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2 speed controllers in parallel to drive a DC brushed motor?

  1. Feb 7, 2015 #1
    I am currently trying to drive a DC brushed motor, but its specs are unique enough that I haven't been able to find a suitable driver. I found one that almost fits our application but our amperage is closer to the peak then the continuous. So is it possible that I could put two of the same speed controller in parallel to diffuse the current? Or would there too much back emf?

    Motor Specs:
    115 V DC
    20 A
    3.5 HP

    Driver Specs:
    40-175 V DC Input/Output
    12.5 A continuous
    25 A peak ~2 seconds
    3 kW

    Thanks
     
  2. jcsd
  3. Feb 9, 2015 #2

    Merlin3189

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    Gold Member

    I'm not an expert here, but my judgement would be no.
    My understanding of DC motor control is that it is essentially voltage control. So you would have two circuits attempting to control the same voltage. Now if there is even a small difference between the values they try to maintain, once the voltage gets between the two values, then one circuit thinks the voltage is too low and increases the current, the other thinks it is too high and decreases the current. They would counteract each other.

    Now your controllers may not work in this way, but it seems to me you are always likely to have a problem like this if there is any discrepancy between the two controllers. You are duplicating the decision part of the control loop and allowing possible disagreement.

    What I think you need is a single controller, but some sort of amplifier to boost its power capability. I might be able to help with that if it were a much smaller motor, but my experience does not extend to kW DC motors. That sounds BIG for a DC motor!

    Have you tried asking on electronicspoint.com? You might find some electronic engineers with experience in this area.
     
  4. Dec 17, 2015 #3

    Just curious, what do you think about a similar setup with say100 amp motor. Two of the 60amp controllers off ebay, now beings they have a simple potentiometer control knob, could one have the potentiometer removed and hooked directly to the control circuit of the other board to act as one, considering same input and out put with a couple diodes on feed side would this still have same feedback problems? And I mean for a bicycle or something nothing dangerous or critical speed control being needed.
     

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  5. Dec 17, 2015 #4

    CWatters

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    Homework Helper

    Most high power speed controllers switch power to the motor on and off rapidly with a variable mark space ratio. That way the transistors/FETs are either on or off most of the time reducing power dissipation.

    If you used two speed controllers the switching wouldn't be synchronised.
     
  6. Dec 17, 2015 #5

    Merlin3189

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    Gold Member

    What you seem to be suggesting, is to disable the control circuit on one board and use the signal from the other board to control it. IF you can do that correctly, I think that would work. Essentially you are just using the extra power transistors of the second board to boost the current capacity. BUT I have no experience of these PWM controllers and am not expert in this area.
    You do need an electrical engineer with experience in this area. Try re-posting in the electrical engineering forum or maybe on electronicspoint.com. You may like to change your title to something like, "3.5HP / 2.5kW DC motor control" as this seems to be the real question. (How to do it by paralleling two controllers is limiting your options.)
    You would also need to have a circuit diagram of the boards, which you might get from the manufacturer's website.
    Yet one more step back to say what you are actually trying to do by speed controlling this 2.5kW motor, might broaden it even further. The more people understand your real problem, the more likely someone will be able to help or advise appropriately.
     
  7. Dec 18, 2015 #6

    CWatters

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    No. It won't work if you connect them together at the control knob. It might work if you add more MOSFETs in parallel but more skill is required.
     
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