Electric Motor Burn Out?

What happens to cause an electric motor to burn up or short out?
I'm currently taking a physics class and this question was posed to
the class by the professor. In the movie "The Grinch Who Stole Christmas" the Grinch has a large toy monkey that crashes two symbols repeatedly. He is letting the toy slam the symbols on his head to block out the noise of the Whos in Whoville singing. At some point he decides to stop the toy and just reaches up and grabs its arms and stops it from moving its arms. Sparks fly and the toy is burnt up or broken.
What happened to the motor? Why did it burn up?
My understanding of the electric motor (DC current) is that there is a metal loop that is between two magnets. When current is passed through the loop the magnet field causes the loop to torque. The amount of torque is dependant on the strength of the magnets and vertical length of the loop in respect to the magnets. When the loop is stopped and current is still flowing through the loop why does it burn up?
Also is there more, less, or the same current on the negative leg when this occurs?
Thanks in advance. If this is not the correct discussion group to pose these questions in then accept my apologies and could someone please suggest a more appropriate group.
TJ
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People like you and I, though mortal of course like everyone else, do not grow old no matter how long we live...We never cease to stand like curious children before the great mystery into which we were born.
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TJ wrote:

alt.homework.answers
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Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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On Wed, 13 Apr 2005 14:52:34 -0700, "Paul Hovnanian P.E."

That group is not one listed by my newsgroup provider. I also did a search for that group on yahoo and google and I didn't turn up anything.
TJ
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People like you and I, though mortal of course like everyone else, do not grow old no matter how long we live...We never cease to stand like curious children before the great mystery into which we were born.
- Albert Einstein -
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Two things are at work. The amount of heat generated in the rotating 'loop of wire' is a function of I^2*R (current squared times the resistance). The amount of heat removed is determined by materials, insulation, and the flow of air around the wire. When heat generated is much higher than heat removed, the temperature rises. If the temperature gets hot enough to melt insulation, then a short circuit develops.
Anytime a motor is 'stalled', it runs the risk of burning out (except for very specially designed motors intended for constant stalling). When the motor stops turning, there isn't any air moving past the coils (or coils moving past the air, it's all relative). So there is less cooling to remove heat from the rotor. Secondly, with a stalled rotor, the current is very high so the I^2*R heat generated is very, very high. The combination of low cooling and high heat generation can quickly overheat the insulation and burn out the motor.
Of course, in the movie, they undoubtedly added some pyrotechnics set off at the right time by someone off-camera. Usually real motors take a few minutes to overheat. The result is a lot of 'acrid odor' and smoke given off. Then simple flames and blown fuses.
daestrom
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On Wed, 13 Apr 2005 22:27:11 GMT, "daestrom"

Thanks for your help. I appreciate you taking your time to respond. I think I have it now.
TJ
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People like you and I, though mortal of course like everyone else, do not grow old no matter how long we live...We never cease to stand like curious children before the great mystery into which we were born.
- Albert Einstein -
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Insulation can also wear out with the time even at normal ambient temperature and normal usage. Varnish is a polymer that is sensitive to thermal aging. With the time, oxygen will migrate into it and cause oxydation, chemical bonds will be broken and crack will appear. Then in these cracks, if voltage is sufficient you can have short-circuit, then then motor will burn.
Martin G.

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TJ: The motor burned out because: He is The Grinch after all.
I guess the movie producers wanted curious viewers to see it this way};-)
The dynamics of a motor are such, that when a voltage is applied and a current starts to flow in the coils.,considering a motor is optimally designed, meaning: It is made to make the best use of electricity to magnetically move the rotor on the stator, if the magnetic field produced by the stator isn't somehow dissipated or expended on the motors rotor function, it will heat up the Mechanism from the induced back emf on itself, Bust an imaginary Wire inside causing it to Spark Off the excess Electricity, brown out then burn up to a smoking halt.
Of course that is just a Hollywood version made-up with special FX.
Only a medium - high powered drive mechanism can possibly spark off and burn out (not necessarily in that order) and probably from excessive current on a Faulty Device or Part, A toy motor intrinsically would not explode from stalling or stopping in itself.
Of Course: we could probably design one in here that could do as well };-)
The real (Toy Scenario) thing would just run your batteries dead and probably ruin the motor in it's course., with only half of the described patterns above.
oy
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On Thu, 14 Apr 2005 00:30:21 -0400, snipped-for-privacy@webtv.net (Roy Q.T.) wrote:

Thanks for response. I appreciate you taking time to respond. With the other two individuals responses I think I understand. Thanks again.
TJ
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People like you and I, though mortal of course like everyone else, do not grow old no matter how long we live...We never cease to stand like curious children before the great mystery into which we were born.
- Albert Einstein -
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TJ: The motor burned out because: He is The Grinch after all.
I guess the movie producers wanted curious viewers to see it this way};-)
The dynamics of a motor are such, that when a voltage is applied and a current starts to flow in the coils.,considering a motor is optimally designed, meaning: It is made to make the best use of electricity to magnetically move the rotor on the stator, if the magnetic field produced by the stator isn't somehow dissipated or expended on the motors rotor function, it will heat up the Mechanism from the induced back emf on itself, ---------------- Where the hell did you learn this nonsense?
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I read a book on it by; D.Kelly
Well maybe it isn't D.Kelly who wrote it, but some of it is in books, like " The Grinch that stole the Christmas "
i see he's trying to steal the newsgroup too
oy
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I read a book on it by; D.Kelly
Well maybe it isn't D.Kelly who wrote it, but some of it is in books, like " The Grinch that stole the Christmas "
i see he's trying to steal the newsgroup too
oy --------- No, I am not trying to steal the newsgroup. It is simply that what you said was quite wrong. Should I applaud?
Quote:
"if the magnetic field produced by the stator isn't somehow dissipated or expended on the motors rotor function, it will heat up the Mechanism from the induced back emf on itself,"
End quote:
I simply wondered where you learned this as it is nonsense.
Somehow, somewhere, someone led you badly astray.
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Don Kelly
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Oh the Hell who you & your accusations Grinch Kelly .....you think just like horrne., everybody is wrong but you.... In time we'll see who is wrong and mislead.... the stench that emanates from your post over a motor being a generator & a restricted motor not overheating is amazing. I don't have to see things your way... you dullard.
You & horne are very adversarial and you can both go jump in a lake somewhere...
State your case on the Topic and Leave me & my analysis alone... I know when to admit I'm wrong without someone trying to steal the sunshine from over me ..... Who Appointed you High Exhalted Mystic Ruler anyhow....
oy
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Oh the Hell who you & your accusations Grinch Kelly .....you think just like horrne., everybody is wrong but you.... In time we'll see who is wrong and mislead.... the stench that emanates from your post over a motor being a generator & a restricted motor not overheating is amazing. I don't have to see things your way... you dullard.
You & horne are very adversarial and you can both go jump in a lake somewhere...
State your case on the Topic and Leave me & my analysis alone... I know when to admit I'm wrong without someone trying to steal the sunshine from over me ..... Who Appointed you High Exhalted Mystic Ruler anyhow....
oy -------- I did NOT say that a restricted motor doesn't overheat. Of course it does but not for the reason that you gave.
Read what I did say. What I did say was:
a) DC and AC motors can be and are run as generators- no switching required. There are many examples of that. Others have mentioned some examples. The equations describing the behaviour of a generator are exactly the same as for a motor- all that happens is that the direction of the current changes (DC machine). Supply design (i.e. rectifiers) may prevent this from happening in some cases but the machine itself inherently doesn't care.
b)Your statements regarding flux dissipation and heating due to back emf are simply wrong. No ifs and buts Note that when a DC motor is stalled, the back emf is 0 but the current and heating is a maximum. The stator (field in this case) flux is not affected unless there is a series winding present. With an AC machine, the mechanism will be different but the air gap flux is not greatly affected by load or the lack of it. No relationship between coil heating and flux exists.
Sorry, but the theory and the facts back me up. Do you want an analysis of either DC or AC motors/generators? - I can give you that, from a simple to a complex approach but rather than take it from me, try a good book on motors and generators.
I'm not trying to put you down and I am sure that there are areas where you know more than I do- this isn't one of them.
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Don Kelly
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I am sorry but I am not in the mood to discuss this any further, the kid wanted to know How they Blew up the toy in the movie., they told him how, I just wanted to expand on a premise TV Land may employ to give some Hollywood Theory on how it happened....
You took this post to serious, that's all, I am not offended, but surprise on the load of carp you can dish out on motors & generators yourself };-) I'lll be reviewing Both subjects to they're entirety soon enough... Thanx
oy
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(DonKelly)wrote::/ No relationship between coil heating and flux exists.
- this isn't one of them. ?????
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You are too conventional to see it at work.
surely there is nothing wrong with seeing things from the bottom up (or
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On Wed, 13 Apr 2005 19:13:30 -0400, "Martin G."

Thanks for the assist. Between you and daestrom I believe I understand.
TJ
----------------------
People like you and I, though mortal of course like everyone else, do not grow old no matter how long we live...We never cease to stand like curious children before the great mystery into which we were born.
- Albert Einstein -
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--------------- What else is involved in the motor? Why do you have a current? What happens when you turn a coil in a magnetic field (hint: Faraday). What effect does this have on the current? What makes you think that the current will be the same when the motor is not turning ?
as for the current on the negative leg- at some time in class there must have been some mention of KCL (what goes in comes out). I'm sure that all the information you need has been given to you- hence Paul's reference to alt.homework.answers. All you have to do is think about it- that's why you were asked the question..
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Don Kelly
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