Brushless vs Brushed motors ... % efficiency gain?

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Brushless vs Brushed motors ... % efficiency gain?

Postby aesmith » Fri Aug 20, 2010 7:27 am

Hi,

The greater efficiency of brushless motors seems to be well established. What I was wondering was whether in the real world this translates into less electrical power needed, and therefore smaller batteries, and if so then could any estimate how much could be saved?

To give a concrete example, assume I am changing from a brushed motor driven by 1100mAh 9.6v NiMH pack. I'm going to change to brushless power with a Lipo battery while I'm at it, but I'm doing so mainly to reduce weight and not to increase power. Given the assumed greater efficiency of this setup, what sort of size pack would be needed to get the same sort of duration at the same power as the old brushed setup?

Thanks in advance, Tony S
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Re: Brushless vs Brushed motors ... % efficiency gain?

Postby RCModelReviews » Fri Aug 20, 2010 8:08 am

I guess that depends on the relative efficiencies of the motors concerned. If you had accurate efficiency data for each then simple mathematics could be used to calculate the required battery capacity to give identical performance on both motors.

One other consideration is that NiMH packs tend to have a higher internal resistance which means that you can't actually realize as much of the advertised capacity as you can with a lipo. So, switching from NiMH and a brushed setup to a lipo and brushless one will see significant improvements in overall efficiency.
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Re: Brushless vs Brushed motors ... % efficiency gain?

Postby jomarx » Fri Aug 20, 2010 8:53 am

Here's another example:

On a typical Lama V4 with stock brushed motor run time was around 5mins on a 800mah 7.4v lipo.
On a dual brushless Lama, run time increased to 13min average.
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Re: Brushless vs Brushed motors ... % efficiency gain?

Postby aesmith » Fri Aug 20, 2010 4:14 pm

Cheers, that seems a huge difference - from 5 minutes to 14 minutes. Just as a wild guess I was assuming that a 1000mAh 3S Lipo would give at least as much energy as 1100mA 9.6V NiMH. Does that sound right? I'm leaning towards getting the motor and ESC sorted out, see how it flies with the NiMH pack and then decide how big a Lipo to go for. I want to keep the plane light, but there might be balance issues since the existing motor and battery are all in front of the CofG.
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Re: Brushless vs Brushed motors ... % efficiency gain?

Postby RCModelReviews » Sat Aug 21, 2010 12:25 am

Yes, the problem with an NiMH pack is that as you draw increasingly large amounts of current from it, more of the energy goes into heating the battery. There's still a degree of battery-heating with a lipo but it's *much* lower so more of the energy gets to the motor.

Also, in terms of total energy a 1100mAH 9.6V battery has a theoretical energy capacity of 9.6x1.1 = 8.7watt/hours

A 11.1V (3S) 1000mAH lipo has 11.1x1.0 = 11.1watt/hours.

Clearly, the lipo wins by a margin of 27%.
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Re: Brushless vs Brushed motors ... % efficiency gain?

Postby ergocentric » Tue Aug 24, 2010 3:07 am

The LiPo also wins in weight.

The energy density is higher, making the battery lighter, the Heli is lighter so it can fly using less power, therefore more time.
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Re: Brushless vs Brushed motors ... % efficiency gain?

Postby aesmith » Fri Aug 27, 2010 7:40 am

ergocentric wrote:The energy density is higher, making the battery lighter, the Heli is lighter so it can fly using less power, therefore more time.


Very true. If I go for similar power and duration, it looks like I'll reduce the all up weight by nearly 20%. Is power usage proportional to weight? Or is it a square root or higher order relationship?
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