Off topic ebike motor watts

[MichaelB]

Apologies for a slightly off topic post but I am working on my first ebike project and some of it is linked to motor and batteries.

Ebike is actually a electrically assisted pedal bike, you have to pedal to get the motor to assist, no throttle allowed. The drive system is usually Shimano or Bosch and a bit like wheelchair manufacture you build from a set of components and the unique part is the frame and suspension system.

The legislation is in place and you have no choice but to comply if you want to build a bike which will be sold. The problem I have is that the legislation states that the motor is limited to 250w and top speed before assist cuts out is 15.5mph. Batteries are measured in watt/hours ot Ah.

I'm sure that I have seen comments on here about why using watts is stupid and could do with them now. I can't move away from the current motor manufacturers but would like to be able to explain to others why the standard is not correct.

[Burgerman]

Watts = Amps x Volts.

A motor watts rating is simpy a measure of maximum CONTINUAL watts allowed. Ie before something melts. So if you were to wrap a towel around a 250 watt motor, it then cant lose as much heat. So it might be good for 125 watts. Or add a heat sink, or water cooling, or just a fan, ot may be safe at 500 watts continuous.

What a "watt" rating does not tell up is how many watts a motor takes! Or how many Amps it will pull at a given voltage.

So you can take 2 identical looking motors, with identical efficiency ratings, that have say 2 turns of the windings, or 20 turns. Both may be 250 watts. The 2 turn one will take 10X the Amps at stall. And so ten times tha watts. And 10x the torque. Yet its still 250 watts AVERAGE.

Take a typical 4 pole wheelchair motor rated at 350 watts. Actual figures may be:
With a 120A R-net controller, it CAN take 24V and 120A at max acceleration, at around half speed, so that is then 2880 watts. With a 60A controller limitation thats then 1440 watts. With no controller limit it may be 3k watts peak or much more dep on impedance. A lot different from its rated 350!!!

So as you can see, max motor power in watts is Max stall amps, x the battery voltage. So that as you can see, impedance, and battery voltage x motor efficiency at a given RPM determine motor power. That 350 watts tells is absolutely nothing useful. Its entirely possible that a 200 watt motor gives more way more power than a 1500 watt motor. Just not continually. Depends on impedance and motor efficiency.

The problem I have is that the legislation states that the motor is limited to 250w and top speed before assist cuts out is 15.5mph. Batteries are measured in watt/hours ot Ah.

So the moron that typed the legislation was a solicitor, or a civil servant. And no clue how motors or controllers work, so its pretty much meaninless drivel. It doesent tell you anything useful! They are not engineers. I suspect that they really mean that the peak power of the whole system, is 250 watt peak. In which case the only way to achieve this is to use a nominal battery voltage of x and a current limited controller of Y which when multiplied together cannot provide a motor with more than 250 watts. So the actual motor used as long as its of low enough impedance is completely irrelivant. A 10,000 watt motor will then be limited to 250W. But its to complicated for them.

And batteries are measured in watt hours because they are all different voltages. A 48V 10Ah battery is the same amount of storage as a 24V 20Ah battery. So we measure them in watt hours. My BM3 is 3240 watt hours. The two batteries in the example above are both 480 watt hours.

They are equally stupid when it comes to power limits on int combustion engines. A new biker that has not passed his test is limited to a bike with 11 horsepower. But in typical idiotic fashion thats all it says! A 18 horsepower engine gives 18hp with no ancilliaries like water pump, or generator etc. Do they mean that? Because it may be left with 11 afterwards. Or do they mean at the wheel? Generally aprox 20% is lost through the transmission. So the same engine loses another few, and reads 9bhp on the dynamometer at the wheel. Is it meant to include windage losses, and does it take temperature and pressure or altitude above sea level into account? Doesnt say. So again another easy 10%variability. And then theres correction factor standards. Which one should I use? SAE 1340? Din 70020? ISO? Again all give a different figure, or P.S. figures? They dont say... I had various cops bring me bikes to test to see if they were "over the legal limit" in the past to my automotive dynamometer. When I asked the these questions I was met with a blank stare? So depending on what I felt like I could give them half a dozen wildly different figures, and all were correct! The civil servants do not understand these things and so cannot write legislation correctly.

[LROBBINS]

Electrical power and mechanical power of a motor are not the same thing, though both are measured in watts. A more sensible motor rating is mechanical watts at maximum efficiency, and many manufacturers do rate them that way. As John says, the electrical power a motor might absorb is not a fixed value - it is equal to mechanical power/efficiency ONLY at the RPM and torque of maximum efficiency; in most cases a motor can run indefinitely at maximum efficiency without overheating, so John is correct to describe this as the power for continuous operation without overheating. However, some motors, such as those in linear actuators, even then have a restricted duty cycle or they will overheat. At other points on the curve, especially close to or at stall, mechanical watts produced and electrical watts consumed can both be much higher than this, to the point of meltdown. Except for motors sometimes called torque motors that can work indefinitely in a stalled state, motors will overheat if held in a stall condition.

[woodygb]

The problem I have is that the legislation states that the motor is limited to 250w

...I think that you should have posted the EXACT wording of the legislation .


A quick Google gives this .

must have a maximum power output of 250 watts

https://www.gov.uk/electric-bike-rules

[Burgerman]

Which means that at 24V nominal voltage for eg, it can never be allowed to pull more than 10.4A + say 7% if its efficient and Brushless. You can test that on a rolling road dynamometer that gives up to but no more than 0.33 horsepower. If its geared, or connected by a belt or a chain then its more complicated as there will be more losses. So you can likely allow say 20% more watts INPUT. But its not possible to know because we dont know if they mean at the motor, at a gearbox, or at the wheel, and does this include tyres losses? They dont specify.

[MichaelB]

Attached screen shot of the actual paragraph from the EN. It is to be measured at the drive shaft. There is a separate EN for motor test conditions.

I understand the motorcycle analogy, HP has been used by a number of companies for marketing without relating to real world use and actual performance.

I always thought that it wasn't an accurate test but as it also has a maximum speed for the motor assistance it doesn't need to have a limit on the watts other than for acceleration up to the max speed. That must be a preset in the system controller but that is tamper proof so can't be altered and I am not aware of the availability of any software or dongle to do that. Plug in inline dongles can remove the 15.5mph limit but the motor makers are now adding systems to prevent this.

It is interesting to look at from a wheelchair perspective and how chairs and bikes are manufacturered from standard parts.

Thanks for the explanations.

[Burgerman]

In other words, unless you have an accurate temperature controlled test facility and a motor dynamimeter, then you can do as you pleas. Because the cops wont have any clue what that acyually means, or any place to test it. So by the time they figure out someway to do that they will need to have gone and learned some physics and engineering. And bought a motor dynamometer. Its never going to happen.

Because if they turn up in court wuthout the correct testing parameters and method, and a measured result, then the court will throw it out as soon as you read out why its invalid...

[MichaelB]

In other words, unless you have an accurate temperature controlled test facility and a motor dynamimeter, then you can do as you pleas. Because the cops wont have any clue what that acyually means, or any place to test it. So by the time they figure out someway to do that they will need to have gone and learned some physics and engineering. And bought a motor dynamometer. Its never going to happen.

Because if they turn up in court wuthout the correct testing parameters and method, and a measured result, then the court will throw it out as soon as you read out why its invalid...

For an individual, when you do it commercially you have no choice but to comply as you have to show compliance, it is only one out of about 100 tests that are needed to get an ebike to market. And you don't need to have the testing equipment, there are a couple of labs in the UK that specialise in testing to CEN safety standards. TBH the system is like wheelchair manufacture, you buy in the parts like a motor, batteries design and build the frame and suspension system then bolt parts in. The parts maker gets the motor tested you get to use their certification. And if you do end up in court with a liability claim you have to prove compliance or look like you tried to avoid testing thus not built to the required safety standards. Not to worry when we Brexit we either adopt the globally accepted EN or create our own, oops we don't have the infrastructure to do that or the people with experience.

[Burgerman]

The parts maker gets the motor tested you get to use their certification.

Its not about the motor. Limiting the MOTOR to 250w output means that it will make less than this at various speeds. Its the wrong way to do it. The more you load up a motor the more amps it will pull. So at 24V for eg the more you load it the more current it takes. And the less efficient it is. A 1000 watt+ motor or rather a low impedance motor, is still only 250 watts output if the controller limits the current correctly v rpm. So you are not talking about motor alone but the system complete designed to limit motor shaft output power to 250 max. In fact the current limited low impedance motor is the most efficient way to go and will have better low speed torque.

So for best performance while having it comply with the rules, you need a motor that is much more powerful than 250 watts, but controller limited to 250watts output shaft maximum, at every RPM from 0 to max rpm limit. This means it will then have 1/3rd hp output from 0mph all the way to its max allowed speed.

[MichaelB]

The parts maker gets the motor tested you get to use their certification.

Its not about the motor. Limiting the MOTOR to 250w output means that it will make less than this at various speeds. Its the wrong way to do it. The more you load up a motor the more amps it will pull. So at 24V for eg the more you load it the more current it takes. And the less efficient it is. A 1000 watt+ motor or rather a low impedance motor, is still only 250 watts output if the controller limits the current correctly v rpm. So you are not talking about motor alone but the system complete designed to limit motor shaft output power to 250 max. In fact the current limited low impedance motor is the most efficient way to go and will have better low speed torque.

So for best performance while having it comply with the rules, you need a motor that is much more powerful than 250 watts, but controller limited to 250watts output shaft maximum, at every RPM from 0 to max rpm limit. This means it will then have 1/3rd hp output from 0mph all the way to its max allowed speed.

You can only buy a complete system for the OEM work that I do, everything is preset by the manufacturer, no way to get into the controller and amend parameters or change anything and you don't know if it is a 250w output or limited 1000w motor. Crazy really but that is how it is, as bike companies very few can afford to develop their own system plus creating a warranty and service network. Output shaft is also the pedal crank with rpm max around 100 rpm, top speed of 15.5mph but that could be with a lot of torque depending upon the motor and controller. Which none of the manufacturers give so you can't tell which system has the best performance. Hmm, think that I should stick to normal mountain bikes.

[Burgerman]

So just like powerchair manufacturers. They are all but clueless about the parts they buy in, and really only know about cycle parts and they make a frame. Thats exactly the same as powerchair manufacturers. Which is why they are so useless.

Theres 10x more tech knowledge about motors, controllers, batteries, wheels and tyres, charge systems, programming, and layout/CG etc on this forum than you will ever find at any normal wheelchair manufacturer. All those do the same thing. With odd exceptions. They make a chassis, and then some plastic covers. They then buy in all the rest from industry "experts" in each field. They dont understand the whole system together. Built and designed by committee.

One exception, was invacares brushless motor/controller system. After speaking to 101 numpties, I finally found the leader of the tech team involved in the brushless motor/controller development. I explained that there's no way in hell that a brushless hub motor will ever zero turn that heavy chair without hitting its 200A limit even when the chair was EMPTY due to design. A gearbox was required. I told them it was super efficient once rolling, (about 2.1A!) But draws a completely rediculous high currents at stall and very low speeds, and so keeps rolling back power to protect itself and simply wont move. This happens very fast, in a second. And theres no fix since its faulty by design. The phone went silent for a full 30 secs. He already knew this himself, but was hoping it would never be figured out by anyone else. And completely agreed. And invacare then refunded the chair and took it back no questions asked ASAP...

[MichaelB]

So just like powerchair manufacturers. They are all but clueless about the parts they buy in, and really only know about cycle parts and they make a frame. Thats exactly the same as powerchair manufacturers. Which is why they are so useless.

Theres 10x more tech knowledge about motors, controllers, batteries, wheels and tyres, charge systems, programming, and layout/CG etc on this forum than you will ever find at any normal wheelchair manufacturer. All those do the same thing. With odd exceptions. They make a chassis, and then some plastic covers. They then buy in all the rest from industry "experts) in each field. They dont understand the whole system together. Built an designed by committee.

Similar but I would like to think that by careful research you build up an understanding before you start designing. My real unknown is how the motor assists with pedalling input. Pedalling initiates motor start up, stopping pedalling stops it, these parameters are preset. How it actually delivers the power affects the design, if it is just when you apply pedal effort ie every 180º or continual, as long as there is some effort the motor continually turns. I didn't think every 180º would be possible but then looked at my FES bike and it fires electricity into the pad at specific places depending upon where the crank is.

And unlike wheelchair makers we'll work alongside say a rear shock maker, developing the suspension inline with how the shock works and its characteristics you can't just build a chassis and bolt whatever you want into it, well you can but it wouldn't sell or get a decent review in a magazine.

[Burgerman]

It will be every time you push, a load cell, but I imagine that it will include a delay so that it covers the crank in between times, say enough time to cover 1/3rd of a revolution. That will need to change time proportional to crank RPM.

[MichaelB]

It will be every time you push, a load cell, but I imagine that it will include a delay so that it covers the crank in between times, say enough time to cover 1/3rd of a revolution. That will need to change time proportional to crank RPM.

That sounds feasible, thanks

[Burgerman]

But guesswork. Never looked at one.

[MichaelB]

But guesswork. Never looked at one.

It is possible and gives me something to explore with the manufacturers. Ultimately not critical to a frame design but it helps if you understand how the components work. I'll not get into batteries suffice to say the same kind of wording.