def215 wrote:thanks dequinox,
my motor is a 250 watt motor so i just went with the amperage calculation the motor would give me at 250 watts, which is 10.4 amps. it didnt last anywhere near an hour . i think it lasted only about 15 minutes flat out, no pedaling, just motor working by itself.
so this is what i came up with with all of the numbers:
Does that calculation sound correct?
amberwolf wrote: Do you have access to at least one of those cheap Harbor Freight meters? They can take 10A for "30 seconds" without melting the shunt off the board (which can be put back on, don't worry...ask me how I know )
dequinox wrote:I would say no, it doesn't sound correct. The reason is you multiplied by an hour, and I'm sure you didn't take a whole hour to go 3 miles! If it took 15 min to go 3 miles...with your amperage value you would multiply by .25hr.
It would look like this: [(10.4A)(.25hr)(24v)]/3mi = 20.8 Wh/mi.
Look a little better than you thought? ...so yeah 20wh/mi is about 1/4th of what you thought you were using!
amberwolf wrote:In your efficiency measurements, your motor is rated as capable of dissipating 250W of power. That doesnt' mean that 10.4A is the limit of the current it would draw; or even the average. So that current could be significantly different from the actual one. Do you have access to at least one of those cheap Harbor Freight meters? They can take 10A for "30 seconds" without melting the shunt off the board (which can be put back on, don't worry...ask me how I know ), and higher currents for shorter times. You could zip tie it to the top tube, go to an empty parking lot, and hit full throttle while watching the meter to see what current starts at and what it is at at different apparent speeds and throttle percentages.
BTW, I think you mean Wh/Mile, rather than Wh/hour.
dequinox wrote:You must keep in mind that the motor absolutely does NOT use 250 watts all the time. That rating is the "maximum continuous" power usage your motor can handle. That, again, is a very GENERAL rating and should not be used in the calculation. Although if you are running full throttle all the time you are probably pushing 200W most of the time. To go 20mph my bike needs about 15A on a flat...so at 24v I am using 360W. My motor is "rated" at 900W...but I almost never use that much. I don't even think my batteries will give it.
amberwolf wrote:To find out the right gearing for what that motor was "designed" for, look at the scooter it came from. I had a trashed ScootNGo I took apart, which had a 24V 250W Unite motor (already damaged, unfortunately), geared about 5:1 with an 11t motor end sprocket and a wheel end sprocket of 54 or so, I think it was, about a handspan across. The wheel was 8" diameter.
So in order to use the motor the way it was designed, I'd have to either use the same diameter wheel, or change the gear ratio to match a larger wheel. I always get confused with gear ratios as to whether I go up or down for things, so this could be wrong, but I think I would have needed to use a much larger wheel end sprocket to make up for the much larger wheel, to keep the motor using the same power and at the same wheel speed. (might be needing a *smaller* wheel sprocket instead).
After you have figured out what size sprocket to use at the rear wheel to make it spin the *same* speed as the original wheel, then if you want the motor to turn the wheel faster than it would have turned the original wheel, you'd use a smaller wheel sprocket than that, or a larger motor sprocket, or both. This results in less torque, but more motor loading and possibly overheating at lower speeds but high throttle (like startup from a complete stop). Pretty sure this is what finally smoked the little one I had.
If you want the motor to turn the wheel slower than the original wheel, use a larger wheel sprocket or smaller motor sprocket, or both. This results in more torque, but also less motor loading at lower speeds.
dequinox wrote:Also, on a side note, your video makes the chain drive sound SUPER loud... you might want to look into what is making that noise because there could be some serious drag there robbing your efficiency.
def215 wrote: ill have to try my harbor freight meter sometime but i think i blew a fuse on the 10A function.
its my chain tensioner thats making the racket, but it has to stay in its position or my chain will derail and fall off. i should find another way to tension my chain but i dont have any ideas at the moment.
dequinox wrote:An idea for you amberwolf. You could expand the capacity of your harbor freight meter by running it in parallel with a short. Measure the resistance of the meter (lol you'll probably need two meters) when it's in "10A" mode. Then wire it in parallel with a shorting wire of similar resistance. You can figure (by proof of Kirchoff's current, or node law) that the meter will read about half of the amperage you're actually drawing. I think most folks can double a number in their head easy enough... cheap solution for a digital display!
def215 wrote:yes. i have the yellow one. when i use the 10A mode, my meter heats up. i think it melted some of the plastic of the case too. i didnt know about the 10 second rule. will it read more than 10 amps when it spikes or will it just go into overload and read "1"? i never got that function to work correctly. ill try that out again when i take it out to get an official amp reading.
the reason for me not pedaling and going full throttle is that i want to push it to its breaking point to see if failure incurs. if not, i guess things are working correctly, for now...lol. i want all the bugs out of it before i use it as my commuter in the spring time.
funny that you mention about tensioners being made from roller blade wheels amberwolf. mine is a roller blade wheel cut in half and modified to fit into my frame.
its fixed in that position and its more like a chain guide like you mentioned. its on the slack side of the drive chain and it rolls smoothly so i guess thats ok, right? i think the noise is only there because it is only the plastic of the roller blade wheel too. not the prettiest thing in the world, but i got it to work
edit: so i hooked up the meter on my bike and i did ground to one wire of the motor and the 10A function to the other wire on the motor. is that correct or am i hooking up the meter wrong on my bike, because the bike wants to go no where when hooked up the wires.
amberwolf wrote:edit: so i hooked up the meter on my bike and i did ground to one wire of the motor and the 10A function to the other wire on the motor. is that correct or am i hooking up the meter wrong on my bike, because the bike wants to go no where when hooked up the wires.
To use the A function on the meter, it has to be in series with what you're measuring. You will want to measure battery current rather than motor current, though, so disconnect the negative of the battery from the controller, put the meter ground/common to that wire, and the meter 10A plug to the controller's negative power input.
If it still doesn't work it means the shunt probably fell off.
amberwolf wrote:Pretty is irrelevant. Working is all that matters. That said, yours looks better than any of the ones I tried to make so far.
If you use as much of the urethane part (the rubbery stuff) as possible left on the wheel, then it will be MUCH quieter. If it's only the plastic part, the chain essentially just slips over it, snap-snap-snap at every link. Most of the higher-frequency noise would be dampened by the urethane, if it was still there.
amberwolf wrote:I didn't crash, just that at least a couple of times an SLA leaped off when riding, due to zip ties breaking during vibration/etc.