Eastern Power?

[Simon Rafferty]

I've been experimenting a bit with a number of different motors & controllers of Far Eastern origin - and found that the stated power of both motors & controllers is, well, at best - made up!

I have an Arduino logging voltage, current (both accurately calibrated) & Temperature.

The nominal power of a motor is usually defined as the continuous output (mechanical) power or in simple terms the motors continuous rated power / its efficiency. There are several definitions of continuous rated power - but it's often the power level at which the motor does not exceed 40C in a 25C ambient temperature.

I can't find much data on Efficiency of eBike motors - but what I can says up to 60% at low speed, dropping to 40% at max rpm. Thus, it might be safe to assume the nominal output power is 60% of the input power.

To test, I have:
350w 36V BLDC Wheel Motor.
250W 36v BLDC Wheel Motor.
500W, 36-48V Speed Controller
350W, 36-48V Speed Controller
250W, 36-48V Speed Controller

Powered from a 36.0V, 50A Switch Mode power Supply.

Driving a torque regulated load.

First using the 350W Motor
500W controller - the continuous power (Volts * Amps) into the controller measured 392W at 36.0V, 10.88A.
350W controller - the continuous power 377W
250W controller - the continuous power 280W

Using the 250W Motor
500W controller - the continuous power 390W
350W controller - the continuous power 369W
250W controller - the continuous power 267W
So, pretty close to the 350W results.

Assuming 60% efficiency (I really need to measure this accurately) the peak mechanical power output of the 350W motor is only 235W and the 250W motor, 234W

At max RPM, these figures potentially drop to somewhere around 156W

I imagine the results would be closer with more 'reputable' suppliers.

Although the Police may use the power stamped on the motor as a guide to whether a bike is legal, the regulation states 'nominal' power which MAY be significantly lower than this value. It means there is a possibility of motors of up to 500W actually being legal based on nominal power, regardless of what it says on the can! [But do not take it as black and white that yours is without measurement!]

 

[trex]

I don't think Chinese motor suppliers lie to us.
I always ask them for the motor test reports.
They usually run hundreds of test cycles, with varied combinations of current and RPM, the output is measured with a normal dynometer.
Here is a typical motor chart for the xiongda XD motor:

http://xiongdamotor.en.alibaba.com/product/1557178214-220560510/New_Invention_2_speed_Ebike_Motor_Amazing_Torque.html

 

[Simon Rafferty]

To be fair, both the motors and controllers are of shady origin - and I did say that reputable manufacturers will be closer!

It was not intended to suggest that any (other than mine) are not accurate. Please accept my apology if you feel it has. I'd be happy for the Moderators to delete it if you think appropriate.

Si

 

[trex]

oh, no, I don't want you to feel bad about it. I have dealt in the past with quite a few Chinese and Taiwanese suppliers and only got shafted just once. More often, it's a case of lost in translation, if I forgot for example to specify a bottom bracket and you can guess, what could happen did happen..

 

[flecc]

I think you'll find efficiency is about 75% for hub motors from reputable makers like Suzhou Bafang and Ezee Kinetics. Apart from past statements to that effect, I've reverse calculated about that from actual hill climb ability of a couple of bikes using motor only on precisely graded hill sections with accurate total weight involved and calibrated speed check.
.

 

[anotherkiwi]

And you have Watt meter readouts which gives Vm, Ap and Wp the latter on my Mxus has shown 548 W peak.

 

[Alan Quay]

When ever I've checked my motors/controllers with my watt meter I've always found the current from the battery to be there or there abouts with stated current limit. I've tested with Bafang BPM/CST/SWX's and Q128, with all kinds of controllers.

Perhaps there's something in your test setup that's altering the results? Just a thought.

 

[Simon Rafferty]

I plan to map the efficiency using a DIY Dyno then use a microcontroller to limit the power in to 250W taking in to account the efficiency at that rpm. However, in practice even limiting it to 250W with a simple current limit, the motor produces sufficient torque & speed for my purpose.

I'm guessing (hoping) that the clause about making it impossible for the user to increase the power to exceed 250W only relates to dongles or codes you enter to override the limits - rather than something that is hard coded into the control software, even though it could be changed (by me) by plugging it in to a PC & re-compiling a different version. There will be no 'magic' override built in to the software.

As much as anything, I'm interested to see how good a genuine 250W drive can be in practice.

Si

 

[Simon Rafferty]

Perhaps there's something in your test setup that's altering the results? Just a thought.

It's not impossible - but I tried to do it in as rigorous a manor as I could. Because of the nature of the measurement technologies (potential divider + ADC for voltage, Hall Current probe + ADC for current), the accuracy at values other than those I calibrated could be +/-10% for each. Likewise for temperature measurement.

My 'reference' is a Fluke 79 multimeter, calibrated 11 months ago to +/-0.1mV & +/-0.01mA. The ADC's were only calibrated at one value and I'm assuming linearity over their range - but they are not, hence +/-10% for each.

Thus, the values could easily be +/- 20% potentially.

Si

 

[Deleted member 4366]

The output power depends a lot on RPM. According to the standard EN15194, you have to determine the efficiency curve before testing the motor, then the motor has to be tested in a short speed range corresponding to maximum efficiency. There's no maximum power allowed. The requirement is that the motor must be no more than 250w nominal continuous rating of output power. There's nothing to stop the manufacturer rating a 1000w motor at 250w because the standard for rating is intended to stop over-rating, so any under-rated motor will pass. Basically, the requirement is that the motor mustn't overheat when used at its rated power. That means that a manufacturer could stick a 250w label on a motor designed for 5kw continuous output. It would be tested by running it at 250w and it won't overheat.