Who is an expert on brushless motors

Discussion in 'OnBoard Electronics & Controls' started by BertKu, May 22, 2011.

  1. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    Hi there, I have written the software for the Microprocessor for the brushless motor, made the printed circuit board and I have a strange problem. It runs beautifully. However, in forward mode it runs faster then in the reverse mode. Immediate one would say, this is the software what has an error. But I swapped and tested all possibilities and came to the conclusion that the problem is in the motor.
    I have glued the 3 sensors in the slots between the stator-metal anchor where the coil is wound around. All three 120 degrees apart. Could it be that I have glued them in the wrong slots and should move them up 30 degrees ? I have 12 anchors. I assume it is a 4 pole as the coils are in star configuration.
    Also I have glued the sensors more towards the end of the coil anchors, should they be in the middle of the coil anchors?
    Bert
     
  2. pistnbroke
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    Many designs of brushless motor ...which are we talking about ?
     
  3. BertKu
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    BertKu Senior Member

    The only brushless motor I know is the real brushless motor, except maybe with 2, 4,8 and 16 pole which dictate the revs per minute of the motor. What do you mean with different designs of the motor? Only the controller differs and the position of the sensors and the type of sensors.
    Bert
     
  4. whitepointer23

    whitepointer23 Previous Member

    check out one of the electric rc aircraft forums. they are gurus on brushless motor technology.
     
  5. pistnbroke
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    Assume nothing ....Squirrel cage mains motors ? R/c motors ...if its RC then look into the posts by Jereme Harris he is he R/C re winding/sensor /comtrol expert
     
  6. BertKu
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    BertKu Senior Member

    Thanks I will do so. But they all buy the contollers and don't develop the software themselves, nor that they open up the motors to put sensors in them. Most of them go for sensorless motors working on the back EMF. I wrote to Jeremy, but he has not replied.
    Bert
     
  7. rxcomposite
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    rxcomposite Senior Member

    Are you using a BLDC motor with permanent magnet? There will be two causes, timing and pulse modulation. I recently made some research and there are loads of info on the internet. I could probably lead you to one.

    I have even come across an article that the program exist in microsoft and can be programmed. I think it is in youtube.

    I am building one out of an old car alternator. Just have to fish one out of the junkshop.
     
  8. Aharon
    Joined: May 2011
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    Location: Israel

    Aharon Junior Member

    oho! diy brushlyesssss!

    "I am building one out of an old car alternator."
    Thank you sir, please share the result please:)
    Converting a motorcycle to electric is a project I have been nurturing for some time. Things went down because of the cost ("you don't convert to electric to save money, you do it to save oil"). Euro2000 for a good german motor, 1400 for a small made in USA one... Wells... If I only knew how to get rid of the "alternating" guts, I would be getting a dc motor at a bargain.
    If you get to that point, rxcomp, please share!:D
     
    Last edited: May 22, 2011
  9. rxcomposite
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    rxcomposite Senior Member

    Hah. I am building a recumbent bike that is arranged so that two can sit side by side and drive it like a car. It will be pedal powered with an electric boost. I have finished the general lay out, weight distribution , and now working on the dual powered system, I.E. the alternator turned electric motor. Lots of ;em around. I am trying to make it street legal.

    I could power it with a grass cutter motor(2 Hp.) but a two stroke will pollute the environment. I am using DC because I have excess supply in my Solar Panel. The batterries just keep charging up.
     
  10. rxcomposite
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    rxcomposite Senior Member

    You have to find an alternator with magnet and the winding is in "Y" connection, not delta. Others do it the easy way, they yank out the rotor and replace it with a new commercial magnet rotor. The hard way is to keep everything intact, including the field winding (less efficient) but with variable supply on the field winding, they say you can get more rpm with less voltage in field winding. Google "building a BLDC motor from alternator".

    China makes sophisticated BLDC controller for about US150. The throttle is about 15 or 20 USD but with forward and reverse. You have to provide the hall effect sensors though.

    Smaller BLDC motor can be found in junkshop from old washing machine or heavy duty office copier. But it is small for my use.

    Building an Electronic Controller is already out of my league. Too much trouble for $150. I will just buy one.
     
  11. Submarine Tom

    Submarine Tom Previous Member

    endlesssphere.com (forum) may be of some help as well.

    -Tom
     
  12. rxcomposite
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    rxcomposite Senior Member

    BertKu,

    Here is a snipet from AN857 motor controller IC. You may download the whole article from the web. Try also AN855.

    "Now consider what happens when commutation is too
    fast. When commutation occurs early the BEMF has
    not reached peak resulting in more motor current and a
    greater rate of acceleration to the next phase but it will
    arrive there too late. The motor tries to keep up with the
    commutation but at the expense of excessive current.
    If the commutation arrives so early that the motor can
    not accelerate fast enough to catch the next commutation,
    lock is lost and the motor spins down. This happens
    abruptly not very far from the ideal rate. The
    abrupt loss of lock looks like a discontinuity in the motor
    response which makes closed loop control difficult. An
    alternative to closed loop control is to adjust the commutation
    rate until self locking open loop control is
    achieved. This is the method we will use in our application."
     
  13. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    BertKu,

    Here is another one from instructables about sensor placement.

    "Two popular Hall Sensor placements exist: 60 degrees and 120 degrees. I glean over this on my website, but the degrees refers to how many electrical degrees apart the sensors are.

    To place Hall sensors properly in your motor, you have to know how many electrical degrees each slot (or tooth) occupies:

    °elec = 360 * p / t

    where p = number of pole pairs. For a LRK motor, this is 7. Likewise, t , the stator slot count, is 12.

    For a LRK motor, the electrical degree of one slot is 210 degrees.

    Now that you know the °elec of your motor, you can technically place the first sensor anywhere. Let's call this the "A" sensor. I have just wedged it between the Aa winding of the first phase.

    You must place the B sensor in a slot that is °elec ahead of sensor A. This may or may not actually end up in the middle of a slot, and it is an iterative process. Each slot is 210 electrical degrees, so start adding. Begin at 0 degrees, the position of sensor A. Keep track of the number of times you add, wrapping around 360 degrees for each result, until the result is equal to 120.

    That is:

    1) 0 + 210 = 210. No need to modulo 360. The number of additions is 1.

    2) 210 + 210 = 420. Subtract 360. The result is 60. The number of additions is 2.

    3) 60 + 210 = 270. No need to modulo 360. The number of additions is 3.

    4) 270 + 210 = 480. Subtract 360. The result is 120. The number of additions is 4. You win.

    Thus, sensor B should be 4 slots away from sensor A, and sensor C a further 4 slots away.

    Conveniently enough , in a LRK motor, a 120 degree hall sensor placement actually results in the sensors being physically 120 degrees apart. Isn't that awesome?"
     
  14. BertKu
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    BertKu Senior Member

    Thank you, that is interesting. What I am doing at present is to place the sensors between the slots, without gluing. I now will tape , with insolation tape around the stator to keep it in place, without that the rotor will touch the tape. I can then test it at lower speed forward and backwards to see whether your solution will be the one. Thanks again. ( I found another article which has in a 8 pole system the sensors 60 degrees apart. All at one side. )
    Bert
     

  15. Jeremy Harris
    Joined: Jun 2009
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    Jeremy Harris Senior Member

    Bert,

    I've attached the circuit diagrams for my home made BLDC controller, as it may provide you with some ideas. The controller chip I used is now out of production, but it is still available from a few suppliers (I bought a batch of them from ebay recently).

    I used small DC DC converters to bootstrap the upper FET gate drivers, as I found that conventional bootstrapping wasn't reliable when it came to starting the motor from a standstill. Had I used a microcontroller I could have had a starting algorithm to switch the FETs and charge the bootstrap capacitors before starting the motor, but the dumb controller chip I used couldn't do this - the motor sometimes needed to be flicked to start it before I fitted the DC DC converters to provide the bootstrap supply. The DC DC converters are a foolproof solution in my view, and work very well.

    The FETs I used are rated for 75V, 195A with an on resistance of 1.85mohms. Realistically, I think this controller, as it stands, is OK for around 60V and 80A, maybe a bit more. The current limit is selected by changing the number of shunts fitted the board - with all 6 fitted the limit is 120A.

    Jeremy
     

    Attached Files:

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