kenne

I have a small enclosure with 110VAC 120mm fan (about 0.2A); need it rigged to keep temperature to reasonable noncritical level for equipment. Had simple reed switch thermostat, so fed triac with it: fine, but fan is TOO loud. So tried simple diac-fired variable control; controls acoustic noise fine but AC noise is FAR, far too bad within enclosure, even with filtering that fit. So: I figure that I can make a zero-voltage-switching control either to set just the motor to a fixed level about half speed and keep the reed switch thermostat, or make a proportional ZVS control that includes temperature sensor. Actually, just any simple, electrically quiet way to reduce the fan to roughly half speed would do adequately, along with my triac/reed thermostat. Size and cost are important - can't need separate power for the control. Figure 17 in app note for AN3004 might do though more complex than I want (at: http://www.fairchildsemi.com/an/AN/AN-3004.pdf ) but neither part AN3004 nor MC33074A appears to be available. I'm not currently into programming PICs &c. So: what is the very simplest thing I can do, please? Thank you, all.
- kenne

stevez

If I wanted to reduce the power to my soldering iron I'd put a diode in series with the load. Not sure how your fan motor would respond to this approach. Put a thermostat in to bypass the diode so it would go to full speed only if it got too hot.

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stevez

Hero999

It's probably uses an induction motor.

If it isn't suitably designed then using a triac control or rectifier to control the speed could cause it or the controling circuit to overheat.

Induction motors are constant speed devices and don't like it when the voltage is reduced, they tend to try to suck more current which can cause them to over heat.

My advice is to buy a smaller fan, possibly one that is rated to generate below a certain noise level.

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kenne

I'm pretty sure that a diode will overheat the fan, but I would think that ZVS control ought to work. I agree that replacing the fan is probably simplest - I have a 120mm low voltage DC fan hiding somewhere. I could probably run it from a wall wart, and wouldn't have much problem slowing that if needed. The current AC fan is secured very well in the enclosure, though, so I'm still looking out for a very simple ZVS control or reducer till I find my DC fan.

Hero999

How are you planning to use a ZVS control for controlling a motor?

If you PWM it, the frequency will have to be well below the mains frequency so it will become jurky.

Also the motor draws a surge when it's turned on doesn't it?

If it's being turned on and of at a 5Hz then it could still over heat because the high starting current will be drawn every 200ms.

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kenne

Well, thanks for the interest but I am not at all an expert in either AC motors nor in ZV switching. If I knew these answers I would not have posted my question. I am talking here of a small fan motor (NMB brand, marked as impedance protected), not an industrial blower - my guess is that it could be slowed to half without overheating, and that triggering the triac with a ZVS would work and keep down the electrical noise. Possibly some simple way to trigger the ZVS on every other full cycle? The fan did not seem to overheat when I controlled the triac with a diac and phase control circuit, though I did not run it very long, really. Anyway, I'm pretty resigned to finding my 120mm DC fan and an appropriate wall wart this weekend. But if anyone knows an answer to my dilemma in the meantime I'd like to hear it. - kenne

mneary

I agree that you're on the right track looking for a DC fan that can be speed controlled.

Speed controlling a shaded pole AC motor is not good. "Impedance Protected" ASSumes that the input frequency shown on the nameplate isn't violated. ZVS can generate unacceptable frequencies with unknown results.

ronsimpson

I control the speed of AC fan motors by adding a capacitor in series with the fan. I use AC capacitors (no + or - lead). A 10uF 160 volt AC capacitor is 277 ohms at 60 hz. You can parallel several capacitors to get the right value.

I am running the fan power through a thermostat. I know the thermostat was built for 24 volts but it works well at 110. (mercury bubble type)

Hero999

I'd still be very careful about doing that.

You need to take into account for the motor's inductance or it you're not careful you'll run the motor at a higher voltage than it's designed for.

For example if your motor has an inducance of 500mH and a resistance of 100hm: when running and you connect a 14µF capacitor in series with it the at 120V current will be 1.2A and the voltage across the motor will be 226V causing it to melt down.

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