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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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