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swbluto

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May 30, 2008
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I've been seeking possible low kV high-power motors using the google search string:

Code:
("(100 OR 110 OR 120 OR 130 OR 140 OR 150 OR 160 OR 170 OR 180) (kv)" OR "(100 OR 110 OR 120 OR 130 OR 140 OR 150 OR 160 OR 170 OR 180) (RPM/volt)") outrunner

and it returned less than 100 posts. Adding on the option of 190 really exploded the hits so I'm getting the suggestion that "ebike ideal" kVs and R/C airplane kVs just don't quite coincide making high-gear ratios almost a necessity. Anyways, while searching on google, I stumbled upon a forum that expressed a similar purpose. It was an "electric vehicle" sub forum embedded in an electric airplane forum: the address is http://www.slofly.com/forumdisplay.php?f=21 . It doesn't seem like the average technical aptitude is as high as it seems to be here, but there are some fellow spirits in the vast etherwebs.
 
I know adding reduction adds complexity. However, running a higher KV motor with further reduction adds power.

There is definately a hole in what we need for bikes versus what RC guys like to run.

Again, I prefer to run higher motor RPM with further reduction for more power.

Matt
 
How does a higher kV add power? I understand that a higher kV typically entails a lower resistance for given motor(Which increases potential current draw and reduces waste heat in the motor at a given speed) but it also seems that necessitating a second stage would also increase drive inefficiency(or so I think). But, I really wonder... how does the drive efficiency of a 7-8:1 pulley compare to a drive stage of two stages consisting of, say, 3:1 and 4:1? I'm under the impression the "ideal gear ratio per stage" for efficiency is something like 4:1 but how much does that get worse at 8:1?
 
For a given motor & voltage...........

Power is torque times rotational velocity.......

I guess it's really a trade off between efficiency/simplicity and weight/cost.

For a single stage of belt drive, the efficiency doesn't fall that much with increasing ratio, certainly compared to adding a second stage.
 
Miles said:
For a single stage of belt drive, the efficiency doesn't fall that much with increasing ratio, certainly compared to adding a second stage. [seen by at one time by swbluto, whether imaginary or deleted] it's usually more of a practical thing...

Like physical size constraints? What about torque limits on a belt? Can those also necessitate a two stage drive? I'm planning on upping my scooter to 2 kW from its current 750 watt, and I'm not too sure if I should be worried if I'll exceed the 5m-15's torque rating.
 
For us, it's not the spec. of the belt itself that's the problem, it's the torque transfer limit imposed by the number of teeth engaged with the driver pulley.

No, you didn't imagine, I was in the process of editing it :)

Yes, size constraints. Two stages means the second stage has to be able to handle the torque multiplication of the first stage.
 
Miles said:
For us, it's not the spec. of the belt itself that's the problem, it's the torque transfer limit imposed by the number of teeth engaged with the driver pulley.

No, you didn't imagine, I was in the process of editing it :)

Ok, so there's like a "per engaged-tooth torque limit" of some sort? Do you have an idea what that is for 5m-15 off hand? N.m. is just fine. And, yeah, I should probably download that gates program you referenced in some post before...
 
D00000000000000000000D, awesuuuuuuuuuuuuuuuuuuuuuuuuuuuummmmmmmmmmmmmmm!!!!!!!!!!!

It seems like 5m belt can take upto 5 kW at "reasonably high rpms" and 2 kW at only 500 rpm. That's well below the design of my scooter which is like 500 watts output at 500 rpm. But, is the HTD Power vs. RPM graph a good enough proxy for whether or not the belt won't slip?

Edit: Wait, I got to page 32. It seems at 500 RPM, a 14 tooth motor sprocket on a 5M-15 belt seems like it can only handle (15/25)*(.258 kW) = 155 watts. According to my graphs, the motor would be putting out 450 watts at 500 RPM. Oh, please no, don't say it's so! I can't physically propel my small scooter at 25 mph up an 8% hill? I feel disheartened. I thought my belt was an HTD so thus it was designed to do nearly anything(Well, it's what that guy in that instructable used(a 5m-15) and he's pumping 1 kW through) but it seems like it can't be powered by an itty-bitty 3 kW motor? Oh, I've been delusioned in thinking the scooter manufacturer over-killed on the components while they were actually getting sticking cheaply close to the performance line of the weak 200 watt motor it originally came with.


Hmmmm... now it makes me wonder if that clicking sound I've been hearing on the scooter when under load is related to the exceeding the power rating of the belt and it's slipping?
 
Yeah. I was originally looking at the graph on page 4 where it plots RPM and "Design power" of the various types(3M, 5M, 8M, etc.) but then I got to page 32 where it specified it for the sprocket value and belt width. Oh, dear god, it'll only handle 1/3 of what I was planning on? Ugh... it seems my scooter is destined to suck. :cry:

So, what can I do... hmmm... I guess I could get a bigger back wheel and remove the fender, but that would make the platform sloping down which might be uncomfortable to stand on(And the front wheel's size is bound by the fork.). Oh, it seems like I've run out of options. There's an actual limit on vehicle size and practical power limits. Dang, and I also have some a123s on the way that I was hoping to use to exploit the motor's potential. :/
 
My recumbent primary stage uses a HDT 5mm pitch 15mm wide belt and can handle 7kw at 6,000 rpm. That is with a 22 tooth motor pulley (10 teeth engaged) run very tight.

With moderate belt tension, 5kw is fine.

You should have no problem with 2kw............

Matt
 
Yeah, my scooter's belt isn't exactly very tight because I get the feeling that strains the motors as it doesn't turn nearly as well(It won't even "roll", it just stops when you stop rolling it). The motor sprocket is also 14t with the second being 60t and it's... 8 inches away(? center to center.) so it only has, maybe, 4-6 teeth engaged.

Anyways, maybe I'll just increase the tension. I'm not sure how my hobby-city motor will fare, though. :-?
 
SW,

That's the rating of the belt for industry, where length of duty is important. It might be ok, you can always up the belt tension...

Edit: You're ahead of me..good, it's well past my bedtime...
:)
 
It is funny how the torque handling goes up with number of teeth engaged. I think there is something to do with the diameter of the pulley as well, reducing how tightly the belt is wrapped. With a larger pulley, the belt is not wrapped in such a tight circle. I do not know if this matters at all, but, it sure seems like doubling the tooth coverage (number of teeth engaged) gives more than double the power handling.

Matt
 
So, basically, you can run more power through it, I just might be replacing the belt more often? Ok, that sounds like it might be doable. I just hope its lifespan isn't below 900 miles(Which would probably be 60 hours of continuous use at 15mph average)! I think that'll probably be what my annual usage is and I'd really like it if I didn't have to replace the belt say, oh, every 6 months.
 
My timing pulley is small, I think, so my power limit is probably less than half than your power limit, matt. I'd venture to say somewhere around 1500 watts, maybe.

So I just think I'll sacrifice performance on my scooter and just shorten the belt's life span and tighten the belt a little and bring down the current limit a little(Possibly 40 amps instead of 60 at 33 volts). I guess 15-18 mph up 8% hills and 30 mph on straight-a-ways is reasonable on something as tiny as my scooter. That sure is a LOT MORE than what I got on my pitiful hub motor, let me tell ya.

Also, the timing pulley is built into the wheel so there's not much I can do about changing technologies. But I'll definitely look into that when I plan to eventually do my bike build.
 
SW,

I doubt you will be down at 1500 watts. When I setup my recumbent, I had a 16 tooth motor pulley and it did not skip at 3kw. :D

Also, belt life is a non-issue as well assuming it is not skipping. I have run toothed belts on my helis for hundreds of hours without wearing them out. Skipping is the issue mroe than anything. Actually, you will wear the aluminum teeth on the pulley before you wear the belt out.........

Matt
 
Ahh, yes Miles, good point. :D

I did not measure RPM at that time. However, I can estimate (based on ratio and speed of the bike) roughly 2,000 rpm when hard throttle was first applied.

I never hit the throttle hard at low RPM so the ESC is not pushed too hard. So, that may explain my experience with this.

Matt
 
:)

Ok, let's assume 3KW at 2000 rpm - that's equivalent torque to 750 Watts at 500 rpm.

If that's about right then SW is covered:
swbluto said:
According to my graphs, the motor would be putting out 450 watts at 500 RPM.

We know the ratings used for industry are pretty conservative, even applying the lowest service factor. The question is, how far can we reasonably up-rate them and at what torque do they start to slip, for a given static tension, tooth engagement etc?
 
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