I need help improving a brushless motor controller

Discussion in 'General Electronics Chat' started by gte, Dec 6, 2018 at 2:39 PM.

  1. gte

    Thread Starter Senior Member

    Sep 18, 2009
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    4
    I have an e scooter that I use in an area that has a 20 degree incline as part of it. The scooters top speed is 15mph on flat ground, but only 7mph on this incline.

    The battery is a 36v battery (red/black wires) and the motor controller has 6 FET transistors (see pics) that share a common bus bar for positive power and grounding is done through the frame I believe.

    I believe there are 3 (green/blue/yellow wires) wires that are + output wires of 18vDc out that correspond to the low/medium/high ranges of the scooter and each FET is paired with another FET, as it appears the speed of the unit is adjusted by controlling the amperage flow. Total amperage rating for the scooter brushless motor controller is 13A.

    I considered trying to duplicate the controllers setup with an additional 2 FET transistors for a boost when I needed it going up hills, but maybe I could take a momentary contact switch and a resistor ladder to get the 18v as well as control current and do the same thing more simply?

    Has anyone done this, or can you at least confirm whether that's a suitable plan or not? A crude drawing is below, the red wiring that goes to the (2) 5ohm resistors and SPST switch on down to the motor is what I would like to add. Thanks

    motorController4.jpg v,
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Most controllers are PWM, if the controller outputs maximum PWM pulse width, this relates to maximum current at the provided voltage.
    The only way to increase that is to go to a higher voltage.
    Max.
     
  3. gte

    Thread Starter Senior Member

    Sep 18, 2009
    353
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    Thanks for the reply. So if I scope it and it's at PWM, I need to manipulate the input voltage? If it turns out it is analog, will my drawing/plan work?


     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It would be extremely rare for it to be analogue, but if so, you would need to indicate or show the method used.
    Max.
     
  5. gte

    Thread Starter Senior Member

    Sep 18, 2009
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    4
    So to test this before I fabricate something to fit, a 9vDc or 12vDc battery in series with the factory battery pack should do the trick? Do you see any probable issues with 48v or 52vDc and the components?
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I would not think that it was designed that tight, the main concern is over driving the motor, current wise.
    Max.
     
    gte likes this.
  7. gte

    Thread Starter Senior Member

    Sep 18, 2009
    353
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    That's what I was thinking and hoping also.

    Here is what I have in my head. It's a 250watt motor and it sees 20v (although as you mentioned my DMM may be reading a PWM average) so that's 12.5A peak current and the controller has a 13A maximum rating on it.

    If I add 12v in series, I'm expecting to see ~26v to the motor. How does this figure into wattage? Would this provide (26v*12.5a) 325w? I'm trying to figure out the math here, do you know how to calculate the new values? I can't find my scope at the minute, so math is all I have.

    motorController5.jpg
     
  8. dendad

    Well-Known Member

    Feb 20, 2016
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    It all looks like a recipe for smoke.
    Generally, it is not a good idea to overdrive electronics with more voltage that it is designed to run on.
    A larger powered motor and associated battery and controller is the way to go, not trying to force the existing one to give more power than it can supply.
     
    shortbus likes this.
  9. gte

    Thread Starter Senior Member

    Sep 18, 2009
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    I agree that's the right (ideal) way to go, none of it is available unfortunately



     
  10. dendad

    Well-Known Member

    Feb 20, 2016
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    If the battery is 39V, then that is what will be going to the motor, less a little for losses.
    The lower reading you are seeing on your meter will because of the phase switching and PWM components.
    As you increase the voltage supply, the current will also go up and either of those can kill your controller and motor.
    Do you have actual data on the controller and motor?
    The controller should have built in current limiting and you may possibly be able to play with that, but do expect to kill it all if you get it wrong.
    It took me a while to get my BLDC controller running reliably, and my first brushed motor controller smoked often in development.
    Actually, come to think of it, so did the bigger second one ;)
    A good test is to put the scooter against a barrier so the wheels can't turn, and wind the wick up full. It should not explode!
    If it does, more work is needed.
     
  11. gte

    Thread Starter Senior Member

    Sep 18, 2009
    353
    4
    If you look at this picture, I believe the bus bar is meant to be a current limitation? What do you think?

    There is definitely some sort of limitation, because the scooter will top out at 15mph with me on it on a flat road and it will do the same when I prop the drive wheel up in the air, keep the scooter stationary and give it full throttle. It maintains 15mph down a hill as well.

    Do you think that bus bar in the picture is an amperage limiter ir that it has a speed sensor somewhere in it? The bar to the left in the picture is the positive one that feeds the FET input pins.

    https://forum.allaboutcircuits.com/attachments/motorcontroller1-jpg.165247/


     
  12. dendad

    Well-Known Member

    Feb 20, 2016
    2,091
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    In a BLDC motor, the controller "knows" the motor speed as it is generating the phases. That said, it probably has current sensing too so as to prevent overloads. The max speed is likely to be set in the firmware.
    And running a couple of 5ohm resistors across the battery for an 18V supply is very poor design. Just get a switch mode regulator from the battery to supply the 18V and you will save a lot of power and heat loss.
    I cannot see what the current sense would be from the pictures.
    The heavy tracks are to conduct the high currents.
     
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