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Q: Reducing/Modulating Starter Motor Torque

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skiri251

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Hello,

Complete newbie to electronics. I have mechanical engineering background but the memory of some electronics I learned in the past is quick to fade..

I have a sidecar rig (motorcycle and sidecar combination).
I built an electrical reverse gear for it.

Honda GL1500 motorcycle starter motor drives the sidecar wheel via friction wheel pushed against tire tread.

It kind of works but the friction wheel slips against tire tread and wears quickly.

Mechanical solutions will be to increase reduction ratio or to use different drive mechanism (e.g. chain/sprocket) but I am wondering if I can solve this electronically by reducing/modulating motor torque.

I googled and found MOSFET based PWM but no off-the-shelf controller (with reasonable price) for my application. I don't think I can design it by myself.

So I wonder what if I put a power resistor to reduce a voltage to the motor. I know this is very crude approach but will it worth a try?

I still need to figure out what resistance and wattage..

Could any one point me the right direction?
 
Using a resistor for something so high powered seems like a lot of heat generated and a massive really hot resistor. It simply might be too crude to use for something so big (especially if it's going to run continuously). Is this too complicated for you?
**broken link removed**

But I don't quite understand the whole drive setup description. You installed a starter motor to drive the sidecar wheel by means of a friction wheeled pushed against the tread? What's an electrical reverse gear supposed to do? Is it supposed to run for continuous periods of time (that'd make using the resistor even worse). Please keep in mind I don't know what the configuration of a sidecar wheel (what direction it turns, whether its reversible, powered, etc).

But it seems to me that the nature of the drive is the problem- belt or sprockets. Reducing motor torque reduces slip and friction but it also reduces the actual torque which you may need.
 
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Sorry.

The motorcycle is a Honda GL1100. I bought GL1500 starter motor specifically to drive the sidecar wheel. It is mounted on the sidecar swingarm. The friction wheel is attached to starter motor output shaft.

I connected the motorcycle battery and the extra starter motor with jumper cable and GL1500 starter solenoid relay. There is a separate button to engage the solenoid relay at the handlebar.
 
I would do both: Implement a PWM controller for the motor, and add gearing to reduce the speed. Don't try the resistance thing; resistors are for reducing current, and they do that by dropping voltage (as heat); you might end up with a "weak" motor that doesn't turn at all (under load), and a resistor that's on fire (or close to it). If you can't implement the PWM controller yourself, then you are going to have to spend the money and buy one. Make sure that it is rated for the maximum (stall) current of your motor, of course.
 
The friction wheel is engaged (pushed against the tire tread) manually with a toggle clamp when I need to back up the rig. So I think the slip occurs mainly at the start up. Rotating the wheel manually moves the rig without the slip.

Yes, it's a mechanical issue but adding belt/chain/sprockets requires huge design change to the single sided swingarm. So I just wondered what if.
 
A power resistor would work but it likely have to be quite high wattage. For example to drop 5V at 50A (probably a typical motorcycle starter current) requires a 0.1 ohm resistor that can dissipate 250W. You would have to somehow determine how much current the starter actually takes and how much you need to reduce the voltage.
 
Don't try the resistance thing; resistors are for reducing current, and they do that by dropping voltage (as heat); you might end up with a "weak" motor that doesn't turn at all (under load), and a resistor that's on fire (or close to it).

Yes, I realise that. But this is for backing up out of parking lot and only requires a short burst (a few seconds). And currently the motor/gearing is strong enough to slip the friction wheel..
 
Ooohhh, I get it now. I guess motorcycles don't need to go in reverse to back out of a parking stall- not so if you had a sidecar!

I'd only try the resistor method if it's only to weaken the motor a little bit (less than 10% maybe? possibly less depending on the initial power levels of the motor) and only because it's a relatively minor short running application. You definately don't want to reduce it anywhere near the point where the motor might stall since batteries leak or explode while motors and wires burn out making it very important to figure out how much weaker it has to be.

I'd much rather go with a PWM circuit though, and obviously mechanical changes at best. Give the circuit a try, though you will need a large screw terminal MOSFET that you can bolt a heatsink too, as well as a gate driver between the circuit and transistor to turn such a large transistor on and off fast enough.
 
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Very newbie question.
Where can I purchase MOSFETs? I guess I only need a few. Are there online retailers with enough spec shown?
I tried DigiKey but they list so many components with little technical data. It's really hard for me to choose.
 
On Digikey, N-channel type MOSFETs (aka NMOS) in SOT-227 packages. One of the only manufacturers that will be listed is IXYS, but there are others.

Do you know the difference between N-channel (NMOS) and P-channel (PMOS)? Just use NMOS for this. More common, more efficient, and cheaper. Other than that the main difference is their ease of gate drive requirements to switch it on and off depending on where you need a transistor in the circuit (either closest to ground or closest to +V). NMOS is easier nearest to ground (aka low-side) and PMOS is easier nearest to +V (aka high-side).

Your application it does't matter so just use NMOS with the transistor nearest to ground (as it is in the circuit I provided). The alternative is to flip things around so that the motor is cloest to ground and the transistor is closer to +V.
 
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Something like this might work. You could build one cheaper, but not a lot cheaper.

**broken link removed**
 
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