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  1. #1
    feets is offline Aluminum
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    Default at what point does 3 phase become more efficient?

    I wanted to know at what point does three phase become more efficient than a typical single phase motor.
    Would it be cost effective to replace the compressor motor on a residential size A/C system with a three phase unit?
    I've got a spare VFD sitting around and my mind started wondering. Would it reduce electric consumption?

  2. #2
    peterh5322's Avatar
    peterh5322 is offline Diamond
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    "I wanted to know at what point does three phase become more efficient than a typical single phase motor."

    Theoretically, the most efficient is 2.7 phases.

    That's theory. And, for uncompensated motors.

    For practical motors, the most efficient is two-phase capacitor start/capacitor run.

    The higher efficiency comes, in part, from the compensation provided by the capacitors which are required to start and to run such a motor.

  3. #3
    feets is offline Aluminum
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    What about carrying a load?
    There would have to be a difference or there would be no point in developing two different methods of operation.

  4. #4
    peterh5322's Avatar
    peterh5322 is offline Diamond
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    "What about carrying a load?"

    Two-phase, uncompensated is about the same as three-phase, uncompensated.

    Single-phase capacitor start/capacitor run, which is a special case of two-phase, has a higher starting torque, and a higher power factor (lower running current) than either two- or three-phase.


    "There would have to be a difference or there would be no point in developing two different methods of operation"

    If you're willing to trade higher complexity, higher first cost and lower operating cost for lower complexity, lower first cost and higher operating cost, then those are the differences.

    As a practical matter, for 10 HP and below, single-phase capacitor start/capacitor run is an option, whereas above 10 HP, three-phase is the only option.
    Last edited by peterh5322; 05-26-2008 at 04:36 AM.

  5. #5
    Pierce Butler is offline Hot Rolled
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    A three phase motor is almost always more efficient. How much?

    here is a chart.

    HP 1Ph 3Ph
    1/3 30% 50%
    1/2 33% 50%
    3/4 35% 50%
    1 41% 52%
    1 1/2 49% 54%
    2 54% 55%
    3 57% 58%
    5 58% 61%
    7 1/2 61% 64%
    10 64% 67%

    As far as replacing the compressor goes, I think you might have some issues there. The compressor and the motor are all one unit. The last time I priced a compressor, you could buy the entire condensing unit for about $50 more.

    Now if you want to put a VFD on your air handler, then a VFD could save you some money. As a matter of fact, that is how they are able to get the high SEER ratings. They put a 3 phase motor on the air handler and add a VFD.

  6. #6
    feets is offline Aluminum
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    I don't understand the chart. What does the percentage represent?

  7. #7
    ttok is offline Cast Iron
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    I am having trouble understanding the above posts. We installed two 3-phase A/C units at the house during extensive renovations in 2003. The local power company encouraged us to do so, even to the extent of installing a second "pole pig" on the pole behind the house at NO CHARGE, since the 3-phase system would use less electricity. They were happy to promote a "greener" alternative for the A/C. My feeling was that the 3-phase system would have less maintenance than a single phase one - easier on the compressors, etc. (My strong secondary motivation was so that we could get 3-phase into the garage for my Sheldon lathe.) Was their argument baseless??

    A.T.

  8. #8
    Pierce Butler is offline Hot Rolled
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    Quote Originally Posted by feets View Post
    I don't understand the chart. What does the percentage represent?
    The chart represents % efficiency.

    Here is a link if you want to know more.

    http://www1.eere.energy.gov/industry...l?news_id=8108

    MotorMaster+ Version 4.0.6

    Release Date: March 1, 2007
    An energy-efficient motor selection and management tool, MotorMaster+ software includes a catalog of over 20,000 AC motors. This tool features motor inventory management tools, maintenance log tracking, efficiency analysis, savings evaluation, energy accounting, and environmental reporting capabilities.


    http://www1.eere.energy.gov/industry....asp?product=5

  9. #9
    Pierce Butler is offline Hot Rolled
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    Quote Originally Posted by ttok View Post
    I am having trouble understanding the above posts. We installed two 3-phase A/C units at the house during extensive renovations in 2003. The local power company encouraged us to do so, even to the extent of installing a second "pole pig" on the pole behind the house at NO CHARGE, since the 3-phase system would use less electricity. They were happy to promote a "greener" alternative for the A/C. My feeling was that the 3-phase system would have less maintenance than a single phase one - easier on the compressors, etc. (My strong secondary motivation was so that we could get 3-phase into the garage for my Sheldon lathe.) Was their argument baseless??

    A.T.
    They are correct and surprised that they ran 3 phase to your house for free. I wish I had three phase power.

    There are advantages to 3 phase motors. Off the top of my head they are:

    3 phase motors are more efficient than single phase motors. - up to 20% more.
    3 phase motors last up to 3 times longer than single phase motors.
    3 phase motors have more torque than single phase motors - up to 600% more torque.
    Depending on the application a VFD added to a 3 phase motor can save up to 30% on energy costs. The payback is typically 18-30 months with 24 months being typical. There is little to nothing that you can do with a single phase motor.

  10. #10
    HelicalCut is offline Stainless
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    It depends on the application, if constant reversing is required a 3 phase motor is better option at any size, if the machine has other 3 phase motors then using a single phase motor is plain silly for practical reasons.

    If you use 440V/ 480V motors then the wiring is much smaller than for a equivalent single phase motor.

    If your application is one which needs variable speed, again the 3 phase wins.

    Starting current for a 3 phase motor is generally less which is more important on larger motors, starting current reduction techniques work better with 3 phase motors and 3 phase motors handle extended start times much better ( not many single phase motors like spinning up a load over a minute or more)

    Peters comment about the running current of a Cap start Cap run motor is correct but that is because the run capacitor provides some power factor correction, it is posible to put power factor capacitors on 3 phase motors and get lower running current especially with the high efficiency units. Never do this if using a VFD.

    If high reliability and high duty is required the start capacitor and centrifugal switch of a single phase motor is the weak link.

    Single phase motors vibrate more and so are a no no for precision grinders unless remote mounted using a compliant coupling such as a flat belt.

    OTOH If you are using manufactured 3 phase single phase motors win on simplicty grounds for many applications and by virtue of good starting torque are a good compressor motor.

    Back to the question about economy, if you are running loads like fans and pumps a VFD can give large savings in power as opposed to throttling or cycling the device. Also 3 phase high efficiency motors are relatively common in scrapyards though not usually small ones. Forget changing the compressor on your AC, Unless you have the gear and can pick up a compressor for scrap price or the like, your cost will be high. It is actually far easier to pick up a commercial 3 phase R22 condensing unit scrap or surplus than messing around changing a compressor.
    Last edited by HelicalCut; 05-27-2008 at 05:22 AM. Reason: did not adress the original post properly

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