Advice about over-driving a chain driven motor

[offGridDownUnder]

I have put together a trike with a 'Stoke Monkey' motor. Here is the build thread: Greenspeed trike with stoke monkey

Currently it is running with 16T at the motor, and 22T at the rear wheel: 11:8. I have a GMAC 8T motor.

Justin commented that I consider down gearing. I'm thinking of 20T at the motor, and 30,32,34,36T at the wheel it drives: 3:2 - 9:5.

This design has no freewheel between the rear wheel and the motor so I can take advantage of regenerative braking, so the ratio over-drives the motor RPM when going downhill.

For the GMAC 8T that I have now, and for the newly released Rear All-Axle, what is the maximum RPM each motor may be mechanically driven at before there is trouble due to back EMF, overcharging the battery, mechanical issues, or? I've written to Grin support, any advice about how to determine this?

 

[amberwolf]

I don't think you'll see any mechanical issues from any speeds you're likely to run the motor at even as a middrive.

But the GMAC is a geared hubmotor, so it's ERPM is higher than a DD motor, and running a lot faster as a middrive could cause problems with a controller trying to drive it properly. You'd have to calculate out what it's ERPM would be with your desired speed, wheel size, gearing from it to the wheel, and motor speed externally vs it's gearing and pole count internally, and compare that to the ERPM of the controller you want to use.

For regen issues on speeds above max drive speed, see these posts:

Adding rear hub motors to a fwd gas powered vehicule ?

I was looking at possible use of hub motors to put in the rear of a front drive thermal car to make some kind of hybrid. Using both energy to take off at a red light, use electric around town and run on gas for the highway. Qs motors seem to have 12kw nominal hub held by a single shaft that...

endless-sphere.com

Hub motor in bike trailer to aid steep downhill braking?

I have a single wheel burley coho and am concerned about steep downhills in high mountains. Anybody ever use a hub motor to regen brake the trailer? Thanks

endless-sphere.com

and there's another I can't find right now.

 

[offGridDownUnder]

You'd have to calculate out what it's ERPM

Please explain, or point to explanation.

I'm using a Baserunner, or possibly a Phaserunner. I'll look at the manuals for those and see if this is mentioned.

I have read those threads, and did read about the alarming problems. I charge to 85% by watching for the voltage in the CA while charging - that gives me some margin to start with.

In a week or two, I expect to hear from Grin, but I also want to learn about this.

 

[offGridDownUnder]

Baserunner v4 manual:

eRPM Limit Not recommended above 60,000 ePRM, though it
will continue to function beyond this.

Phaserunner v2.1 manual has the identical statement.

 

[offGridDownUnder]

A tape measure told me my rear (driven) wheel is 26.5" in diameter. My math now tells me that at 50km/hr, this will turn at 394 rpm (happy to have someone else calculate that as a sanity check).

I have a very vague memory that the gearing is 5:1 internally for the GMAC, so does that mean 'effective RPM' is 5 * 394 = 1970?

Nope, I forgot the physical gearing multiplier as well - at it's current 22:16 ratio, that means 5 * 394 * 22 / 16 = 2709

Does this match what you are suggesting I find out about this set up? If not, can you explain?

 

[amberwolf]

The ERPM is the Electrical RPM, so it is how fast the phases have to be commutated by the controller to spin the motor at the physical RPM.

I can't recall the rest of the formula, but you need to multiply the final number by the number of poles (or pole pairs) in the motor; this is the number of magnets (poles), or magnets divided by two (pairs). I forget the GMAC's poles; a typical DD hub has 46 magnets (23 pairs), if your geared hub is similar it'd be 46 or 23 x the other number(s). If we go total worst case that's 46 x 2709 = 124614, which is probably wrong (too high). If it's pairs then that's 62307. If it's only using hte motor's gearing and not the chaindrive's, then that's 45310.

I'm too wiped out ATM to come up with a good google search term for the ERPM formula...but i bet it's on the Grin site somwehre.

 

[offGridDownUnder]

The docs for the motor say "Magnetic Pole Pairs (Phaserunner) 80", and I also recall reading this is 16 pole (pairs?) * 5:1 gearing.

394 rpm (wheel) * 80 effective pole pairs (16 * 5) = 31520

Current gearing: 16:22 - 43340

Extreme gearing: 20:36 - 56736

Likely gearing: 20:32 - 50432

With the likely gearing, max speed would top out at 59.5km/h! I wouldn't want to ride that fast and I'd be on the regen or mechanical brakes well before that if I can.