motor/controller efficiency at 24/36v 50% 90% Duty cycle

I have an option to enter a motor race class, and they have different current limits at different voltage.

I can only use this one motor that is rated to peak power 24v 600w and 36v 1400w.

In this power limiting classi have to limit to either 24v 10a i.e 240w battery watts. or either 36v 7a i.e 252w. i.e 12w more at 36v.

the issue is the motor is way over powered so limiting down from 36v 1400w down to 252 battery current limit the duty cycle may be only like 50% ??

whereas at 24v I would get a lot closer maybe not 100 but maybe 90% duty cycle to get the power down from 600w to 250w.

now the question is what is the difference in losses ? and is going to 36v worth the free extra 12w ? or will I loose more than 24v going from say 90% duty cycle at 24v vs 50% dity cycle at 36v ?

note efficiency is very important for this so I need the most efficient setup, will be using an FOC controller or trapezoidal controller

Am I understanding correctly that it's the same motor, but it has different power ratings at different voltages? That seems odd, but OK.

The thing to remember is that for the same motor, at the same torque and RPM, the current and voltage the controller supplies to the motor are exactly the same regardless of battery voltage. Likewise, the losses in the motor will be exactly the same.

However, the higher battery voltage will allow a higher top speed, and you're allowed to use 5% more power at the higher battery voltage, so this seems like a no-brainer to me.

Am I understanding correctly that it's the same motor, but it has different power ratings at different voltages? That seems odd, but OK.

The thing to remember is that for the same motor, at the same torque and RPM, the current and voltage the controller supplies to the motor are exactly the same regardless of battery voltage. Likewise, the losses in the motor will be exactly the same.

However, the higher battery voltage will allow a higher top speed, and you're allowed to use 5% more power at the higher battery voltage, so this seems like a no-brainer to me.

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Hi cyborg yes the motor has different power ratings at different voltages.

Yes agreed for the motor their is no different in the losses, what i am wondering about is what is the difference in the losses in the controller ? and if they are more than this free 5% extra power? i.e since the controller maybe running 50% duty cycle vs 90% to maintain this lower power limit ?

Alex07 said:

what i am wondering about is what is the difference in the losses in the controller ? and if they are more than this free 5% extra power? i.e since the controller maybe running 50% duty cycle vs 90% to maintain this lower power limit ?

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Controller losses are negligible compared to motor losses in any case, but duty cycle doesn't matter. PWM frequency has some effect due to switching losses, but duty cycle doesn't change the frequency.

Controller losses are negligible compared to motor losses in any case, but duty cycle doesn't matter. PWM frequency has some effect due to switching losses, but duty cycle doesn't change the frequency.

Click to expand...

Oh cool so if a controller is never running at 100% duty cycle and only at 50% it wont make the controller losses too high? so in that case i might take the free 12watts and run a 36v system thanks for the advice !!

I am in the final process of putting a electric motor that weighs a bit under 20 kilos into an ATV quad bike. Among other reasons, because its kept at a place where I don't live I have managed to put off finishing it for over a year.
The thing that struck me is that I am seeing a lot of small ebike mid-drive motors that claim they are 3000W and are around 1/10th the weight etcetera.

And that got me wondering about it all. While I am not an expert in electric motor design surely it would be more accurate to measure a motors performance similar to how combustion motors measure there ultimate performance in CC/cubic centimeters displacement size.
https://en.wikipedia.org/wiki/Engine_displacement
And for those who think there's a counter argument -> https://www.reddit.com/r/cars/comments/4f3hpc/what_are_your_thoughts_on_the_phrase_no/d25rhok/

I was thinking about that final episode of Braking Bad where that guy says the line "There is no replacement for displacement"

https://youtu.be/pHb4KH08bZs

Anyway I was thinking maybe thats technically true for electric motors as well as in the displacement would be the size of the copper winding and neodymium magnets.

So the line of the future will be "There is no replacement for copper winding displacement"
And it could be measured by by placing the winding in water and seeing how much water is displaced to make the comparison.
I came up with the meme for it..




What do you think?