Shaded pole motor got destroyed as I unplugged it. I want an explanation for it.

I'm puzzeled? Why did you unplug it while it was running? Was the on/off switch broken?

Al

Short circuit caused by old/faulty insulation material on the "high" winding, which was destroyed by the inductive voltage kickback, caused by the unplugging. This must have occurred at the exact instant when the current was at its maximum. Gene

Wow. That's a *LOT*. Six hundred volts at forty kilohertz!

I think that the old girl was ready to go to fan heaven and would have died if you had used the on/off switch.

Please stop melting the insides of your wall outlets by unplugging running fans. Someone may want to use those outlets later.

Why not? I don't see what the big deal is, at least not with a fractional ampere 120V AC load.

What difference does it make whether you use the cheapo switch on the fan, auxiliary power switch(on power strips, accessory switch gadget that goes to outlet for appliances w/o switch and etc) or simply unplugging?

What about items without a power switch such as aquarium aerators, refrigerators and George Foreman grills?

So, if I want to turn off an aquarium pump or an aerator(similar in current draw to mini fan and all three are inductive) w/o a switch, I'm supposed to shut the power off at breaker, unplug, then turn it back on?

Just because it's got a power switch doesn't mean using the plug as a switch isn't fatal to the equipment.

I unplugged to turn it off, because I was going to relocate the fan. It doesn't make much sense to turn off, unplug, plug then on again.

I'm still thinking about the problem as reported, but wouldn't unplugging accomplish the same effect of turning it off?

Examining the reported evidence, if the connection between common and neutral was melted, and the high-speed windings were destroyed, it sounds almost like a lightning hit just at the time he unplugged the fan.

Anybody else have a better explanation?

Harry C.

I'm not sure if you interpreted the way I intended, so let me clarify.

The connection between neutral and common: Neutral comes into motor and neutral connects to common connection of the motor through a thermal fuse. Thermal fuse was intact, but one of the leg on fuse was melted and there was artifiacts of evaporated metal around on the paper wrapping.

After reconnecting the melted connection, applying power to neutral and low speed got the motor going.

An alarm clock, a TV and a telephone connected to the same power tap didn't suffer anything, so I doubt it was lightning.

Anyhow, I'm off to buy me a new fan, but I sure like to find out how the heck this happened.

It's all a matter of degree. Yes, pulling the plug on an aquarium pump while it is running will melt the insides of your wall outlet a bit, but you probably move your fan more often than you move your aquarium. BTW, I don't believe that you are too stupid to figure out any other way other than using the breaker. I suspect that you know about outlet strips.

"Then I saw a flash and heard a loud pop at the plug. It charred the outlet a bit."

It seems that on that day doing what made sense to you didn't work so well.

Yes, but switches are designed to minimize arcing and last through many disconnects, and plugs/outlets are not.

Gene alreade posted one:

|Short circuit caused by old/faulty insulation material on the "high" |winding, which was destroyed by the inductive voltage kickback, caused |by the unplugging. This must have occurred at the exact instant when the |current was at its maximum. |Gene

It was probably designed to act as a fuse.

What do you find unsatisfactory about the explanations given? They are guesses, but marginal winding insulation breaking down because of inductive voltage spikes during turn-off is common, and an overcurrent/melted lead would be the likely result.

Just buy a new one and forget it. It was made over seas anyway. What did you expect? After all you get what you pay for.

-- If it is not broken, I cannot fix it. If I screw with it, it will kill me. Payday is Friday.

It's Volts that jolts, but Mills that kills!

Wanting to know what the heck just happened, I removed the covering form motor winding. There are three wires. Common, low and high. The wire between thermal fuse and common winding end was melted, but fuse wasn't open. Applying power directly across common and low made the motor run once again, but there was an open circuit between common and high connection.

Wanting to track down the location of breakage, I applied 600V 40KHz across common and low, then common and high. No arc between com and low. I saw an arc a few layers below the outermost layer of the winding when I applied to voltage between common and high indicating a broken winding.

Long story short, fan was running fine on high. Lots of arc'n spark at the plug when I unplugged it. The connection between neutral and common was melted. high-speed winding got instantly destroyed. I didn't smell anything and there is no evidence of overheating.

Anyone know what could have caused this? I sure don't. . .

That is not what he said - you are being intentionally obtuse. When a device has an on/off switch, use it to turn power off before unplugging. That will prevent unnecessary arcing at the plug/receptacle.

I don't think the unplugging caused your failure - I suspect the failure was going to occur in any event, and the multiple make-break created by the unplugging just pushed it over the edge.

When you plug in, or unplug, a device that is on, you will get arcing at the plug/receptacle contacts. Does it make sense to allow that to occur when it is simple to avoid?

Probably aluminum windings anyway; good riddance...