Variable speed control of multiple dc motors

[dubya_80]

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?

 

[Nidum]

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 .

 

[dubya_80]

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?

 

[dubya_80]

After a bit more reading after your advice I've come across this:
http://www.ebay.com.au/itm/DC-10-50V-40A-Motor-Speed-Control-PWM-HHO-RC-Controller-12V-24V-36V-2000W-MAX-AU-/371120426645?hash=item5668804e95:g:MXwAAOSw~FNUZWHa#shpCntId
As far as I understand, I could use this in conjunction with my existing relays?

 

[NascentOxygen]

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?

 

[dubya_80]

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.

 

[CWatters]

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.

 

[NascentOxygen]

The car has steering, does it? Or are you instead going to speed up the left wheels to make it turn right?

 

[CWatters]

Good question. I assumed it had steering.

 

[dubya_80]

Yes it has steering but that's 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.

 

[dubya_80]

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.

 

[meBigGuy]

I don't understand how the wheel rpms are matched with motors in parallel. Do you just depend on motor matching?

 

[dubya_80]

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?

 

[CWatters]

If the rpm isn't matched one will draw more current than another but it might not matter.

 

[Nidum]

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 .

 

[dubya_80]

Ok that sounds lije something i should look at. Can I use inductors alone or would it need to be more complex?

 

[Nidum]

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 .

 

[dubya_80]

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

 

[NascentOxygen]

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...

 

[dubya_80]

Quite possibly! The wheels that aren't on the ground will pinch all the power? Or is there other issues I'm not considering?

 

[Nidum]

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 .

 

[dubya_80]

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?

 

[dubya_80]

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!

 

[Nidum]

Could you post a copy of any technical data that came with the module ?

 

[dubya_80]

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.

 

[NascentOxygen]

I am uneasy at your plan to use relays to reverse the load. Without spike suppression, opening the motor under load may produce switching transients which could kill the PWM controller.

Some controllers apparently have an inbuilt reversing switch, if I read this advert correctly. http://www.ebay.com.au/itm/Hot-Reversible-3A-Pulse-Width-PWM-DC-Motor-Speed-Regulator-Controller-Switch-New-/331614500577

 

[dubya_80]

I guess I'll just test this unit in one direction and if it tests out ok just order a second one for reverse. .

 

[NascentOxygen]

Did you ask the seller for a link to a data sheet? It's hard to believe he would be buying these from a supplier who has no additional information on them.

 

[dubya_80]

Well i just went by the info on the ad itself. I added a cooling fan to be safe and crossed my fingers! I got lucky with the 4 motors seeming to cancel each others electrical noise! I put the car on crates and when I put load on two wheels the other two just slowed to match and there was barely any voltage spike at all!
Thanks everyone for your help!