Advice welcome on how to control the speed of a large(ish) 12v dc motor

Discussion in 'General Electronics Chat' started by harlydog, Jul 10, 2018.

  1. harlydog

    Thread Starter New Member

    Jul 10, 2018
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    Hey there,
    So I'm building a dog lure for our rescue greyhound.

    Its basically a great fun way to build fitness and confidence in a controllable way for a rescue dog that was mistreated.

    It's a simple system - a large reel of thin dacron cord like a builders brick line, a few strips of a plastic bag attached to one end of the line as the lure. You switch the motor on and the dog chases it.

    I've seen several people use a starter motor directly attached to a car battery. BUT you don't have any control over the speed obviously.
    I'd like to have a little more control so I can vary the speed of the lure and thus build the dogs fitness back at a steady and safe pace.

    I have this motor here - https://wheelspinmodels.co.uk/i/210...MI4JP0kJ2S3AIVSF4ZCh0O3AAmEAkYASABEgJeUvD_BwE

    It's a 12v starter motor for small RC engines and it will be plenty powerful to use to drag the lure even through longer grass. I have a 12v motorcycle battery that I'm planning to use.

    What would be the best variable speed controller for a motor like this? I know its probably a simple question for many of you but any advice would be gratefully received!

    many thanks
     
  2. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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  3. jpanhalt

    Expert

    Jan 18, 2008
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    I have made 12V winches using the Ford long-shaft starter or the purpose-made luring motor from India. The latter was a copy of the Ford starter, but I believe the company is out of business.
    At 12V with that starter, you will draw more than 30 A when pulling the lure at doggie speed. My first controller used 5 paralleled mosfets. A later version used only 4 in parallel.

    I would have doubts whether the smaller hobby starters (made for much smaller motorcycle and yard implement engines) would be very successful.
     
  4. harlydog

    Thread Starter New Member

    Jul 10, 2018
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    Yes I'm guessing these motors can pull a lot of amps! I'm a beginner when it comes to circuits, although I've recently built two Bottlehead audio kit amplifiers, a crack and a mainline, so my soldering is, I would say OK. But I'm not at a stage where I could even begin to design a controller like you suggested! Is there a resource where I could find a circuit like that or is it simple enough for you to sketch it out with the components I would need to buy?

    OR would this controller (thanks for the link Alberthall) be suitable?

    12-80V 30A (CCM6DS-D)

    Features:

    • Incorporate a 30A fuse, and with the protection function of reverse connect power supply
    • High Power, Digital Display
    • Potentiometer with switch function
    • Large 0.56 inch 3-digit LED display numbers allow viewing under the most adverse conditions
    • The display range of 3-digit LED display is "000"-"100". (For example, displayed "055", it means that the PWM output duty cycle is 55%.)

    Specifications:
    Model: CCM6DS-D
    Input supply voltage: 12V-80V DC
    PWM frequency: 21kHz
    Duty Cycle adjustable: 0%-100%
    Suitable current range:current rating: 30A
    Module Size(with case): 110 * 77 * 35mm
    Display device: 79 * 42 * 24mm
    Potentiometer cable length: 0.79inch
     
  5. jpanhalt

    Expert

    Jan 18, 2008
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    We used the winches for launching sailplanes (generally 2 m to 3 m wingspan and <5#). Launch line was limited to 200 m. You could stall the motors, but of course, you tried to avoid doing that. Estimates of the current were up to 200A (competition rules limit). For luring, the values were not much different as I understand from correspondence with the company I mentioned.

    The usual way to control speed was tapping on a foot peddle switch. Switching was with two starter solenoids in series so a single failure would not create a permanent on (something you also need to be concerned about). Some people added an additional manual switch or an ax as the ultimate shutoff. I built the variable speed control thinking it might have an advantage for the lighter models. It didn't. However, I did use the same design with a slightly smaller treadmill motor to control the speed of a line retriever.

    Again, I don't think 30 A is going to cut it, unless your line and lure are very light. A few tangles of grass will change the pull. How much line are you pulling? Will it be suspended or in grass? Do you know what current is drawn by the usual starter motors?
     
  6. harlydog

    Thread Starter New Member

    Jul 10, 2018
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    Great stuff! thanks for your reply...
    Yes 30 amps might not work ha! I think you are right, we will be pulling through fairly short grass lengths as we introduce the dog back to harder shorter grass surfaces ( our well tended local parks ) as she heals... Theres a definitely a danger of snagging.

    I guess it would just be easier to find a nice quick action on and off switch that I can handhold and try to feather the power like you say.

    I can't find any detailed specs on my particular motor, even from their website and they don't answer emails but I'm pretty sure it will pull 150m of builders line and a light lure made from strips of plastic bags. These little motors seem to be used in few similar diy builds I've researched on youtube and even in a couple of the more compact commercial units available.

    I'll have a look for a good simple switch now.

    Thanks again
     
  7. jpanhalt

    Expert

    Jan 18, 2008
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    Hand or foot operated switches are fine and are easily controlled. The line has a lot of stretch, which dampens pulsations effectively. However, you want something to handle the current of the drive motor and a hand switch will probably not suffice. For that, use the operator's switch to control a starting solenoid. Then the solenoid controls the motor. Automotive starting solenoids are relatively cheap new, or can be gotten from recycler yards for almost nothing.

    Do not underestimate the danger of a "light weight" line going at 20 mph or so. It will cut steel and fingers with almost equal ease. That is another reason to use a solenoid. I used a heavy duty truck solenoid and a cutoff switch (I don't remember the rating of either, but each was 100+ A). Solenoids in series was another solution. Takes a little more space, but you can put LED's across each solenoid, and if the LED is off when the solenoid should be off, you know its contacts are fused.
     
  8. harlydog

    Thread Starter New Member

    Jul 10, 2018
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    Had a good look online at what you are suggesting, I'll be using a solenoid and hand operated momentary push button switch.
    Many thanks for the advice.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The best DC motors for this application is the P.M. type, the older starter motors that were series wound field operate in a run away condition when used with small loads.
    They are designed for very high loads to keep the RPM to a safe limit.
    Max.
     
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  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    I have another take on this. Why not hack one of those electric scooters? Instead of a wheel , replace it with your drum for the line. The speed controller is already there for you built by the scooters maker, as are the batteries.
     
  11. harlydog

    Thread Starter New Member

    Jul 10, 2018
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    Ah this is an interesting and lateral approach - thank you... I'll have a good look at some second hand ones now
     
  12. Bernard

    Expert

    Aug 7, 2008
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    Random thoughts.
    Old Ford 12 V starters were rated @ about 2 HP.
    Wench , 12V, replacement motor,
    50 A no load, 8000 RPM.
    3.8 in. dia. drum @ 1760 RPM = 20 MPH. Hmm- might consider speed reduction, belt & pulley gears?
     
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