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Old Jul 17, 2011, 03:26 AM
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Buzzkill2's Avatar
United States, MI, Grand Blanc
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how to test motor efficiency?

I've been doing it for a while but, apparently I've been doing it wrong.

For those that know, how do you do it?
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Old Jul 17, 2011, 04:00 AM
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yomgui's Avatar
France, RA, Cormaranche-en-Bugey
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Hi !

How do you did it ?

You'll need a tachymeter to know the rpm.

Talking about brushless:

The motor (+esc) is converting electrical power (input power) into mechanical power (output power, power at the shaft).

So for the input power you measure volts and amps (measured between batterie and esc): volts*amps=power (Watts)

The output power is more complicated. You can measure it with a torque meter and a tachymeter, power (W) = torque (N.m) * prop's revolution speed (rad/s),
but this is very difficult.

Or, you can use a software, like drivecalc, in order to see what's the output power for a given prop at given rpm (example: apc 9x6E at 8000 rpm needs 80 Watts at the shaft, more or less!). Drive calc should be quite correct for well known props.

Efficiency is = output power / input power

It is depending upon load (ie amps) and rpm.

guillaume
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Old Jul 17, 2011, 07:35 AM
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Germany, lake of constance
Joined Oct 2010
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Hi Buzzkill2

As guillaume has mentioned, first you have to measure the input power of your motor. You need i.e a watt meter to measure voltage
applied to the motor and the motor current. Input power is U [Volt] * I [Ampere], equal input power Pin [Watts].

Next step is to measure the prop speed [rpm]. You need a prop with known n100 constant.

The n100 constant is the prop speed [rpm] where the prop consumes 100 W input power, shaft power on motor.

P_prop [W] is (measured prop speed / n100 speed)^3 * 100 W.

motor efficiency = P_prop / Pin

But due to variation of n100, accuracy of watt meter and accuracy of speed measurement, the result can easily
variate in a range of +/- 5 %.

Best regards
Micha
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Last edited by snoopy_schulz; Jul 17, 2011 at 07:46 AM. Reason: typo
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Old Jul 17, 2011, 07:39 AM
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eflightray's Avatar
South Wales U.K.
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Motor efficiency.

Quote:
Originally Posted by Buzzkill2 View Post
I've been doing it for a while but, apparently I've been doing it wrong.

For those that know, how do you do it?
First define what you mean by efficiency.

Do you mean basic power in, to shaft output power, on a test bench using a dynamometer ?

Then you may need to consider at what specific voltage, or range of voltages, the timing requirement, the load requirement, perhaps cooling requirement.

Sorry, but efficiency, (not a particularly useful word in my vocabulary), when it comes to model motors in model planes, can mean many things to many people. What about the 'efficiency' of the ESC, and probably the battery as well ?, minimized cable length and suitable gage ?, connector quality ?. There can be many factors that could influence the results.

If it's just a test bench exercise to get accurate data, then you probably need some pretty sophisticated test gear.
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Old Jul 17, 2011, 08:07 AM
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Ken Myers's Avatar
Commerce Township, MI
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The OP wanted to know how to calculate motor efficiency, why, I have no idea as it is not a very useful number, but most respondents have NOT noted how to measure motor efficiency, they mention things like a power meter between the battery and ESC. That does not measure the voltage at the motor, but the voltage at the ESC. A dyno to calculate the power out and some means of calculating the power in at the motor is needed and then the power in at the motor can be used to calculate the motor efficiency.

All things being equal in quality and build, a small mass brushless outrunner is less efficient than a larger (mass) outrunner of the same quality and build. For the same size (mass) outrunners by the same manufacturer, the highest Kv motor in that size (mass) is more efficient than the lowest Kv motor in that size.

What knowing the motor efficiency could possibly be used for, I honestly have no idea as there are many, many more important variables when choosing a power system for a specific task.
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Old Jul 17, 2011, 08:10 AM
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eflightray's Avatar
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I think I read somewhere in a thread, -

'If you put your finger on the motor after a good run and don't scream in pain from the burn, then your motor is running fairly efficient'.

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Old Jul 17, 2011, 09:09 AM
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Germany, lake of constance
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Hi

Shure a watt meter measures the input voltage of the ESC. But with a good ESC selection the internal Ri is low compared to Motor Rm.
So the power loss at WOT within ESC is in most cases negligible. To measure motor temperature
by finger is no means to quantify motor efficiency. It is maybe an indication if motor is overloaded. Measurement of motor efficiency is
only valid for a certain test setup. Aim of measurment of motor efficiency on a test rig is to obtain efficiency data for similar motors, equal test setup.

BTW, the motor efficiency is not the propulsion efficiency. Using a wrong prop wrt to the plane data will kill your
drive efficiency rather than the motor efficiency.

Best regards
Micha
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Old Jul 17, 2011, 09:19 AM
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Quote:
Originally Posted by Ken Myers View Post
The OP wanted to know how to calculate motor efficiency, why, I have no idea as it is not a very useful number, but most respondents have NOT noted how to measure motor efficiency, they mention things like a power meter between the battery and ESC. That does not measure the voltage at the motor, but the voltage at the ESC.
I agree of course, sorry, i always watch for brushless + esc since any complete motor needs a commutation device...

guillaume
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Old Jul 17, 2011, 07:10 PM
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The reason I want this information is so I have sort of concrete data to establish a motors performance capabilities in comparison to other motors at or close to the same specs.

For example I love the latest Hyperion motors, the HP-Zs and HP-Hs motors.

These motors are the best I personally have ever owned. Granted I have not owned every brand out there and have never owned what most would call "the best" because I just can't afford them.

So my quest is when I say a motor is great and it performs better than another one id like to have some proof.

I assumed a motor that had an efficentcy of 80% would be better than a motor that was 75%. Of course im talking about motors that are the same specs or as close as possible.

Is there something else that may establish a motors capabilities other than the efficency?
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Old Jul 18, 2011, 04:26 AM
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eflightray's Avatar
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Quote:
Originally Posted by Buzzkill2 View Post
................................................

Is there something else that may establish a motors capabilities other than the efficency?
How about how well the plane flies ?

Very little is talked about the (I wont use the word 'efficiency') 'choice ?, capability ?, of the props.

A motor test may find that elusive 90% of something on a test bench, but put the wrong, (even slightly wrong), prop on that motor and any testing could become a bit of a waste.
Even going up or down 1" on the pitch and/or diameter, using a different manufacturers prop, could make a noticeable difference when flying.

I often wonder how many people decide on an 'upgrade', and just mean the motor, (usually just more watts capability), but don't consider the best prop for the plane and then try to find a motor suitable to turn it. Just sticking on a prop a motor manufacturer list/suggests in the motor spec, isn't always the best way of using any motor, and especially for any plane.

The prop does all the work to make a plane fly well, the motor is just the thing that needs to turn it at a suitable rpm.
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Old Jul 18, 2011, 05:49 AM
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Quote:
Originally Posted by eflightray View Post
How about how well the plane flies ?

Very little is talked about the (I wont use the word 'efficiency') 'choice ?, capability ?, of the props.

A motor test may find that elusive 90% of something on a test bench, but put the wrong, (even slightly wrong), prop on that motor and any testing could become a bit of a waste.
Even going up or down 1" on the pitch and/or diameter, using a different manufacturers prop, could make a noticeable difference when flying.

I often wonder how many people decide on an 'upgrade', and just mean the motor, (usually just more watts capability), but don't consider the best prop for the plane and then try to find a motor suitable to turn it. Just sticking on a prop a motor manufacturer list/suggests in the motor spec, isn't always the best way of using any motor, and especially for any plane.

The prop does all the work to make a plane fly well, the motor is just the thing that needs to turn it at a suitable rpm.
That's kinda my point and reason for wanting to find out how good a motor is.

The prop, esc, battery and plane remain a constant and only the motor is swappedvout it stands to reason a motor with higher efficency will perform better than a lower one.

Im pretty competent when it comes to picking the right power combination for my planes. Sure I play around occasionaly and offer over power my planes from time to time buy, im trying to differenciate between motor quality and performance in and of themselves not in conjunction with the rest of the power system.

I have to believe that efficency, higher efficency' means better performance. Im pretty sure efficency does not nessecarily mean quality.

If a motor uses the best materials available at least within reason, then I would think design would play a big part.

Let me ask this, which is a better motor Scorpion or Neu? Why is one better than the other? How do we know one is better than the other? Im just using those two brands for the sake or argument. Keeps in mind im not trying to figure out what particular motor is the best for a certain plane/setup, im trying to understand what makes one motor better than another based soley on the motor alone.
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Old Jul 18, 2011, 06:56 AM
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Quote:
Originally Posted by Buzzkill2 View Post
That's kinda my point and reason for wanting to find out how good a motor is.

The prop, esc, battery and plane remain a constant and only the motor is swappedvout it stands to reason a motor with higher efficency will perform better than a lower one.

............................
Sorry, that's were I disagree. I still maintain the prop is the most important part of the plane, (unless you fly gliders ).

How often have you tried a different prop before swapping out a motor to see if you had the right one to start with.

Everyone on these forums, (sorry, nearly everyone it seems to me), changes the motor to go faster.

I wonder how many would have tried to go up one pitch size to see what affect that has to the top end speed ?, but just changed the motor in stead, (hopefully they also check the current draw to see if it is still safe).

How many have would have tried dropping a pitch and perhaps going up one diameter if they wanted better vertical acceleration, but weren't too interested in the final top speed ?, but just changed the motor instead.

Go ahead by all means with you motor test to find good motors, (is cost equal to performance ?), but please don't forget to include your prop recommendations if you intend publishing the results.
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Old Jul 18, 2011, 10:44 AM
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Yes it's true that the prop choice can be very painfull for the overall efficiency, far more than the difference between a "good" and a "bad" motor. Peak efficiency of a prop can vary from less than 30-40% to 75-80%, depending on rpm, diamètre and pitch, etc...
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Old Jul 18, 2011, 11:17 AM
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Moab, Utah, USA
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Having the proper propeller for an airplane and its flight regime has absolutely nothing to do with motor efficiency. Let's say propeller x/y spinning at z RPM is the perfect propeller for an airplane and the propeller has a shaft power of 500 watts. Now a brushed motor spins the prop at z RPM with 55% efficiency and a brushless motor spins the prop at z RPM with 85% efficiency. Since this ideal prop is spinning at the same speed with both motors the airplane performance is the same with both motors. However the brushed motor at 55% efficiency requires 909 watts input power to develop the 500 watts shaft power, whereas the brushless motor at 85% efficiency only requires 588 watts input power to develop othe same 500 watts output power.

Chosing the correct propeller for an airplane is very important. Chosing the most efficient motor to spin that propeller is also important.

Larry
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Old Jul 18, 2011, 12:17 PM
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Germany, lake of constance
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Hi Buzzkill2

In the attachment there is a description how to measure/calculate the max efficiency of a motor in an easy way.

Best regards
Micha
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