downrating a motor instead of gearing it down?

[morganw]

John Tetz's low power assist seems like a perfect way for me to shorten my 10 mile commute by giving me a kick to cruise at 20MPH instead of 15. I've run across so many posts about detonating gearboxes (such as the Bane Bots P60s I was looking at) that while Tetz came up with a current limiter to avoid that, I'm still worried.

Looking at designs that use hubmotors to drive through a chain, I was intrigued, but want something smaller & lighter.

What about using a motor at 1/10th of it's rated output at a low speed? Something like a 3kW Turnigy with a <20 tooth freewheel driving the main drive chain. I ride a recumbent, so I'd like to mount the motor so the existing idler works to increase wrap around the motor drive cog.

It looks like the RPMs would just be too too low, though. A 52 tooth chainwheel at 90 RPM pedaling would only have an 18 tooth motor cog going at 260RPM or 1.5V at, what 80 Amps? That's only 120 Watts and needs some fat supply wires. Is the motor so inefficient at this low speed that it's effectively a heater that happens to rotate?

Is there any point in this line of thought? Using a single instead of dual stage reduction with easy to get pulleys for only 8:1?

 

[mwkeefer]

Are you asking about running a motor at 1.5v and 80A?

#1 - It's not the watts that smash up gearboxes it's the torque which is a result of
a.) Reduction amount, more reduction is a higher torque multiplier
b.) Current - 80A @ 1.5V (if it could spin a motor, I've never seen a 1.5v controller?) would produce the same torque as 100V @ 80A... The constant of a motor which is used to calculate the output torque is: kT

Now the motor you refer to has a kV of 170 (no load) and I would assume efficiency around 85% (give or take) and they list the Maximum Voltage as 37 but recommend 6-10S (10S would be 42v)... Let's stick with low power idea you had...

First - lets figure out the real loaded kV you can expect...
170 * .85 = 144.5 RPM per Volt (lower than you had expected eh?)

Now that kT figure... lets get it:
1355 / 170 = 7.9705882352941176470588235294118 in oz per Amp input
for argument sake lets call it 7.97 (too much math)

6S @ Nominal (working) voltage would be: 24.96v...

Assuming you want somthing like 250w of power (basically another riders power)... I would suggest you look at 10A limit (little 6S controllers will do this fine I believe)...

24.96 * 10 = 249.6 watts

The Nominal Speed of the motor shaft will be:
24.96 * 144.5 = 3606.72 RPM

To drive your chainline directly as you are seeking would require a total reduction to the cranks of:
3606.72 / 90 = 40:1

Your time may be better put to mounting a motor beneath your saddle as Recumpence does, then driving the left side of the rear wheel via an ISO adapter... an alternative would be a single stage style system but replacing your rear with somthing internally geared to give your legs and motor a chance to match cadence.

Unless you can lock the cadence in, no 250w system is worth the work.... if you can dial in your own power, even 250w will make the ride more fun

-Mike