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Old Sep 21, 2007, 11:45 AM
Detail Freak
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Harbor City, CA
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Operating over or under Max Efficiencies?

Hi-

I'm relatively new to the E-power calculator program available. I'm wondering what the advantages/disadvantages are of picking two different motor/prop/battery combos for a given airframe, and running one UNDER the motor's rated current M. E. and one OVER the motor's rated M. E. current?

Specifically, I'm looking at Mega 16/15/x verses Mega 16/25/x....

For a little weight penalty with the longer motor, I'd be running likely under the max efficiancy amperage.

With the shorter motors, I'd be running well over the rated max efficiency.

Any advantages/disadvantages?

Here is the airframe in question:

Thanks,
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Old Sep 21, 2007, 12:50 PM
Giz
Tony Rogers
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First of all, keep in mind that the maximum efficiency current varies with applied voltage. It usually (always?) increases with voltage.

Anyway, I think it is better for your full throttle current to be above the maximum efficiency current. Then, when you fly at part throttle, your motor efficiency will get better. If your full throttle current is already below the maximum efficiency current, it will only get worse at part throttle.

Having said that, it does depend on the actual efficiency of the two motors and the shape of the efficiency curves. It would be possible to have a motor operating below the maximum efficiency current that has a higher actual efficiency than a motor operating below the M.E. current.
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Old Sep 21, 2007, 01:05 PM
Detail Freak
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Harbor City, CA
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Quote:
Originally Posted by Giz
Anyway, I think it is better for your full throttle current to be above the maximum efficiency current. Then, when you fly at part throttle, your motor efficiency will get better. If your full throttle current is already below the maximum efficiency current, it will only get worse at part throttle.

Thanks, Giz.

I hadn't even thought of the fact of being over M.E. at full throttle vs. partial throttle. That's a pretty good point.

Having said that, I did notice with the 16/15 series, the M. E. was close to half of the max amperage allowed, where as with the 16/25 series, the M. E. was closer to the max amperage to begin with.
All of these observations were with 3S pack in the calculator.

T
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Last edited by target; Sep 21, 2007 at 01:39 PM.
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Old Sep 21, 2007, 01:22 PM
Giz
Tony Rogers
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Bath, UK
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When you're thinking about efficiencies, you need to think about why efficiency is important.

Motor efficiency affects motor temperature and therefore how much power you can get out of it and it also affects wasted energy. The first one of these is usually the most important and the second almost irrelevant.

A motor will generally get hottest at full power because that is where most power is wasted. A motor that is 80% efficient at 300 watts is wasting 60 watts as heat and giving 240 watts to the prop. At 1/3 throttle, even if the effciency is a lot lower, the heat lost will probably be less. If the efficiency drops to 70% at 1/2 throttle, 30 watts is lost to heat and 70 watts gets to the prop. So, the motor still gets hottest at full throttle. So, part throttle efficiency is probably not worth worrying about from this point of view.

The second point is that lower efficiency motors will require higher throttle settings to fly the same way as a higher efficiency motor. This will have an effect on flight duration but it will probably be tiny. A 70% efficient motor will require 143 watts to output 100 watts. AN 80% efficient motor will require 125 watts to do the same. So, the duration of the less efficient motor will be 87% of the better motor. Not a lot really, especially as the larger motor is carrying more weight which will reauire a higher power setting etc.

The main thing is to pick a motor that will give you the power you need driving the prop you want with the battery of your choice and which does not get too hot.
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Old Sep 21, 2007, 01:50 PM
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Max efficiency figures (often stated as in the 90%'s) are typically at low voltage and at very low current - so they don't mean much in real life since we have to run our motors at well above these levels.

As Giz says - choose something that works well as far as YOUR motor/prop/pack go - if the motor doesn't get too hot then you know you must be at a respectable level of efficiency (not max obviously, but as good as you can hope for, for your purpose - maybe 75% or better).
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Old Sep 21, 2007, 03:53 PM
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Be very careful about all this max efficiency malarkey. Its perfectly possible to have on a highish voltage, a maximum efficeincy point well beyond the heat limits to the motor..or its RPM limits.
\
I did a spreadsheet - serach for MUMTATS - that identifes the intersection of maximum efficieicy with the maximum allowable heat loss, and shows you how to max out a motor.

Having said that, its generally better to run them on a lower voltage than that, otherwise they get pretty warm even at part throttle.

I generally pitch for max efficiency at about my cruise level of power - generally half voltage and half current compared to WOT, and pitch it so that WOT static is just within allowable limits.

That gives me great cruise duration, and plenty of takeoff power.
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Old Sep 21, 2007, 04:17 PM
Detail Freak
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Harbor City, CA
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Roger that, all points duly noted.

I'm surprised that the 16/25/x motors aren't more popular. I guess that is because they are 480, not 400 sized.

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