Electronics Forums > Electric Motor Burn Out?

# Electric Motor Burn Out?

TJ
Guest
Posts: n/a

 04-13-2005, 09:34 PM
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

----------------------

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 -

Paul Hovnanian P.E.
Guest
Posts: n/a

 04-13-2005, 09:52 PM
TJ wrote:
>
> 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.
>

--
Paul Hovnanian (E-Mail Removed)
------------------------------------------------------------------
Just say 'No' to Windows.
-- Department of Defenestration.

TJ
Guest
Posts: n/a

 04-13-2005, 10:06 PM
On Wed, 13 Apr 2005 14:52:34 -0700, "Paul Hovnanian P.E."
<(E-Mail Removed)> wrote:

>>
>> 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.
>>

>

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

----------------------

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 -

daestrom
Guest
Posts: n/a

 04-13-2005, 10:27 PM

"TJ" <insciens@_REMOVE_yahoo.com> wrote in message
news:(E-Mail Removed)...
> 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.
>
>

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

Martin G.
Guest
Posts: n/a

 04-13-2005, 11:13 PM
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.

"TJ" <insciens@_REMOVE_yahoo.com> a écrit dans le message de news:
(E-Mail Removed)...
> 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
>
>
> ----------------------
>
>
> 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 -

Roy Q.T.
Guest
Posts: n/a

 04-14-2005, 04:30 AM
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

Don Kelly
Guest
Posts: n/a

 04-14-2005, 04:48 AM

"TJ" <insciens@_REMOVE_yahoo.com> wrote in message
news:(E-Mail Removed)...
> 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.
>

---------------
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
All you have to do is think about it- that's why you were asked the
question..

--
Don Kelly
(E-Mail Removed)

TJ
Guest
Posts: n/a

 04-14-2005, 04:57 AM
On Wed, 13 Apr 2005 22:27:11 GMT, "daestrom"
<daestrom@NO_SPAM_HEREtwcny.rr.com> wrote:

>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
>

Thanks for your help. I appreciate you taking your time to respond.
I think I have it now.

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 -

TJ
Guest
Posts: n/a

 04-14-2005, 04:59 AM
On Wed, 13 Apr 2005 19:13:30 -0400, "Martin G."
<(E-Mail Removed)> wrote:

>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.
>

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 -

TJ
Guest
Posts: n/a

 04-14-2005, 05:07 AM
On Thu, 14 Apr 2005 00:30:21 -0400, (E-Mail Removed) (Roy Q.T.) wrote:

>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

Thanks for response. I appreciate you taking time to respond. With
the other two individuals responses I think I understand. Thanks
again.

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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