DC motor speed control problems

Discussion in 'General Electronics Chat' started by RLS56, Dec 18, 2014.

  1. RLS56

    Thread Starter New Member

    Dec 18, 2014
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    This is my first post here, and by intro I am a complete noob. Have played around with various electrical things, no real electronics experience, and no education in this area. So please be gentle, and especially please use simplest terminology.

    In my work I am rebuilding a device that uses a small DC motor at low rpm. It is labeled "Motor DC - Filter RF - HP .002 - RPM 24 - Volts 27 - Amp .5 - For use on 28 volt DC system". I attempted to use a cheap PWM controller that I bought on Amazon: http://www.amazon.com/DROK-Controll...=UTF8&qid=1418916526&sr=8-1&keywords=DROK+pwm. I supplied 12VDC to the controller. Regardless of the pot setting, the motor runs at one speed. Using a variable resistive controller the speed adjustability is excellent. What am I doing wrong here? Thanks for any thoughts.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Have you any way of confirming the mean level out of the PWM controller, even a DC VOM should work with the motor connected.
    Also does it appear to be a plain DC brushed motor with the supply wired direct to the brushes?
    Max.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Welcome to the forum!
    Is the motor brushed or brushless?
    You applied 12VDC to the controller, but what voltage to the motor?
     
  4. RLS56

    Thread Starter New Member

    Dec 18, 2014
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    The motor out connections on the PWM controller showed 12V - exactly the same as the power in connections - regardless of the pot position. I've tried 2 of the DROK controllers, thinking the 1st was a bad one, with the same result. The motor doesn't have externally accessible brushes, so I'm not sure if it's brushless or not. can't find any info on-line as it's an old motor, circa 1985.
     
  5. MaxHeadRoom

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    If you back feed the motor (spin by hand) does it generate a rpm dependent DC voltage which changes polarity with direction?
    If so indicates a simple brushed motor.
    Also try a Automotive lamp on the PWM output in place of the motor, it should dim.
    Max.
     
  6. RLS56

    Thread Starter New Member

    Dec 18, 2014
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    The motor housing must contain reduction gearing, as I can't turn the motor by hand. I don't have the controller with me here at work today, but will try the lamp trick this evening - thanks! If the motor is brushless, could that explain the failure of the PWM controller?
     
  7. MaxHeadRoom

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    If it was brushless, I would expect some kind of controller internally as brushless typically has 3 winding's.
    Also if the case, I could not see the PWM controlling it?
    At 24 rpm I would think the gearing would be considerable.
    Max.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If the motor spec is 28VDC, are you hopeful the PWM will work with only a 12V supply?
     
  9. RLS56

    Thread Starter New Member

    Dec 18, 2014
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    Yes? Do you think this is where the problem lies? I thought of perhaps using a higher-voltage input (I'm using a laptop power supply and could swap in a 19V) to see if that made any difference. I might have a 24VDC power supply floating around here somewhere...
     
  10. RLS56

    Thread Starter New Member

    Dec 18, 2014
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    I tried a Silverstar headlight bulb and also a 12V blower/inflator. No change in output regardless of pot position. Are these DROK controllers just junk?
     
  11. MaxHeadRoom

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    Your's appears to be shot!
    They appear to sell lots one ebay.
    Max.
     
  12. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I bought some similar boards on amazon and ebay a while back for evaluation for integration with a project and IIRC the ones branded DROK all performed well. Boards by other names that failed, their failure mode was output=input, no regulation or pwm. It was usually a shorted diode in the buck boost converters; not sure if a pwm board has or needs the same diode.

    The board you bought has mostly positive reviews on amazon so the chances of two duds is low, but certainly not impossible. Are you sure you hooked up everything correctly? Didn't (even momentarily) reverse the polarity or connect power to the output instead of input? If you even momentarily connected power with polarity reversed, you probably shorted something out; I wouldn't count on such a cheap China specimen having reverse polarity protection diodes.
     
  13. RLS56

    Thread Starter New Member

    Dec 18, 2014
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    The first board was treated pretty brutally by Dremeling the heat sinks to fit the project box. The 2nd was handled with care and not mis-wired. I hate to keep buying these things, but maybe one more. Thanks for all the help and insight.
     
  14. Alec_t

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    Your motor is rated "0.5A", but that is almost certainly the optimum running current, not the stall/starting current. The latter will be several Amps and may have exceeded the capabilities of your PWM circuit (advertised as '10A', but perhaps those are Chinese Amps? ;)). Can you let us know the DC resistance of your motor?
     
  15. MaxHeadRoom

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    You can buy it off ebay direct from H.K. , for $4.50 P.P. , it states at 10amp it goes into O.L. and current protection.
    Max.
     
  16. RLS56

    Thread Starter New Member

    Dec 18, 2014
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    The DC resistance is 14.4 ohms, as best I can tell. I took some readings off the output of the PWM controller with nominal 12VDC and 21VDC input voltage (two different power supplies). With 12.98VDC at the input, the output varies from 12.94V to 12.98V over the span of the pot. With 21VDC input (~24VDC actual), the output varies from 28.8 to 29.4 VDC over the pot span. So there's some minimal voltage control, but nowhere near the advertised range. And I wonder why the output at higher voltage is so much greater than the input?

    I agree the motor draw may be >0.5A especially with load, but I didn't see any effect on the 12V bulb or the 12V blower motor either...

    I'm half-considering buying a model train transformer off ebay...
     
  17. Alec_t

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    Your meter may not respond as expected if you apply a PWM input to it.
     
  18. MaxHeadRoom

    Expert

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    I would delve into the controller first, these are simple 555 based PWM controllers usually by a single Power Mosfet output, which in your case could be shot, it may be worth doing a check of the semi's first and find out where the problem is?
    For that small a motor I would not have expected it to overload the controller.
    Max.
     
  19. Alec_t

    AAC Fanatic!

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    Agreed, if the DC resistance is indeed 14 Ohms and the controller is rated (IIRC) 10A.
     
  20. RLS56

    Thread Starter New Member

    Dec 18, 2014
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    So I bought a new PWM controller (identical) on ebay. Connected a 21VDC power supply to the power in side and the motor to the well, motor side. Glory be, variable speed, off at minimum pot position. Tried the same test using the 12VDC power supply that I have wired to the project box. All is good. Then came the mistake, but I can't isolate it. I need to grind the corner off the heat sinks on these boards to clear the project box internal dimensions. Just a bit, nothing excessive. The heat sink barely got warm. Then, when I assembled the board into the project box, I connected an LED indicator light on the power in side, in parallel with the power in leads. One of these two steps - grinding or the LED - fried the board. All I get now is one speed, just like the previous boards. Thoughts?
     
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