"Arny Krueger" wrote in message
...
"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".
**OK. Fans 101:
AC motor fans use a 'shaded pole' motor system. They are synchronous and
THEORETICALLY their rotational speed is dependent upon the number of poles
and the frequency applied. They are under constant, heavy load (from the
air). As such, they can never reach their synchronous rotational speed.
Reducing the input frequency will reduce their speed (obviously). Reducing
the input Voltage will also decrease their speed. Since the fan is usually
under a constant, heavy load, the variation in speed will be reasonably
predictable. If the air load varies, then the result will be unpredictable
and simple Voltage reduction systems are not recommended. Variable frequency
schemes would be a better idea, in this case. PLEASE NOTE: A series resistor
not a perfect solution. It is, however, simple, cheap and effective, if the
air load is reasonably constant.
DC motor fans come is a very wide variety of flavours. Too many to address
individually here. Broadly speaking, they fall into several groups.
DC brush motors: I have not seen one of these for quite awhile. They can be
easily and effectively changed in speed, by the simple matter of altering
the input Voltage. This could be by using a resistor, zener diode, linear
regulator or a switch mode regulator. Such motors don't last very long,
under continuous duty, however. Brushes wear out.
DC brushless motors: These come in a bewildering array of types. Some are as
simple as a couple of transistors, which set up an AC signal to drive an AC
motor. Others have sophisticated motor control systems, with hall effect
devices, and constant speed regulation circuits. Some have external access
to speed regulation circuitry, allowing thermal sensors to vary speed. It is
these last types, which have the best success in speed altering. Other types
are very much hit-n-miss affairs. Some respond to varying input Voltages,
reasonably well. Some do not.They don't because the internal circuitry
'expects' to 'see' an input Voltage of a reasonable range (say; 10 ~ 12
Volts). Voltages outside this range may cause the internal oscillator to
malfunction. Gogging usually results.
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.
**An excellent idea, but well beyond the requirements of the original
poster. A series resistor of the appropriate value, is STILL the best option
for him. That may not be the best option for someone else.
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...
**It depends on the fan. I just grabbed two, different 12 Volt DC fans. One
operated fine (albeit with cogging noise) down to 3 Volts. The other ceased
operation at 7.5 Volts. I also grabbed two, different 240VAC fans. One
stopped operation at 60 Volts and would not start up until 70 Volts. The
other stopped at 65 Volts and started up at 70 volts.
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...
**The only limitations of AC fans, are their higher costs (more materials,
etc). Their reliability is somewhat higher than any DC fan.
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!
**Then you're doing it wrong. My power amp uses a (continuously) variable
speed AC fan. When the fan is just ticking over (about 200 RPM), the noise
from the fan is lower than the (potted, shrouded) toroidal power
transformer. And the transformer is VERY quiet. I measured the noise spectra
using my LMS system. Even with the amplifier and measurement mic fully
enlcosed with several blankets and a doona, the ambient noise at 4:00 AM was
higher than the fan noise.
I guess I should take my own advice and pop in some 12 volt 120 mm fans
with
a regulated variable power supply.
**I guess you should.
--
Trevor Wilson
www.rageaudio.com.au