"Trevor Wilson" wrote in message

"Ron" wrote in message
...

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 synchronous
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.

The irony is killing!

First, Trevor makes up the fiction that 12 VDC motors are fixed-speed and
"cog" if you try to reduce their speed, and then he denies that synchronous
motors are unhappy when run way below synchronous speeds. Next thing you
know he'll tell us they don't "cog".

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.

And still another way, run synchronous motors from a variable-frequency
power supply.

Unfortunately, the results can be rather hit and miss,
depending on the inductance of the fan motor. Start up problems,
however, are largely gone.

It's like people don't pick the resistors they put in series with AX fans by
heuristic methods. Double irony!

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.

Not nearly as well as DC fans running from regulated power supplies...

What is more, they run for a very long time, with high levels of
reliability.

In the history of cooling technology, first there were AC fans and when the
limitations of those became fully apparent, they developed DC brushless
fans...

Truth be known I've been trying to run AC fans at variable speeds for years,
pushing air though my fireplace insert. When I try to turn them way down
they make the most horrible noises... ...always did!

I guess I should take my own advice and pop in some 12 volt 120 mm fans with
a regulated variable power supply.