# Factors that determine motor efficiency



## major (Apr 4, 2008)

Spence said:


> Does the RPM of you electric motor affect efficiency? If so how?


Increases friction, windage and core loss (hysteresis and eddy current).



> How does Amps affect efficiency?


Increases I²R.



> How does Volts affect efficiency?


In your typical EV motor, no loss directly associated with voltage comes to mind


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## Yabert (Feb 7, 2010)

Hi

I think it's the good thread to ask.
Wich factors that determine motor efficiency between smaller and bigger motors of the same type under high load?
Or another manner to ask...

Like you can see on my build thread, I'm interested to know if my small Impulse 9 will suffer directly of inefficiency by his smaller size at high load (100-120 Kw at motor terminals) compared at my last 11'' motor.



Yabert said:


> Compare to my last 11" motor (the green one), the Impulse 9 have only:
> 
> -75% of the copper area on the armature bars.
> -90% of the brush area.
> ...


I think all depend of efficiency and it's why I doubt than the impulse 9 will produce same power under high load compared to my 11'' motor. What do you think?

Both motors are similarly build (49 bars, 4 pole, 8 brush, etc...)

Thanks


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## major (Apr 4, 2008)

Yabert said:


> Hi
> 
> I think it's the good thread to ask.
> Wich factors that determine motor efficiency between smaller and bigger motors of the same type under high load?
> ...


Hi Yab,

I think you guessed the answer yourself. At the same load (same V and I applied at motor terminals), the larger motor is bound to be more efficient given what you told me and assuming similar quality materials and sound motor design principles were applied to each 

At the same current, the larger motor has greater cross sectional area on the conductors but somewhat longer length conductors. The net will undoubtedly be less resistance and therefore less I²R loss.

Assuming both motors run at the same flux density would mean that the core loss per unit volume is the same for a given frequency. The smaller motor has less core but increased frequency most likely causing the smaller motor to have greater core loss.

Larger brush cross section in the bigger motor means lower brush voltage drop. But maybe greater force on the springs and larger comm radius, so greater friction. Larger core would mean more rotor windage loss for the bigger motor but the higher speed of the small guy will more than offset that for higher loss there. But higher windage due to higher RPM will cool better and temperature will increase resistivity of copper windings.

So throw all that into the mix and I think the big guy wins the efficiency war by a fair margin.....my SWAG....maybe 5 % points higher at 100-120 kW range. Back down at rated load (1 hour or S60), maybe just a percent or two. And at lighter than rated loads, the smaller motor might just be the same or slightly better.

Regards,

major


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## Spence (Nov 18, 2011)

So the ME0709 according to this chart the motor has maximum efficiency of 87% at 60-105 Amps at 24-48V (CW)?

http://www.evdrives.com/Manuals/Mars_Electric/ME0709 Performane Data.pdf

What controller should I use with this if I am going to only go up to 48V and 300A max? I want to have max efficiency when using this motor normally.


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## major (Apr 4, 2008)

Spence said:


> So the ME0709 according to this chart the motor has maximum efficiency of 87% at 60-105 Amps at 24-48V (CW)?
> 
> http://www.evdrives.com/Manuals/Mars_Electric/ME0709 Performane Data.pdf
> 
> What controller should I use with this if I am going to only go up to 48V and 300A max? I want to have max efficiency when using this motor normally.


Hi Spen,

I don't know what you're building and what your goals are. The DIY method is great in my opinion because you get to choose those things. Do what you want and what you perceive as important.

But I look at those charts and see eighty plus percent efficiency over 85% of the load range and say that's cool. Only at light loads does it fall and there you have little power so percentage of less is little loss. I think you put too much credence in a "peak" number. That number likely came off a single dynamometer test in some lab with an engineer tweaking it years ago and may or may not be representative of a production product you buy today. Every process has tolerance, especially motor assembly manufacturing. Specific performance would likely be +/-5%, maybe greater.

High efficiency is great. But it can be counterproductive when you focus on a single attribute. Consider the system. Where to get the most bang for your buck. How could you expect to run that motor at the exact load point where the chart shows 87% without adversely compromising the system as a whole? I don't see it. But like I said, it's your project 

Regards,

major


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## Spence (Nov 18, 2011)

Thanks Major, This is what I was thinking of building:

a custom built motor cycle (frame) or donor cycle that would weigh no more than 500 lbs (with me on it) that can go 0-60 mph in 4 secs (just cause  ) 

In a perfect situation I would need no more than 20394.62W of energy to do a 0-60 in 4 secs but there is RR and Cd and this motor can only do 21600W of energy in a perfect situation as well and it is probably more like 19000W because of efficiency,
but can I overcome this using a gear ratio less than 1? 

Now the efficiency part would be for normal riding where acceleration would not matter, I would limit the amount of watts my motor would put out to 7560-9000W (72v 105-120A)to give the maximum efficiency and acceleration time of 0-60 in about 10-12 secs, my gear ratio would limit the bike to 60 mph; that calculation would be based off the motor rpm and wheel size.


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## Yabert (Feb 7, 2010)

major said:


> I think you guessed the answer yourself. At the same load (same V and I applied at motor terminals), the larger motor is bound to be more efficient given what you told me and assuming similar quality materials and sound motor design principles were applied to each


Thanks Major for the great explanation!
It's sad....5% = roughly 5000w more heat inside the motor and around 5 hp less to the motor shaft.

I liked my green beast of 200 lbs ...


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