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I'm planning to install a 5 ton central AC. I'm deciding between a condensing unit that runs on 220V and a costlier unit that runs on 207Y three-phase.

My understanding is that three-phase motors use less power, and that this is their primary advantage. But no one seems to be able to tell me how much less power. Without knowing that, it's hard for me to make a decision.

I'm sure the answer is some form of "it depends," but can anyone give me a ballpark? Should I figure 10% less power consumption, or more like 50% less. Any guidance is greatly appreciated.

Also, the wikipedia article on three-phase power claims that three-phase motors vibrate less. Is this something I should care about? Will a three-phase motor therefore last longer than a single phase motor?

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3 Answers 3

Single phase

1hp = 756 watts

5hp = 3730 watts

3730 watts at 230 volts = 17 amps

Three Phase

3730 watts = 3730(watts) / (207(v) x 1.73) or 3730 / 358.11 = 10.4 amps.

Three phase is 40% less amperage.

There are other things to make this accurate, like motor efficiency and power factor. I used 1 for each.

Another savings is that with the reduced amperage you can reduce your conductor size for your feeder. With the price of copper today that could help matters too.

I used 5hp for no special reason, just a nice round number.

I suggest using three phase.

Formula found here.

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Less amperage seems like a good thing, but I'm really concerned about wattage, because that's what I pay for every month. –  Andrew Cone Jun 2 '12 at 17:07
    
Wattage is wattage, can't help you there. However I think three phase will let you do in less horsepower also, and less horsepower is less wattage. Does a 5 ton unit use the same motor on three phase as it does in single phase? –  lqlarry Jun 2 '12 at 22:24
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If you have the condensers already selected, you should be able to get the two pieces of data you need to make a decision.

1) price increase for 3 phase condenser
2) operating power for single phase and 3 phase units.

I suspect they will quote you that the two units consume the same amount of operating power.

Yes, theory says 3 phase motors run smoother just like V8s run smoother than 4 cylinder engines. But the practical difference in motor life is something you will probably never see.

One possible reason that a 5 ton unit is available in 3 phase is that the amperage is lower and thus you could save some money on the wiring cost. This comes at the cost of an extra circuit breaker.

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Conversely, if three-phase did not provide a significant benefit for a 5 ton system, why would manufacturers bother to make both three-phase and single-phase versions. I mean, they do for 5 ton, but not for 3 ton. I have three phase service already, so there is no additional wiring cost to use three phase. –  Andrew Cone Jun 1 '12 at 15:08
    
@AndrewCone You have three-phase in residential? Really? –  Tester101 Jun 1 '12 at 18:09
    
@Tester101 it's plentiful in older homes in the higher end country clubs in the area I live in... or if your home is 27,000 square feet. –  lqlarry Jun 2 '12 at 1:13
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@Tester101 The building in question is a 35-unit apartment building with seven retail spaces. The three phase may have ben installed for the washers and dryers, which are the only things currently using it. The AC is for a common area of that building, with lots of people coming and going. –  Andrew Cone Jun 2 '12 at 17:09
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Think I'll chime in here and add my two cents to these other good answers. There are two components to the original question:

  1. How much less power does will the 3 phase A/C compressor unit use?
  2. Longevity of three-phase motor versus single phase

Power

Motor Efficiency = Power Output / Power Input

It takes a certain amount of power to run the compressor regardless of single or three phase power... this is the power output. The power input is what you are trying to minimize, thus you want to increase efficiency as much as possible. The trouble is this metric (efficiency) is a hidden combination the components inside the compressor unit (compressor, fans). It is not necessarily true (but it usually is true) that three phase is more efficient than single phase.

I would look at the SEER rating which 'automatically' includes the motor efficiency for a better indicator of "cost to cool".

Longevity and Reliability

Three-phase motors and compressors are generally more reliable than their single-phase couterparts. But like anything else, there is more to it than that one attribute. Using Philps' automotive engine example, a 4 cylinder Honda might be more reliable that that old smallblock V8.

Vibration

Certainly less in the three-phase motor. And generally less vibration equals greater reliability.

Motor Starting

Three-phase motors have lots of starting torque and don't (typically) require any special circuitry to start (capacitors, centrifugal switches). And of course fewer components means fewer things to go wrong.

Cost

Is disappointing that the three-phase equipment is more expensive. Almost always, three-phase motors are actually cheaper than an equivalent single-phase motor.

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