"Ron" wrote in message
...
On Tue, 12 Aug 2003 18:53:57 GMT, "Trevor Wilson"
wrote:


"Arny Krueger" wrote in message
...
"WindsorFox[SS]" wrote in message
news:jW_Za.10115$qf.4685@lakeread06

Right, I wondered what a 12 Volt fan had to do with your question,
it's so cross posted now I thought I'd missed something and was just
replying to the 12V comments.

I plead guilty to bringing up 12 volt fans. I did so because 12 volt
fans
are plentiful and readily adjustable. They can be adapted for use where
the
original fan was 12 volt when DC power is readily available, such as in
a
power amp.

I've experimented with fan speed controls on 120 VAC fans, and had
disappointing results. 120 VAC fans are based on synchronous motors and
IME
they get pretty ugly at low speeds if you turn their speed way down.

**Then you're not doing it right. With simple series resistors and a
decent
fan, speed reductions of at least 40% may be obtained very easily,
cheaply
and reliably.

Further proof that little knowledge is worse than no
knowledge at all. He was taking about SYNCHRONOUS MOTORS.
Putting resistors in series with a synchronous motor is
folly.

**No, it is not. It is a popular and effective method of reducing the speed
of such fans. It is very important to realise that a synchonous motor, as
used in a fan, is under constant, heavy loading. It can never reach it's
theoretical maximum speed, which is determined by the frequency and number
poles used. If the motor was under light, or no load conditions, then a
resistor WOULD be a dumb idea. Fortunately, on this planet, we have air to
load the fan blades.

Actually, there is another way to lower the speed of the fan, with no hot
resistors. That would be to choose an appropriately sized capacitor.
Unfortunately, the results can be rather hit and miss, depending on the
inductance of the fan motor. Start up problems, however, are largely gone.

And, for the record: I have used all manner of fans and speed controls, over
the years. A resistor in series with an AC fan works just fine. What is
more, they run for a very long time, with high levels of reliability.


--
Trevor Wilson
www.rageaudio.com.au