not understand anemometer

Because they shouldn't call them "DC Brushless Motors", they should call them "permanent magnet synchronous motors". But nobody would understand that, so they stick to the misnomer.

Inside of a "normal" DC motor you'll find a little gizmo called a commutator. It's job is to take the DC from the power wires and switch it around to the coils -- making it AC with just the right frequency to drive the synchronous AC motor that's inside of nearly every DC motor*. A brushless "DC" motor takes the commutator out of the motor and puts it into a circuit, eliminating one of the major wear points of a DC motor.

• except for the homopolar motors, but they're weird.

that is how you drive a brushless motor; with an AC power that is synchronized to the shaft rotation.

They're going to amend the Constitution to ban those.

Coils create magnetic fields and attract the magnets. If you apply DC to a coil, the motor moves to some position and then stops; you noted this yourself. To get the motor to spin, you have to keep changing the current in the coils to keep the rotor moving; you have to keep changing the "place where it would stop"

If you use the motor as a generator, by externally spinning the shaft, the magnets move past the coils and generate voltage. As a magnet approaches a coil, the voltage swings positive, and then as it moves away, it swings negative. This happens over and over as the shaft spins, so the average voltage induced into the coil is zero. So there's no DC to measure.

Actually, as I said, AC is just moving DC. If you were to turn the shaft very slowly and look at it with a sensitive DC voltmeter, you'd see the slow positive and negative swings. But the voltage would be very small. As the speed goes up, a DC voltmeter can't follow the swings and reports the average value, 0. An AC voltmeter essentially rectifies the voltage before measuring it, so you get a number.

John

The internal circuitry of our brushless DC motor consists of 12 coils, and a permanent magnet ring that spins around them. These motors are NOT like a normal DC motor...they require a special driver circuit to make them spin. If you apply plain DC current to the motor, it will simply seek a point aligned with the coils, stop, and burn out. The motors have 3 leads...a common in the center and 2 outputs (well, actually inputs!). For connection to measuring equipment, you need only connect to the center common lead and one of the side ones. The output of the motor when spun as an anemometer can be measured with a multimeter set for AC volts, or by counting pulses with a frequency meter or BASIC stamp. We had the best results using a Fluke 87 multimeter set for measuring Hz (cycles per second). There are 12 internal coils in the motor, but we only measured the output of half the coils (since we connected to only one power lead). Therefore, a meter reading of 6 Hz equals one revolution per second (60 rpm). Both frequency and voltage readings from the motor are quite linear, making for easy calibration. The other advantage of counting frequency over measuring voltage is that the length of the data cable would affect voltage readings; when counting frequency it can be any length and the calibration will stay the same.

why you use the AC volts to measure a DC brushless motor? solar power arm board development team

"developer" ?ÈëÏû?ÐÂÎÅ :csn95d$ snipped-for-privacy@imsp212.netvigator.com...

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still do not understand

I do. Thanks John, that makes much more sense to me than Tim's answer :-)