A Honeywell HT-904 tabletop fan stopped working. I tried the speed switch in multiple positions and listened carefully. Not even a gentle hum from the motor. [Incidentally, the line outlet was, indeed, live.] Here are the steps I took to debug it.
Set to start debugging. I have a work area, tray for small parts, screwdriver, multimeter, and pen and paper to take notes. More things will be added later, but at this point, I have no idea what the problem is.
Since the fan appeared completely dead, power is probably not gettting to the motor. There was no burn smell to suggest that the motor’s windings burned up. So the first thing I did was put an ohmmeter across the prongs of the power plug, and turn the switch on. It read infinity. Something was disconnected. Just a minute, this plug looks strange – it’s a little bigger and longer than most.
Turns out that the plug had a fuse in it! There was actually even a spare fuse in the slide-out cover. Aha, likely cause! Nope, the ohmmeter showed zero ohms – the fuse was good.
The fuse was ruled out, so the next most likely problems were a bad switch or bad connection. Had to take it apart. Six screws held the cover in place.
It could use a jolly good cleaning, but that can wait.
One pan head screw holds the cover for the junction box.
That black thing is the power and speed switch. After pulling the knob off the outside, the switch slides right out.
The switch is out, but notice now that the white line cord is joined to the black and blue wires with simple wire nuts. Perhaps it’s just a loose connection.
When a wire nut was removed, the connection within was revealed to be soldered. Continuity was checked with the ohmmeter all the way from the power plug pins to the other end of the black and blue wires (at the switch). As expected, there was no problem.
Earlier, I had guessed it unlikely that the fan was simply mechanically sticky, as there was no hum when I attempted to turn the fan on. In turning the blades manually, I found there was a very slight resistance, but not enough to stall the fan, in my opinion. I had also guessed that the motor winding hadn’t burned out, due to lack of a burn smell. But a wire might have worked loose, somehere. The only way to find out for sure was to remove the motor and look.
The strain relief neededto be disengaged to give the line cord some slack, to remove the motor.
After rotating the strain relief 90 degrees, it could simply be pushed into the chassis.
Two screws to remove to get the motor out.
With the motor out, I could see visually that there is no charring on the windings, consistent with the no-burn theory. But the motor had dust all over it, and hair wound around the shaft. Removing two screws released the rotor from the housing for cleaning. I noticed that the motor had no ball bearings whatsover, the shaft went right through raw metal. I cleaned everything up and put some silicone/graphite lubricant on it. After doing that, the fan blade spun freely and kept spinning. It occurred to me that over time, the shaft would probably have gummed up and either mechanically siezed, or overloaded the motor and blown the fuse. It was a cheap fan, but if someone wanted to make such a unit last a long time, they might consider cleaning and lubricating the motor periodically. With the components on the table, I plugged the fan back in and turned the switch on. It remained nonfunctional.
Below is a sketch of the wiring of the fan.
There must be a completed circuit for the fan to run. With the fan off, the black wire already connects to windings in some way, coming out as gray, yellow, or red. This means that with the switch on, the blue wire must be shorted to one of gray, yellow, or red – there’s no other way for current to escape. The current must return to the plug via the other blue wire.
No matter how I fiddled with the switch, I couldn’t get anything to connect to the blue wire on the plug. What were those two blue wires, anyway? I suspected they were a winding of the motor, perhaps a field or stator winding. Perhaps a blue wire came off the winding inside?
The only way to see what the blue wires were doing inside was to cut the tape wrapping the motor windings. I might have irreparably damaged the motor by doing so, but then again, it was already broken. The tape was tough, hard to cut even with a razor blade.
After opening the outside tape, I saw that the blue wires didn’t go to a winding at all. But whatever that device was, I knew it had to have continuity for the motor to run – and it didn’t.
A close-up view of the device. It appears to be a thermal fuse of some kind. 115 C would be a reasonable overheat temperature for the motor. Whether it was an actual over-temperature, or simply wear-out over time, I couldn’t tell. I suppose that over time, as the bearing on the motor gets stickier and sticker, the mother would turn hotter and hotter, leading to the thermal fuse opening. The addition of this device was probably justified by calling it a safety meature, but it certainly looked like planned obsolesence to me.
I cut the thermal fuse out, and connected the blue wire coming out of the switch directly to the wire nut leading to the plug. The fan then worked normally, and I reassembled everything. If there is truly an overload, there is still the electrical fuse in the plug for protection.