Measuring motor output

Following some recent threads on the forum discussing the likely output of electric flight motors, and one in which I suggested that building a dynamometer for testing this would be easy, I decided to give it a go. Some horrible wet, windy weather helped with the motivation as our flying patch is completely waterlogged, and, sadly, my S6B is still but a dream in the plans box!

A chat with a mate who builds hill climb race cars suggested that a “reaction” dynamometer would be the simplest, although this is limited to measuring the shaft output of motors with a single load – e.g. a prop. This type of dyno will not vary the load and provide a complete torque curve like an engine tuning dyno will. But, for most of us, this is what we want to know – the output power of the motor in a given setup – motor, battery, ESC, prop etc. And the result was the test rig you see here.

All that is needed is a method of mounting the motor so that the mounting is free to rotate in the same axis as the prop shaft. For this, I found an old Irvine 40 diesel crankcase and bolted the motor mount to the crankshaft. I did clean and oil the crank bearings to ensure smooth friction-free movement.

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A 250mm aluminium torque arm is bolted to the motor mount. The motor output torque therefore reacts, through the torque arm, against the digital scales which gives a measurement of actual motor output torque in units of “grams x 250mm”. The only other thing needed is a tacho, since Power = Torque x Speed.

The equipment I used was a digital tacho, digital scales, DVM and clamp type ammeter. The motor in the photo is a Scorpion 3026-10 with a 6S 90A ESC and two 5S 4000mAh 20-30C Li-Pos in parallel.

To test the whole setup I bolted a small 8×6 APC-E prop to the motor and ran this up with 3S 6000mAh 20-30C Li-Pos so as not to generate too much load for the first runs in case the whole thing was unstable.

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The rig worked immediately and the first result was instant:

  • Torque 66g (on the 250mm arm); Speed 10800rpm; 11.4V; 18.0A, giving an input of 205W and an output of 183W. Efficiency was therefore 89%.

    For this setup, Scorpion-calc gives 217W I/P and 166W O/P, with a motor speed of 10567rpm, but these are calculated figures, not measured. [For those who don’t spend half their lives number crunching, all the data have to be in SI units to make the equations work, so that Power (in Watts) = Torque (in Nm) x Speed (in radians/sec). So grams on a 250mm bar has to be converted to Newton metres, and RPM has to be converted to radians per second to get power in Watts. One radian = 2pi or 6.283 x revolutions. Roughly speaking, divide RPM by 9.5 to get Speed in radians/second and divide grams on a 250mm arm by 408 to get Torque in Nm, then multiply both numbers together to get shaft Power in Watts. So, for this first set of data, output = 66g/408 x 10800rpm/9.5 = 183W. Input is simply volts x amps = 11.4×18 = 205W. Efficiency = output/input = 183/205 = 89%]

    The rig showed no signs of instability, so I decided to give it some more juice, and changed the lipos for the 5S packs and the prop for a 9×6 APC-E. The suggested prop for 5S on this motor is a 10×7, but this was the closest I had. This gave the following figures:

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  • 58A: 19.0V: 222g torque: 15900rpm which works out at I/P = 1102W; O/P = 906.5W; Eff = 82%.

    This current is 8A higher than shown in Scorpion-calc, but, again, Scorpion-calc is based on calculated estimates, not measured data. The Eff figure of 82% matched well, though. For a motor of only 190g weight, this is quite an impressive output – over 1.2bhp – equivalent to a good 46 two-stroke.

    One thing I have wanted to know for some time is whether Axi motors are better than Scorpions. I tend to prefer Axis as the build spec’ is clearly higher, but I recently bought a few Scorpions because you get a lot of power for the weight and they’re about half the price of Axis.

    So I dug out my Axi 2826-10, this being the suggested equivalent to the Scorpion 3026-10, and ran them both on the dyno with 3S Li-Pos. Results were:

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  • Scorpion 3026-10 (12×6 APC-E) 8800rpm: 177g torque: 44A: 10.8V.
    O/P = 400W: I/P = 475W: Eff = 84%

  • Axi 2826-10 (Jeti 40A ESC soft timing; 13×6.5 APC-E) 7500rpm: 175g torque: 39A: 10.7V
    O/P = 337W: I/P = 417W: Eff = 80%

    I then repeated these tests, but with the 9×6 APC-E.

  • Scorpion: 10300rpm: 87g torque: 22.5A: 11.4V
    O/P = 230W: I/P = 256W: Eff = 90%

  • Axi: 9300rpm: 72g torque: 16.4A: 11.3V
    O/P = 172W: I/P = 185W: Eff = 92%

    I then repeated the Axi test on hard timing which increased the O/P to 197W, with an efficiency of 90%.

    Allowing that this dyno is probably not 100% accurate (as it relies on four pieces of measuring equipment, each with its own tolerances), it is very consistent. For each repetition of the same set-up, I got virtually identical data.

    The efficiency figures measured for both motors were higher than quoted by the manufacturers, which implies that either there is a consistent error in the measurements somewhere, or the manufacturers are conservative with their quoted specs. Whilst it is easy to confirm the I/P data (volts and amps) using calibrated equipment, and I think the torque figure is pretty accurate as well, since you can calibrate scales easily, I do not know how accurate the tacho is. It is a standard type that everyone seems to have, but if it over-reads it will over state the output, and hence the efficiency. Mine reads 3000 (rpm) dead when pointed at fluorescent lights, which is exactly what you would expect. But maybe it drifts at higher RPMs – it certainly seemed to give higher figures than those predicted by Scorpion-calc. I haven’t worked out how to test this yet.

    So this dynamometer is, at least, a useful tool for comparing motors, ESCs, set-ups, measuring gearbox efficiencies and other system losses. It is very simple and costs almost nothing to make, assuming you have the gear that most aeromodellers seem to have as standard these days. It is also very consistent and produces numbers surprisingly close to those expected. It is particularly useful for evaluating motors at the cheaper end of the scale that don’t seem to come with much performance data. And, perhaps best of all, it provides something interesting to do with electric flight gear when the weather’s too bad for flying! But if you simply want a reasonable estimate of the actual shaft output of a direct drive motor, then multiplying the input power by the manufacturer’s quoted efficiency is probably as good as anything.

    My conclusion regarding the comparison tests is that the Scorpion is certainly more than equal in performance to the Axi, and (arguably) marginally better in terms of efficiency. And if you ran an Axi 2826-10 at 58A on 5S Li-Pos it would probably melt as the max current specified is 42A, whilst it is 68A for the Scorpion, so they are certainly superior in terms of power to weight. But, in my opinion, the mechanics are engineered to a lower spec compared with the Axi.

    I hope this is of some interest. It kept me amused and out of the house for the weekend, at any rate!

    Ed Kershaw.

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