# not understand anemometer

Discussion in 'Electronic Basics' started by developer, Jan 20, 2005.

1. ### developerGuest

http://www.otherpower.com/.html

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?
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2. ### John LarkinGuest

The rotor spins. AC is just DC in motion.

John

3. ### developerGuest

still do not understand

4. ### no_oneGuest

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

5. ### Tim WescottGuest

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.

6. ### NSMGuest

| developer wrote:
.....
| 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.

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

N

7. ### John LarkinGuest

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

8. ### Derek BroughtonGuest

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