Measuring motor output

Tag : Electric Motor Bearing

Allowing that this dyno is probably not 100% accurate (as it relieson four pieces of measuring equipment, each with its owntolerances), it is very consistent. For each repetition of the sameset-up, I got virtually identical data.
The efficiency figures measured for both motors were higher thanquoted by the manufacturers, which implies that either there is aconsistent error in the measurements somewhere, or themanufacturers are conservative with their quoted specs. Whilst itis easy to confirm the I/P data (volts and amps) using calibratedequipment, and I think the torque figure is pretty accurate aswell, since you can calibrate scales easily, I do not know howaccurate the tacho is. It is a standard type that everyone seems tohave, but if it over-reads it will over state the output, and hencethe efficiency. Mine reads 3000 (rpm) dead when pointed atfluorescent lights, which is exactly what you would expect. Butmaybe it drifts at higher RPMs - it certainly seemed to give higherfigures than those predicted by Scorpion-calc. I haven't worked outhow to test this yet.
So this dynamometer is, at least, a useful tool for comparingmotors, ESCs, set-ups, measuring gearbox efficiencies and othersystem losses. It is very simple and costs almost nothing to make,assuming you have the gear that most aeromodellers seem to have asstandard these days. It is also very consistent and producesnumbers surprisingly close to those expected. It is particularlyuseful for evaluating motors at the cheaper end of the scale thatdon't seem to come with much performance data. And, perhaps best ofall, it provides something interesting to do with electric flightgear when the weather's too bad for flying! But if you simply wanta reasonable estimate of the actual shaft output of a direct drivemotor, then multiplying the input power by the manufacturer'squoted efficiency is probably as good as anything.
My conclusion regarding the comparison tests is that the Scorpionis certainly more than equal in performance to the Axi, and(arguably) marginally better in terms of efficiency. And if you ranan Axi 2826-10 at 58A on 5S Li-Pos it would probably melt as themax current specified is 42A, whilst it is 68A for the Scorpion, sothey are certainly superior in terms of power to weight. But, in myopinion, the mechanics are engineered to a lower spec compared withthe Axi.
I hope this is of some interest. It kept me amused and out of thehouse for the weekend, at any rate!