How do hub motor power ratings work?

[Fordulike]

I know that wattage power ratings on a hub motor can peak way above the quoted hub rating.
Could someone explain what the power rating actually relates to.
I.e. on a 250 watt motor, would this mean that it averages 250 watts with an average weight rider on flat ground?

 

[DBCohen]

Well, I am sure that Flecc will weigh in with an expert opinion, but I always thought it was purely a measure of the electricity it consumes, rather than a power output (due to efficiency losses and other variables).

Even if not, you can probably NOT count on the figure being an average. Marketing is all about having the highest number - it is possibly a peak measurement, not an average.

 

[NRG]

It the continuous electrical power rating of the motor, 250w on a nominal 36volt system drawing 7Amps.

 

[Deleted member 4366]

This has been debated before. Here's my opinion. I think that it's the other way round. Manufacturers know that the 250w limit is a bit low for what most cyclists want, A test ride will soon show whether one motor is more powerful than another, so they're all trying to get away with the maximum possible. The actual ISO standard involves an acceleration test, which measures the time to accelerate from zero to I can't remember how many km/hr and then calculate the power from the time and the mass. I've never seen a result from one of these tests, probably because most motors exceed the limit. It would be easy to reduce the result by changing or limiting the controller, so that's probably also a meaningless test of the actual motor power. The Bafang 250w motor, which is a good value very common motor in good value bikes can pull a continuous 700/750 watts from the battery up a long hill because I've measured three different one - although the amount of that power that's actually translated into the movement of the bike and rider would be something less. I don't believe that other 36v motors are much different from comparisons that other people have done on the forum. One of the main things that the authorities are interested in is the speed of the bike, which is much easier to measure, so the manufacturers can easily design the motor to reach the maximum allowed and no more, whilst having plenty of power for hill climbing and everybody's happy.

 

[banbury frank]

Hi

We now only use 48 volts as you cant climb steep hills with 36 volts

with a 18 / 20 stone rider . All our controllers are now limited to 15 MPH


as standard with a link that can be unplugged for off road use

Basic controller 33 A optional 50A

then 28 MPH +

Frank

 

[Biged]

Wattage is calculated by multiplying the voltage by the amperage. In something like an electric fire this is simple 240v x 4.2A = 1Kw or thereabouts.
With a battery it becomes more complicated, a battery is rated at X amp hour, which means the battery can supply that amperage (X) for 1 hour, but a battery will try to supply whatever load you put on it, for instance a car battery is 12v and say rated at 48ah but will supply 300-400 amps to the starter motor for a short while.
The hub motor will no doubt be similar, its down to the controller to sort it all out.
250w is the legal limit and your hub motor will run at that, but it can easily exceed that if required.
Hope i have explained this clearly enough, i know what i mean but it does not always come across that way

 

[jbond]

Just found this set of motor curves for a Bafang 250w/36v motor SWXK-5.
Bafang SWXK5-36V250W185R - Part Three

I've seen very similar curves for Cute motors as tested by the factory. I'm curious though how the torque is measured and hence how Pout is measured. And to what extent the controller settings affect this. You can't run these BLDC motors without a controller and the controller must affect the results. And it's hard to measure the actual phase currents. It's not really clear if the I current figures are for battery average or phase current.

If you're just measuring the motor, then I would expect the controller to be set with a very high current limit and the test bench runs to be adjusted to avoid getting into areas with dangerously high values.

These graphs are typically for full throttle 100% PWM runs at the high end of the rpm range. There's enough reverse EMF in that region to limit current so that the apparent 9A at 36v is as much as the motor will ever take. However, halve the speed and lug it up a hill and I guess it will draw considerably more current if the controller can supply it.

The French testing procedure is properly confused. Part of it is a low speed acceleration test from stationary and part is a no-load speed test. So it looks like you could be legal with a controller that starts the motor slowly, stops at 25kmph +10% but is much more than 250w in the middle.

 

[flecc]

I agree with d8veh, and like him and member Tiberius I've measured the consumption on some Bafang motors. The continuous figure capability on the most powerful eZee controller setup with the Bafang is easily 750 watts and short term peaks of 1000 watts are evident. Tiberius (Nick) measured over 1000 watts.

It is what can be got away with, of necessity. In the earliest days of e-bikes they actually conformed to the limit of 200 watts in Britain, and they were useless, completely gutless, despite only having to be geared for the assist speed limit of 12 mph at that time. Some were drive-through-gears throttle bikes to make better use of the low power. It was only when, a decade later in the 1990s, that manufacturers started to push limits that e-bikes started to have some more universal use. In some cases though the still lowish powered brush motors used gained as much in speed as in power, the odd drive-through-gears ones capable of 30 mph in favourable circumstances, which was an OTT side effect.

If ever there is a real clampdown (unlikely), the outcome will be lots of Panasonic type systems where the drive-through-gears makes limited power still useful, and hub motor bikes rather like the Powacycle Salisbury/Windsors in performance. Those at around 300 watts are the closest to legality at present, and were even closer when with NiMh batteries. OK on up to 10%, but any real hills, forget it.
.

 

[jbond]

Getting away from legality for a moment (!) a question comes up every so often about what controller current limits are possible and sensible with the QSWX range of Bafang motors. I've seen people say that 36v/20A is OK, and even 48v/20A but that anything over about 15A can produce problems with stuttering on startup with sensorless motors. The two weak points (somewhat related) seem to be heat and the internal gears on the freewheel. Heat problems can probably be avoided by just not lugging the motor up long hills, but even if heat doesn't soften the nylon gears, banging too much power though them, especially on rough tracks may be a problem. There are lots of posts on ES when the first Bafang motors appeared of people breaking gears, but I think they're tougher now and also people are no longer trying to vastly over-run them with power like 72v-30A.