It has been many years since I had a college course in AC machinery but
I believe there is enough information in the dark recesses of my memory
to give an explanation of what is happening here. If there are flaws in
my explanation please feel free to correct them.
Most small AC motors are synchronous induction type. They have poles
on the armature and stator. When 60 hz power is applied, they assume a
rotational speed where the armature poles are under the stator poles at
precisely the moment when the AC current peaks and the speed is a
function of the number of poles and the line frequency.
If an excessive load is applied, the speed decreases and the
efficiency decreases because the line current pulses are not applied
when the poles are perfectly aligned. I believe this has been called
pole slippage. If the motor is operated at full voltage it will
overheat. If it is operated at reduced voltage, the motor can reach some
reduced speed for the load applied and survive. I suspect that, under
this condition the rpm will vary as the load varies. In our application
the load is relatively constant so some constant reduced speed is
achieved.
I have operated a pair of 115 volt fans in series with no ill effects
as have many others.
73, Dan, N5AR
Steve Katz wrote:
>
> > I'm sure this is all true, but in certain situations, a muffin fan in
> > suction mode can be a great help. I got a couple of 3" diameter 115 volt
> > muffins, wired them in series, put one behind my transceiver to help
> > exhaust air from the final heat sink, and the other atop the power supply
> > section of my SB-220 to augment the main muffin fan which blows in across
> > the tube envelopes. In both instances, temperatures went down noticeably,
> >
> > and because of the reduced voltage the fans are inaudible.
> >
> //What "reduced voltage" are you talking about? I'd like to hear
> more about operating AC fans on reduced voltage, I was never successful
> doing this. (I do operate DC brushless fans on "reduced voltage" to reduce
> rpm and noise, and this works, with reduced airflow, of course.) -WB2WIK/6
>
> > 73, Pete N4ZR
> >
> > Check out the World HF
> > Contest Station Database at
> > www.pvrc.org
> >
> >
> >
> >
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