# Metal Choice in Motors ( AL , Cu, Ag, etc. )



## IamIan (Mar 29, 2009)

Just in response to a request ... Here's a thread to expand on the concept.

The concept is the material used in motors ... copper is common but not exclusive and given the different characteristics of different materials ... I anticipate there are contexts and applications where other materials can actually be a better choice.

Of course at the extreme end we would have advanced materials like superconductors ... in the more near future though , we are probably more likely to be limited to the big three metals ... Silver , Copper , Aluminum ... and alloys of them.

Of the three:


Silver has the best / lowest electrical resistance per length of the same cross section of wire.
Silver has the best / lowest thermal resistance per length of the same cross section of wire.
Aluminum has the best / lowest electrical resistance per weight of wire.
Aluminum has the best / lowest thermal resistance per weight of wire.
Aluminum has the best / lowest cost per unit weight of metal.
Copper tends to fall in the middle not the best at any of the above , but in 2nd place out of the 3 at just about everything.


One other interesting phenomenon is the effects of the three in a AC and fluctuating magnetic field ... Where both Silver and Copper are Diamagnetic , but Aluminum is Paramagentic ... this leads to an interesting effect in the AC Ohms of equal length and cross section size gauge wire as the frequency changes ... bellow is an example graph of this effect for Cu and Al ... in that narrow band even with the same length and cross section the test transformer AC resistance is very close to the same value for either Cu , or Al.

In that narrow frequency band larger cross section of aluminum wire could weigh the same , but would have a lower AC resistance than copper , or silver of the same weight.... or that same cross section gauge wire Al transformer would weigh less and cost less.

There are also some other criteria designers need to be mind full of ... for example , the 3 metals are not all equally hard or soft to physical stresses ... the 3 metals do not have the same temperature operating windows ... the metals do not have the same metal fatigue characteristics ... the 3 metals do not have the same expansion and contraction characteristics ... and the 3 metals do not cost the same.

I think there is a knee jerk reaction by some people to always go with copper ... when I think a careful consideration of the context and application can sometimes leads to other choices that might be a better fit to the context and application.

On some scales this is already done ... things like aluminum core but copper clad wires ... for skin effects , weight and cost considerations ... etc.

And for the large % of most BEVs ground up or conversions ... weather it is ideal design for their context and application or not ... most usually stay restricted to what the market offers them ... it is often very expensive in terms of time and money to start designing and fabricating devices from scratch to be the best ideal fit for a specific context and application.

The $ listed for each of the metals in the attached is a bit dated ... but still the same ranking ... Silver most expensive of the 3 ... Copper 2nd / middle expensive of the 3 ... and Al least expensive of the 3.


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## rochesterricer (Jan 5, 2011)

I'm interested to see what comes of this thread. Even if EV's seldom or never use all aluminum motors(or other metals), the implications from a theory standpoint are pretty interesting to me. Thanks for starting this thread


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## Stiive (Nov 22, 2008)

Interesting thread. 
I was thinking the other day, if money wasn't an issue, how much (if any) better a silver stator/rotor induction motor would be. Probably not worth dwelling on too long though because money obviously is *THE *issue 


Your graph of Al vs Cu is great, but unfortunately that 'narrow frequency band' is a bit too high to be practical. You do see 400hz machines, but 50khz? way too high. Perhaps there's a way to reduce it to under 1khz?

Normally copper is used in stator, and aluminium in the rotor bars for low inertia (and cost). Though IMO copper rotors are taking off in a big way and can have significant advantages with the right control. They'd suck with no VSD as there is minimal rotor resistance and therefore low-end torque, but have much higher power and efficiency. With the right control and good overloading capacity, low end torque is not an issue - but the higher efficiency and power remains. 


I was gonna ask in another thread, has anyone on this forum built their own EV (>~60kW) sized motor? It'd be good to see a well documented DIY thread. I've often considered giving it a crack.... Maybe early next year...


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## IamIan (Mar 29, 2009)

rochesterricer said:


> I'm interested to see what comes of this thread. Even if EV's seldom or never use all aluminum motors(or other metals), the implications from a theory standpoint are pretty interesting to me. Thanks for starting this thread


You're welcome.



Stiive said:


> I was thinking the other day, if money wasn't an issue, how much (if any) better a silver stator/rotor induction motor would be. Probably not worth dwelling on too long though because money obviously is *THE *issue


The first couple things that occur to me to expect from silver vs copper ... assuming same length and cross section gauge of wire ... etc ... Feel free to double check this crude quick snap shot bellow in case I missed a detail.



With it's lower electrical resistance per volume ... The Silver should convert about ~8% less of the electrical joules of energy to heat joules of energy... which is a increase in motor efficiency.
With it's higher density ... The Silver of the same length and cross section of wire should weigh more ... about ~17% more for the part switched from copper to silver.
With it's higher atomic weight we will have about ~41% fewer Silver Atoms in an equal weight object ... adjusting for the difference weight from the difference in density we should still have about ~31% fewer Silver atoms in an equal volume ( cross section wire size and wire length )
With it's higher Molar Heat Capacity Silver needs about ~3.7% more joules of heat energy to have the same temperature change for the same number of atoms ... with the fewer atoms present from the above ... this means about ~28% less joules of heat energy will be needed to raise the temperature of the equal cross section and length of wire.
Sense the same cross section and length of wire will produce about ~8% less joules of heat energy , but needs about ~28% less joules of heat energy to increase it's temperature the same degree Celsius ... I expect about ~22% more net degree C temperature change for a given Watt of electrical power.
With it's better thermal conductivity the silver of equal surface area under equal temperature gradient should conduct or remove about ~7% more joules of heat energy away.
So I expect under the same Watts of electrical power flow for the same cross section and length of wire , the Silver will heat up about ~13% faster.
With its Lower Melting point , and higher thermal expansion ... I expect the Silver wire wound motor will have to have a much lower maximum operating temperature.
With its lower Hardness , Young's Modulus, Shear Modulus , etc ... I expect the Silver wire wound motor will have to have much lower physical / mechanical stresses on the wires... or a lower power and peak torque for the motor of otherwise equal size and design.
Last I checked Silver was around ~$34/oz ... vs Copper at about ~$0.29/Oz ... so I would also expect just the raw materials to build the motor to cost about ~137 times as much $... if there is no significant fabrication cost difference.


So that small bump in motor efficiency seems to come at a high price ... not just in $.



Stiive said:


> Your graph of Al vs Cu is great, but unfortunately that 'narrow frequency band' is a bit too high to be practical. You do see 400hz machines, but 50khz? way too high. Perhaps there's a way to reduce it to under 1khz?


I agree ... unless there is a practical and cost effective way to operate up an AC motor windings around that ~100khz frequency range ... Aluminum for the same cross section and length of wire will have more electrical resistance.

But ... If some cleaver chap someday does manage to develop a motor controller that can do that ... I think would have the potential to be a would be a major boom ... It would mean being able to use a lighter and less expensive aluminum to wind a motor with no additional loss in electrical resistance ... Or a larger cross section size gauge of aluminum could be used and still be lighter or the same weight as the Copper ... but in that narrow band of operating frequency it would now be less electrical resistance and potentially still lighter , all while still using less expensive metal for the windings.

The other potential ... is to skip the high frequency and just exploit the much lower density of aluminum ... thus allowing for a larger cross section gauge of wire thus having less electrical resistance , and still not weighing any more ... potentially weighing less... this has to be balanced a bit though ... and can not just be taken to any arbitrary level ... because larger cross section of wire not only decreases the electrical resistance for a given length of wire ... but it also increases the distance separation of the magnetic field generated by the current carried by that wire from one turn to the next ... So there is a tipping point where the larger wire gauge cross section becomes more of a negative than the increased electrical current flow becomes a positive (Diminishing Returns).

As we saw with Silver ... Aluminum windings would probably also have lower max operating temperatures , and lower mechanical stresses for an equal sized motor.

Bellow just a Similar to the above for Aluminum ... without the high frequency , or changes in wire size:



Aluminum would have about ~71% more electrical resistance.
Aluminum would be about ~70% lighter.
About ~235% more Al atoms for the same weight ... adjusting for the difference in density ... about ~30% few Al Atoms in same volume.
Aluminum has about a ~1% lower Molar Heat Capacity of equal number of atoms ... adjusted for different number of atoms is about ~29% less joules of heat are needed to raise the same volume of AL wire the same degree C.
With about ~71% more electrical joules of heat being produced at the same Watts of electrical power flow ... and about ~29% less joules needed per degree C ... I'd expect about ~245% faster degree C increase for a given watt of electrical power.
With Copper being a roughly ~69% better thermal conductor ... that means that the Aluminum of an equal volume and size will be that much less able ability to dissipate heat in the same temperature gradient... so I expect it to heat up about ~414% faster.
With Aluminum having a larger thermal expansion rate... and a lower melting temperature the device would have to be limited to a lower operating temperature
With Aluminum having Lower Young Modulus , Shear Modulus, Hardness ,etc... I expect it would also have to be limited to less mechanical loads and stresses.
Last I checked Copper at $0.29/oz but Al at only $0.06/oz ... making the Copper about 4.8x more expensive than the Al.


Now if we could get into that sweet spot of frequency for Al where the AC Ohms are virtually the same for the same size wire.



Aluminum would have about the same electrical resistance.
Aluminum would be about ~70% lighter.
About ~235% more Al atoms for the same weight ... adjusting for the difference in density ... about ~30% few Al Atoms in same volume.
Aluminum has about a ~1% lower Molar Heat Capacity of equal number of atoms ... adjusted for different number of atoms is about ~29% less joules of heat are needed to raise the same volume of AL wire the same degree C.
With about the same joules of heat being produced at the same Watts of electrical power flow ... and about ~29% less joules needed per degree C ... I'd expect about ~29% faster degree C increase for a given watt of electrical power.
With Copper being a roughly ~69% better thermal conductor ... that means that the Aluminum of an equal volume and size will be that much less able ability to dissipate heat in the same temperature gradient... so I expect it to heat up about ~49% faster.
With Aluminum having a larger thermal expansion rate... and a lower melting temperature the device would have to be limited to a lower operating temperature
With Aluminum having Lower Young Modulus , Shear Modulus, Hardness ,etc... I expect it would also have to be limited to less mechanical loads and stresses.
Last I checked Copper at $0.29/oz but Al at only $0.06/oz ... making the Copper about 4.8x more expensive than the Al... plus whatever the difference in cost would be for the fancy high frequency needed to get this effect.


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## PStechPaul (May 1, 2012)

Quite interesting. It may be worthwhile also to look at non-metallic materials for motors. The magnetic components now made of iron and steel might be replaced with powdered iron and ferrite. These would allow the use of much higher frequencies. Perhaps it would be possible to design a ferrite motor with several hundred poles and run it at 10kHz. If it was a SRM design, no windings would be needed in the rotor, and it could be molded or machined with hundreds of tiny pole pieces. The stator would need a comparable number of pole pieces, and would need windings, but they might be only a few turns at such high frequencies.

For the windings, perhaps a conductive ceramic or graphene might be possible. Graphene seems quite promising, with current densities a thousand times (or maybe a million times) greater than copper, although at this point I think it is only practical for use as bonding conductors in ICs and transistors. Here are some links:

http://www.zdnet.com/the-10-strangest-facts-about-graphene-3040093050/
http://www.sciencedaily.com/releases/2009/07/090729210454.htm
http://www.sciencedaily.com/releases/2011/04/110418114206.htm
http://www.futurity.org/top-stories/ripples-warp-graphene’s-conductivity/
http://www.rsc.org/ebooks/archive/free/BK9780854049844/BK9780854049844-00001.pdf
http://www.ionotec.com/conductive-ceramics.html
http://en.wikipedia.org/wiki/Graphene
http://en.wikipedia.org/wiki/Indium_tin_oxide
http://en.wikipedia.org/wiki/Conductive_polymers
http://en.wikipedia.org/wiki/Electroceramics


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## aeroscott (Jan 5, 2008)

IamIan , very nice . I only looked at Ag verses Cu on (Ir) conductivity and thermal conductivity . 
I would think strength could be compensated for with support . And temperature would be limited by insulation , unless using ceramic insulation . It's just the $4500 for 10 lbs. of Ag for a Prius motor . + 17% for added mas $750 .


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## IamIan (Mar 29, 2009)

PStechPaul said:


> Quite interesting. It may be worthwhile also to look at non-metallic materials for motors.


Interesting idea ... I'll have to look more into it ... Thanks for the starter links.



aeroscott said:


> IamIan , very nice . I only looked at Ag verses Cu on (Ir) conductivity and thermal conductivity .
> I would think strength could be compensated for with support . And temperature would be limited by insulation , unless using ceramic insulation . It's just the $4500 for 10 lbs. of Ag for a Prius motor . + 17% for added mas $750 .


Well ... If you have the 'just' , $5,250 for the Silver + $ , time, and skills for fabrication ... then by all means have at it and enjoy the about ~8% increase in efficiency ... But that seems like a big price to pay to me for about ~8% ... lots of other things I think would give a bigger bang for the buck ... although if you do do it ... please post ... that would be interesting.

Yes some of the strength / mechanical differences can be offset by some support ... but ... the force is on the wire that carry the electrical current that produces the magnetic field ... so yes support can help ... but ultimately even with support the current carriers themselves are just not able to handle as much physical stresses ... ie no matter how much support you have for a 'wire' made of play dough , it just doesn't itself have the strength ... yes I know this isn't play dough ... I just thought it would be an easy example of the concept of the limits support can help.

The lowered mechanical stresses might not be significant ... that will depend on the application / context ... but I would expect it to be lower by some amount.

As for temperature ... yes the electrical insulation temperature is also an issue to be considered ... but that doesn't really help the temperature issue of the metal difference itself... The silver melts at a lower temperature softens at a lower temperature has a large rate of thermal expansion and will heat up toward that temperature faster , under the same watts of electrical power flow.

Again the lower max operational temperature might not be significant ... depending on application / context ... but I would expect it to be lower by some amount.


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## TigerNut (Dec 18, 2009)

Apologies for not reading the linked material... but is consideration given to the alloys of the base metals that would typically be used? The mechanical, electrical and thermal properties of copper, aluminum and silver are each significantly affected by even small amounts of alloying element, and while the chemically pure materials often have exemplary qualities, the stuff that we can buy and use practically usually are less ideal (or at least, a lot more alike than what you'd expect from their CP characteristics) and thereby make it not as enticing to try the exotic solution.


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## IamIan (Mar 29, 2009)

TigerNut said:


> Apologies for not reading the linked material... but is consideration given to the alloys of the base metals that would typically be used? The mechanical, electrical and thermal properties of copper, aluminum and silver are each significantly affected by even small amounts of alloying element, and while the chemically pure materials often have exemplary qualities, the stuff that we can buy and use practically usually are less ideal (or at least, a lot more alike than what you'd expect from their CP characteristics) and thereby make it not as enticing to try the exotic solution.


100% agree.

So far I don't think anyone posted the properties of a specific alloy to consider... Did you have one in mind?

Another angle on this non-pure issue ... is even with buying initially pure metals ... Copper will not stay 100% pure Copper , nor will Aluminum , etc ... just exposure to the air prior to assembly will have an effect ... this is a factor that I expect would increase the fabrication side costs by another factor that would be in addition to the above mentioned material difference in costs themselves.

Although ... it is also possible as it is in many other applications that the right mix of an alloy might yield a net overall better combination of characteristics than just a pure metal would ... that's one of the reasons we use alloys in so many applications.


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## Jamie EV (Oct 3, 2012)

rochesterricer said:


> I'm interested to see what comes of this thread. Even if EV's seldom or never use all aluminum motors(or other metals), the implications from a theory standpoint are pretty interesting to me. Thanks for starting this thread


Has anyone considered weight as a factor in this discussion? Aluminum might pose a great advantage there....?


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## IamIan (Mar 29, 2009)

Jamie EV said:


> Has anyone considered weight as a factor in this discussion? Aluminum might pose a great advantage there....?


Agreed.

Aluminum for the same volume is about ~70% lighter than Copper.
That is potentially a great advantage... especially in mobile applications.

But there is a bit of a pull in both directions as well... because it also has more than 70% more electrical resistance , heats up faster , is softer, etc.

I would expect It to be rated for less kw of mechanical power per volume ... but more kw of mechanical power per weight... at least in that difficult high frequency range where the AC Ohms are nearly the same for Copper and Aluminum.

If you don't operate the Aluminum up in that very difficult high frequency range I think that you would also have less mechanical power per weight as well as the less mechanical power per volume.... although due to lower cost of the metal , I'd expect it to be more kw of mechanical power per $.


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## PStechPaul (May 1, 2012)

I think most of the motor weight is iron. Aluminum is often used in rotors where it is held tightly by the laminations so mechanical strength is not an issue. But high efficiency motors usually use copper. However the greatest weight saving may be switched reluctance motors which do not need rotor windings. See separate threads for more discussion.


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