From everything I have read, Kv actually comes from the back emf. The back emf voltage will be slightly lower than the supply voltage due to the winding resistance and some other small factors. My understanding is that motors that are rated at a Kv value are measured this way. I guess I was just trying to come up with a way to measure the actual back emf and see if that is what these devices use. But the Motolyser gives the test voltage so I can just compare that. But now, I still want to see the actual back emf.