Creep of copper windings above 135C
Creep of copper windings above 135C
(OP)
When we think about temperature limits, typically we think about the insulation.
One thing I have always wondered about. If you look up creep in a materials textbook, you'll see that there is a threshhold temperature:
Iron and steel 270 °C
Copper 135 °C
Aluminium 7 °C
Below the threshhold temperature, creep does not occur. Above the threshhold temperature, creep can occur, although very slowly. The further above the threshhold, the faster the creep.
Aluminum can creep at room temperature. Copper can't creep at room temperature but can creep above 135C.
Can this cause any problems if temperature raises above 135C? I know aluminum transformer windings have a bum reputation for this reason.... can it affect the reliability of rotating machinery if we exceed the copper creep threshhold temperature?
I tried to think through the possible vulnerable areas:
1 - loosening of any compression connections in the connection end? I have seen winders use a metal ring to hold the conductors together but then it's soldered/brazed... ... I guess we don't rely on compression connections and I'd think creep is not a concern here, but I'm not positive.
2 – loosening of endwinding blocking tieing ? It seems that we assume the coils are rigid for endwinding bracing. Does it make any difference if they can bend over time or contract at pinch points over time? My gut feel is "no", since the vpi-impregnated tie materials are rigid, but I'm not sure.
3 – Slot section? I tend to think if it's wedged properly, there is nowhere for material to flow to.
4 - Random wound machines? I would think the endwinding tieing and the slot section loosening might be bigger vulnerabilities for random wound machines since there are inherent void areas between the conductors... if conductor bundle is squeezed copper will tend to flow into those voids over time and the coil bundle would become smaller / less tight.
Do you have thoughts on the above 4 or other areas or the overall subject? Any experience of poorer reliability with high temperature motors not related to insulation failure?
One thing I have always wondered about. If you look up creep in a materials textbook, you'll see that there is a threshhold temperature:
Iron and steel 270 °C
Copper 135 °C
Aluminium 7 °C
Below the threshhold temperature, creep does not occur. Above the threshhold temperature, creep can occur, although very slowly. The further above the threshhold, the faster the creep.
Aluminum can creep at room temperature. Copper can't creep at room temperature but can creep above 135C.
Can this cause any problems if temperature raises above 135C? I know aluminum transformer windings have a bum reputation for this reason.... can it affect the reliability of rotating machinery if we exceed the copper creep threshhold temperature?
I tried to think through the possible vulnerable areas:
1 - loosening of any compression connections in the connection end? I have seen winders use a metal ring to hold the conductors together but then it's soldered/brazed... ... I guess we don't rely on compression connections and I'd think creep is not a concern here, but I'm not positive.
2 – loosening of endwinding blocking tieing ? It seems that we assume the coils are rigid for endwinding bracing. Does it make any difference if they can bend over time or contract at pinch points over time? My gut feel is "no", since the vpi-impregnated tie materials are rigid, but I'm not sure.
3 – Slot section? I tend to think if it's wedged properly, there is nowhere for material to flow to.
4 - Random wound machines? I would think the endwinding tieing and the slot section loosening might be bigger vulnerabilities for random wound machines since there are inherent void areas between the conductors... if conductor bundle is squeezed copper will tend to flow into those voids over time and the coil bundle would become smaller / less tight.
Do you have thoughts on the above 4 or other areas or the overall subject? Any experience of poorer reliability with high temperature motors not related to insulation failure?
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RE: Creep of copper windings above 135C
Might be worth a putting a post with a link to this thread in the materials forum?
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If we learn from our mistakes I'm getting a great education!
RE: Creep of copper windings above 135C
The failure mechanism for aluminum wire under a screw head as I understand it it:
1> Load cycling causing thermal cycling.
2> Thermal cycling causing cold flow of the aluminum wire trapped under a screw head with a lower index of expansion.
3> When the wire is hot it expands and cold flows. When it cools down there may be an air gap under all or part of the mating surfaces.
4> Air contact with the metal surface causes slow oxidation.
5> This oxidation leads to more heating and higher temperatures, which leads to more oxidation and more resistance and complete failure.
This failure mechanism is fairly common with aluminum, but I have seen similar failures with copper connections occasionally. Once heat corrosion or oxidation starts with either material, failure is almost inevitable.
Bill
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"Why not the best?"
Jimmy Carter
RE: Creep of copper windings above 135C
There are three stages to creep which normally occurs at elevated temperatures coupled with load.
I would of thought that electric motors are designed in normal use to be below 135 degrees centigrade so that creep didn't occur, or if creep does occur then the strain in the copper components is such that the motor lasts for its designate life.
(see this link for brief description of creep)
http://www
A bigger problem that might occur if the copper exceeds 135 degrees centigrade is thermal expansion or differential thermal expansion which can generate high mechanical stresses, in fact thermal expansion is present during normal stop/start as this article describes:-
h
creep+in+copper+motor+windings&source=bl&ots=6cSWGFTIby&sig=0CpOPwD0RGKMbrH3wGsQTIQz3_k&hl=en&ei=GCw5S4HOLsKrjAetk9CmDg&sa=X&oi=
book_result&ct=result&resnum=3&ved=0CA4Q6AEwAjge#v=onepage&q=&f=false
Finally creep resistance of copper can be increased slightly with cold working and also by alloying the copper with other metals.
Check this link out:-
http://ww
When you choose a material click on mechanical properties and the creep data.
desertfox
RE: Creep of copper windings above 135C
Muthu
www.edison.co.in