What happens when an electrical motor dies?

If I set up an exhaust fan in the crawlspace that runs non stop, eventually the motor would wear out and fail. Would it go up in flame? Or are all motors used in the usa required to shutdown gracefully?
Alot of fans/motors are used in commercial applications. I can't imagine them going up in flames at the end of their lives.
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many have a thermal fuse built into the motor, if it gets too hot the thermal fuse blows, cutting off power
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On 10/1/2013 10:06 PM, bob wrote:

dry out, and they over heat. Often, that causes them to break a wire.
I'm sure motor fires have happened, but I can't say I know of one in my life.
. Christopher A. Young Learn about Jesus www.lds.org .
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bob;3128932 Wrote: > If I set up an exhaust fan in the crawlspace that runs non stop, > eventually

>

No, if an induction motor (which is an electric motor that doesn't have brushes) stops turning for whatever reason, then you effectively have a short circuit through the stator. In that case, current through the stator will quickly exceed the amperage the circuit is fused down to, and you'll simply blow the fuse or trip the breaker that motor is on. As long as it's on a circuit that's fused down to 15 amps, the motor will just blow the fuse and then you no longer have power available to anything on that circuit, including the motor.
Take a look at the electrical schematic for an induction electric motor:
http://tinyurl.com/qarvcqk
This one is a capacitor start motor, but a split phase motor's schamatic would be exactly the same, except that it wouldn't have a capacitor.
You have a Main winding and a start winding. The start winding is taken out of the circuit as the motor comes up to speed. If that were a DC circuit, you'd have a dead short through the main winding. The only reason the current through the stator in a real AC electric motor doesn't become excessive is that the main winding sets up an electric field through which the rotor windings spin. That action creates a current in the rotor windings, and the current in the rotor windings creates a magnetic field that opposes the current through the stator windings. This is why electric motor only draw a lot of current when they're starting. Once they're up to speed, the running current is actually quite low, making them highly efficient mechanically.
But, if you were to grab onto the rotor and prevent it from turning, you wouldn't have the electrical current generated in the rotor windings and there'd be no opposition to the current flow through the main winding, and the result would be that the current through the main winding would act like a dead short and quickly exceed the amperage of the fuse or circuit breaker that motor is on, shutting down electrical power to the circuit.
The motor would only go up in flames if it wasn't fused down to a lower amperage. If it were the convection fan motor in a convection oven that was on a 220 volt 50 amp circuit, then there could be 50 amps going through the stator, in which case it would probably get so hot the flammable parts inside it would probably catch fire. I've never had a convection oven, but I expect the motor of the convection fan has it's own fuse or fusible link so that if current through that stator got excessive, the fuse or fusible link would burn out. Otherwise, it'd be kinda dangerous and a potential fire hazard.
--
nestork


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On 10/1/2013 10:57 PM, nestork wrote:

Locked Rotor Amps rating. I think some motors do, also.
. Christopher A. Young Learn about Jesus www.lds.org .
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Cheap fans usually have a fusible link in them that blows. Better fans have a thermal switch that may latch "off" or may keep trying to start.
I would be careful of fans in unattended spaces and be sure it is isolated from flammable materials.
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On 10/1/13 9:06 PM, bob wrote:

The motors I'm around are three phase, 480 volt. I've seen wiring going to them burn but not the actual motor. You might want to put the wiring in conduit. The motors sometimes get awfully hot when they fail. More knowledgeable people here might be able to recommend some sort of motor starter as an additional safeguard.
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They go to motor heaven.
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On 10/3/2013 6:29 AM, Meanie wrote:

I've been wondering if I should float that answer.
What if the motor is religious? Do Jewish motors go different than Hindu or Buddhist motors? Would a Jehovas Witness motor door to door, looking for Heaven?
--
.
Christopher A. Young
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...and give all your money to yourself?
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It goes to electric motor heaven.
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On Tue, 1 Oct 2013 19:06:20 -0700, "bob"

The motor in a swamp-cooler of mine quietly stopped in the middle of the night a few years back. I woke up and happened to notice that it wasn't running, and went to investigate. Thick, dark smoke was roiling from the swamp-cooler. I immediately unplugged it and went back out to take the shrouds off. I got the hose and started squirting the motor down, but it took about 15 minutes before it stopped spitting and steaming.
This cooler was on a 1950s 15a circuit shared with other outlets and lighting. The breaker didn't pop.
I don't leave swamp-coolers on overnite anymore!
--
croy

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can't speak for the rest of teh country but here in AZ, at least in the "old days" when most people had swamp coolers, they were not hardwired. There was a hardwired receptacle placed on the wall where the cooler was hung and the cooler was plugged into that. Even for two speed coolers they had a multiple prong outlet that the cooler plugged into. It was not unusual to buy a new cooler every ten years due to them corroding away. You bought a new one, plopped it into the metal frame that hung on the side of the house, screwed the duct back on, plugged it in and away you went.
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wrote:

Ten years? Just how hard do you think it is to rewire a box?
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On 10/6/2013 10:25 AM, Oren wrote: ...

Virtually none were anything but window units back then other than commercial...
I've never seen anything but the wall outlet for one in a home; the load is simply the fan and a tiny little pump, not a compressor so there's no need for anything more than 15A service.
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I guess it's awkward to permanently wire something into a window unit. Just routing the cables would be a pain. Even our thru-the-wall ACs (one 240V, one 120V) were plugged in. OTOH, I'd expect a roof-mounted unit to be permanently wired.
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wrote:

It was a "built-in" unit, added sometime after the house was built, by the previous owners. It was stuck in a hole they cut in the side of the house (cutting right thru a diagonal brace, no less). It had a typical appliance-type power cord that plugged into the wall outlet. I had replaced the service entrance panel and installed all new breakers, but the wiring remained original.
I opted to replace it with a roof-mounted unit, hard-wired with its own 20a circuit, with all new wire.
Now we'll see what it does to try to get me.
--
croy

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