Testing a Hunter attic fan controller?

May 20, 2008 7:27 PM

How do I test a Hunter attic fan controller? I want to be thorough before I climb in the attic.

My mom's old (1980 ish) attic fan won't go at any speed beyond low. I pulled the controller, put in a regular light switch, and it still ran at the same low speed. I told her it was probably the motor (pain in the ass - tiny attic in a 1.5-story house), but she asked if I could test the controller itself. Not thinking much, I agreed to do so.

I got it home, and figured, hey just test the resistance at various settings on the knob. I get no resistance whatsoever across the terminals no matter what setting I put the knob to. Then I recalled, hey, the cover plate said "solid state" which means, well, if there's one thing I can't wax lengthy about, it's electricity. I suppose to test it I'd have to have 110V across the terminals, and a load (i.e. the fan motor) on the neutral leg of the controller? I'm not sure how to set this up in a test situation, without effing up my own house electric, and I'm not sure how the hell the fan changes speed.

The Hunter web site doesn't even list attic fans. I guess they aren't cool any more.

posted by notsnot to home & garden (6 comments total)

Solid state probably means it works like a light dimmer, i.e. uses a triac to cut off part of the sinewave in each cycle of power; the lower the power setting, the more it cuts off each cycle. You won't be able to measure a resistance or anything like that from it.

Easiest way to test would be to plug in a low-power lightbulb - no more power than the fan is rated for. Say 20W. Have a look on the controller and see if it has an output power or current rating on it and don't exceed that with your test load.

If the fan only runs at low speed with AC connected directly to it, it's probably gunked up with something - grease, dust, etc. A good clean might restore it to attic-y happiness.

Or maybe it's really running at high speed and you just think it's slow - I suspect an attic fan doesn't have to be very powerful. Maybe the controller is blown (always at full power) and the fan is running at its (not very) high speed.
posted by polyglot at 8:10 PM on May 20, 2008

Ouch. Your first paragraph sounds likely.

I know the fan *can* run faster. The speed it runs at now barely opens the louvers partway, and I remember how loud and powerful that thing was when I was a kid. The fan is b no means running full blast.
posted by notsnot at 8:38 PM on May 20, 2008

Many variable speed fans use a type of AC motor called a universal motor, with an inexpensive SCR (silicon controlled rectifier) or triac (back to back, full wave, symmetric SCR) control. In simplified form, they work like this, and can be distinguished by the fact that they have brushes. The brushes eventually wear out, and often create visible sparks as they do.

Another kind of single phase AC motor, that eliminates the brush problem, at the sacrifice of starting torque and efficiency (but which would still be suitable as a multi-speed fan motor) is the shaded pole motor. Older fans with two or three speeds often used this motor type, because all that was required for control was a simple 2 or 3 position switch. If your control was a switch type control, you'd be able to check it with a continuity checker. But this type motor is terribly inefficient (maybe 20 to 30% efficient), and usually fails when one or more of its field windings eventually shorts out under heat and mechanical vibration wear. There is nothing to do then but replace the motor.

You might want to re-install the controller, and go ahead and crawl up in the attic, and see what kind of motor the fan uses. If a universal motor, the brushes could be shot, or the armature windings worn through, and there is nothing additional voltage from the triac controller can do to produce the additional torque needed to get the fan up to higher speed. In the case of a shaded pole motor, typically, one or more winding sections may have opened. Typically, in that case, setting the control to those positions will effectively stop the motor, but in some case, if the winding is shorted to ground, higher speed settings simply cause the motor to continue to run at the same speed, through the armature short.

In either case, there are now much higher efficiency systems for attic fans, and the rising cost of energy may make it advisable to install a better system.
posted by paulsc at 11:14 PM on May 20, 2008

well, if there's one thing I can't wax lengthy about, it's electricity. I suppose to test it I'd have to have 110V across the terminals, and a load (i.e. the fan motor) on the neutral leg of the controller?

Testing involves live connections to AC line, so real caution is required. You aren't inspiring confidence :P

However (and the rest of this answer is all just informed guesswork).. The fan probably uses a shaded pole motor, and the speed control is probably similar to a standard triac dimmer.

I think you could test it by connecting a standard incandescent light socket. Only use a bulb with the same or lower power rating as the fan or controller. If the brightness changes as you adjust the fan speed control, you know it is working. It would require quite a bit of live AC electrical wiring, which would have to be done with great care, so I'm not sure you really want to do it.

Also, if the speed control works on low, it probably just works. More likely that the fan's oil saturated brass bushing is seizing up, in which case the fan needs replacing because you can't properly duplicate the lubrication applied in the factory.
posted by Chuckles at 11:24 PM on May 20, 2008

Damn you paulsc!! :P
posted by Chuckles at 11:24 PM on May 20, 2008

I was being self-deprecating on the "I don't know anything" comment. I can hold my own on house wiring and control wiring (used to build car washes), but anything with a transistor might as well be Martian.

Looks like I'm gonna have to replace the motor. Damn. I'll look into replacing the whole unit, just on account of the bearing problem, and the newer technology.
posted by notsnot at 4:06 AM on May 21, 2008