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  1. #1
    mark thomas is offline Titanium
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    Default 3-phase vs 1-phase motor efficiency

    I see it stated all the time that 3-phase motors are "more efficient" than single-phase motors. For example:

    http://www.3phasepower.org/3phasemotors.htm

    But it's never explained HOW they are more efficient.

    I understand 3-phase motors are cheaper to buy and maintain, so total lifecycle economic efficiency is better. But is running a typical motor more efficient, in the sense of fewer watts consumed per work done? Suppose I power up two identical machines, but one with a 1 phase motor and the other with a 3 phase motor (same HP, same ), and I do the exact same work on each. Will the 3 phase motor consume fewer watts?

    Is the answer simply in the efficiency rating of the motor itself? I looked on grainger for examples, and typical 3P motors are listed as 86% efficient, but 1P motors don't have a rating (or not the dozen or so I sampled) Isn't about 85% typical for 1P motors too?

    Or is this one of those things that's way more complicated than it appears?

  2. #2
    Tony Quiring is offline Hot Rolled
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    Default V-8 to 4 banger

    The simple explanation is there are three different phases providing power to the magnetic field.

    You also do not have the starting winding or control, makes for a smaller package, looks are deceiving.

    If we have 400 cubic inches and divide it by 4 you have more angle between the power stroke, so less time doing work. Now divide by 8, less power more often, but now we are more constant in work output so a smaller motor may produce same usabe work.

    It is a little more complicated but it gives the general idea.

    If using RPC you loose energy in the overhead running the RPC so there may be smaller advantage.

  3. #3
    SAG 180's Avatar
    SAG 180 is offline Titanium
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    Default

    The problem with a single phase capacitor run motor is they have a secondary motor winding and a capacitor in series to provide delayed supply of AC sort of like a pseudo two phase AC motor. The ideal value of the motor run capacitor varies with the mechanical load to keep the phase delay right for maximum electrical efficiency.

    If you could get a really high quality single phase motor and rig up some microprocessor circuit to switch different value capacitors in and out depending on loading, you'd have much better efficiency than a normal single phase motor with a "Best guess" value of capacitance that can be a compromise unless the mechanical load is constant at just the ideal amount.

  4. #4
    MBensema is offline Hot Rolled
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    Default

    A quick check of the Baldor website shows many of the 1 hp single phase motors have an efficiency of about 65-70%.

    Other then losses due to the capacitor, I would expect higher losses due to the higher current in the windings for single phase motors vs three phase. For the same HP and voltage, a three phase motor requires 43% less current per phase which means less heat and less power loss.

  5. #5
    roysol is offline Cast Iron
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    Default I'm pretty clueless,

    but I think the answer is "more than meets the eye".

    I frequently read articles that I don't understand, and I just happened to recently browse through http://machinedesign.com/article/hea...tt-crunch-0807 . The chart at the end showing all the different motor types was particularly eye-opening.

    Cheers!

    Roy Solomon

  6. #6
    9100's Avatar
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    Default

    If you put an ammeter on a typical circa 1/2 hp single phase motor and on a similar sized 3 phase one, you will usually find a much greater difference between the idle and full load current on the 3 ph. The full load efficiency is only one criterion. In an application where the motor is run continuously at full load, there is a fairly small difference favoring the 3 ph one. In an intermittent service, like a table saw, the 3 ph will use a lot less electricity and run cooler. Single phase motors are mostly pseudo multiphase ones ( at least in startup ) and none of them work really well. There is a reason that there are few single phase motors larger than 5 hp. Having suffered with single phase and a couple different converters, in my, in this case not so humble, opinion, nothing works like a real 3 ph line.

    Bill

  7. #7
    Bill's Machine Shop is offline Hot Rolled
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    Default 1 ph vs 3 ph

    I remember asking an engineering professor that same question. I.E. If a three phase motor is more efficient than single phase, how huch more efficient is it? He said without hesitation, "86 percent". I don't know if that is true, and that was in 1980, so things may have changed somewhat since then. However, you still can't get something for nothing. Compare full load amps on each machine. That's where the HP calculation comes from, so they can't be too far away from each other. One way of looking at the whole question would be to approach it form the "green" point of view and consider how much copper and iron are going to produce the same effect. As Tony says, "Looks can be deceiving". Generally, the 1 ph motor will have twice as much copper and iron as the 3 ph motor. That extra stuff is devoted exclusively to starting the 1 ph motor. When it gets up to normal operating speed, the centrifugal switch takes it out of the circuit. And as said earlier, the 3 ph motor runs cooler. I would attribute some of that to the start function in the 1 ph motor. WWQ

  8. #8
    J Lauffer is offline Hot Rolled
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    Default

    Here is an example using amperage values from the NEC motor tables (tables 430-150 and 430-148). Real-world values may be slightly different, but you get the idea. Both are 1Hp, 230V:

    Single-phase motor is rated 8 amps. 230V x 8A = 1840 watts.

    3-phase motor is rated 3.6 amps. 230V x 3.6A x square root of 3 = 1434 watts.

    So, at full load, the 3-phase motor uses 22% less power to do the same work.

  9. #9
    Lloyd H is offline Aluminum
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    Default motor wattage

    I think that if you compare apples to apples you will find under a full rated load with comparable quality motors single and three phase motors operate very close to the same efficiency. Application, ie. starts and stop, short run intervals, part load, tend to favor three phase. However purpose built motors for say refrigeration and air conditioning that operate at near full load do quite well in single as well as three phase.

  10. #10
    JL Sargent is offline Titanium
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    Default

    Its kinda interesting how motor technology continues to evolve. There for while it seemed you could buy more efficient 5hp single phase motors than you could 3 phase. Not all motors are created equal. I believe you might still find higher efficiency ratings on some single phase motors but thats not the typical.

  11. #11
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    you might still find higher efficiency ratings
    Motor efficiency isn't the Holy Grail, motor reliability probably takes that honor. One major reason that all motors today are more efficient than motors from 50 years ago is that they run a tiny air gap between rotor & stator. The smaller the air gap, the higher the efficiency (all else being equal).

    The problem with a small air gap is not apparent when the motor is new & the bearings are unworn. As bearings wear, the rotor has more radial play, and will rub the stator. Older motors, though less efficient, had a relatively huge air gap & would continue to run with worn out bearings.

    Federal law now mandates minimum motor efficiency. But efficiency & long motor life don't always go hand in hand.

  12. #12
    peterh5322's Avatar
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    "There is a reason that there are few single phase motors larger than 5 hp"

    Single-phase motors are now available up to 10 HP.

    Single-phase capacitor start/capacitor run motors offer higher starting torque, and higher power factor, than three-phase motors of comparable power.

    Three-phase motors offer the lowest installed cost (a "capital account" item) and the lowest total operating cost (an "expense account" item), if three-phase service is already present.

    If three-phase is not already present, single-phase may be the lowest cost.

  13. #13
    mark thomas is offline Titanium
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    Default

    Quote Originally Posted by J Lauffer View Post
    Here is an example using amperage values from the NEC motor tables (tables 430-150 and 430-148). Real-world values may be slightly different, but you get the idea. Both are 1Hp, 230V:

    Single-phase motor is rated 8 amps. 230V x 8A = 1840 watts.

    3-phase motor is rated 3.6 amps. 230V x 3.6A x square root of 3 = 1434 watts.

    So, at full load, the 3-phase motor uses 22% less power to do the same work.
    That approach was my original analysis, but I did not get my values from NEC tables, but instead from sampling actual motors on graingers.com.

    I found 1 HP single phase 230V was 5.8A, which is 1334 watts, and 3-phase 230V at 3.2A, which is 1273 watts, so a <5% advantage for 3-phase. But I also found 3-phase 1HP motors rated up to 5.8A, which is 13% more watts than the single phase. It's not clear to me what the range in FLA for a given voltage and HP correlates to, but sampling a variety of real-world specs for 20 plus motors, at several HP levels (1,3,5) didn't lead to any conclusive advantage for 3-phase in watt calcs.

    In any event, it sounds like for my use (lots of start/stop, mostly running well under full load, etc) there's probably some advantage to 3-phase.

    btw, one of the reasons I'm asking is I wonder if I should buy a 1-phase or a 3-phase motor for basic things like a buffer or grinder, given that I am using a Phase Perfect. The Phase Perfect claims 95-97% efficiency. So if I get 5% efficiency gain from 3-phase, it's a wash, in terms of elec costs.

    My tentative conclusion is that there is no significant disadvantage to using 3-phase motors where I could as easily use 1-phase, like a simple bench grinder, and there is both some short-term and long-term advantages to 3-phase in my situation. The short-term being that used 3-phase things often sell for less than equivalent single phase, and the long-term being the greater reliability of 3-phase.

  14. #14
    J Lauffer is offline Hot Rolled
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    Default

    Amperage values can vary greatly, for a given HP, depending on the type of motor. There are things like "farm duty" motors for example, that will have much higher current ratings than "general purpose" motors, etc.

    Another example: I have 2 sump pumps in my house. They are different brands/designs, but both are labeled as 1/2 Hp, and both have similar GPH ratings. But one is rated 9.4 amps and the other is 4.3 amps. I could understand a few tenths of amps difference, but over 50%...WTF???
    Last edited by J Lauffer; 01-07-2009 at 12:14 PM.

  15. #15
    skrity is offline Aluminum
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    Default i went to grainger and i carnt work it out lol

    Metric Motor, 3-Phase, IP55, 1/2 HP, 0.37 kW, 1700 RPM, 230/460 Volts, 1.8 Full Load Amps, D71D IEC Frame, Service Factor 1.15, Ambient 40 C, 60/50 Hz, Nominal Efficiency 74.0, Insulation Class F, Thermal Protection None, Rotation CW/CCW, Inverter Duty, Rigid Base Mounting, B3/B5 Foot/Large Flange Mount Type, Ball Bearings, Aluminum Frame Material

    thats a grainger discription. even using power factor how did they get 0.37kw or are they referring to a non loaded motor?

  16. #16
    Jim Caudill is offline Stainless
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    Default

    Peter's post in #12 hit upon a very important consideration: the cost of electricity. 3-phase service here is usually demand-metered, and this has cost me all kinds of grief at my car wash. So, even if your motor uses less kwh, you may lose that gain due to the rate you pay for those kwh.
    Last edited by Jim Caudill; 01-07-2009 at 12:21 AM.

  17. #17
    skrity is offline Aluminum
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    Default demand metered

    have u looked at soft starts and vfd,s they help with demand metered costs.

  18. #18
    peterh5322's Avatar
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    "Peter's post in #12 hit upon a very important consideration: the cost of electricity. 3-phase service here is usually demand-metered, and this has cost me all kinds of grief at my car wash"

    At this nation's largest municipal electric utility, where I was an EE in a former lifetime, we had a number of residential three-phase customers, all of which were "grandfathered".

    Should they discontinue their three-phase service for any reason, they could never get it back as our rules for residential services were strictly single-phase, unless "grandfathered".

    None of our residential customers had demand meters, whereas most of our commercial customers had demand meters.

    So, for those relatively few residential customers with three-phase services, they really received a gift, whereas none of our commercial customers received did.

    The predominate reason for having three-phase in a residence was the presence of elevator(s), most often just one, but occasionally more than one (man-type for the residence, freight-type for the service personnel). Obviously, these were older, multi-level residences.

    And, these elevators were usually of the Ward-Leonard System type, which has the prime-mover motor running at a constant speed, and the functional motor running at a variable speed (most often off).

    Now, if the Ward-Leonard System died, and was replaced by a modern replacement, possibly an ac drive, then perhaps a conversion to single-phase would be made, and the three-phase service would be removed, at the customer's request.

    Another reason for having three-phase in a residence was the presence of private motion-picture screening rooms, usually with arc lamps and possibly selsyn-interlocked sound.

    As one might imagine, in and around Los Angeles, there were quite a few private screening rooms within residences.

  19. #19
    wrenchhead531's Avatar
    wrenchhead531 is offline Aluminum
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    Default

    Quote Originally Posted by precisionworks View Post
    Motor efficiency isn't the Holy Grail, motor reliability probably takes that honor. One major reason that all motors today are more efficient than motors from 50 years ago is that they run a tiny air gap between rotor & stator. The smaller the air gap, the higher the efficiency (all else being equal).

    The problem with a small air gap is not apparent when the motor is new & the bearings are unworn. As bearings wear, the rotor has more radial play, and will rub the stator. Older motors, though less efficient, had a relatively huge air gap & would continue to run with worn out bearings.

    Federal law now mandates minimum motor efficiency. But efficiency & long motor life don't always go hand in hand.
    amen....Dave

  20. #20
    Georgineer is offline Hot Rolled
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    Skrity -

    The figure puzzling you is the output power. One horsepower equals 746 watts, so the output in 'mechanical' watts is 0.5 hp x 746 W/hp = 373 W, which is roughly 0.37 kW.

    This is (again roughly) 74% of the input power in 'electrical' watts, so the input must be about (373/74) x 100 = 504 W, say 0.5 kW

    The difference appears as 'heat' and 'noise' watts.

    I hope this helps,
    George

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