I couldn't get the title to make much sense, so hopefully the following explanation will.
Today I tore apart an air purifier we've had forever that's going to the trash, and harvested the two-speed C-frame shaded-pole fan motor out of it. Nice little motor, I took it apart, cleaned and oiled it, and it runs absolutely silent. Can't be more than 1/50 HP.
I want to use it in something, and my original idea was a smallish box fan, no more than 12". But then I got an even more awesome (crazy?) idea - build the fan, but also include inside the fan cavity a fluorescent lamp that, with the flip of a switch, is connected in series with the motor, using the motor as a ballast, and powered on. This is a two-speed motor, too, so the lamp could run at two brightness settings.
The trick, of course, is finding the right lamp, or combination of lamps, such that they get the right amount of current on high speed (as long as they get the right current on high speed, I won't worry about them being too underdriven on low speed). The motor itself draws 0.23A on low speed and 0.85A on high speed. I spent a couple of hours trying various lamps, and I've concluded the following:
A single T5 gets way too much current. A single F14T12 gets 0.41A, probably not enough overdrive to hurt anything, but I always like to avoid overdrive if possible. An F15T12 gets 0.35A, still a slight overdrive. An F20T12 gets 0.28A, which I would be totally fine with, but such a lamp is far to large for this application (any T8 or T12 lamp is, really). Two F14T12 lamps in series get far too little current.
I found that two F6T5 lamps in series get 0.16A - perfect. (Although oddly enough, I got 0.20A a different time I tried it. I'll do more tests to see what I get for current.) However, preheat current is some 0.30A, far too high - I blackened my modern Sylvania lamp as a result. (I also used my generic 1960s F6T5 lamp, as I don't have any more expendable 6 watt lamps (will get another next time I'm at the surplus and salvage store), but luckily it didn't seem to sustain such damage.) I would stick a resistor in series with the starting switch to fix this. I found that current on the low speed in this setup is 0.08A - quite low (each lamp is doing only 3 watts), but as long as the lamps are started on high speed I wouldn't fear of sputtering.
There is one insolvable caveat, though - on low speed on this setup, the motor stalls - simply not enough current. Shaded pole motors are completely mechanically resistant to damage from stalling, but they do need active cooling. However, perhaps the current is so low that not enough heat will build up to damage the motor. I will run some tests and see. I could possibly set it up such that both lamps run on high, but only one lamp runs on low - thus the motor sees enough voltage to continue running, and the lamp would still get an acceptable amount of current. But that's probably asking for more complication than I'm willing to go for.
As a last resort, if the above-mentioned setup doesn't work, I could use a single lamp of the desired size, and simply insert an appropriate resistor in series with everything when the lamp is powered on. All-in-all, though, the dual-F6T5 setup sounds promising. The only thing I need to figure out is the fan blades. The air purifier used a squirrel-cage fan, which is no good for what I want to do. Grainger appears to have some blade sets that may work; only thing is the motor has a keyed shaft, while those blade sets have a perfectly circular bore. I'll keep looking.
Trent
