Brushless motors have a lot of advantages over traditional brushed motors. However, testing them can be a bit of a pain. Because the resistance of the motor’s coils is usually very low, a standard resistance check isn’t likely to be useful. Some people use LC meters, but those aren’t as common as a multimeter or oscilloscope. [Nils Rohwer] put out two videos — one two years ago and one recently — showing how to test a brushless motor with a multimeter or scope. Oh, you do need one other thing: a drill.
You don’t have to drill into the motor, instead you use the drill to spin the motor’s shaft. Since a motor and a generator are about the same thing, you can read the voltages produced by the spinning motor and determine if it is good or not. The first video shows the technique and the second, more recent video shows a scope reading a bad motor. You can see both videos, below.
In the second video, you’ll see that a pair of coils have shorted — probably due to a failure of the insulation, perhaps due to heat — and are putting out about 1/10 of the voltage a good coil produces. In the end, he opened the faulty motor and looks at the suspect coils, even though there wasn’t much to see visually.