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Variable speed control of multiple dc motors

  1. Jan 15, 2016 #1
    I'm building a hybrid car for my kids. I'm up to speed control and hitting a few hurdles.
    The set up is 4 x 12VDC motors, one to each wheel through a reduction gearbox.
    I've wired them to a single bus, and have used 4 standard relays to deliver forward/reverse.
    I have tried varying the speed with a 200w 10ohm pot, (otherwise it wheel-stands!), but there isn't enough current to get the car moving at anything but almost 0 ohms.
    The power supply is from an alternator run by a lawn mower engine, producing around 14.5 volts.
    Individual motors were fused at 4 amp (I figure to be safe, assume 5 amp), so around 300 watts in total.
    It seems I could do away with the relays and use a high power IC?
    Would the pot still be useful put across the motor bus?
     
  2. jcsd
  3. Jan 15, 2016 #2

    Nidum

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    Direct wired potentiometers do not give good control of motors and they are usually very wasteful of power .

    Some simple electronic control modules will do a much better job . Build your own from published plans or buy ready made .

    Have a look here for some ideas :http://buggies.builtforfun.co.uk/index.php?domain

    Please come back if you have more questions .
     
    Last edited: Jan 15, 2016
  4. Jan 16, 2016 #3
    I've done some reading and I understand the point of pulse wave modulation over the pot and how it will give me torque at low rpm.
    I have dug around in my junk electronics and found about 400 Watts worth (according to their datasheets) of various transistors.
    The question I have is can I use them all together, or is it important to use all the same type?
     
  5. Jan 16, 2016 #4
  6. Jan 16, 2016 #5

    NascentOxygen

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    I think you'll need either 2 or 4 of those controllers. That advert is a bit short on details, but it would be handy if you could operate all controllers with just one of the knobs. Can you email the seller and ask is there a data sheet for that device with more info?
     
  7. Jan 16, 2016 #6
    My existing relays are wired to deliver a variable input in forward or reverse. I tried using a pot to vary the power but i had no start up power except on full. I think this unit will deliver variable speed to the motors.
     
  8. Jan 16, 2016 #7

    CWatters

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    I think given the low cost of that controller it would be worth giving it a go. One PWM controller driving 4 motors in parallel will probably produce a lot of electrical noise and some high current spikes but it might survive.

    PS I doubt it has electronic braking.
     
  9. Jan 16, 2016 #8

    NascentOxygen

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    The car has steering, does it? Or are you instead going to speed up the left wheels to make it turn right?
     
  10. Jan 16, 2016 #9

    CWatters

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    Good question. I assumed it had steering.
     
  11. Jan 16, 2016 #10
    Yes it has steering but thats another headache actually! If i was to build another vehicle i think i would use skid steer!
    I have ordered the pwm module so I'll let you know how it goes.
     
  12. Jan 16, 2016 #11
    forward-reverse relay diagram.jpg so you can see what I have done with the relays here is a circuit diagram I did crudely with Paint.
    This part at least works perfectly, just the pot wasn't useful to vary the supply.
    The PWM module claims 480W at 12V, I expect to peak around 300 or so.
     
  13. Jan 16, 2016 #12

    meBigGuy

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    I don't understand how the wheel rpms are matched with motors in parallel. Do you just depend on motor matching?
     
  14. Jan 16, 2016 #13
    The motors are matched pairs front and back. They all run the same speed under no load. Not sure but i hoped this setup might also act like a differential?
     
  15. Jan 17, 2016 #14

    CWatters

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    If the rpm isn't matched one will draw more current than another but it might not matter.
     
  16. Jan 17, 2016 #15

    Nidum

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    It comes down to a bit of dubious thinking which nevertheless sometimes works out reasonably well - if all wheels are in firm contact with the ground then they and the motors will all be going at the same speed .

    Problems can arise though if there is likely to be loss of grip on one or more wheels while vehicle is moving on rough ground .

    Personally I would have some simple feedback in the system as a safety feature .
     
  17. Jan 17, 2016 #16
    Ok that sounds lije something i should look at. Can I use inductors alone or would it need to be more complex?
     
  18. Jan 17, 2016 #17

    Nidum

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    Can you supply a picture or sketch of the assembly of one wheel and motor ? Need to see what it looks like to suggest a simple and rugged feedback device .
     
  19. Jan 17, 2016 #18
    I'm on my way to work atm. The motor and reduction boxes I'm using are straight off a few kids electric cars . I can upload a picture this arvo, including an unassembled view, as i have spares
     
  20. Jan 17, 2016 #19

    NascentOxygen

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    This is a buggy for the kids to burn around in, right? So expect half the time 2 wheels will be barely contacting the ground....
     
  21. Jan 17, 2016 #20
    Quite possibly! The wheels that aren't on the ground will pinch all the power? Or is there other issues I'm not considering?
     
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