DerStrom8 said:
I'm sorry, I assumed the OP was a newbie to electronics, but reading back he didn't really say anything of the sort
Actually... looks like he did say he was "green to electronics". Me assuming that he may know enough already was a bad call on my part. Sorry.
DerStrom8 said:
Anyway, as for the circuit, I have a couple reservations about it. The first is the "control" setup. A 5k+ pot on the control pin of the 555 timer will not give full, steady PWM. I'd sooner use a circuit like this one:
Using diodes to stabilize the timing is a useful possibility. And my circuit does vary in frequency from max to min, and also doesn't fully stop at min pot setting either. Both our circuits use a 555 as the timing element, so we can use whatever topology gets us the best results.
DerStrom8 said:
My second issue is the freewheel diode. I think it would probably require a lot more protection than just a single diode, especially considering the kind of current these motors probably draw.
They make some beefy diodes, Digi-Key Part Number 150EBU02-ND for example. Or 300U40A-ND if you don't mind spending $30 for just one part. I have used such things with great success. But we could use a TVS or something similar just as easily. Another possible option is making a circuit that feeds the freewheeling energy back into the battery pack. This would create regenerative breaking and eliminate the need for the protection altogether. In any case, if you leave the motor open as is shown in your schematic, the back EMF will almost certainly build up enough to murder the FET's.
DerStrom8 said:
Also, finding high-power MOSFETS to drive it might be a problem, and will rack up the price a bit.
I don't think it's a problem. You can easily get 100's of Amps @ 12v with just a few FET's. I have just two FET's controlling a 300~400 Watt 12v DC motor right behind me as I type this and it works perfectly. And the FET's I used are nothing special, I recovered them from an old motherboard. If the OP specifically purchased switches with the application in mind, he could get better matched parts and maximize power-cost ratio. Just the FET's would probably cost less that $20 and would provide more than enough power.
ronv said:
I would try to retain the gear box if I could. Getting down from 12,000 rpm to something you can use will be a problem. I think 2 motors have twice the torque of one motor but will be hard to interface.
I agree, using the gear box to some extent can be very useful. And I also agree that two motors would be better than one *IF* it can be made to work mechanically. Two motors that are drawing the same amount of electrical power will be making twice the mechanical power of a single unit, as is necessary to be in accordance with the First Law of Thermodynamics. But it is a big "if". To make them work together mechanically, rather than expend mechanical energy fighting each other would require a variable gear system, similar to what you would find in a snowmobile. Another option is to have precision torque gauges on each motor, and vary the electrical power to each independently so that the torque matches at all times. This quickly becomes electronically difficult, and such torque gauges are quite pricey.
The whole problem is analogous to attaching two switch mode power supplies together in parallel and making them share current evenly. If there is even a small difference in the two motors RPM(voltage), then torque(current) will get lost going from one motor into the other and not add to the systems mechanical power. Even using the two independent tires will give problems because they are mechanically coupled together though the ground.
This isn't going to be something easy for the OP to get around I don't think. For simplicity's sake I would just go with one motor and over power it. Most simple electric motors are solid as a rock electrically will put out just about as much power as you can throw into them without problems.
How I do/did power plant to mechanical interfacing for all my bike projects/plans is use a direct interface to the tire it's self, rather than try to integrate something into the gear/chain. This gives you a great step down plus has the added benefit of a safety slip clutch in the event things should bind. But most of my projects involved attaching small gas engines to bicycles. And even a good 2 stroke chain saw engine will only get up to about 8k RPM. So even with my method but an electric motor, the OP may still need a gear box of some kind.