Aluminum windings for generators

[mdxinv]

All of the generators and alternators I know of use copper windings. However, aluminum is used as a conductor in some applications, most notably for high-tension/high-power transmission lines.

Aluminum would be much lower cost than copper for armature windings in a generator because of its lower density (2.8 g/cc for Al, 8.0g/cc for Cu) and lower cost per unit weight (~$1/lb for Al, ~$4/lb for Cu), which more than offset the 40% lower conductivity of aluminum versus copper. So, if aluminum works successfully for some applications, should it be used in generator windings?

For example, a generator with 5 lbs of copper windings can use aluminum instead for a $17/unit cost save. It seems that whatever special terminal ends or manufacturing considerations are necessary to make this work would be well worth it.

 

[itsmoked]

Except for the fact that you need far more cross section which means the iron structures then need to be larger. The iron is no longer cheap. Furthermore, small is beautiful.. In an applicational way.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[electricpete]

Aluminum does find common application as a conductor in the rotor of induction motor rotors below perhaps 250hp. The advantage there is primarily the ease of manufacturing a cast aluminum rotor (including oddbal bar shapes that are very important to motor performance characteristics).

I think keith is exactly right on the main reason why it doesn't find frequent application in stators. Let's say we want to keep the flux density the same and the current density the same. Then we need a bigger slot and the only way to make it bigger assuming we keep the stator I.D. the same and tooth width the same (to maintain flux density) would be to make the slots deeper which will cost more tooth iron and more back iron and also will likely inrease the leakage reactance. Another question I would have would be about the ability of the conductors to stay tight within the slot over time.

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[electricpete]

When I said keeping current density the same... I was talking about amp-turns per meter of airgap cirfumference (not amps per meter squared within the conductor). By keeping the full load current density the same and flux density the same we would achieve the same rating.

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[mdxinv]

Package size is certainly a consideration, but when the commodity cost of a device is a large percentage of the unit cost, there are bigger drivers than package that can influence what is best. Copper supplies are limited so prices increase as the world economy grows and demand increses, whereas aluminum and iron supplies are virtually inexhaustable.

In the case of an automobile alternator (I'll use 5 lbs of copper/unit as a base value), I beileve that if a new device with aluminum conductors were designed clean-sheet, the size might increase in diameter by ~10%, while the weight is ~20% lower and the cost is ~30% ($17 per my previous post) less. For this cost save, I am sure that there would be many millions of potential sales in the Auto industry with the only issue being a 15mm increase in diameter.

I don't agree that the larger stator would have as significant an increase in cost as the savings of having aluminum conductors insted of copper:

cast iron $0.40 / lb $0.0062 / cc
aluminum $1.00 / lb $0.0065 / cc
copper $4.00 / lb $0.0704 / cc

The stator structure should be cast aluminum since the cost per unit volume is the same as iron whereas the mass would be 60% less. The other benefit is that aluminum windings on an aluminum stator would stay tight since the thermal expansion coefficients are the same.

Thanks 'electricpete' and 'itsmoked' for your coments.

 

[rmw]

Aluminum has that evil wicked characteristic called 'cold creep' which wouldn't so much matter where the bars fit in the iron slots, but it would every where you had a clamped connection which there would be more of IMHO.

Actually, thinking about the topic of the OP, I kind of like the idea for rotor windings assuming the math would work out on the current carrying capability of the windings within the rotor slot geometry limitations. Sure would have a profound effect on the retaining rings and wedges.

rmw

 

[electricpete]

All good points. I think we all agree that the cheapest design which is still reasonably "reliable" is what is produced.

The material prices seem to provide a good starting point. Some other items that might be rolled into the thought process:

It is not a swap of one material for another. So using cheapest available material per volume doesn't necessarily win if you have more volume. Making the slots bigger makes the core bigger and the frame bigger.

The iron is not cast iron, it is laminated insulated steel sheets which have been processed for low electric losses.

Copper is readily available in magnet wire form which has various forms of strand insulation already applied. I don't think the same is true of aluminum.

Larger conductor also means more groundwall insulation expense. I think that eddy current considerations will also drive you toward similar strand size for aluminum as copper, meaning you invest more in turn and strand insulation as well.


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[electricpete]

I have heard of aluminum being used in transformers. Not as much as copper, but it's out there.

By and large the considerations for motor material economics are not hugely different than for transformers.

If we ask the question why aluminum finds more useage in an transformer windings than in motor stator windings, where does it lead us?

Beats me. Possibly the cold creep which has been mentioned twice. I think it has a bigger impact for motors than transformers (although it would not be good for either). Motors perhaps vibrate a little more. But most importantly the winding system of a motor is perhaps more complex and more delicate. The slot has special considerations and maintains it's mechanical integrity based on being tightly packed. The end turns are somewhat cantilevered and may present some unique requirements as well.

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[electricpete]

I'm not sure why I said cold creep since the device operates at elevated temperature where we all know creep is worse. Should've just said "creep"

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[davidbeach]

The biggest difference between transformers and motors, aside from rotation, is that transformer windings don't go into slots the way that stator windings do. For a transformer, aluminum windings just mean a physically larger coil. The larger coil will create more leakage impedance, but that may not be a problem as most transformers are ordered with a specific impedance and the design needs to provide that impedance so as long as the minimum impedance possible with aluminum windings is less than the required impedance it matters not that there could be less impedance with copper windings.

 

[electricpete]

An application perhaps along the lines of what rmw mentioned:

http://www.gepower.com/prod_serv/products/tech_docs/en/downloads/ger4212.pdf

Page 9 of 28:
Generator Rotors with Aluminum Alloy Windings
As conventionally cooled generators were increased to larger sizes, the generator rotor stresses increased to unacceptable levels in the rotor and retaining rings. Aluminum alloy (condal) windings were incorporated on some generator rotors, enabling the rotor size and ratings to increase and still allow conventional indirect cooling to be used in the design of these units. These units have provided many years of reliable operation. However, to ensure future long-term reliability when they are rebuilt, the design of these units requires special design and process considerations.

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[MJR2]

Aluminum magnet wire is available. Most of the DC electromagnets we build are made with aluminum. Generally speaking we don't have space issues and the aluminum works fine. As a rule of thumb an aluminum coil will weigh one half that of a copper coil having the same function. There are issues with creep at connections that have to be properly dealt with. Long term reliability is not a problem when designed properly.

Mike

 

[itsmoked]

MJR2; are you speaking of electromagnets like the scrap metal variety or tokamak, accelerator styles?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rcw retired EE]

mdxeng- You suggested using cast aluminum for the stator structure. That can be done for the frame, but steel laminations are still needed to channel and carry the magnetic field in the stator. Aluminum can't do that.

 

[mdxinv]

Here is the recent price history for copper and aluminum:

2002 2007

copper $.75/lb $3.30/lb
aluminum $.62/lb $1.15/lb
price ratio 1.20 2.87

I believe that only in the last 2-3 years copper has been so much more expensive than aluminum that there is a significant cost save for using aluminum coils for generators and motors. Perhaps this is why the industry hasn't come out with more aluminum windings in these devices because they haven't had time to change their designs and retool. The question is will copper remain this much more than aluminum? I suppose copper will continue to be three or more times the price of aluminum because copper reserves are limited and so price is sensitive to demand, whereas aluminum is inexhaustable and price depends mainly on energy costs.

good site for metal price data:

http://www.kitcometals.com/

 

[electricpete]

You might be right.

Some more thoughts.

Global VPI winding would go a long way toward easing concerns about windings becoming loose in the slots.

There is also the matter of differential thermal expansion, especially important for big machines. The difference between aluminum conductor and steel slot is bigger than the difference between copper conductor and steel slot.

http://en.wikipedia.org/wiki/Coefficient_of_thermal_expansion#Thermal_expansion_coefficient

Steel ~ 10 E-6/C
Copper ~ 17 E-6/C
Aluminum ~ 23 E-6/C





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[edison123]

Copper is preferred to aluminium in machine windings because

1. Aluminium cannot withstand stresses due to various coil forming operations like pulling, forming, spreading etc. That is the reason aluminium overhead lines are steel armored and electromagnet coils are normally simple circular shape not involving much stress.

2. Since aluminium is not easily weldable (porosity, hard aluminium oxide coating due to its large affinity to oxygen), making good coil connections requires a complex process.

3. Larger machine size (due to higher resistivity) leading to higher material, machining and handling costs.

4. Higher differential expansion as compared to copper leading to earlier insulation fractures (VPI or not).

Until these problems are solved, I don't foresee aluminium replacing copper in machine windings in the near future.



* I would go green if only I were not yellow *

 

[MJR2]

itsmoked-
All types of large industrial DC electromagnets. Scrap handling being a small portion of that. No tokamaks.

All of the comments made by many here are generally correct. However...

We wind thousands of very large aluminum coils each year. Most are simple round coils but many are not. When they are not extra steps are necessary.

We do weld aluminum wire dozens of times a day. Technique is important.

It does take more room for an equivalent aluminum coil. When space is an issue copper is the first choice. However square aluminum wire can sometimes get you by.

On a space for space basis copper will weigh about three times as much as aluminum. But usually the same functionallity can be had with copper at about twice the weight. So the copper coil ends up costing roughly five or six times as much. This is significant. But when you consider the other aspects of the design it is not always so.

Some of our larger individual aluminum coils weigh in excess of 8000 pounds and as coil assemblies in excess of 50,000 pounds.

Higher differential expansion is an issue but we have coils in the field in excess of 30 years.

Copper and aluminum wire are different in handling and design. Understanding of all the aspects allows one to chose the proper material. It was a big leap to go to aluminum years ago. My view is that copper will probably increase in price with respect to aluminum.

Mike

 

[motorspert]

I saw a generator in a repair shop about 2 years ago with aluminium windings. It was of AEG manufacture, 4 pole about 3MVA - apparently they didnt make many, we suspected reliabilty problems

The coils were about 200 turns of 80mm x 0.5mm strip. the aluminium was treated to oxidise the surface (aluminium oxide is a good electrical conductor with good thermal conductivity). These particular generators were in from the desert somewhere and had never been reliable - they had earth faults and inter-turn faults, I think they were manufactured in the early to mid 90's. We replaced with copper winding. I think the mechanical issues mentioned above contributed to their downfall.