# using multiple (!) DC motors with ring gear...



## blackie (Mar 9, 2008)

OK, I'm no engineer, just joined to post really dumb idea and find out if it's even remotely feasible....

assuming one can use a car with a working trans (manual or auto), and the flywheel/torque-converter has a ring gear on it....

small DC motors can be had surplus cheap. What about making an adapter plate that allows a bunch of motors to be mounted in a circle, each with a brass or bronze gear on it, so the ring-gear is being driven by a bunch of small motors at the same time?

the big advantages of this is: cheap. cheap. and each motor, being small, can air-cool itself (I think). Brass or bronze is weak, yes, but the load is divided up a bunch of times, and it's quieter than steel, and wouldn't require the lubrication steel on steel would require. 

DC control is an issue, yes, but certainly doable. With the gearing inherent in the trans, and the large advantage of many small gears on one large ring gear, the motors don't have to rev above 1 to 3k-ish. It occurs to me, for example, that on-off pulse-width modulation might work fine...

I know the general trend is toward fancy controllers for 3-phase AC motors, etc., and there's probably a good reason I've never heard of using many small motors rather than one large one, but I'm just interested in what could be cobbled together cheap in one's backyard, using a drill-press, surplus parts, some care and scoring surplus...if a bunch got together and made a group buy, a few zillion smaller motors could be bought dirt cheap.

blackie (old flat-rate fart)


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## 3dplane (Feb 27, 2008)

Hey blackie! The only way I could see that work maybe like a small tractor or so.Imagine the gear reduction from the motors pinion to the ring probably 20:1 unless the pinions would be so big that the whole motor assembly would be 4 foot diameter So even with the tranny in overdrive just to much reduction.Also the straight gear teeth even soft metal contact would make lots of noise.In short I believe technically it could be done however it would be highly impractical(for road use)Good thinking though and never hurts to get different opinions.Btw general trend is DC motors atleast in home conversions.Look over to EV Performance there is a thread on multiple motors ,read to the end.


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## blackie (Mar 9, 2008)

thanks, 3Dplane, you're right. 

although....we could fasten a smaller diameter gear onto the flywheel, use larger pinions, and then it wouldn't be so low geared. Can you tell I don't wanna give up my idea of little surplus motors? I also have a hunch that smaller motors have more efficiency, but perhaps that's wrong.

another possibility, admittedly complex, is to have a long shaft from the tranny with mutiple pulleys on it, and the small motors spaced along a frame so those round elastic urethane belts could be used. I know from experience that a few dozen of those urethane belts, even pretty small diameter, add up to a ton of gripping ability/torgue, and they have the advantages of being almost silent, easy to repair, easy to install, and the elasticity absorbs shocks well. waddya think about that?

blackie


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## rankhornjp (Nov 26, 2007)

blackie said:


> another possibility, admittedly complex, is to have a long shaft from the tranny with mutiple pulleys on it, and the small motors spaced along a frame so those round elastic urethane belts could be used. I know from experience that a few dozen of those urethane belts, even pretty small diameter, add up to a ton of gripping ability/torgue, and they have the advantages of being almost silent, easy to repair, easy to install, and the elasticity absorbs shocks well. waddya think about that?
> 
> blackie


I've seen this idea a couple of times and I wouldn't recommend hooking a SERIES wound motor to a chain or belt type setup, because an unloaded series wound motor has a habit of destorying itself, if a belt broke under load, the motor would spin up too fast and it would fly apart. I would only try this if I had a rev limiter setup on each motor.


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## headrush (Jan 20, 2008)

Would a setup like that become a torque monster? From what I under stand is the DC motors create a huge amount of torque, especially compared to the IC engine. I would think that you would have to really limit those motors to keep the amps down so that the torque didn't kill the whole setup.


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## rankhornjp (Nov 26, 2007)

headrush said:


> Would a setup like that become a torque monster? From what I under stand is the DC motors create a huge amount of torque, especially compared to the IC engine. I would think that you would have to really limit those motors to keep the amps down so that the torque didn't kill the whole setup.


I would think it would depend on how you wired them. Parrallel: they would all have the same voltage and split the amps, Less torque, more speed. Series: they would all have the same amps and split the volts, more torque, less top speed.


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## blackie (Mar 9, 2008)

Thanks, that's good info about the over-revving danger. If one of the urethane O-ring belts broke, that motor would certainly rev out!

So, the key to this idea is a simple, cheap circuit that cuts power to the motor when it starts to rev out of sight, meaning its belt has broken. It could be mounted right at the motor, but probably better at a central power distribution PCB; a mosfet for each motor, either on or off, not in its linear region, would be a cheap switch...I would think the power to each motor could be monitored and the tach extracted from the noise fed back from the motor windings. It's not necessary to accurately measure the tach, all that's needed is frequency to voltage converter and a threshold way above what the motor would ever rev to in normal use, because all that is needed to catch it in runaway mode. The central power distribution circuit could also feed an array of LED's on the dash, and you could see all of the little buggers working just fine...or not. An LED goes out, and you know power to that motor has been cut, and you then check it to see wassup. Or green LED's that change to red, or whatever......

Mind you, the motors I have in mind are small...so even if one did fly apart, I don't think it would do much. The motors I have in mind normally have a no-load rpm of about 5500 rpm; setting this up so the motors don't normally rev above 3K would give a large margin (but the over-rev detector would be set for, say, 5K, or below whatever rpm they would rev to with full power and a busted belt...)

And, because they're cheap, it's no big deal to replace them as they blow or just wear out. I don't know if they would have brushes, it's been decades since I knew the slightest thing about electric motors, although I don't remember a DC motor working without brushes if the power is just DC, not a stepper. Would a mabuchi 5-pole DC motor be a stepper...or have brushes?

While I'm asking dumb questions, how much torque in gm-cm would the motors all have to total up to for a minimum amount of power to drive a car? (Let's say for the sake of the example that they are all revving up to 3K in each gear, the ratios taken care of to be the same as if there were a gas motor in there revving to 3K, and it's an automatic with a torque converter so there's some rev space at the bottom).

Oh, 3DPlane I just saw your note on the multiple motor thread, sorry, I missed that somehow, I'll check it out, thanks.

blackie


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## Mastiff (Jan 11, 2008)

You could use inexpensive forklift motors and link the power shafts together(end to end), this would probably be the most practical way.

If you wanted Parallel/Series motor switching with a dual forklift motor setup but didn't want an expensive controller all you need is some extra contactors to switch them.

You'd let off the throttle so the controller isn't pushing any juice(to be safe) and then trip the contactors to convert from Series to Parallel.


Also about the over revving motors problem.

You can attach a centrifugal break unto the motor shaft.

Basically it would be tuned so that if the motor reached the RPM limit, say 5000RPM the spinning breaks would contact the breaking drum and thus put a load on the motor, giving you time to disconnect the motor.


Or instead of a break you can literally create a centrifugal switch that once the motor reaches 5000RPM the switch engages and triggers a servo that disconnects the main battery contactor. (probably the best way)


So personally I think the simplest and cheapest way would be to link end to end two forklift motors.


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## blackie (Mar 9, 2008)

Thanks Mastiff....

how cheap would two forklift motors be, at all?

blackie


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## Mastiff (Jan 11, 2008)

It depends on where you get them from, sometimes you can get them for very cheap locally.

Jim at High Torque Electric refurbishes and modifies forklift motors and sells them:
http://hitorqueelectric.com/gallery/v/MOTORS+FOR+SALE/rebuilt+modified/

The main factor you'll have to be concerned with is just getting two motors of the same type.


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