What's the real difference between a BLDC and a 3phase AC motor?

Discussion in 'General Electronics Chat' started by strantor, Jul 15, 2011.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    This is an off-shoot from bobcart's post.
    Shortbus Enlightened me about the Brushless DC motor (BLDC).
    I have questions.
    What is the real difference between a BLDC and a 3phase AC motor?
    I found this which gives some insight, but I cannot find anything on the net that specifically compares the two.

    What I know (or think I know) so far:
    BLDC uses hall sensors or other sensors to determine the position of the rotor to fire the next phase; 3phase AC motor controllers (VFDs) just fire away phase after phase, assuming that the rotor is keeping up (slip).
    BLDC pulses straight-up DC into the phases, rather than a PWM simulated sine wave.

    Could one be used in place of the other? I.E. could you rip the circuitry out of a BLDC and run it with a VFD? Conversely, could you use BLDC circuitry (hall sensors, et.al.) to run an AC motor?

    Page 10 of shortbus's reference shows the phases of the BLDC to be wired the same as a 3phase AC motor.
    Page 11 shows the the inverter drive section of the BLDC to be exactly like the inverter section of a VFD
     
  2. justmark50

    New Member

    Aug 14, 2010
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    So why does the BLDC motor have to have a halleffect sensor for positioning? And why would the 3ph AC motor not need something similar? Also there is a difference between a 3ph Delta wound and a 3ph Wye wound motor. Shortbus' answer is only partially correct. A 3ph delta wound uses only 3 connections(L-1, L-2, L-3), while a 3ph wye wound uses four connections(L-1, L-2, L-3, L-0). In the wye configuration the L-0 is the common and each phase is referenced to this. I work on systems that use low voltage pulsed DC in a 3ph delta wound AC motor to control electric forklifts. For speed and direction control the motor rotation is monitored by a dual channel encoder. Just my 2 cents...have a great weekend!
    Mark C.
     
  3. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    I've never seen an 'L-0' in a 3ph motor, nor have I heard of bringing out the neutral on motor windings.

    If you consider a vector drive, there isn't much difference
     
    Last edited: Jul 16, 2011
  4. praondevou

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  5. strantor

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    Me neither, but praondevou's first link seems to lend it some credibility. It says that "the torque developed by energizing two windings simultaneously is 50% greater than energizing only one winding" - and - "with most motors the neutral is not accessible" - so I guess that's why we've never seen them. they suck.

    Yes, very much, thank you!
    I went through the whole first document (that's a lie; I skipped over most of the equations) thinking that there really is no difference. Even at the end where he sort of does a side by side comparison of the two. I had come to the conclusion that 3 phase AC is more efficient. Then I read the second document and it turns out that the BLDC motor has permanent magnets on the rotor. Both documents were written by the same guy (apparently a motor genius but doesn't know how to spell check) but nowhere in the first document does he mention that the BLDC has permanent magnets.

    So, in answer to my first question: the magnets,
    and to my second, no: you can't use the two motors & controls interchangeably.

    thanks again
     
  6. strantor

    Thread Starter AAC Fanatic!

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    here's an excerpt from praondevou's 2nd document:

    They don't need them unless you want to run in closed loop (for more precision). Usually a high PPM encoder would be used. almost all VFDs have an option to run in open loop vector (no feedback device) - I believe the drive either measures back EMF for a rough estimate of position/speed or just doesn't care; I'm not sure

    I don't doubt that you have seen this, but its by no means usual for wye-connected motor to have the common connected to anything. Sure, it's accessible, but the only accessing it should be the ends of 3 windings (in my experience). If you have been connecting it to a common you might want to make sure you're doing it like the plans call for (not saying you're wrong, just saying you might want to make sure you're not wrong).

    check out this link:
    specifically, check out "preparation: step 4" and "Wye wiring: step 1"


    Thanks for the input!
     
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  7. GetDeviceInfo

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    I think that the questions are too vague to be answered. I've worked on 450hp permenant magnet motors that are run with VFD. Is the motor then AC or brushless DC.

    AC induction motors operate on a principle of slip, while DC systems are typically synchronous. VFDs typically employ a DC link, so are they DC or AC. If the VFD employs vectoring, then it utilizes positional feedback. Modern VFDs use sophisticated sensing to operate in sensorless vector modes. I believe the terms VFD and BLDC indicate the origins of certain technologies, rather than catergorize thier current use, as techniques are borrowed from the discoveries of each development.

    If the first question was AC induction motors, then the answer would be slip. The second question could be yes, depending on the mode of operation.
     
    Last edited: Jul 16, 2011
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  8. shortbus

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    The VFD takes AC, rectifies to DC, and converts back to AC. This is what allows them to operate a three phase motor on single phase. And to allow the VFD to vary the frequency to the motor.

    The BLDC can be either two or three phase, but is strictly DC.
     
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  9. justmark50

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    Aug 14, 2010
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    My point of asking about the hall effect sensors, was not to dispute their use, only to say that there are other devices that can acheive the same result. So, how would you measure the back EMF? Probably through some sort of current sensor that is incorporated into the drive control. Not all BLDC motors have to have hall effect or some other type of feed back. A good example would be a muffin fan that is in your computer...brushless, permanent magnets and no sensor. Thank you for your feedback, I appreciate it very much! Have a great day everyone!
    Mark C.
     
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  10. strantor

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    BLAH! grey area. I hate grey area. no defined line between the two. Dang it, I guess I'll just have to settle for a "maybe" because all situations are differnt.
     
  11. strantor

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    Your point wasn't wasted on me, I got it; and you are totally right.
    That's a good point. maybe you could just fire the windings one after another with a timing circuit and assume the rotor is keeping up. In the case of the PC fan however, it has a hall sensor. I just annhilated one in the quest for knowledge and found a hall sensor in it.
    check it out:
    [​IMG]
    see attached data sheet.
     
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  12. BillB3857

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    Feb 28, 2009
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    One of the key features of a BLDC motor is that it can maintain a holding torque and shaft angle at zero speed, when used with the proper controller.
     
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  13. THE_RB

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    I don't think that's key to being BLDC, it's more key to having permanent magnets. A PMAC motor is still an AC motor but will also have that characteristic at zero speed.
     
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