not understand anemometer

From: developer (noemail_at_nowhere.net)
Date: 01/20/05 

Date: Thu, 20 Jan 2005 11:39:30 +0800

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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