On 6 Jun 2006 18:21:19 -0700, wrote:

One respondant mentioned duty cycle. I kind like that, don't bother
rectifying anything. Just use an SCR. I could trigger it from a single
stage flip flop then to get the 30 Hertz. Heck I could even do 20 Hertz
with flip flops, and just cut the duty cycle accordingly.

A BELT drive fan? The sears I was talking about was direct drive . .
.. Why not just change the size of the pulley, or put an adjustable
speed pulley in - that's a low cost easy solution even if you to
change the belt size.

I used the words duty cycle, but as an analogy to make a distinction
between RMS and square wave power consumption, and again in making an
ersatz sine wave for a VFD.

You can't just cut down the duty cycle or use SCR/Triac type
controllers for FHP Induction motors. They overheat, unless the
pulses approximate a sine wave - the two efficient ways are with
stepped voltages (usually six levels, plus and minus - and relatively
complicated to pull off) or varying the widths of several pulses for
each half cycle of the sine wave (still relatively complicated IMHO -
but better than dealing with voltage levels)

Then there's that pesky starting winding - you're driving a fan. A
fan (unless it is seriously broken) turns freely with no real
resistance (unlike wash machines with transmissions seals and gears to
get moving, compressors, or most anything else . . .) until the fan
gets up to speed - torque required tracks shaft speed.

Run the motor at half speed and the starting winding will kick back in
even if you start it at full speed.

DC brush, or "universal" AC/DC motors are ideal for speed control
applications - but the longevity of the brushes is a limiting factor
and one reason they are seldom used to drive fans. (also noisy,
create sparks and ozone, radio interference)

We had this centrifuge at work - big one horse brush type motor with a
variable transformer to change the speed. Probably dated back to the
40's. The transformer burned out one day and they wanted $400 for a
new one. I went to Grainger's and got a high power lamp dimmer for
$20 - no problems.

A VFD drive that uses single phase power and drives a three phase
motor is probably the ideal if cost is less important than longevity,
excellent speed control, and efficiency.

Plenty of appliance repair/junk dealers around with two speed wash
machine motors - and a new one might cost $50- $70.


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