I watched a youtube video http://www.youtube.com/watch?v=hT3jJRGVdW4 The man says : by spinning the motor to see whether the output is AC or DC, then it can tell whether the motor is generating AC or DC voltage. It also tell whether the motor is AC or DC
==== What I think is below===
What I think is that "For a DC brushless motor, there is a permanent magnet rotor. If I spin the rotor, the current at the stator should be AC."
Now, if we talk about normal DC brushless motor spinning, we need to provide DC power (as power source) to the motor. That means the motor itself should have a circuit to convert the DC voltage to AC voltage to change the magnetic field at the stator to spin the rotor.
If there is a circuit to convert the DC power input to AC power to really spin the rotor, why the youtube video shows the circuit could reverse AC voltage (generated by the guy moving the rotor) to DC voltage for voltmeter measurement
Then it goes to two questions
Does the youtube video right?
Do the built-in circuit in the motor allow backward voltage convention? What is that circuit?
Here is a transcript of the relevant part of the video:
The first thing that you need to know is that only DC motors can be used as a generator. So that makes this a really easy test. So we're going to connect our pins up to our multimeter, and then we're just going to lightly spin the shaft. Doesn't really matter which way we connect it. We've got the multimeter set to 2 volts. I'm going to spin and you see I'm spinning it clockwise and we get a positive voltage about 1.5 volts if I spin it really fast. If I spin it in the opposite direction I get a negative voltage at about the same amount. I can actually spin it really hard and get past 2 volts. This means this is a DC motor. So now we can move on to figuring out how to drive this motor and whether or not we can use it for some cool project.
I don't know the operating voltage of the motor, but I'm going to assume with a motor this size that it's either 9 or 12 volts. Most DC motors have a faily safe operating range. I've seen 5 volt motors be listed as 3 volts to 6; sometimes to 3 volts to 9. So even if this is a 9 volt motor, I'm going to, in the next video, connect it to a 12 volt power supply and we're going to see if we can drive it. And you know what, if we smoke the motor, it doesn't matter.