BLDC Efficiency at different voltages

[cwah]

Hello there,

I have a little 24V direct drive mini hub motor of 4.7kg:
http://www.conhismotor.com/ProductShow.asp?id=76

I overvolted it to 74V (3 times magnitude) in order to reach 30mph on a 16" bike.

But I realised that the motor is about 20% less efficient than my 26" bike. I did 6,6km at 25-35kph to go to work and used 100wh. Then I came back home and also used 100wh. The voltage of my 74V8AH lipo (zippy 5S 8AH) dropped from 80V to 76V.

For a 74V8AH lipo, I believe that I loose 1V every 50WH, so 4V is 200WH (confirmed by the CA)


I suppose that the motor is overspinning and not anymore on his efficiency band? Similar to a car at first speed at 30mph? But that has not been displayed on the simulator.

So overvolting the motor leads to lower efficiency because it is not on his efficiency band?

 

[Jeremy Harris]

Motors don't have a voltage rating/limit as a rule. Efficiency is primarily a function of current, until you get to motor commutation frequencies where hysteresis losses in the motor core start to dominate.

For hub motors it's almost always current that dictates efficiency, as you're pretty unlikely to get to speeds where hysteresis loss exceeds the I²R loss in the windings.

Some basics:

- Voltage determines motor rpm

- Current determines motor torque

- Power losses are roughly proportional to the square of the current (so double the current = four times the power loss)

In short, increasing voltage for a given power pretty much always IMPROVES efficiency, as the current will be lower. The exception is for non-hub motor set ups (or, perhaps, geared hub motors under some conditions) where the motor rpm increases to the point where core (hysteresis) loss becomes significant. This isn't generally an issue for direct drive hub motors.

 

[cwah]

Thanks jeremy for this very nice explanation. Looks like my motor should have 78% efficiency at 9 amps (the power I'm using) but somehow I don't know why I'm using so much power.

I know I have a slight miscalibration from the CA, but it really drain my battery faster than my other bike, for similar speed. The CA indicates 700W for 22mph (34kph). For a similar speed, my full size bike uses 470W.

Going to check the battery voltage drop from my full size bike tomorrow and see how much difference I have between the 2 bikes

 

[MadRhino]

It is not because your motor is spinning faster that it is less efficient, it is because at higher voltage its efficient speed is faster. Getting to that higher efficient speed, your small motor suffer extra energy loss during acceleration. All the time spent riding less than half its top speed is wasting a lot of watts into heat, and if it takes a long time to accelerate because it is not powerful enough, it is even worse.

 

[John in CR]

You're probably accelerating a bit harder using the higher voltage. Also, the friction of the gears increases linear with rpm, so spinning the motor a lot faster at higher voltage will have greater losses in those gears. Lastly, the tiny motor inside isn't designed for that voltage, so a bit more demand during acceleration and a bit more losses in the gears is making it work harder and quite likely hotter, which increases resistance in the windings and decreases efficiency. Also, so far above design voltage may be kicking you into far greater iron losses due to the much higher rpm than the motor is designed to operate at. I think those motors have the 2 stage plastic gear reduction in them, so the motor is spinning at pretty high rpm at design voltage, so hysteresis losses could easily be the difference.

If it is the little motor with the 2 stage reduction with plastic gears, then it's probably a bad idea to run it at such a high voltage anyway, because the risk of a gear failure is higher despite the lower torque load of the smaller wheel. Geared hubs aren't nearly as flexible in operating voltage as the direct drive motors, because the actual motor inside is already operating in it's optimum range when running at the high end of design voltage, which is probably 36 or 48V.

John

 

[cwah]

Madrhino, I'm uses 700W for 30-35 kph continuous, when the acceleration is over. Seems a bit low for me. During acceleration it's limited to 700W anyway to limit the amount of power used.

John, i have a direct drive motor, so no gear. Efficiency should be higher, that's why I'm surprised the bike isn't performing that well

 

[John in CR]

Sorry, I saw mini and thought of the little geared Cutie hubbies, and just missed that your said direct drive. I get a zero for reading comprehension on that post. That DD looks like one GoldenMotor sells that comes in a 12" built on rim, just a different cover. See the performance graph at 48V here http://www.goldenmotor.com/hubmotors/hubmotor-12cure.pdf. One of our members got one to test and opened it up, and it's really just a kids bike motor, thick laminations in the stator, etc, just not a great motor overall. It shows in the performance graph too, with peak efficiency not quite making it to 80%. Too bad too, because a really well made motor in that size, and wound for a higher Kv would make an ideal mid-drive.

Your biggest problem with efficiency is the slow acceleration. Power during acceleration equaling power at cruise is going to give you terrible efficiency. The current limit is too low for the motor to get past peak power and into the meat of the efficiency curve. What is the no-load speed of the motor? I'm guessing that your cruising speed is well under 70% of no-load speed. If you try it with a higher current limit, it should fair better, but being such a small motor it may require an even smaller wheel at that voltage. Definitely check it for heat, and give it some pedal assist on take-off to reduce the load. If I'm right, then you should see more speed too, but if you don't then be even more careful about the heat, because it may simply be unable to generate enough power to overcome the wind load at 30mph.

I could be wrong and it's a better quality motor than the Golden, and if so then a boost in current should net you better overall efficiency because it will accelerate quicker and spend less time at low efficiency. 700W at 30mph is a great number, and the 15wh/km at 25-35kph isn't too bad unless you're adding significant pedal assist. If you're after max efficiency, then a lower voltage is warranted, so your cruising speed is WOT. Also, add some aero treatments to your ebike.

Look on the bright side, my primary ebike would need over 300wh each way for your commute. I would be an exhilarating commute though, because my bike is one of the best amusement park rides there is, and almost every time i climb aboard I try to use a much electricity as possible to get where I'm going. That includes both trying for ever high wh/mile numbers as well as going the long way to get there just so I can ride for longer.

John

PS- If you ever get a chance, pop that puppy open and give us some close up pics of the stator. Don't forget to put some alignment marks on the covers and magnet backing ring first, so you get them back on exactly how it came apart. Doing so could also be part of an efficiency improvement strategy by installing larger gauge wiring, so you lose less power in the wires.

 

[neptronix]

9 amps x 72v = 648 watts... you are starving it for amps! i bet it wants more like 30a.

If that's the case, then you are not getting the best efficiency because you're not hitting the 80% loaded figure, which is where most electric motors are at their highest efficiency.

 

[cwah]

Guys, I was wondering about that but actually the peak efficiency if this motor is around 7 amps (or lower). You can check on the performance curve chart (for both conhismotor and goldenmotor).

To reach 30mph I use about 1400W continuous.
At 1000W continuous I'm at 25mph
At 700W continuous I'm at 22 mph

I do slow acceleration to preserve the wh used lol.


I think I'm probably below 70% unless i reduce the voltage. Never thought I would need that much power at 30mph (or even at 25mph)

For now I prefer not to open my motor as it's running without problem and I'm not good at mecanic. But my wires are all quite thin and about 2m long (as my controller is on the handlebar bag), I however think that it shouldn't be the main reason of this low efficiency

 

[John in CR]

What's the no-load speed, and no-load current at that speed?

 

[cwah]

No sure about the no load speed. On conhismotor website it's written 227 RPM at 24V. I still have to see the speed with a speedometer. Going to install one this weekend and see the result

 

[cwah]

Just checked the no load speed at 77V: 60kph at 1250mm wheel circumference. I use 0,7A and 50W. Is it good?

 

[rscamp]

FWIW, another thing higher voltage does is move the maximum efficiency of the motor to a higher output power level. You can use the motor equations for a DC motor to see this...

 

[cwah]

I'm have the feeling now that I'm using a lot of power at 78V. I draw around 2000W on peak and probably 1500W to sustain 25mph (for few second as I have a lot of stop and start in london)