Join Date: May 2008
Location: San Diego, CA
Bikes: '72 Schwinn Sports Tourer, '73 Schwinn Super Sport, '79 Schwinn Twinn Sport 10sp tandem
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The charts I've been getting from these companies, don't list Amps, Torque, Efficiency etc. as a function of RPM. Rather, they list Amps, RPM, Efficiency etc. as a function of torque. At first I wondered how the chart had been made, but finally fingered it out.
My guess is, they mount the motor/wheel combo on a device with a brake. Then they apply 48V and let it spin. At first, they don't apply the brake at all, so the motor spins up to its max RPM. Sort of like your taking your E-bike, lifting up the motorized wheel, and opening the throttle all the way - the wheel just spins in midair. Then they apply enough brake to slow the wheel down a little and hold it there, still with full throttle. They measure how fast it's turning, how many amps, how much drag the brake is applying. Then they apply a little more brake to slow the wheel down a little more, and measure those things again. Then a little more brake, measure again etc. That's my guess, anyway.
Those three quantities (Amps at 48V, RPM, brake drag) will let you figure out torque, Watts of (electrical) power taken in by the motor, Watts of (mechanical) power the motor puts out, efficiency, and power wasted as heat/noise/whatever. They put all that info on their chart, with torque along the bottom of the chart, and the other things along the vertical axis. In a few cases I've found lists of the data instead of a pretty chart. But sometimes the list is more useful than the chart.
Yes, all the measurements are at fairly high RPM, which translates to a pretty high bike speed, where most people frankly won't ride. I have a hunch this is because they used a standard controller to apply power to the motor. These being brushless motors that run on three-phase AC, you can't just hook the battery directly to the motor, you have to use a controller that changes the battery's DC into AC.
Most standard controller have a current limit, they can't put out more than 30 amps (or whatever their limit is). When these guys were testing, they found that as they applied more and more brake, the motor drew more and more current (and produced more torque). When they got it down to 200-something RPM, still with the throttle wide open, the motor was demanding more current than the controller could supply. And the controller automatically started supplying LESS than the full 48V, to keep the current below 30A, even though the throttle was being held wide open by the testing guy. This probably happens a lot when you ride your E-bike.
But the people doing the tests detected this, and stopped the test at that point, so they could present accurate data where the motor really was getting the full 48V. So we only get data at the higher RPMs, that is, where current draw was low enough that the controller's current-limiting function hadn't kicked in yet. Remember, the controller limits current by reducing the voltage to the motor, even if you're holding the throttle wide open and demanding full power.
In real life (i.e. not on a test stand), this probably happens a lot, as I said. If you're riding slowly, maybe 5 mph, and you want to speed up, you open the throttle all the way. Your batteries are supplying 48V to the controller as they always do, and that doesn't change. But the controller can only supply 30 amps to the motor, so it automatically reduces the voltage it gives to the motor, and you're only accelerating with 35 volts or so at the motor itself. As you speed up (still with throttle wide open), the motor isn't demanding so much current, so the controller automatically adjusts the voltage it's giving so that the current stays at 30amps. Eventually you get up to 30mph or so, and at that speed the motor is only demanding 25 amps at 48 V. So by then, the controller is giving the motor full voltage again.
This brings up an interesting experiment. Suppose you had a BIG controller that could supply, say, 100 Amps? And batteries that could do the same? And you pedaled the bike to 5mph and then opened the throttle all the way, as before?
Two things would happen:
1.) The bike would accelerate like a bat out of h*ll, and
2.) The motor would start heating up pretty fast. Maybe along with the wires.
If you're VERY careful with that full-throttle business (and had that big 100-Amp controller), you might get away with this, briefly. Most people don't treat E-bikes like dragsters, or push them hard up long hills where the motor is forced to produce maximum torque at a full 48V with no current limiting.
If you had your 100-Amp controller, and went up a long hill with the throttle wide open, your motor might draw 50 or 60 Amps or more. At 48V, that's 2000 or 3000 Watts or more - far more than your motor was designed to handle. Some motors would keep going and put out the huge torque you could expect from that kind of power... but as I said, they would start heating up pretty fast. If you kept the throttle wide open, soon your motor would get so hot, you'd fry it.
There's a reason these vendors supply a controller with current limiting, and it's not just because they're cheap. It's so you won't set your motor on fire going up a long hill.
If you never ride ANYWHERE except on perfectly flat ground, you might get away with using a super controller like that 100-amp one. If you opened the throttle at low speed, it would rocket you up to 30mph pretty quickly, with the motor heating up rapidly. But then at 30 mph, you'd back off on the throttle to maintain speed at 30, and feed a lot less current to the motor. And with the air flowing over the motor at that speed, it would have a chance to cool back down, before reaching the point where you fry it.
When a manufacturer says its motor is a 1000-Watt motor, they mean that's the most power the motor can take in, pushing your bike at decent speed for a LONG time, without getting so hot that it damages itself. If you have a super 100-Amp controller, you can easily feed it more power than that - but it will start heating up a LOT, and you will destroy it pretty quickly.
Of course, it's also true that most batteries can't supply that much current either, even if you have a super controller that can. Lead-acid batteries can, for a brief time (that's why they are used to start car engines), but if you do it for a long time, then their voltage will start falling off in less than a minute, and if you discharge them too fast too far that way, you can damage them.
Applying more than 1000 Watts to your 1000-Watt motor, can be done for a BRIEF time without damaging the motor, if you back off pretty quickly afterward and let the motor cool down. Even 30-amp controllers can do this, at 48V. Accelerating your E-bike up to speed, can briefly draw more than 1000 Watts, and the motor will start heating up. But if you back off as soon as you reach that speed, you will be OK.
Pushing it up a long hill at full throttle, where you NEVER back off, is a different matter, and you can fry your motor if you just open the throttle all the way and keep it there forever, while the hill prevents the bike from getting up to full speed and you never back off the throttle. Even checking the motor temperature by feeling the outside of the motor shell, isn't real accurate, since the heat is produced in the internal parts of the motor, which will get hotter more quickly than the outside shell.