields high quality, balanced 3 phase power.

    So first, am I understanding these devices correctly(if oversimply...)? Second, how much of that marketteer bs is really bs - how good are these devices? How efficient? I realize it can't really be as good as real 3ph power from the power company, but is it a viable option or is it something that makes electrical engineers cringe?
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  3. Apr 5, 2006 #2


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    I think you are understanding the two types correctly. I'd stay away from the inverter option myself, since all that switching power supply noise can cause many other interference problems in the house. Stereo, TV, phone, WiFi can all potentially get stepped on by the noise, which would put a crimp in the vacation home lifestyle, eh?

    I'd also look at how much power is being run to the home already on the 240Vrms feed, and calculate what fraction of that power will be consumed by the two 3-phase generator options. If it's a modest fraction of the available power to the home, that would be another reason to motivate the homeowner to run the separate power feed in 3-phase from the power company.
  4. Apr 5, 2006 #3


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    One of the reasons I wouldn't use an inverter (and I've worked on both types) is that power devices have a higher likelyhood of component failure due to the heat involved. One needs to look into the maintenance cost before making a decision.

  5. Apr 6, 2006 #4
    I am having a similar issue on exacly what goes on with the conversion. As i understand it, there are two types as you talked about: Rotary and Static. Static can be anything though just about. AC to DC back to AC in three phase, or a direct AC to AC (through a cycoloconverter). The issue arises as I see it is whatever motor you have driving your fans can only run at about 2/3 horse power if you do a static conversion compared to the rotary conversion. Some of the terms at least may help.

    From Power Electronics and AC drives by B.K.Bose.
  6. Apr 7, 2006 #5


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    Russ, even though i am an EE, i don't venture here very often from the physics forums. i'm about as sympathetic as your boss is. this guy must have boatloads of $$ to burn to be building a house 4 times larger than mine and it's not even his primary residence.

    i don't know why you don't want to put in multiple single-phase air conditioners in, but i would think that would be the least of evils here. any efficiency gained by having one big chiller is likely lost with the [itex] 1 \phi \rightarrow 3 \phi [/itex] conversion, even the solid state bugger. the return on investment that the $70,000 would earn would outstrip any savings by a more efficient industrial grade A/C than a few smaller units placed around the house.
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