Determining how adding motors raise efficiency ?

[Vanarian]

Hi guys,

The title says it! How do we determine the way power efficiency is affected by multiple motors?

For a given Kv, maximum RPM no-load rpm won't change no matter how many motors are running. But since the weighted RPM is directly relevant to efficiency, this means that adding motors reduces strain on each motor as long as power fed is multiplied by x motor number.

So by adding motors and power, you raise the max weighted RPM closer to the no-load max RPM limit.

Have any of you already tried to measure it? This could be useful when used with calculation software (you know, Kv / voltage / gearing / wheel size ==> max speed).

 

[WeeChumlee]

"So by adding motors and power, you raise the max weighted RPM closer to the no-load max RPM limit. "
Sure that is true but has little to do with efficiency which is the ratio of work done to to energy supplied.
Are you trying to get the closest possible "real life" speed with regards to voltage and KV ratings or do you wish to know the amount of energy supplied by the battery to go a specific distance at a specific speed? (With different numbers of motors)

I am about to rebuild my dual with new 6355 motors and am interested in the efficiency question.
What I will do, if I get round to it, is go a measured distance on both motors and see how much charge goes into the battery.
Then do the same distance with only one motor - remove the belt on the second. This will give a close enough idea but will be far from exact due to many reasons.

 

[Vanarian]

You're right, I'm trying to get the "closest possible real life speed" and, to be honest, I'm also interested in the second topic which is : do you use "less" current when you move 2 motors instead of one ?

That's true efficiency is not the best term but I was refering to the difference between no-load speed and real life speed.

 

[WeeChumlee]

The end speed on a dual will not be any faster than a single in a straight line on a flat. (Unless so loaded that the single motor is struggling and cannot reach end velocity)
What will obviously change is the acceleration and speed up hills - purely due to the extra power.
I popped a belt on my dual and did not even realise it until I got home.
I use a Excel calculator I downloaded from this forum and use a "load factor" of 90%.
Speed is pretty much bang on as far as I can tell. (If you are using low gear ratios or high voltage then I cannot say as I just cannot go that fast - scares the brown smelly stuff out of me)

 

[CoolRextreme]

Thanks for posting this question Vanarian. I have been wondering about this exact topic myself.
Might it also improve a tad bit of motor efficiency, as they (hopefully) would not be getting as hot?

Also I had another question, quite similar to this one.

Would dual 500 KV motors give around the same torque of a single 250kv motor (geared the same as the dual 500kv), but with twice the top speed of said 250? Or do I have KV all wrong in terms of torque power?....

 

[melodious]

I don't have answers, I'll put that out.

The only reasons why Eboard folks who need to know #motors & efficiency, are racers who need the edge to keep up with the lead pack. :lol:
Or
Folks who under specced their system to begin with and are worried about catastrophic heat failure.

In my case, I use two motors because I have hills to climb. 2x 50mm cans to be exact. I've a feeling that I could "get away" with a single 60mm can. I could really get going with 2x 60mm cans!

 

[Vanarian]

Haha you're right, personally I am a speed freak, going slowly is boring!

On topic, the only clues I got about efficiency were about hub motor... One single tiny hub motor as 1:1 weighted with human load may have very low efficiency of 33%. This would explain why okp was unsatisfied with the 1WD Carvon kit. Double it and you supposedly raise it around 55% and you're also doubling power. This also explain why people seem so happy with the 2WD Carvon? I guess 4WD brings it around 80%?

That would explain why Guinness record was made with a AWD board.

Maybe by refining motors like Hummina, can the losses be diminished and efficiency raised even more.

 

[liveforphysics]

If copper loss is dominant, added motors improves efficiency.

If core losses are dominant, adding motors decreases efficiency.