Motor Efficiency
I recently saw a post regarding propeller quality and I figured I would contribute my motor-selection knowledge as a nice complimentary post.
This is my first post, so a quick intro. I'm an electrical design engineer for BLDC drives, known in the RC community as ESC's. Most of our applications involve fans and pumps, so there is a lot of similarity in what I do in my day job and what you all are doing here. I found DIY Drones a few months ago and I now catch it nearly every day looking for something interesting. I always find something :). I will be doing my first UAV soon with the standard (for many of you) Bixler drone. It is on its way :).
Now, to my topic. Above, I have a very typical series of curves (credit to these guys for the chart). These curves are generated by applying a constant input voltage to the motor and ramping the torque from 0 oz-in to locked-rotor.
There are two ways to choose a motor. For performance, you would want to look at the power vs. torque (dotted red). In this case, you would want to choose a prop that places about 0.325 oz-in of torque in order to maximize the power output of the motor.
The second way to choose a motor, which applies more to drones, is to look at the efficiency curve and to select a motor/prop combination that puts you just to the right of the peak efficiency point. In this case, a load of about 0.1 oz-in would be ideal. If you select a point to the left of the peak, even light variations in load will cause vast efficiency swings (notice the steep slope) and if you select a point too far to the right, then you will be losing power continually with little recourse.
One more thing. If the voltage (PWM duty cycle) is reduced, these curves also scale to the left. The scaling isn't perfectly linear, but it is close enough for estimation purposes. Also, your propeller draws torque in a very non-linear fashion (vs. speed). It is *very* likely that your max efficiency point with a particular motor/prop combination is not at maximum throttle, but at some other point. So you might need to do some testing on your current setup to find out where you are and go from there.
It is likely that you don't have a dynamometer (test equipment that records torque and speed by applying a load to a running motor). I certainly don't, but I do know that there are ways to make a DIY version. Most motors on the market have a torque/speed curve like the one above, or maybe enough data that you can generate a rough estimate and draw a graph using Excel or LibreOffice. Basically, torque/speed curves are attainable from the manufacturer.
The prop is a different story. Each prop does have a typical torque applied to the motor vs. the prop speed... but I can't seem to find one. This might be b/c a prop behaves differently at standstill and while moving, but it would still be nice to have a starting point. The ideal way to get this would be to get a torque transducer and simply run the prop through its speed range. Unfortunately, that is expensive. This is where our motor torque/speed curve comes in again. Note that one of the curves is current vs. torque (blue). You should be able to measure the DC current draw of the motor and get an idea of the actual torque being applied to the motor shaft. It would be best to take this measurement in-flight, but a static thrust shouldn't be too far off the mark. Remember to keep the voltage steady at the voltage at which the curve was taken. These curves are only valid at one particular voltage!
Now that you have the torque, go vertically on the chart to the motor efficiency to see where you are on the efficiency curve. At 100% throttle, the ideal point is just to the right of the peak efficiency for maximum endurance or the peak power point for a performance or aerobatic design. Or you might decide that you want to run at some other operating point for most of your flight, in which case, you would want to choose a motor/prop combination that puts you in that sweet spot. I would stress that you want to be just to the right of the peak. To the right of the peak, efficiency drops off slowly, so a small error won't cause a big loss. To the left of the peak it drops off very steeply. Manufacturing variations, measurement errors, etc. will likely add uncertainty to your data, so be sure to stay in the safe zone rather than on the edge of the cliff.
There are loads more to this. People have written PhD's on several of the topics touched here and my knowledge only scratches the surface, however, this should help you make a more informed motor/prop selection.
Good luck,
J
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Comment by Martin Szymanski on January 11, 2012 at 10:46pm
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I wish I had the words to explain as you have.
In dealing with water pump curves they seem to be most efficient at 80%.
A vacuum cleaner motor goes faster without a flow around it. A person hears this if a hand covers the hose.
There are certain fluid powers that work best at a given rate with resistance.
Some industrial motors have a service factor of +25% and some pumps depending on slippage and flow resistance try to grab that + 25%. In certian applications placing a resistance to flow lowers the amperage to where the motor runs efficient.
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Comment by Toby Mills on January 11, 2012 at 11:36pm -
This is really useful!!!
It would make a great demo video.
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Distributor Comment by UnmannedTechShop.co.uk on January 12, 2012 at 2:48am -
Thanks that is a very useful bit of information about prop/motor selections. Thanks!
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Comment by Sami Finnila on January 12, 2012 at 3:21am -
http://www.drivecalc.de/ <-- A really good tool for estimating your efficiency/power etc. It has a really large database of hobby motors, propellers, escs and batteries.. Makes matching your motor/prop/battery combo quite a bit easier.
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Comment by Peter Seddon on January 12, 2012 at 5:27am -
Try http://www.motocalc.com/ as well. Huge range of motors and props are built into the database and it also offers comparative plots for a powertrain and estimates of flight performance for a given airframe.
Peter
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Comment by Jason on January 12, 2012 at 5:43am -
I'm so glad that I posted! drivecalc is *perfect*. I went to the motocalc website and it appears that their product is great as well, but I didn't want the 30-day free trial and all of that.
Something that I should have emphasized is that motor designers can't seem to design for peak efficiency. They are always designing for peak power or peak torque. Often, this is known as power density (most power in the smallest volume). When you are choosing a motor for peak efficiency, you will often have a larger motor than you might normally expect for a given load.
Get the DriveCalc software suggested above and play around with it. Thanks for posting!
J
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Comment by Rob Nelson on January 12, 2012 at 8:27am
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Thanks for the quick tutorial Jason. With all the prop/motor/airframe combinations available, your input on motor selection is helpful.
R
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Developer Comment by R_Lefebvre on January 12, 2012 at 8:36am -
Yes, the goal of motor design is different between vehicles and industrial applications. In industry, efficiency and longevity are key design parameters. That's why a 150hp AC motor weights several thousand pounds. In a vehicle, the motor weight plays a big role in the overall system efficiency. It's not just the energy efficiency of the motor that matters, but you also have to consider the weight of the motor that is being lugged around. Thus, they design for weight and power density. A 150hp motor for an electric car only weighs a few hundred pounds.
I think it may be true that a given propeller will be more efficient (that is, turning shaft power into thrust) at lower RPM's, with allowance for a drop off in efficiency at very low speeds. Unfortunately I think that means that propeller is more efficient when the motor is less efficient. So there will be a sweet spot in the middle somewhere.
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Developer Comment by R_Lefebvre on January 12, 2012 at 8:39am -
Oh, and I'm curious, are you an designer of industrial BLDC drives? Or variable speed AC drives? I work with AC drives a lot (mostly SSD/Eurotherm) and I'm always curious to see how an AC motor compares with a BLDC hobby motor. The only real difference I can see is a BLDC hobby motor has permanent magnets, whereas my industrial AC motors have a wound stator and rotor. I haven't actually been aware anybody making industrial BLDC motors (with permanent magnets), is that what you're doing? And then all of this gets confused when you bring "servo drives" into the discussion as I am not really sure what the difference between them and variable speed AC really is.
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Comment by Jason on January 12, 2012 at 11:42am
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In industry, they say that I design 'fractional horsepower drives for permanent-magnet synchronous motors'. These aren't for industrial use. They usually go on specialized equipment and vehicles. There are larger industrial motors that are very similar to hobby BLDC motors. They are permanent-magnet synchronous motors (PMSM). You may be more familiar with these. The difference between BLDC and PMSM is the shape of the back-driven phase voltage. In BLDC, each phase looks 'square', whereas in PMSM, each phase is sinusoidal.
AC motors have much less power density because there is a current on the rotor and on the stator. Current causes heat and since there is no direct path for the heat on the rotor to get out of the motor, the AC motor must be thermally de-rated. A BLDC/PMSM motor has a permanent magnet that generates the magnetic field required. There is no heat generated within the rotor, therefore, the only heat source is the stator. Since the stator is much more accessible (thermally) in most motors, the power that you can get out of a similarly sized physical motor is much greater than an AC motor.
There are also finer, less complex drive methods required to run a BLDC/PMSM motor. You can apply a square wave and the motor will run just fine (though there will be a mild efficiency hit). This is why you can buy a cheap drive for RC motors but not for AC motors. AC drives are just more complex.
The reason you probably don't know what 'servo motors' really are is b/c the usage of that term is not consistent. With sensors, it is possible to run a BLDC motor at very low speed, which is useful for loads such as servos where the motor isn't actually moving, but it is applying some torque in order to hold a particular rotational position... just like a servo. One of the groups that I work with designs lightweight servos with very high power density for military aircraft and they prefer to put a gear head on a standard BLDC/PMSM motor because they can operate in a 'zero-speed' condition.
J
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