On 5 Jun 2006 22:42:35 -0700, wrote:

I have this fan I use most of the summer months. It is older than I am
and is a Sears. NOT a Sears-Roebuck. It was powered by I think a 1/6
horse two speed motor. That motor eventually burned up. It is now
powered by basically a washing machine motor at 1750 RPM.

The old motor was seven hundred something and eleven hundred something.


My basic idea here is to build a 1/2 voltage and 1/2 frequency
generator to power it at 1/2 it's normal RPM. Power transformers that
will rectify into 70-80 volt rails are out there. I can find them. Then
we are not talking about any class A amp stage here, switching
transistors, driven correctly should do the trick.Thirty hertz, sixty
volts. Would that be efficient ? I don't see why not. I could probably
even find a way to cut back the duty cycle for more savings.

See, I don't need this thing to do 1750 RPM, half that would be fine. I
could easily divide the line voltage and maybe even do it with discreet
components (IIRC how to build a flip flop). Of course now I am dating
myself. You might ignore me figuring I'll die soon, but I assure you I
will never get that lucky. I was meant to live.It is going to get hot,
I am not allowed to die.

Anyway, I used to use this fan in lieu of AC for a long time, only turn
the AC on once in a while since I had it. But that motor is too fast
for it. Looking for cheap and dirty.

My major concerns are starting torque (it has a centifugal start
switch) and if I do have to start it on 120V I will need protection
diodes. Also, should I put some inductance in the path ? or will the
motor windings takre care of that ? I know if I use any capacitance I
need to use inductance, but do I need any capacitance ?

I don't see any reason to screw around with waveshaping for a coil the
is going to turn a shaft, do you ?

T
I'm no expert in variable frequency AC drives, but have used a few in
some applications . . .

First - if the motor is capacitor run or capacitor start it won't work
with a VFD - If you're lucky the capacitor will vent and leak all
over. It just won't work - that type of motor is unsuitable for your
app.. Capacitor start might work - after the cap is removed.

Straight induction motors can be used with VFD's, but even that isn't
a perfect application.

The ideal situation is to drive a three phase motor with a variable
frequency drive.

From reading between the lines I get the impression that you just
figure on halving the output speed, and figure there's some cheap easy
way to do it . . .

A transformer isn't necessary - decrease the voltage by keeping the
pulse width low. If you were to use a square wave you would be
dissipating more power than the RMS sine wave uses - a 70% duty cycle
square wave on each half cycle would be the equivalent of full power.

There will be some extra heat generated and some lost efficiency due
to the high frequency component in using a square wave - but it is
done all the time.

Pulse width and frequency track each other.

Fans don't require starting windings - but your motor almost
undoubtedly has a centrifugal switch in there that will be active
whenever the speed is below a rather high threshold (1/2 speed would
be too slow for the switch) - so you'd need to do some work inside
the motor or connection box if it has one.

I wanted a variable speed fan for my house and after checking the
options I settled on an array of 8" "Patriot" brushless DC fans
driven with a 6-48 VDC scr controlled power supply. Works well - but
a little loud at high speeds.

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