Is all magnet wire the same?

Kingfish

Kingfish

100 MW
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Feb 3, 2010
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Redmond, WA-USA, Earth, Sol, Orion–Cygnus Arm, Mil
Actually what I really want to know is where to locate the best magnet wire for crafting custom windings.

Greetings.
I took a break from motors and magnets over the holiday and began thinking about the winding material for my AF motor designs. In the past we’ve discussed magnetic wire, flat wire, and Litz wire. The conclusions in the reference book on AF motors suggest tightly-twisted wired is likely the best solution.

There are also coatings, cores, and plating that can be applied to the wire, cable, and insulation which allows for higher than nominal currents. I recognize that this could be the province of hyper- and superconducting wire, and obviously this path is impractical. Yet I think it worthwhile to investigate high-current low-resistance conductors that operate in hot conditions.

Are there folks here on ES that have experience with using these conductors or highly effective magnet wire in windings?
For reference: Magnetic Wire & Nd Magnets Reference

Thanks, KF
 
Toorbough ULL-Zeveigh said:
not sur what to make of this.
sales pitch sounds good tho.
klik on the link repeatedly & Dr. von Braun, errm... Levine is likely to show up.
http://www.sodiumwire.com/

not taking a break for hanukkah?
Click to expand...

For a motor, volume vs conduction is most critical. Sodium = fail.





If the motor is coreless, you MUST use litz. No way around it. You will notice every high efficiency core-less motor uses litz. This is because the core normally guides the flux through the tooth to prevent it from being a massive eddy current loss, and coreless doesn't have this protection.

If it has a core (and using super-conductor wire is out), ideally you would want bare silver wire through the slot as a solid chunk with just enough air gap or stator insulation to keep it from shorting. The next best thing would be the same design, but with copper. The next best is copper in a shape that enables it to achieve the highest fill%. Generally this means square, and the insulation ideally would be infinitely thin with infinitely high electrical resistance, superconducting thermal resistance, and infinitely high melting point. Square is extremely difficult to wind though, so folks settle for round, and the 240C temp insulation cracks so easily, most folks settle for 200C insulation. While both of those things are compromises, they are the best option of what is feasible for a motor builder with resources closer to $1 than $1M dollars.
 
Good points Luke: I forgot to mention that this is coreless AF.

In the Reference provided I struggled to find a wire of suitable size to match the particular winding. (Admittedly, I originally spec’d flat wire). Can you tell me if you have a manufacturer of preference, or if there is one local shop that could be contracted to fabricated windings with Litz?

Thanks friend! :)
~KF
 
I buy my litz from ebay. The sizing is odd for it, but equalivant gauge sizes from around 9-11awg up to >30awg are generally always available in some auction.

If you want it wound right, you're gonna need to make a winding mandrel, wind it yourself, lay it all in a mould with the intented hardware parts, and pour the encapsulating material yourself.
Production shops are gona be cost prohibitive to do it the right way.
 
OK, understood.

Luke, I’m digging around in fleaBay and I have a question:
I get that Litz can be several strands of a particular gauge. However I didn’t expect to see strands in bundles. This raises a new idea...

  • If I have a “cable” of Litz wrapped around a shape for a given length, do I treat that as one winding, or would it be possible to use each bundle within the assembly as windings (however I join them – series or parallel)? Clear as mud?
  • Secondly, how would I rate the current carrying capacity at temperature for any given cable since the chart that I’ve been using has generally been laughed out of significance? :roll: :wink:
Thanks, KF
 
Kingfish said:
“…how would I rate the current carrying capacity at temperature for any given cable…”
Click to expand...
Since the question remains unanswered I have gone to the trenches trying to sort this out.

Allow me to work up a case study. If this seems to be familiar territory for some… it is; I’ve borrowed this out of Plan-D from Doing the Math. :wink:

  • One Winding, carrying 25.4 A @ 112 Hz (discounting PWM) and 34.6 V. I have already determined the length of the conductor to be 448.3 mm, and using 20 AWG flat wire this could be achieved with 22 turns relatively easily. But we’re no longer interested in flat wire and we have to convert this analysis somehow into a Litz wire solution.
Caveats:
  • The solution can be multiple columns and rows to achieve a like end-result.
  • These columns need not be in the same stator if higher flux-density can be encouraged.
In digging, I found a couple of differing sources of information for determining current-carrying capacity. However – I feel compelled to make a brief statement for clarity:

  • I am doing this diligence in a vacuum. I am asking for help; if you have better reliable sources or math please by all means forward it on. I’d hate to get far down the path of calculation, commit to purchase, only to discover it was foolhardy. Make sense? :)
There are two Litz wire manufacturers that lent some illumination on this topic, one being easy to follow and the other too abstract. Allow me to demonstrate the findings:

LitzCurrent.P1.png


Part 1 seems easy enough to grasp. If we compare and contrast with the Powerstream tables it doesn’t look too promising, in fact it is even more conservative.
Source: http://www.mwswire.com/faqs.htm#mw6


LitzCurrent.P2.png


Part 2 doesn’t make sense to me at all and therefore the formula must be in error or the explanation too cryptic. The Source describes two versions which have been denoted as A & B (hopefully this is readable).
Source: http://www.coonerwire.com/Products/Litz/DesignD_2.html

So the question remains open:
Kingfish said:
“…how would I rate the current carrying capacity at temperature for any given cable…”
Click to expand...
Confuzzled, KF :?
 
The continuous current carrying capability simply depends on resistance, insulation temp rating, ambient temperature and your application's temperature delta. It is the last part that can be difficult to estimate.

But to be clear about the others first, let us say that, at 100K delta your machine is able to shed 1KW.
By simple estimation, if it produced only 500w of heat, it would be 50K above ambient.

Copper has a resistance of 16.8mOhm/mm^2/m.
Let's say you run 100 meters of 1mm^2 inside the machine, that would be 1.68 Ohm.
Power loss Watt = I^2 * R
1000w = 1.68 Ohm * I^2
1000/1.68 = I^2
sqrt (1000/1.68) = I
I = 24.4 Ampere is allowed if insulation rating is 200C, ambient is 50C, delta is 10w/K, and you want a safety margin of 50C.

So, probably not much help at all, since knowing the w/K value for the machine is probably more guessing work than anything. Litz would have slightly higher delta than flat wire just getting through all the tiny insulators and air spaces, I guess.
By utilizing the thermal conduction values of your materials, crossections and lengths, fudge factors for joins, radiation and convective losses, it should be possible to estimate the true current rating within a few hundred percent or less.
 
If you wanted to know the transient current rating it'd be much easier.
Copper has thermal capacity of 4 J/g.K, meaning, if you've got a gram of copper, and want to heat it by one K, you must apply 4J (watt-seconds) to it.
volume of 100m 1mm^2 wire is 100 cm^3, according to density 9 g/cm3 it's 900g heavy. If the heat had nowhere to go, you'd apply 4*900*100 joule to rise it by 100K. That is 360KW for one second.
I^2 * R = watt
sqrt (w/r) = I
sqrt (360 000/1.68) = 463 Ampere for one second.

Divide by as many seconds as you need the rating for.
 
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