multi strand vs single strand inductor winding

[hajj]

So here's the problem,
Some time ago I started using an ultrasonic cleaner to clean my LPs, it worked flawlessly until one day, the cleaner made some weird noise, and after investigating, it turns out one of the big ferrite core inductors in the circuitry was fried.
I sent it to my favorite winder and he told me that it's going to cost a ton to rewind it using the multi-strand wire that was used originally, but would cost much less (10x less actually) if it could be substituted for single strand wire with the same cross-section.
Now, I do not know what the original inductance of the coil was, and have to rely on reverse engineering the number of windings to get it right.

Now my question for the inductor specialists here:

Can single-core wire be used instead of multi-strand while maintaining the same number of windings and expect the inductor to be the equivalent of the original one? bearing in mind that it's most probably a high frequency type of application.

What other options could I have knowing that the company that sold me the Ultrasonic cleaner is now defunct and I can't find any literature about the machine online?

Thanks all

 

[jcx]

insulated multi strand or not?

multi strand individually insulated wires in "bunch windings" are an intermediate step to Litz to reduce high frequency loss

 

[hajj]

insulated multi strand or not?

"bunch windings" are an intermediate step to Litz to reduce high frequency loss

Strands do not seem to be individually insulated.
It does in fact look like Litz wire without the individual insulation but rather with one external insulation jacket for the "bunch".
So I'm guessing it's bunch winding...

Would losses (which I'm guessing are due to higher resistance at higher frequencies) be substantial enough to justify the extra cost?
What if I use thicker single core wire?

 

[AndrewT]

Resistance increases as frequency increases.
The HF wants to migrate towards the outside surface of each conductor leaving a lower current density in the core.

 

[abraxalito]

It depends what the frequency of operation is. What's the gauge (thickness) of the wire (the copper part, not including the jacket)? Given that your winder has said the price for the stranded wire is 10X more, he must have identified the wire as Litz type. Otherwise the cost difference between solid core and stranded (non-insulated strands) would be relatively negligible.

 

[hajj]

The wire must be around 1.8mm+ in diameter made up of strands in the 0.1-0.2mm range.
I have no idea what frequencies these are expected to work at, but it should be around 40khz.

 

[abraxalito]

Well it sounds like if you re-wire with solid core, it'll get rather hot - the maximum 100% penetration freq for 1.8mm wire is around 5kHz. The Litz must've been there for a reason so if I were you I'd not try to cut costs on this.

 

[hajj]

I see,
Thank for the replies!
I guess I'm gonna have to pay for rewinding the inductor as much as I paid for the whole cleaner...
What's the thickest gauge I could get away with?
Any formula that might help here? This way, I do my own homework instead of freeloading on your time? 🙂

 

[abraxalito]

We'd need to know more details of the ferrite core to make suggestions about possible cost savings. It might be possible to replace the inductor with a number of smaller paralleled inductors.

There are formulas known to the designers of high frequency magnetics (read SMPSUs) but they're fairly complex. As far as I'm aware they're more heuristics than closed-form equations as this stuff gets solved by numerical methods.

 

[hajj]

I found an online calculator for skin effect, and plugged in the 40khz number which gave me a skin depth of 0.38mm, according to the calculator, this is the "depth below the surface of the conductor at which the current density decays to 1/e (about 0.37) of the current density at the surface"
What can I use this number for?
Is it of any use?

 

[Elvee]

Is your "inductor" actually an inductor, or is it a transformer?
What shape is it (cylindrical, U-core, ....?)

 

[abraxalito]

I found an online calculator for skin effect, and plugged in the 40khz number which gave me a skin depth of 0.38mm, according to the calculator, this is the "depth below the surface of the conductor at which the current density decays to 1/e (about 0.37) of the current density at the surface"
What can I use this number for?
Is it of any use?

Its of some use, not much because you most probably have a multi-layer inductor. Proximity effect tends to domininate the losses - that's the tendency for the current to crowd into the areas of the winding which are closest to areas on adjacent windings carrying current. The skin depth alone would suggest you could wind with wires around 0.7mm diameter but in practice this would still give significant losses due to proximity effect.

 

[hajj]

Is your "inductor" actually an inductor, or is it a transformer?
What shape is it (cylindrical, U-core, ....?)

Hello Elvee, thanks for chiming in,
I'm pretty sure it's an inductor (I might not be able to do the calculations for it, but I can tell an inductor from a transformer 🙂 )
It's an EI ferrite (or powder) core inductor, or to be absolutely exact, it's a EE ferrite core inductor (used as a choke probably).
Hope it helps.

P.S: Belgium seems to have quite the promising team in the football worldcup

 

[Elvee]

Hello Elvee, thanks for chiming in,
I'm pretty sure it's an inductor (I might not be able to do the calculations for it, but I can tell an inductor from a transformer 🙂 )
It's an EI ferrite (or powder) core inductor, or to be absolutely exact, it's a EE ferrite core inductor (used as a choke probably).
Hope it helps.

If it is an inductor, the reason for multistrand is probably more dictated by eddy currents in the bulk of the copper than the skin effect, for two reasons:
-these losses are caused by the flux escaping the magnetic circuit, which happens in open-circuit or gapped inductors
-they are a proportion of the reactive power seen by the winding, which is normally large for an inductor unlike a transformer for example, in which the magnetising current is relatively small

This means that solid copper will probably heat up much excessively

P.S: Belgium seems to have quite the promising team in the football worldcup

I don't really care, football is not my cup of tea....

 

[hajj]

If it is an inductor, the reason for multistrand is probably more dictated by eddy currents in the bulk of the copper than the skin effect, for two reasons:
-these losses are caused by the flux escaping the magnetic circuit, which happens in open-circuit or gapped inductors
-they are a proportion of the reactive power seen by the winding, which is normally large for an inductor unlike a transformer for example, in which the magnetising current is relatively small

This means that solid copper will probably heat up much excessively

Thanks for the clarifications, (it does seem to be a gapped inductor btw)
I guess there's no way around a multi-strand version then....
I'll take Abraxalito's advice and try a DIY multistrand with individual strands in the 0.3-0.5 mm per strand... (give it a bit of margin)

 

[AndrewT]

The cost of a professional rewind is probably 95% made up from the time the Technician spends on the job and other overheads (office, electricity etc.)

The material cost of the rewind wire is tiny, almost certainly <5%.

If they order up a wire that costs 10times the solid core, then that adds 45% to the wire cost to make 145% of the cheapest way of rewinding the inductor.

Even if the Litz wire were 50times the cost of the solid core, that still only adds 245% to the original 100% giving a total very expensive rewind of ~3times the cheapest.

 

[hajj]

The cost of a professional rewind is probably 95% made up from the time the Technician spends on the job and other overheads (office, electricity etc.)

The material cost of the rewind wire is tiny, almost certainly <5%.

If they order up a wire that costs 10times the solid core, then that adds 45% to the wire cost to make 145% of the cheapest way of rewinding the inductor.

Even if the Litz wire were 50times the cost of the solid core, that still only adds 245% to the original 100% giving a total very expensive rewind of ~3times the cheapest.

I do understand the universal cost breakdown you present, but it does not seem to apply to my part of the world.
For example, I can get a very well made, custom wound, 3-4 secondaries, 650VA EI transformer for a bit over 45$.
What's making the price skyrocket in the inductor application is the scarce availability of Litz wire, making it much more expensive than common sense would price it at.

 

[abraxalito]

Thanks for the clarifications, (it does seem to be a gapped inductor btw)

You'll want to pay attention to the fringing flux around the gap then - this can lead to conductors in close proximity getting hot. A keep-out area is advisable close to the gap.

I'll take Abraxalito's advice and try a DIY multistrand with individual strands in the 0.3-0.5 mm per strand... (give it a bit of margin)

I'm not at all confident that this will have low enough losses. But if you're willing to 'suck it and see' (having a lot more time than money) it's worth a try.

 

[AndrewT]

Enameled copper wire is used in HF transformers in all the SMPS I have looked inside.
None have used Litz wire.

Thin multi-parallel individually insulated wire should be good enough to lower the losses due to skin effect.

Ask your re-winder what minimum thickness they can handle. Ask how many wires they can parallel.
Based on that, give them a new specification and ask them to re-quote for the re-wind.

 

[jneutron]

The flux is strong in this one...

You'll want to pay attention to the fringing flux around the gap then - this can lead to conductors in close proximity getting hot. A keep-out area is advisable close to the gap.

I was wondering if anybody would point that out. Excellent...

Proximity will indeed toast single conductors in this app.

For winding density, litz is absolutely the best. If that is not an option, use as many strands of insulated magnet wire as possible, while retaining the overall effective gauge. Two parallel is better than one, three, four, eight, gets better as the number goes up.

edit: my gallery has a comparison of solid 15 gauge vs litz 15 , note how the proximity losses scream up at the higher frequencies by two orders of magnitude. And that is with an air core. Don't remember how to link to it at the moment...got it..

Note that at 10Khz, it just takes off. at 20Khz, it's 80 ohms.



jn