How much RPM is too much for a hub motor

[cwah]

Hello there,

I'm planning to convert a Brompton with a mac motor. Cell_man can have motor with 110mm dropout so that's perfect for the Brompton. However, for a 16" wheel I'd need 800+ RPM to reach 30mph and it's too much for the motor.

I've also read the for the Bafang with 500+ RPM it's too much.

So is it the same for all hub motor? More than 500 RPM leads to motor death?

 

[LI-ghtcycle]

If you are running such a small wheel using a geared hub motor seems to be working against you.

Why not use a small DD instead?

You might need to run higher voltage but the geared hubs are already running high RPM to make them efficient in a larger wheel but you already have a small diameter wheel so you might be surprised how close to the RPM you need with just 48V.

 

[cwah]

Do you mean DD motor can handle 1000+ RPM without problems? Any idea of light weight DD motor (less than 4,5kg) that can do that?

 

[neptronix]

Electrical RPM can get nasty high on a small wheel plus a geared motor, i see why that could be an issue. At 800rpm, a geared motor is usually spinning at 5x, thus 4000rpm. I'm not sure how that converts to electrical RPM, but it's a very high value for sure!

I'd think that the problem would be the controller more than the motor, but maybe the bearings and gears in geared motors are not rated for that high speed either.

A DD would definitely do what you're looking for, but the weight will suffer unfortunately. If you can get your hands on a Crystalyte 4xx, that would be ideal. Crystalyte is also going to release a small & light hub motor in the near future. You might want to hold out! I think it's called the Crystalyte G ?

I'm not sure if you can get those in a small axle size.. so you'll have to do some research.

Here is an idea though - with a 16 inch wheel, you have a prime candidate for a chain drive setup with an RC motor or something along those lines. A motor that would normally run at 6000rpm would only need a 6:1 gear reduction to hit 1000rpm.. This opens up new possibilities if you're willing or able to fabricate some stuff up.

 

[cwah]

I have absolutely not enough knowledge/skill to set up a mid drive bike :lol:

There is also the crystalyte G209 motor that's going to be out soon. But it's a front motor that doesn't fit the Brompton.

Anyone tried to go for more than 800 RPM on geared hub motor?

 

[dogman dan]

Is the rpm issue gear related, or more watts related.

If the wattage is under 1000 to reach 30 mph on that motor in 16" wheel, then that should be ok. Others do 30mph on the motor, so the wattage would be similar I think, and sustainable for at least awhile. 30mph sounds doable to me at 800-1000w depending on wind and slope. You'd be at higher volts of course, but the wattage to go 30 wouldn't be very different.

Maybe you just have to guinea pig this one for us. It's how it works here. How did I find out what dd motors can take going up steep hills in the desert. Easy, I went out a melted a few motors. Cost of extending the knowledge base at the time was ok with me. In 2008, real data was a bit scarce, and hype was rampant. Justin was great, but couldn't do desert testing in Canada.

So your turn now, to let the other brompton owners know what worked, or didn't work, for you. Somebody's gotta do it, and I don't remember anybody else writing about 30 mph, mac, and 16" wheel. Lotsa data for 20" but not for 16".

 

[cwah]

Damn! I don't want to be a guinea pig :lol:

But you can be sure I'll have a small folder able to reach 25-30 mph. Future of e-bike is the MINI MOPED. A moped you can carry in the restaurant and in the bus

I don't really know what the issue is. I shall ask Cell_man. It's probably gear related because I don't need a lot of watts on 16" wheels to achieve high RPM.

 

[dogman dan]

In any case, you should be able to reach 30 mph on 1000w, 30 on the street, not no load. Just needs the right combination of voltage to go that fast on your winding and rim size, then amps enough to have 1000w of that voltage.

I think it's definitely doable for the motor, heat wise, for rides not longer than 30 min. And gears got better since the bafang peanut butter gear threads.

 

[cwah]

The problem is not the "amount" of watts, but the high number of RPM. The Mac (and most geared motors) are not made to go over 550 RPM. With 16" wheel I'll be at 800 RPM which is too much for the motor. Not sure what would happen though.

 

[MadRhino]

It is the Chrystalyte 209 that you need, or the newer model of it. This DD hub motor is made especially for the Brompton, with 28 spoke flange and a perfect fit for the dropout width. It would need to be overvolted to attain your target speed, but is likely to do it.

I guess that some of those Brompton specific kits might still be available at some resellers in UK, and Clyte is supposed to have a new Brompton kit for 2012 that should be hitting the market by the spring.

 

[cell_man]

The Mac is a 16 pole, 32 magnet motor. It has a 5:1 gear reduction. That's 80 electrical cycles per wheel revolution. At 800 wheel rpm, you will have 64,000Hz, 4000 motor rpm. Stator losses is the issue at this high a speed. The Mac is intended for good torque at low rpm, not small wheel, high speed, high rpms. I have some new stators with higher grade and thinner laminations that I've had specially made and will be trying these out soon. It would be interesting to see how much the operating range can be extended, although that is not why I am looking at these higher grade stators. It is instead to see what efficiency gains can be made in the normal operating range and possibly in the future they could enable other aspects of the motor to be optimised for further gains.

Anyways, as others have said, the standard Mac may not be the best choice. A small DD motor might work out better

 

[cwah]

So it's maybe better for me to get something like this 350W motor?
http://conhismotor.com/ProductShow.asp?id=85

Then overvolt it to 74V lipo? I'd have peak at 2000W and 1000W continuous but the DD motor should handle it?

 

[bjosta]

So it's maybe better for me to get something like this 350W motor?
http://conhismotor.com/ProductShow.asp?id=85

Then overvolt it to 74V lipo? I'd have peak at 2000W and 1000W continuous but the DD motor should handle it?

That sounds a bit much for such a small motor, it sure can handle more than the rated 350w, but 1000w cont. with 2kw peak, not sure about that.
So doing the math, if you say 2Kw peak at 74v, does that mean you have a 27A controller? Do you need that much?

 

[cwah]

The 500W Bafang weight 4 kg and can handle 1000W continuous and 2000 peak. And the Mac at 4.3 can also handle 1000+W continuous and 3000+W peak.

So I suppose a heavier and gearless direct drive motor at 4.6kg should be able to handle as much watts as the lighter Mac and Bafang?

ps: No I don't have my controller yet but yes I need that much!!

 

[dogman dan]

Only going to need 1000-1200 for 30 mph in any case. So why not guinea pig that motor? With a 20 amp 72v controller, you'd see peak watts at about 1500w. Sounds like it would work to me. Lots of people ran smaller dd motors at 1500w for years. This one might be a bit smaller, but I bet it can stand 1000w continuous for 10 miles.

Looks like the axle has room to shave the left side, and then dish the rim to get your dropout to fit.

 

[cwah]

isn't dishing the rim only to make sure the wheel is straight? Why do I need to dish the rim to make the dropout fit

 

[Scottyf]

To make it fit centrally that is all.

 

[dogman dan]

The motor you are pointing to is 135mm. You'll have to cut a deeper notch on the axles to make it fit the folder.

If you take more off the left side, so you can still have a few gears on the right, you will have shifted the whole rim to the left. So you'd have to dish it back to the right to center the rim.

You might be able to keep the rim centered, if you just ran a single cog freewheel. Then you could cut the notch on both sides of the motor the same. 7.5mm deeper axle shoulder on both sides.

 

[cwah]

Ok thanks guys. I'm trying to get information from the seller then.

 

[LI-ghtcycle]

Have you considered Ebikes.ca or Golden Motor?

They have several options in the smaller wheels all ready to go. I would chose that over taking a 135mm hub and making it fit.

 

[cwah]

ebike.ca and golden motor propose motor that weight more than 5 kg.

Having a heavy motor on a Brompton is completely against its philosophy and the bike is no longer "portable". I'll probably overvolt a 250 or 350W small direct drive motor that weight less than 5 kg. Just hope it's not going to melt :lol: