[major]

Well, there's the economic factor. Silver is approx. 40% more conductive than copper but the spot price is $9/ troy ounce while copper is $1.70/lb.

Hi Tesser,

My chart shows resistivity of silver at 9.9 and copper at 10.37 (annealed) and copper at 10.7 (hard-drawn). Units for both are Ohm*circular mills per foot at 20 degrees C. A far cry from 40%.

Regards,

major

 

[major]

Don't know where you got your information from, maj, but here's just one reference I pulled at random from the web:

http://www.reade.com/Particle_Briefings/elec_res.html

Cu is 2.65 x 10^-8 and Ag 1.59 x 10^-8 ohms per meter.

Hi Tes,

I think that web site is in error. Several others I just checked put it at 1.72 x 10^-8. I originally posted numbers from a copy of a page from a text book I've had nailed to my desk for about 10 years. Don't remember which book it came from. Also, it might be that the web site was giving a range. It says something like "1.673e2.65e-8", which is confusing. The resistivity of copper will vary with the specific alloy. But electrical grade copper should be close to 1.72 to 1.78.

Regards,

major

 

[major]

The link I posted also suggested that there are other reasons to use silver - and I quote:

"Coils of silver enamel wire produce a higher degree of electromagnetic flux creating more energy generating greater torque as compared to copper coils of the same weight."

and

"With almost same prices, imported silver enamel wire has an advantage over copper wire which is also its most important feature — interestingly only one third of silver enamel wire is enough to substitute copper wire. So this wire has not only reduced the cost of production but also the weight of electrical goods to a considerable extent."

Can you guys confirm this, or are they incorrect?

Hi uncle,

I'd say the guy that wrote that is full of crap.

Does a conductive carbon nanotube create a magnetic field the same as a metal wire?

It is not the conductor which creates the magnetic field. It is the current, or flow of electric charge. Actually, a conductor is not even needed if you have a flow of charge, like an electron beam.

Regards,

major

 

[major]

For once, major and I are in full agreement...

Hey Tesseract,

Sounds like the first time. Imagine that.

The guy that wrote that stupid article must own a sliver mine.

Have a nice day

major

 

[major]

The cost may be 77X but if the cost of copper is small compaired to the motor and the windings last forever and the power and weight savings are good than it may be worth it.

How many feet of copper wire with what type of insulation and what gage is needed to wind a particular motor, say 20 hp?

Hi kool,

How many pounds of wire is the correct question. Let's say 20 lbs of copper in the motor. And if you went to silver, for the same resistance, you could use 19 lbs. At today's spot prices, that would be an extra $5900 to save that one pound.

This is just the raw material cost. What do you think would be the cost to get the mill to pull silver in the correct dimensions and then coat it with the proper insulation? And to do copper, they usually have a 1000 pound minimum order.

You might as well buy the silver and put it in a safety deposit box hoping the price increase will do better than other investments. Putting it in the motor is a no-brainer.

Regards,

major

 

[major]

The question is not how many pounds because the wire is not sold by the pound but by the foot.

When was the last time you bought custom size magnet wire? I think it does boil down to pounds. And if memory serves me correctly, a required 1000 lb minimum buy.

Is the amount of wire determined by the number of turns of a given diameter with a given gauge of wire?

Sure, for a particular size coil. Realize that almost all the DC motors used by this DIY crowd use rectangular copper ribbon in armatures and series fields. The few that have shunt fields will use round wire, but that boils down to a handful of guys.

 

[major]

The other question is what type of efficiency gain do you get with the 1 lb gain in weight. Certainly 1 lb is not a good return on $5000. But if you get other gains because of lower resistance, heat dissipation, horsepower gain. $5000 may not be a bad investment. Especially if the part you invest in will have a very long life. How many times can you rebuild an electric motor?

If you keep the mass of silver equal to the mass of the copper, then you will reduce the resistance approximately 5%. Resistive losses account for maybe about 2% of input at rated load. So that equates to a 0.001 (0.1%) increase in motor efficiency or put another way, an improvement in losses of about 1%. So for $5000 (material alone) you make a 88% efficient motor 88.1%.

 

[major]

However, I *think* the main reasons Al is so popular in this application are it's low density and that it's easy to cast directly into the rotor slots. You can't do that with either Ag or Cu and, of course, both metals are much denser than Al so they would substantially increase the rotational inertia.

I like the idea of copper cages. A lot of makers use them, but insert drawn bars and weld the shorting rings. Casting aluminum has been a big cost reduction with just a small reduction in efficiency. Recently a lot of work has been done to develop copper casting methods with some good success. I think several makers adopting the method for premium motors. I haven't heard of any progress with casting silver into rotors

 

[major]

Any answer yet?

See post #13.