Now that my utility is calling Cu a "precious metal" in its anti-wire theft campaign what is the status of Al wound motor development?
Supposedly the WWII era Al motors burned up easily but there shouldn't be any reason why they couldn't be protected very cheaply nowdays.
If a motor costs several cents/watt and develops 1 W/gm a motor and the cost of copper is 1 cent/gm maybe a third of the cost is copper.
Bret Cahill
One reason is no one is going to give up even a little of the 95% efficiency of Cu windings for just one cent/watt, the cost advantage of Al.
Bret Cahill
Aluminum wire requires different connectors than copper. It expands and contracts more with temperature changes. Check with the EEs over at alt.engineering.electrical. They could give you a definitive answer.
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Figure the cost of the electricity is an order of magnitude greater than the cost of the motor, maybe 30X more than the copper at current prices and even a 1% decrease in efficiency isn't worth the capital cost savings.
Al will probably never be more cost effective.
Bret Cahill
than copper. =EF=BF=BDIt
Al also melts a much lower temperature but these issues might all be resolved with proper design and controls.
At least one killer is the conductivity of Al is 30% less than Cu which means the coils in an aluminum motor wouldn't pack as tight for the same current & field and the motor wouldn't be as efficient as copper. Since the cost of electricity over the lifetime of a motor might be an order of magnitude more than the motor itself, the 30% increase in capital cost of a Cu motor over Al can be justified with only a 1 or 2% efficiency increase.
Copper probably can provide that efficiency increase.
OK.
I'll also ask about those "pegleg" transmission line towers that have one wood pole leg and one steel with a wood pole running across the top.
Bret Cahill
Perhaps silver could be justified in this application.
What application? Swindling the ignorant?
A 5% increase in conductivity won't result in anywhere near a 5% efficiency increase, certainly not enough to make up for an order of magnitude or so higher capital cost.
Maybe there is some really coincidencial situation where the only motor that will work must be slightly smaller that what is possible with Cu.
But that's a small market.
Bret Cahill
Al also melts a much lower temperature but these issues might all be resolved with proper design and controls.
At least one killer is the conductivity of Al is 30% less than Cu which means the coils in an aluminum motor wouldn't pack as tight for the same current & field and the motor wouldn't be as efficient as copper. Since the cost of electricity over the lifetime of a motor might be an order of magnitude more than the motor itself, the 30% increase in capital cost of a Cu motor over Al can be justified with only a 1 or 2% efficiency increase.
Copper probably can provide that efficiency increase.
OK.
I'll also ask about those "pegleg" transmission line towers that have one wood pole leg and one steel with a wood pole running across the top.
Bret Cahill
Using aluminum, larger conductors to keep the same resistance means that the motor becomes larger. This affects core losses and to counter that means more turns. In addition, the motor would have to run a bit cooler -this also adds to the size problem. It all comes down to a balance between the performance (including efficiency) and costs. going to aluminum may be OK in some cases but the overall savings may not be there. It would require a hard nosed analysis of all the factors. I expect that manufacturers have already explored this option considering today's copper prices.
With transmission lines it is easier- wrap the aluminum around a steel core and go with it- this was recognized in the late 1930's.
I've not seen the "pegleg" construction but I suspect that either the steel tower or the wood tower was there earlier and it was the cheapest alternative when upgrading transmission capacity to simply add the other tower and the crossbar to get the needed spacing between phases at a higher voltage. --
Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer
It might not even be lighter than a Cu motor, Al's only major selling point besides being a somewhat less desirable target for thieves.
If the cost of electricity went way down then Al might make sense.
If it were close they would be fine tuning their spreadsheets.
Sounds plausible. No one here would waste much time sourcing a matching pole just for aesthetics. Add the cheapest support and call it a day.
Maybe they could save some ground wire with the metal poles.
Bret Cahill
The resistivity of copper at 20C is 1.673 microhm-cm and its density is 8.96 g/cm3. Therefore if the cross section area of a copper wire is
1.673cm2 it will have a resistance of 1 microhm/cm and the mass will be 1.673 x 8.96 = 15.0g/cm.The resistivity of aluminium at 20C is 2.655 microhm-cm and its density is 2.70 g/cm3. Therefore if the cross section area of an Al wire is
2.655cm2 it will have a resistance of 1 microhm/cm and its mass will be 2.655 x 2.70 = 7.17g/cm.It is therefore clear that Al is a far better conductor on a mass basis. Connection is a problem but I was recently reading about a company that claims to have broken all power to weight ratio records with an Al motor. Each coil was a 3/4 turn and built up of 4 custom shaped rods of aluminium. Two of the rods where shaped to exactly fit into the holes in the magnetic laminations. These rods had threaded holes in both ends. The other 2 rods were screwed to the first two rods to connect them together on one side and to the neighbouring coils on the other side.
Sorry but I cannot find the link at the moment.
Well, none actually. The only thing that's been done with motors of any kind in the last 50 years is making A.I., Robots, Fiber Optics, and Digital Motor Controllers. Since that not only reduces the motor cost, it's get rid of the wiire theft problem too.
That doesn't necessarily mean a lighter motor. The efficiency might drop a lot.
Maybe there is a future for electric airships.
Someone else will.
Bret Cahill
Where do the efficiency losses come from? The IR losses in the wire dominate. In the example above I made the wire resistance the same in both cases.
The next biggest loss comes from eddy currents in the magnetic material. Why should there be a difference between an Al and a Cu motor? The magnetic field will have to travel a bit further in the Al motor but spinning a little faster will make up for that difference.
Of course there is. Electric motors already dominate the radio controlled aircraft business. That was probably where I read about the Al motor.
That's not transcontinental transportation but that's not necessarily an argument against all long range turbineless flight either.
Just last week they announced that they discovered a bird that flies
7,000 miles nonstop, the world record for the animal kingdom. Maybe it eats flying bugs on the way but it's still amazing. If a bird can do it . . .A bird might have to go 40 - 60 knots/hour to get its Reynolds number up to the magic 80,000 -- 100,000 where the drag coefficient drops precipitously. Some birds' feathers must act like dimples on a golf ball.
Have they tried dimples on radio controlled aircraft? The size and speed could designed around N re = 100,000 and dimpling could vastly extent the range of such craft.
Widebodies reach Nre = 100,000 going 0.1 knots so dimples won't work there.
If you could get the specific power up to 5 - 7 kW/kg even sacrificing some efficiency might be acceptable.
Bret Cahill
Aluminum is used in the cast rotors of squirrel cage induction motors to replace copper where cost is more important than efficiency or where the higher resistance of aluminum is indeed more important.
What would concern me the most about aluminum is its apparent tendency to fatigue quicker than copper.
Copper prices have dropped to 40% of their value 2 months ago due to the world credit crisis.
So there is always _some_ flexing or movement of magnet wire no matter how tightly wound?
Al has no fatigue limit. Sooner or later, with enough cycling, it will crack.
Steel would be even cheaper and the strains could be designed under its fatigue limit. It could then cycle forever without breaking.
Does copper have a fatigue limit?
Same if not a bigger drop for Al.
Bret Cahill
There are physical stresses on the coil due to the magnetic forces.
I presume you meant it HAS a fatique limit.
Unfortunately its conductivity is vastly lower; it is however common to include steel wire in the multistrand core of overhead transmission lines. Motors are however wound with single strand wire.
I understand what you mean about the lack of fatigue limit. It would no doubt be possible to make a steel wired motor as well as an aluminum unit. The question is: what will happen to the wieght, size, cost and efficiency of such a motor?
all metals do bar steel I think. Copper has greater malleability and ductility. Consider the case of copper wiring for housing which once had a reputation for starting fires (presumably becuase installation procedures hadn't been followed or developed). It can be pulled around tight bends and can be clamped into a crude and rather nasty screw terminal without wire protection. Aluminum can cause house fires if treated similarly. At the bare minimum the wire terminal needs protection from the screw (by a flat) and in reality should be ferruled within a corrosion protection paste or grease. I might add that the lastest Airbus A380 transmits its power electrically using aluminium wiring instead of hydraulically.
A fatigue limit just means the material can be stressed an infinite number of times below that limit, i. e. a steel in a watch spring. If a material doesn't have a fatigue limit, i. e., an Al in an aircraft wing, then, no matter how low the strains, with enough cycling, it is 100% guaranteed to crack.
At first it sounds crazy to intentionally design a plane with wings that are 100% guaranteed to, sooner or later, fall off, but the stress profile over the life of a wing is hard to predict, microbursts, rough landings, etc., so using a material with a fatigue limit doesn't help a designer much in predicting reliability. Al structures can easily last longer than steel that occasionally cycles above its fatigue limit.
In actual practice aircraft designers just keep the strains so low that 99.999% of the time the wing will last ten thousand years. Do a cost/benefit risk analysis to optimize your overall odds at survival and wing fatigue failure will be at the bottom of the list.
In fact, lot of lives probably could be saved _overall_ by shifting toward fuel efficiency and away from overly robust wings. The big picture must be spread sheeted.
Alloys seem to have lower conductivities than their pure element parent metals. The conductivity of iron might be higher than a steel but it's too brittle.
What about carbon fiber?
I once rewound a fan motor with copper of the same gauge. I couldn't get the coils tight and the fan never quite had the power of the original. I'm guessing it drew even more current than the original because there was less of an inductive effect to restrict current. Basically it was a short. The number of turns was less than the original but this would also be true for a tight Al coil using the same motor frame.
The effect on the field should be similar for the tight Al as the loose Cu using the same motor frame.
Maybe this could be mitigated somewhat if a motor could be specifically designed for a lower conductivity wire, but there still is no way to avoid at least some loss in efficiency.
. . .
No one mentions it because it was so gradual but the reliability of electric motors has gone way up. It's hard to find a vehicle without power windows and the windows always seem to work.
The relative cost of electric motors for power has always been low. That's why efficiency won't be easily sacrificed for cost. The cost of the electricity passing through a stationary motor running 24/7 is easily an order of magnitude more than the motor itself.
In contrast a rechargeable battery costs at least twice as much as the electricity it will ever hold in its lifetime.
To be sure an EV doesn't run 24/7 but if an EV lasted 100 years the cost of supplying batteries to an EV might cost 20X more than the electric motors.
The place to sacrifice efficiency to reduce costs is the battery.
Either the initial cost must be very low or it must cycle tens of thousands of times.
Bret Cahill
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