What do controller and motor ratings actually mean

[karotto]

Hi,
If a hub motor is rated at 250W what does this actually mean? I would assume that the voltage applied to motor windings can be somewhat arbitrary. (Even windings from a transformer made from very thin wire is often rated at 110V). So if this is correct then I assume it's only the current that a motor should be rated for. Too much current through the winding wires and the motor get's too hot. And that's independent from the voltage as long as we are talking about the same amps, correct?

If a controller is rated at 24V 250W does that mean the maximum current it ever supplies to the motor is 10.4A? What happens if you run a 24V 250W motor with a controller which is rated at let's say 36V or 48V and 250W? What if you run it with a controller rated at 24V but 500W? Or 48V and 500W? Trying to get a graps. Thanks for your input

 

[Old_Dave]

Good questions karotto

A meaningful motor rating is normally the 'output' expressed in watts at a certain voltage (whilst amps are also a good thing, they are not used as a reference to power as are watts)

The good news for us is that motors can be given extra voltage which will increase the speed of the motor...and you choose your controller to match your battery voltage (for a couple of good reasons) but that alone does not allow or necessarily mean extra power / watts, whats needed for that is extra amps / watts which can be achieved by either fitting a different controller that can supply a higher current, or by modifying the existing controller (soldering the shunt for example)..

So extra pulling power / power output, requires allowing the motor to have more amps, extra speed requires more volts... put the two together and you get happiness.

But happiness isn't free the cost is the risk of overheating (if your not careful in your modifications) and reduced miles per charge.

 

[wurly]

Power ratings on electric motors are there to determine the output of a motor without damage, usually to the insulation of the windings. If you put a an underrated electric motor to drive something above it's power rating, it will get hot, sometimes enough to burn off the insulation of the copper wire which will lead to short circuits (eg 'burn out') i think the ratings are determined when a motor reaches a temperature for a given power input (thermal equilibruim). There is a class rating for motor windings also.
So a 250W rated motor can (and does) see a lot more power than it's rated especially on ebikes, but not continuously so not much over heating is likely.

The transformer voltage you refered to is determined by the ratio of primary and secondary windings to transform voltage value from to another value) they also have ratings (VA) same as power W.

Controller rated power is, like you say, the amount of output current the controller can deliver to the motor. Ratings on all the componants are a bit vague thb. If you were to use a higher voltage battery you will find the motor will spin faster and the controller would be certainly outputing more power this way.
So expect a 250W rated hub running on 48V to be running hotter, faster and under more stress than on 24V.

 

[flecc]

Hi,
If a hub motor is rated at 250W what does this actually mean? I would assume that the voltage applied to motor windings can be somewhat arbitrary. (Even windings from a transformer made from very thin wire is often rated at 110V). So if this is correct then I assume it's only the current that a motor should be rated for. Too much current through the winding wires and the motor get's too hot. And that's independent from the voltage as long as we are talking about the same amps, correct?

If a controller is rated at 24V 250W does that mean the maximum current it ever supplies to the motor is 10.4A? What happens if you run a 24V 250W motor with a controller which is rated at let's say 36V or 48V and 250W? What if you run it with a controller rated at 24V but 500W? Or 48V and 500W? Trying to get a graps. Thanks for your input

In practice with e-bikes, the rating means practically nothing. All of them can deliver over 250 watts net in practice, and don't believe the argument that some make that it cannot be continuous, it can.

For example, there are two permitted ways an approved laboratory can gain the EN15194 certification for manufacturers and suppliers when measuring power, one of them being an acceleration test over a specified short period of time. If the e-bike takes over the specified minimum time, it's taken as read that it delivers under the 250 watts. Clearly the maker can specify the controller characteristic to ensure a power delivery gradient that makes the e-bike take long enough, while allowing it to go on to deliver very much more. By that method there are some e-bikes which can even deliver double and more the legal 250 watts continuously yet be EN15194 certified.

I've owned an e-bike of a well known brand that was long sold as legal yet delivered up to just over 1000 watts gross and over 700 watts net power.

 

[103Alex1]

So all you have to do is introduce a power delivery lag via the controller so the acceleration isn't instant and then you can happily run 1k - 2k continuous power up your hills as the trade-off and provided this tails off at 15.5mph EN15194's your badge ? If so then what a joke ... makes the rules completely meaningless !

 

[flecc]

So all you have to do is introduce a power delivery lag via the controller so the acceleration isn't instant and then you can happily run 1k - 2k continuous power up your hills as the trade-off and provided this tails off at 15.5mph EN15194's your badge ? If so then what a joke ... makes the rules completely meaningless !

In short, yes. It's the practical realisation that the legislators 250 watts is insufficient, while recognising that the legislators are unlikely to agree to more than an average fit cyclist can produce.

Pragmatism at work!

There is an example of the Lynch commercial e-trike motor which is 200 watts rated but at a very slow speed outputs up to 5 kilowatts maximum! Ideal for it's 5 cwt van version.
.

 

[Old_Dave]

In short, yes

But... lol

In order to be 'legal' the bike would have to have a EN15194 certificate of compliance, now that's going to be the problem.

 

[wurly]

I think karotto wanted to have more of an understanding of power ratings from the electrical point of view and not the legallity issue........

 

[mountainsport]

I think karotto wanted to have more of an understanding of power ratings from the electrical point of view and not the legallity issue........

Are you saying that people are "drifting" off to sleep again?

Mountainsport.

 

[flecc]

But... lol

In order to be 'legal' the bike would have to have a EN15194 certificate of compliance, now that's going to be the problem.

No problem, as long as the controller response is adjusted to produce the right very short term response to pass that acceleration test, easy to do.

 

[flecc]

I think karotto wanted to have more of an understanding of power ratings from the electrical point of view and not the legallity issue........

Probably yes, which is why my answer began with the qualification "In practice with e-bikes," covering that aspect in case he wanted to know that. The right lines vis-a-vis current that karotto was on have limited practical relevance in our sphere, useful to know when selecting an e-bike.

 

[Mrke]

By that method there are some e-bikes which can even deliver double and more the legal 250 watts continuously yet be EN15194 certified.

I've owned an e-bike of a well known brand that was long sold as legal yet delivered up to just over 1000 watts gross and over 700 watts net power.

So how would anyone (me) know which bikes are more powerful when all manufacturers and retailers quote the same motor wattage?

Is there a simple way to work this out?

Thanks..........

 

[Marctwo]

But... lol

In order to be 'legal' the bike would have to have a EN15194 certificate of compliance, now that's going to be the problem.

I don't think so, it only has to comply. As far as I'm aware there's no such official certificate. Although some manufacturers may issue their own cert's, it's not a requirement of EN15194.

 

[Deleted member 4366]

So how would anyone (me) know which bikes are more powerful when all manufacturers and retailers quote the same motor wattage?

Is there a simple way to work this out?

Thanks..........

As a general rule-of-thumb, you only have to ask what is the maximum controller amps and multiply that by the battery voltage. The higher the number, the more powerful the motor. Most ebikes these days are 36v, so you only have to compare the amps directly.

You can change/adjust a fair proportion of controllers to a much higher rating, so you can then use another general rule: The bigger the physical size of the motor, the more likely it is to be able to handle substantially more current.

Finally, if all this is still confusing, you can ask on this forum.

 

[Oxygen Bicycles]

And once you multiply voltage by amps remember to multiply the the final score *0.8 then you deduct the efficiency as least 20% of total power turns into heat.

 

[trex]

10mph wrote a very interesting piece a couple of years ago to show how manufacturers can get EN15194 certification despite having apparently powerful motors:
http://www.pedelecs.co.uk/forum/electric-bicycles/8743-en19154-power-test-gives-incorrect-high-values.html
Jemery also posted something similar last December:
http://www.pedelecs.co.uk/forum/electric-bicycles/13377-approved-ebike-list.html
Apparently, the power is determined by measuring acceleration. So if your bike (equipped with a 350W BPM motor) accelerates 0.1s more slowly than it is capable of, it'll pass. With a slow power ramp curve and a 250W sticker, any bike will pass this test. It's easy but not cheating.
The second feature of EN15194 is to limit assistance to under 15mph. Many kits do not come with speed limiters like a king meter, you may break the law without being aware.

 

[karotto]

So very interesting. Thanks for all the replies. So what I got is that a motor manufacturer thinks approximately like this: First they send increasing current through the motor and measure how hot it gets and when it get's as hot as they think should be the maximum heat they read the amps sent through the motor. To determine the voltage rating I assume the figure would be rather arbitrary since even a high voltage us unlikely to zap through the insulation of the wires. The probably choose 24V for a motor that does not need high speed and for ease as it takes less batteries. Or they decide to market a motor as high speed in which case they would simply pick a higher voltage. But how would they arrive at the watts rating of a motor? they cannot simply multiply volts x amps as the controller controls the amps to the motor by reducing the voltage regardless of battery voltage. Meaning that if a motor draws 10A in a 24V battery and controller system, in a 48V system when 10A pass through the motor the voltage to the motor would be identical than in a 24V system (since the resistance of the windings is the same and current = voltage divided by resistance).
So how would a manufacturer then decide what wattage a motor is rated for? Seems like the amps it can draw or the mechanical force it produces would be a more reasonable way to describe the power of a motor.
Am I getting a grasp in all this? thanks again

 

[trex]

So how would a manufacturer then decide what wattage a motor is rated for? Seems like the amps it can draw or the mechanical force it produces would be a more reasonable way to describe the power of a motor.
Am I getting a grasp in all this? thanks again

Manufacturers measure torque and speed (in RPM) of their motors at different voltage and current with a dynamometer. The power in W at the shaft is roughly = torque in Nm * 2 * pi * rpm/60.
EN15194 measures power after friction loss (of the whole bike), a little lower than at motor shaft.
All popular ebikes on this forum have motors capable of 500W+ but restricted to comply with the law, that enables them to affix the 250W sticker to the motor. The restriction is usually done by programming the controller for smooth slow power delivery and cutoff at 25kpm (15.5mph).
Most motors can thus be made legal, the best are probably the Bafang BPMs.
If you want to build a legal bike for sale, you'll need a legal controller and a speedometer.
A legal controller has 7A at 36V programmed in. The cutoff amp is not important. Some manufacturers take advantage of this and have upper limit of 20-22Amps, the most poplar cutoff is around 15A.
Derestriction is done by modifying the controller or fooling (using a dongle) or switching off the speedo.
Bikes with speedo are easier to derestrict and can just as easily be reverted back to their original state.

 

[Deleted member 4366]

So very interesting. Thanks for all the replies. So what I got is that a motor manufacturer thinks approximately like this: First they send increasing current through the motor and measure how hot it gets and when it get's as hot as they think should be the maximum heat they read the amps sent through the motor. To determine the voltage rating I assume the figure would be rather arbitrary since even a high voltage us unlikely to zap through the insulation of the wires. The probably choose 24V for a motor that does not need high speed and for ease as it takes less batteries. Or they decide to market a motor as high speed in which case they would simply pick a higher voltage. But how would they arrive at the watts rating of a motor? they cannot simply multiply volts x amps as the controller controls the amps to the motor by reducing the voltage regardless of battery voltage. Meaning that if a motor draws 10A in a 24V battery and controller system, in a 48V system when 10A pass through the motor the voltage to the motor would be identical than in a 24V system (since the resistance of the windings is the same and current = voltage divided by resistance).
So how would a manufacturer then decide what wattage a motor is rated for? Seems like the amps it can draw or the mechanical force it produces would be a more reasonable way to describe the power of a motor.
Am I getting a grasp in all this? thanks again

Most of that is incorrect.
The insulation on the wires is sufficient to run substantially higher voltage than the ratings. People often run their motors at double the voltage rating and some at triple.

The controller doesn't adjust voltage to the motor. Instead, it fires pulses of current at the windings. When you operate the throttle, the pulse widths are changed so that the amount of energy in the pulses varies.

When the motor starts to turn, it becomes a generator as well as a motor. As the speed increases, the generated power (back EMF) increases until it reaches the same voltage as the battery/controller, so no current can flow. This is then the maximum speed of the motor. Therefore. power from the battery decreases as speed goes up. The speed depends on the number of turns of wire on each pole of the motor for any given battery voltage. If you increase the voltage, the maximum speed of the motor will increase in proportion.

When the motor is under load at full throttle, the efficiency starts to drop off after the speed of the motor drops below about half the maximum rpm. The lower the speed, the lower the efficiency. This means that the motor has a sweet spot about 2/3 maximum rpm, where efficiency and output power are highest. Maximum current comes at zero rpm because at that speed is no back emf, so you get maximum power consumption at that speed. but very little power output. Motor ratings have to take this effect into consideration because nearly all the maximum consumed power has turned your motor into an electric heater.

At anything less than full throttle, slightly different rules apply.

Most hub-motors use speed-control controllers, so the pulse width doesn't relate directly to the throttle setting. Instead, the controller uses an algorithm that looks at the difference between your actual speed and the calculated speed from the throttle setting.

So, power consumed is easy to measure = volts x amps from the battery. Maximum output power depends on speed, so easier to measure mechanically. The rated power takes into consideration heating effects, which change with speed and the speed of the motor windings.

It's torque that makes one bike feel more powerful than another. Torque has a close relationship to current, so, if a bike has a controller that allows a lot of current, then torque will be high as long as the motor can handle it without over-heating.

 

[flecc]

So very interesting. Thanks for all the replies. So what I got is that a motor manufacturer thinks approximately like this: First they send increasing current through the motor and measure how hot it gets and when it get's as hot as they think should be the maximum heat they read the amps sent through the motor. To determine the voltage rating I assume the figure would be rather arbitrary since even a high voltage us unlikely to zap through the insulation of the wires. The probably choose 24V for a motor that does not need high speed and for ease as it takes less batteries. Or they decide to market a motor as high speed in which case they would simply pick a higher voltage. But how would they arrive at the watts rating of a motor? they cannot simply multiply volts x amps as the controller controls the amps to the motor by reducing the voltage regardless of battery voltage. Meaning that if a motor draws 10A in a 24V battery and controller system, in a 48V system when 10A pass through the motor the voltage to the motor would be identical than in a 24V system (since the resistance of the windings is the same and current = voltage divided by resistance).
So how would a manufacturer then decide what wattage a motor is rated for? Seems like the amps it can draw or the mechanical force it produces would be a more reasonable way to describe the power of a motor.
Am I getting a grasp in all this? thanks again

Your query shows why I introduced the legal factor. Most e-bikes are sold as e-bikes, only a tiny minority of sales is through self build kits. Laws commonly refer to the power of the e-bike and the way in which it works, for example in UK law an e-bike is supposed to have a plate on the bike (not the motor) showing amongst other things the wattage. In Japanese law the power phase down slope is strictly specified. In EU law a power phase down is specified, and the EN15194 specification tests can only apply to complete e-bikes, it's impossible to test a motor to that standard. So a specified controller must always be present in every one of these cases.

Therefore the majority makers of complete e-bikes specify the e-bike's wattage, which of course includes the controller as well as the motor, so it's a system measure. In any case, it's arguable that there is no such thing as a PWM motor in isolation, the essential nature of the controller means the motor without one isn't a motor, it's just a non-functional part, so a "motor" in isolation cannot have a fixed wattage specified.
.