# Re: Measuring the efficiency of a motor ???

"Jim" <jim@xxxxxxxxxxxxx> wrote in message
news:FLouk.100\$Dj1.98@xxxxxxxxxxx
Absolutely. Watts are somewhat meaningless because of all
the of the factors that can vary it widely and depending
on the efficiency and quality of the motor, a lot of the
watts go to heat rather than power. I gave up on watts a
long time ago and use a digital fish scale hooked to the
tail of the airplane and anchored to the ground to get
actual static thrust. I at least know it will fly the
airplane! Much more meaningful information.

Watts of power output is the correct measure that I was
trying to obtain. Watts of power output divided by watts of
power input yields the efficiency of the whole system.
Thrust is not a good measure because the thrust may not
provide enough airspeed. Even tons of thrust enabling 10 MPH
is useless if stall speed is 15 MPH.

"Peter Olcott" <NoSpam@xxxxxxxxxxxxx> wrote in message
news:vz2rk.2010\$Rs1.641@xxxxxxxxxxxxxxx

"The Natural Philosopher" <a@xxx> wrote in message
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Peter Olcott wrote:
The efficiency of a motor is the ratio of the power the
motor produces in Watts divided by the power that it
consumes, also in Watts.
How do I measure the power that a Prop/Motor
combination produces?
With great difficulty.

If at all.

The classic engineering way is a dynamometer, to measure
output torque: that times the rpm with appropriate
constant is the output power.

It tells you nothing about propellor efficiency though,
and must perforce include ESC losses and possibly
battery losses as well as motor losses.

If you want a a really quick and dirty approximation,
though, which for quite subtle reasons works fairly well
in MOST cases, just take the prop off, measure the RPM,
and then put the prop back on. The efficeincy is very
close to the ratio of the on load RPM to the off load
RPM, if the battery doesn't sag too much.

The better approach is to do some fairly tedious
mathematics involving measuring the current drawn under
zero load conditions as well, and that way you can get a
better figure.

That will tend to give you the motor + ESC efficiency if
you have a volt/ammeter (whattmeter) between the battery
and teh ESC.

You have to do more maths to separate out the controller
losses..

ACTUAL power OUT of the motor AND prop is a function of
the amount of air being moved by the prop, and the speed
its accelerated to, and is a fiendish thing to measure.

and almost useless when you do, since it changes
according to the models airspeed.

You best bet is to gear the motor, and use a big coarse
pitch prop. The quieter it is, the more efficient is is.

If you are used to glow engines, forget everything you
thought you knew: very little of it applies with
electrics ;-)

Ah the speed and volume of air moved, good! What about
simply hooking the prop/motor to a scale to measure how
many ounces it pulls? I am guessing that this will only
measure thrust, and not Watts because it does not take
into account the speed of the air.

Maybe my measure above combined with some way to measure
the speed of the air (little paddles turning a wheel
hooked to a gear) would derive a reasonable
approximation?

.

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