Introduction to measuring the electric brushless outrunner motor constants Rm, I0, and Kv.
Accurate motor constants are needed to analyze the performance of our electric motors. You need to know them in order to use my free electric power system calculator, for example.
It is a sad fact, but more often than not the constants supplied by the motor manufacturers are wrong. Some of this is just due to the natural variability in manufacturing each motor. But other times numbers are published to make a motor look better or to match a competitor’s numbers. The only way to find out the truth is to measure the constants yourself.
I wish I could tell you that I have come up with a very accurate and very easy method for measuring the constants in our motors. I did not, because it cannot be done. But I have done my best to break down the process into easy to follow steps.
This series of videos will only cover brushless outrunner electric motors. Brushed motors are a little different and I will not be discussing the differences.
Because of the challenges involved, there are many different ways of measuring the motor constants. They will all give you slightly different results. Each method involves trade-offs in accuracy, safety, and convenience. Safety is very important to me. I will explain first the method that I use myself. It is very safe, reasonably accurate, and easy to do.
The tools that you need for the different methods also vary. I will devote a video just to talk about the different tools you might already have or need to buy.
There are three key constants. First is the no load current (I0), which is the minimum amount of electric current needed to turn the motor. Second is the winding resistance (Rm), which is the electrical resistance of one loop in the motor’s copper windings. These two constants are critical to measuring the efficiency of the motor.
Finally we have the voltage constant (Kv), which relates the input voltage to the motor’s RPM. This is the constant which you are more likely to be familiar with, since it also helps determine what size propeller you need to use with the motor.
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