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Efficency of motors

  1. Feb 16, 2005 #1
    Wich are more efficent, Dc motors or Ac motors? Say for instance you had 2 motors 24V Dc and a 240V Ac. Wich would be more efficent?
  2. jcsd
  3. Feb 16, 2005 #2
    That is a difficult question to answer. You don't specify the type of motor, horsepower, etc.
  4. Feb 17, 2005 #3
    also is the DC motor brushless or not? that make a big difference, all ac motors are brushless i believe
  5. Feb 17, 2005 #4
    The DC motor is not brushless. they are both 1/3HP motors.
  6. Feb 17, 2005 #5
    Split phase? Capacitor start? 3 phase? How about if you multiply the voltage and the full load current on both (for watts) and make the decision yourself?
  7. Feb 19, 2005 #6
    I have never learn AC motors, so I cannot be of much help.

    But one way of determining which is more efficient is to do a simple test, if you have the money to buy those motors. Then input a DC voltage of 24V and observe the rotational speed of the motor, (don't know if there a a meter out there to measure rotational speed). Then input a 240V AC volatge to the AC motor, and observe the rotational speed. The faster one is the efficient one I believe. But one question, why compare a 24V DC to 240V DC?
  8. Feb 20, 2005 #7
    Ummm, no. You are only measuring speed. You need to test each one under load. How many horsepower is each one developing and how many watts is each one consuming.
  9. Feb 23, 2005 #8
    The suggestion of testing them is not exactly what I was hoping for. I need to select the most efficent motor I can and the documentation you get from suplyers is almost entirley wrong.... I have tested several motors already. What I wanted to know is... If you had to select a motor based on efficeny... Which would you look for? Keeping in mind its a 1/3HP motor.
  10. Feb 23, 2005 #9


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    How is the efficiency of the motor more important than the efficiency of the system? If you need a 1/3 HP motor to turn a shaft with x load and y rpm that's just fine. Then the real efficiency would be about how much electricity you need to purchase to get that. But if you have to convert 24VDC to 240VAC or vice versa those losses would need to be accounted for and could negate any efficiency gains.

    Wildly guessing based on a constant load at a constant RPM a 240VAC operating with a good power factor sounds like a winner. Vary either the load or the RPM or go to a poor power factor and now the 24VDC might be well ahead.

    As has been said, not enough variables have been defined.

  11. Feb 23, 2005 #10


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    It's probably hard to say in general. A google shows a study where brushless DC motors were compared to DC motors with a brush, and the brushless DC motors won.

    (This is the HTML version, I can't read the original powerpoint version).

    But this study didn't compare AC and DC motors. On theoretical grounds, the DC motors might have a small advantage because they use permanent magnets that don't have any I^2 R loss. On the other hand, since you specified a DC motor with a brush, the inefficiences here might well overwhelm any such theoretical savings. A lot depends on the load - the brushless design gave the best power savings at light loads and low RPM, so if your load is light or variable the AC motor with the appropriate efficient controller is liable to win. If the load is heavy and constant and the RPM's are high & constant, the race will probably be a lot closer, and it's possible the DC motor might win because of the permanent magnets.

    That's my best guess, anyway, take it for whatever it's worth.
  12. Feb 24, 2005 #11
    My dirt devil vacuume , which i took apart the other day , had brushes..
    I have never before seen a commutator with so many contacts as that one had ..
  13. Feb 25, 2005 #12
    The motor is used to run a Compressor. I am trying to increes the efficency of the (motor + compressor)system. We decided to try and first find a better motor to run the compressor; because the compressor is highly inefficent. The compressor itself can not be changed at the moment.
  14. Feb 25, 2005 #13


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    You misunderstood - I was referring to the electrical system, a portion of the entire system. The watts-to-torque aspect if you will.

    A compressor that runs at a constant speed and load, correct? And you have a 240VAC electrical service available for power, correct? Sounds like as long as you can get an appropriate AC motor with enough startup torque that should be the optimum choice. Beyond that, a conversion to a DC motor would require a controller/PS that adds to the costs and there would be losses to account for in the power supply too.

    Not all motors typically thought of as AC motors are brushless, more or less the induction type like in a small room fan are brushless. A universal motor like in a corded handheld drill or other power tools has brushes and will operate on DC in addition to AC.

    What does this unit cost to operate, $200-300 a year? What is the efficiency right now? What was the efficiency of the motors tested? What is the power factor?

  15. Feb 25, 2005 #14
    1. The compressor is not runing at a constant speed or load.
    2. Power imput we want to keep running off 12V DC or 24V DC though we can use AC if it gets us a better efficency.
    3. Any improvement to the motor is going to be an improvement to the system(I hope)

    I can post more details about the system if you would like... But my main consern is the motor. My goal is to have a more efficent compressor as a result.
  16. Feb 25, 2005 #15


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    If you are interested in effeciency, why are you saying that the DC motor is not brushless? A brushless DC motor would be the obvious candidate for improving effeciency. You'd only see a big increase in effeciency with a brushless motor if your compressor runs at a low torque or low speed a lot of the time - but it sounds like that might be the case.

    It's hard to say for sure whether or not an AC induction motor with an appropriate inverter/speed controller would offer any advantage over a brushless DC motor, you'd really have to test it. My personal guess would be not, but that's a *guess*.
  17. Feb 25, 2005 #16
    From my experience any brushless motor has to be AC. A DC brushless motor is simply making AC through the use of a switcher of some kind.
  18. Feb 25, 2005 #17


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    1. Varying kinda rules out AC, efficiency drops on induction motors outside their optimum RPM range.
    2. Converting won't really help anything for efficiency. Even the best FETs and torids are going to be 90% at best but could also be much less.

    24V should help over 12V by cutting the current in half, the I^2R losses will be 1/4 on the wires to the motor (assuming same gauge) and the internal losses would likely be lower too.

    But those are smaller gains of a few percent, bigger gains would be had in a precison machined rare-earth magnet motor but even there the gains might not be worth a bother. If you have 75% at the motor and 20% at the compressor, making the motor 85% only nets 17% overall instead of 15% previous. It could be looked at as a 13% improvement but it still leaves much to be improved at the more lossy stage. You already know that, but while we're throwing numbers around...

    According to this site (second chart) the range over which their motors are above 80% efficiency is quite large (especially when the peak efficiency is only a couple points higher) and the 75% range is still pretty good too. Pretty reasonable price for the HP, size, and apparent build quality. Hopefully you can find something like this in your RPM and HP range (that lives up to claims) if this really is what you're looking for.

  19. Feb 28, 2005 #18
    Those robot motors look quite good from what I can tell at first look. I think somthing like that will do the job. THank you for your help =-)
  20. Feb 28, 2005 #19


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    Clif_J mentioned this before, but since you pay for electricity at 120/240 or 208/277VAC, you have to convert it to 24VDC. There will be significant losses in the conversion and even if the efficiency of the 24V motor itself is higher, you probably can't overcome that loss.

    Most compressors I've dealt with are connected to a tank and run at constant rpm and relatively high load factor. The motor doesn't ever slow down, but cycles on and off as needed to maintain pressure in the tank. That's a near-ideal situation for an AC motor.

    I'd recommend sticking with A/C for those two reasons. And like others, I think the efficiency of the compressor itself should not be neglected: it likely plays a much bigger role in your energy costs than the motor.
  21. Feb 28, 2005 #20
    Though you are right... We are only conserned about the Power supply after all convertions are done. Ie: after 240AC-> 12DC
    Most compressors do run like that. The compressor I need is far from that operating condition. No Tank. Variable imput pressures and varible output pressure.

    And yes... The efficency of the compressor is the biggest problem... Wich we can not fix at the moment :mad:

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