I am a very new owner of an oscilloscope, as in it just arrived yesterday. So the first thing I did with it, (aside from calibrating the probes) is to measure the bemf of a slave bldc motor. A little background. I am into RC cars and racing them. My main focus is stock and mod 1/10 indoor touring cars. I have lots of gadgets for taking all sorts of measurement of these motors but I am wanting to test the validity of some. When we get motors they don't come with a Kv value, they come as simply the windings. Stock for my class is a 21.5 540 can size sensored bldc motor. I have a tool called a motolyser that measures the sensor positions, rpm, Kv, current, and phase current. It gives a test voltage used and the Kv is simply the rpm divided by the test voltage. My understanding is the Kv should be calculated using the bemf, so I am wanting to measure the bemf and compare to motolyser.
I created an experiment to measure the back emf of a slave motor coupled to a motor being driven by the motolyser. I used a tachometer to verify that the rpm that the motolyser was reading was accurate. I then used the rpm value and divided by the rms voltage seen by my scope attached to the slave motor. After that I ran the slave motor on my motolyser and compared the Kv values. I thought I had a win, but I overlooked that we can adjust the timing on the can and my test will always provide the same results no matter the can timing. Whereas the motorlyser will have different results based on the timing.
So I need a way to measure the back emf of a motor being controlled and running by a controller, and I would like to see it on my scope. From the reading I have done, it looks like I will have to use 2 channels and invert channel 2 and do a math calculation. I am still a little hazy on this. But even then, how can I isolate the bemf from the supply?