Originally Posted by
gigaplex
Yes it's possible to calibrate it, but you need a way to measure it in the first place to build that model. It's possible to build a dyno out of a slave motor but the math is a lot more complicated than using a flywheel. When I asked what they were trying to achieve, I was wondering if they were trying to make a dyno or just a loaded analyser.
If you are trying to get actual dyno numbers out of the test motor, then yes it would be complicated. But if you are just trying to get relative numbers it isn't that difficult. The good thing about the slave motor is the timing it is set at doesn't make a hill of beans difference to the back emf, or voltage it generates at the phases. When we hook the resistors up to the slave motor in this fashion, the voltage will drop to 0 over the resistors giving us the ability to calculate the power dissipated by the resistors. You now have relative power to work with. But the end user doesn't really need this info, because it is all based on the rpm of the system. The rpm is what matters here. You can then plot the rpm at different voltage steps (throttle input). You can then compare this to different loads, or no load.
As to the load choice itself, my initial thought was to build my own analyser with built in motor controller. Well, that is a challenge, (one that I want achieve, but it will take a while). So I had to work with something that is out there in the wild. And I had a Motolyser so why not use that. The Motolyser is capable of up to 20a on the phases, which will translate to a lower number from the battery. So I needed to use a load that would stay under the current cutoff for the Motolyser.
I recommend the 1 ohm 100 watt resistors for higher turn spec motors like the 21.5 or 25.5. I recommend the 2 ohm resistors for 13.5 or 17.5. Also the choice of a slave motor makes a difference. Higher turn motors are better inductors, which also have a lower kv and will generate a higher voltage on the phases than lower turn motors. I actually measured the voltage on the phase of the slave using a USGT being ran by a 13.5. It was close to 13v! This makes the 1 ohm 100 watt resistor not a good choice here since you are dissipating nearly 170 watts. Short bursts are fine, but the resistors will get rather warm. I had an "oh duh" moment when I was first testing this and the resistors got up to 85* C.
When evaluating the above, you come to the conclusion that you can't make one load work for all motors. So I decided on a couple of values that would produce a noticeable load but would stay within the operating parameters of the controllers.