Variable speed control of multiple dc motors

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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?
 

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  • #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 .
 
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  • #3
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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
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?
 
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  • #6
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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.
 
  • #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.
 
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  • #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?
 
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  • #9
CWatters
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Good question. I assumed it had steering.
 
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  • #10
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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.
 
  • #11
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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.
 
  • #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?
 
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  • #13
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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?
 
  • #14
CWatters
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If the rpm isn't matched one will draw more current than another but it might not matter.
 
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  • #15
Nidum
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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?

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 .
 
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  • #16
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Ok that sounds lije something i should look at. Can I use inductors alone or would it need to be more complex?
 
  • #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 .
 
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  • #18
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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
 
  • #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....
 
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  • #20
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Quite possibly! The wheels that aren't on the ground will pinch all the power? Or is there other issues I'm not considering?
 
  • #21
Nidum
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The wheel off the ground will tend to speed up considerably unless there is provision in the control system to detect the overspeed and reduce power to that wheels motor .
 
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  • #22
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These are pics of the sort of motors and reduction gears I'm using
I have removed the inductors and capacitors from the ones I installed, and now I am realising what they do! What sort of things should I consider using caps and inductors?
20160118_184428.jpg
20160118_184240.jpg
 
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  • #23
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My PWM module arrived today so i went about getting it ready to install and it's got a common +ve! So I'm off to rewire the relays to separate the circuits!
20160122_182915.jpg
20160122_182907.jpg
 
  • #24
Nidum
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Could you post a copy of any technical data that came with the module ?
 
  • #25
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it didn't come with any!
all I have to go by is what's on the sellers Ebay page.
none of the transistors or the IC have any markings either.
I'm mounting it in a fan cooled box before I test it out.
 

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