Re: Three wired DC Motor question.
From: PaPaPeng (papapeng_at_yahoo.com)
Date: 09/14/04
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Date: Tue, 14 Sep 2004 20:38:15 GMT
On Tue, 14 Sep 2004 11:37:08 -0500, "jakdedert"
<jdedert@bellsouth.net> wrote:
>Most CPU fans these days have three wires...one, I guess would be common,
>and the other two: + and feedback.
>
Now that you mentioned it CPU fans are brushless. I did wonder how it
managed to rotate without the brushes you would find in any ordinary
motor. OK the 60Hz AC supply was a quick if uninteresting answer. But
because its such a cheap item and there is little else one could use
the motor for - too weak, too slow and the fan is molded into the
rotator unit - I never cared to find out how it worked not that I knew
brushless motors was a new technology anyway (circa the 1980s.) But
then again I did wonder why a simple motor fan would cost several tens
of bucks back then and still cost more than brushed motors even now.
With the hair thin magins in PCs wouldn't they cut costs especially
for a low tech item like a fan?
>
>The URL www.allegromicro.com/techpub2/compumot/a17a19.pdf gives a
>very clear explanation of the brushless motor technology involved. It
>does require three wire leads and its obvious how the direction of
>rotation is derived.
Brushless motors can function only with three leads to handle AC
polarity pattern generated by an electronics circuit necessary to
drive the motor. There is no + - and certainly no feedback. Why
would a simple cooling fan need to provide feedback anyway? The
beauty of a circuit generated AC is that it can be minituarized and
the cct can be programmed for speed control as well as for rotational
direction. It also means that the brushless motor have a controller
PCB. You can't just take the two leads off a battery and apply it to
the brushless motor.
In the URL http://www.mikromodell.de/index_e.html mini circuits and
motors have been used to RC scale plastic models, a project I am doing
now. I was puzzled when one of the contributors said he wrote a
control program for his model. I was think WOW these guys must be
rocket scientists who knew the innermost secrets of electronics
miniaturization, programming, etc. Surely they must have custom built
their toys because no manufacturer would cater to such a small niche
market. Well things are beginning to look a lot more achievable now
although I cannot program anything.
Do hop over to Radio Shack and take a look at their latest miniature
RC toys the one that comes complete with a little docking bay battery
charger. Among the minicars they have aftermarket motors with gear
sets that you can use to exchange with the setup of a standard model.
Those motors are exquisite with a diameter of perhaps 3 mm and a
length of 10 mm. How anyone can manufacture such a small precision
motor is amazing. This size would have been impossible with regular
brush motors.
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