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Re: [TCML] Controlling Speed of an AC (Muffin) Fan
Brandon Hendershot wrote:
Alright,
So I plan on purchasing this:
http://www.mpja.com/prodinfo.asp?number=17780+FN AC muffin fan and I
need a way to vary its speed. I was thinking of using a rheostat, but
Paul suggests otherwise. Then I thought of using a dimmer switch, but I
remember trying to control my drill press with my variac (kinda the
same, right?). It wouldn't work because you need to control the
frequency of the electricity, not the voltage like you do with most DC
motors.
Depends on the kind of motor. A lot of AC induction motors (especially
split phase/PSC types) will vary speed with a variac or triac dimmer,
without changing the frequency. Sometimes, you have to start the motor
at full speed first, then slow it down. Sure, they're running with a
lot of "slip" at low speeds, but the output horsepower is low too, so
the fact that it's horribly inefficient isn't a problem. Fans are great
because they are a very nonlinear load (somewhere between square and
cube law).. cut the speed in half and the power required to drive the
blades is a LOT less than half.
If you're using a lamp dimmer, putting a lightbulb in parallel with the
load helps make the action smoother (the inexpensive dimmers don't like
the inductive load).
Anyway, what do you guys use to control your AC motors on your rotary
gaps or spark gap fans?
For spark gap fans.. physical restriction. Leave the fan running full
tilt and choke off the input.
Thank you,
Brandon
P.S: I only opt for AC motors for their immunity to the coils magnetic
field. One more thing on the topic of fans, should 200 CFM be good
enough to quench my RQ gap? I'm running a 15/30 NST.
Maybe, maybe not. You'll have to try it. Not every fan rated at 200 CFM
will actually put out 200 CFM in your system. There's a performance
curve for a fan that gives flow vs back pressure. But even with known
flow, the cooling/quenching behavior is idiosyncratic. Your primary
inductance and capacitance will affect it too.
Don't try to over analyze.. just hook it up and see how well it works.
Run it, then pull the plug on the fan while it's running and see if it
gets better or worse as the fan slows down.
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