Terminology and Explanations of Common Misconceptions
Needs all terminology added, explanations of how stuff gets confused. Ah vs A, W vs Wh, power vs current vs voltage, why a controller is what limits power and not battery "amperage", etc.
[quote="oldskoolhead"]also to note if you have a 20 amp battery you also need a 20 amp controller, i discovered this to my expense when i tried paralleling a second battery to my bike and it fried my controller almost instantly, so as a rule i would buy 2 smaller batteries and swap when 1 is dead or make absolutely sure your controller can handle a 20 amp battery[/quote]
The amps the battery is capable of putting out don't have anything to do with whether the controller can handle that. A battery doesn't output any current except what is "pulled" from it by a load (like your controller).
A "20A" battery would be one that can only output 20A before the BMS shuts off the output. I suspect you really mean a "20Ah" (Amp-hour) battery, which simply means that the battery could output a total capacity of 20Ah, which has nothing to do with how many A (amps) it can actually supply, despite the similar terminology.
A "20A" controller would be one that should pull a maximum of 20A continuous from whatever battery is hooked to it, at the maximum loading of the motor attached to the controller (like when climbing a hill or starting from a stop). (it may pull more than 20A during very short peaks).
--Drkangel 14:12, 8 February 2013 (UTC)
Sadly, it seems prevalent, even among the supposedly intelligent, that the watt rating of a motor refers to how many electric watts can be put into it.
The "watt" rating of a motor refers to how many watts,(a measure of power), it can output, at a specified voltage. 746 watts = 1hp (horsepower)
The perfect motor would output at a 100% efficiency. Typically Brush motors have a ~80% peak efficiency, Brushless nearing 90%. (ICE, gas only 15-20% and diesel at 20-25% efficiency.)
Sound reasonable and convincing ... but Wrong!
For example - Do the math on this motor. "MY1020 36 Volt, 750 Watt, 2800 RPM, 27.4 Amp, permanent-magnet motor. 11 tooth sprocket for #25 chain." *36V x 27.4a = 986.4w input to produce 750w (1hp) output Which, shows a motor efficiency of 750/986.4 or approx. 76% peak efficiency - reasonable for a brush motor. My eZip motor. "MY1018Z 24 Volt, 450 Watt, 3000 RPM, 24.0 Amp, permanent-magnet motor." *24V x 24a = 576w input to produce 450w (6/9hp) output. 78% peak efficiency. Find your motors v & a ratings and do the math.
Edit: At maximum motor output, efficiency is almost exactly and universally 50% ... with sufficient amps! 1000w electrical energy input to produce 500w peak physical energy output.
And, these are the peak efficient ratings for the motors. If you "lock test" a 24a motor, you might be able to dump in 100a. This is similar to a heavy throttle start from a dead stop, just prolonged. Smoke and flames are possible.
That is why the motor v & a specs are listed with the rpm also. The peak, rated output at the optimal v & a & rpm / efficiency.
Efficiency Dead stop - efficiency = 0%, no matter how many amps you dump in! Efficiency gradually increases till reaching reasonable at 50-80% of no load speed. A more powerful controller, typically, only increases performance in this inefficient, sub 50% zone. It is very feasible that a 1000w rated motor will be using 2000w of input electricity to produce 500w output when "dogging it" up a hill.
"At speed" the motor will draw no more amps than rated! --Drkangel 14:12, 8 February 2013 (UTC)
Thought I'd graph a direct comparison of the electrical watts supplied into a motor and the actual motor output (watts) power.
Based on a 750w motor at full throttle ...
Flat spot at low rpm "Watts in" is a current limit of ~50A @ 48V (54.6V full charged)
Added a 50A vs 30A controller comparison.
Further explanations of motor watt ratings: http://www.endless-sphere.com/forums/viewtopic.php?p=701464#p701464
[quote="justin_le"][quote] From the start of this, I've always stated that the rating is for continuous usage, not a max power rating. That was one of the points being debated as far as I know.[/quote]
On this point that's how it should be for sure. To the extent that there are power ratings on motors from a manufacturer, it is generally for the continuous and not the peak power output, at a known set of operating conditions.
This is how it has been at least in industries where the motors are sold for engineer types. But when the watts becomes a selling point number for consumers, then it usually shifts over to first being the peak power and then the peak input power, to even a worse case surge input power from the moment a machine is plugged into a load. If you look at electric rider's site, they take the very simplistic [b]power = volts*amps[/b] approach, regardless of the actual motor being run: http://www.electricrider.com/crystalyte/index.htm
I do see the appeal of this because it more closely matches the watts that someone sees if they have a watt-meter display on their bike, even though the maximum output watts will at most be about 65-70% of the quoted system power. When dealing with the masses, this means way less explaining to do. And, it also means a bigger number. My hunch is that over time this will be the norm for how 'watts' is used in the ebike industry in USA. In Europe, the situation is the opposite where it is a disadvantage to have a motor advertised as more than 250W, so you'll see vendors adopt the most conservative rating scheme possible.
[u]For the sake of those confused: [/u]
[list] [*]Motors by themselves do NOT have a power rating without a context of an operating voltage and RPM. You can't just say "this is an XXX watt motor". It doesn't mean anything. [*]When supplied as part of a system with a controller and battery, then there IS a very exact peak wattage and less exact continuous wattage that can be determined. However, determining this is not very easy without dyno data which most people and most vendors don't have. [*]When people have a watt-meter on their system, they can see the input watts. This requires very little equipment to get, and the peak input power of volts X controller amps stays steady for large portions of a ride. Therefor, the norm for rating ebike systems in this industry will probably wind up being the max input power. [/list][/quote]