Just putting an unknown load on it at a steady state RPM isn't that much better than running unloaded. How do you know what resistor value is appropriate? Why would you use less load to simulate the straight? That's when friction and aerodynamic drag is at its peak. Adding more load to the slave and seeing where RPM tops out doesn't actually tell you how quickly it accelerates.

An analyser has a lot of limitations due to being an unloaded test. But just adding an arbitrary load to the analyser and checking final RPM has most of the same limitations.

If you could calibrate the driven motor with several loads it is possible to get real numbers. Also if you slowly raise up the revs you can still make a dyno like power graph and if you take such a graph from both sides and put them over each other. There is still a lot of info created to work with.

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 can learn such a setup and translate it to the perfect setting it will be better than setting a motor to the magic 6A on the tester.

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.

Careful with that assumption. Lowering the voltage will not be the same as maintaining the voltage at a reduced duty cycle. It won't simulate reduced throttle like you'll expect. Also, steady state RPM at 1/4 throttle for example doesn't really tell you much about how well it'll accelerate so I'm not sure what conclusions you'll hope to gain from such readings.

Are you being intentionally obtuse? You keep misrepresenting what the use of this is for. Of course you can’t measure acceleration. That is the change in velocity with respect to time. Nowhere in this system are we measuring time. Again, this is a relatively low cost unit designed to put a load on a motor to test.

If you want a Dyno there is one available for about $1000, and you will still need an esc and a computer to run the software. Oh wait, you have issues with that too…

Now if you have something constructive to say, then I welcome your input. But if you are just going to be a naysayer, then don’t bother.

Did you miss the part where I was warning you that lowering the input voltage is not the same thing as changing the duty cycle? That's information you may need to get relevant data.

Where the hell did that come from? Why do you think I'd have a problem with an ESC and a computer?

Here is a quick run I did with a Slot Machine 21.5.



The spread sheet is available from R1 site.
You can see the RPM plot in blue. Look at how the rpm stops going up as fast when timing goes up. This doesn't happen when running the motor on the analysers without a load. RPM increases in a more linear fashion and you have to look at amp draw. Looking at this data, I would time this motor at 45*. And that directly compares to dyno runs I have made with the MiniPro dyno.

You have an issue with the MiniPro dyno. You have argued its validity with me a few times...

I plan to use an R1 Digital 3 ESC to show me real world rpm/amp draw/temps/gearing in my touring car, with that data I can then set an appropriate load on the slave to then allow me to plot timing/amp draw/temps in my basement to set my motors for my conditions. In the end will it make much difference? Who knows but it will be a fun experiment here in the winter when it rains non-stop and I need something to do. Hoping to learn something while I’m at it.

I questioned your setup because the low RPM part of the curve didn't look right. It looked like a soft start was being used. You confirmed you were doing a soft start. I never said the minipro was an invalid tool.

I have come up with a board design using Kicad. I have never designed a board before and this is just a prototype. Eventually I will break out the 328p from an arduino and just have that with the required components instead of using a full nano. There will be some other changed as well, I'll probably end up using the ACS724 sensor instead of the big ACS772. Also I will try to reclaim space. My routing probably isn't optimal, but it should be fine for a prototype.

I still need to figure out all the requirements for where I choose to get it made. And, I will be moving very shortly, so the time I can put into this is going to be minimal for the next few months.

Also I am working on getting timing. I think this can be achieved by comparing the rising edge of sensor inputs to each other. I just haven't sorted out the code. But it should be fine using the digital pins I am using for rpm, so it is just a matter of coding.

If you want to bounce any ideas off someone, let me know. I've done a decent number of custom boards using microcontrollers. Some general suggestions - I'd use something more modern than a 328P. There are AVR devices with better internal peripherals, more flexible and simpler to implement that cost half the price. Not quite sure how you'd establish timing from the sensor signals alone, since they'll always have a fixed relationship relative to each other.

I actually just bought an STM32 Nucleo to start tinkering with it. But I will probably take you up on your offer as well. As to timing, I was going to start with just getting sensor angle offset from 120*, I hadn't quite worked out what to use as a fixed reference to compare that to.