hemo wrote: ↑20 Sep 2022, 4:55pm
At the end of the day the rating is what ever the manufacturer puts on the motor and as Cugel says for sure 250w or 350w labelling is the same motor, not sure about 500w but wouldn't surprise me the motor being the same if the voltage is the same. A 350w labelled one being the same as 250w laballed one, the only difference making it illegal is the labelling.
It'll be the same motor, but a different battery voltage. The fact is a DC motor can be operated at a range of different voltages and will have a different output characteristic at each voltage. And at any voltage, the power output varies depending upon the speed at which the motor spins and how much current the controller allows it to draw. So the manufacturer's labelling has to consider not just the motor, but the system as a whole: motor, battery and controller. The continuous rated power will be measured when the motor is running at or close to peak efficiency. Demanding more torque, eg by going uphill, slows down the motor and causes it to draw more current. Power = torque × rpm (×PI/30) and in those conditions it is likely that torque will increase more than speed reduces, so that a bit more than the rated 250W is output. Whatever: the increased current at constant voltage greatly increases electrical power consumption, hence the drop in efficiency.
Electrical input is a simple thing to meter, so that's what e-bike tinkerers and internet bloggers do. Most of the tinkerers are after more power and speed and some of them have an agenda to subvert the EU rules on e-bikes. They seize upon these sometimes large, transitory power inputs and use them to spread confusion by allegations that EU-approved e-bikes regularly and greatly exceed their 250W rating. Figures as high as 800W are bandied about. Figures
that high are probably mere speculation, but it's not unlikely that some approved systems could occasionally draw as much as 500W whilst still outputting only 250W - or perhaps a little more, as may be permitted by the way the regulations are written. The difference goes into heat of course, which the system may nevertheless be able to withstand - at least for a while.
Mechanical energy is far more difficult to measure than electricity, but that's what matters when it comes to the performance of the bicycle. We measure the performance of a human cyclist after all, by how many watts they can produce at the pedals, not the number of calories they must consume in order to do that!