# enamelled wire properties

#### Keit

i want to pre-calculate the self capacitance of some transformers I need to design. To do that I need the thickness and dielectric constant (permitivity) of the enamel. When wire came from major western manufacturers, one could get that from the manufacturer. But I need to get data for wire of uncertain provinence (ie bought on eBay from Chinese sellers).
Is there an easy way I can measure thickness and dielectric constant of a sample of wire?
It is not sufficient to just measure the overall diameter with a micrometer, as the manufacturing tolerance on copper wire diameter is too large. I tried scrapping of the enamel so I could measure the copper diameter accurately but got inconsistent results - it is hard to tell whether I am scrapping off copper. I tried burning off the enamel with a gas flame, but that gave inconsistent results too.
Keit

#### Osvaldo de Banfield

Take two pieces of, say, 1 metre and twist them not too tightly. Thus, measure its capacitance with any known method: Q-meter, RLC digital, capacitance bridge, etc. Thus you will get an idea.

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#### basreflex

RFcafe has a list of dielectrics and lists enamel as 5.1. Still your winding method is a bigger influence.

#### Keit

Osvaldo, that's obviously true, but I need better than a vague idea. It is very costly to design a transformer and make it up to make it up, only to find that the self capacitance is twice or whatever that you thought it would be.
The capacitance measured is a function of the enamel thickness, enamel permittivity, wire diameter, and the permittivity of the air or transformer impregnation varnish surrounding the wire. It is surprisingly hard to twist two wires one metre long in a consistent way that matches the neat near zero spacing achievable on a coil winding machine, which has controlled wire tension.
Inter-tune and interlayer capacitance is the air or varnish capacitance in series with two lots of enamel, plus in series with any interlayer wrap. Thus In a transformer, each turn is not next to one turn, it is surrounded by turns on all sides. This means the ratio of the enamel capacitance to the other contributors is completely different to just two wires twisted together. Electric flux tends to concentrate where the wires touch, as enamel permittivity is much higher than air. You need to know the enamel thickness to sort it out.
I have been experimenting with ways to wrap 6 wires around a central wire, so that I can measure the capacitance between all six outer wires and the central wire. This is more simply translatable to the conditions in a transformer. But it is not an easy thing to do.

#### Keit

BaseReflex, a number of websites give wire enamel permitivity, varying over a 3 to 1 range. As do a couple of books I have.
RF cafe lists ""enamel" as 5.1, but also lists "polyamide", which is what most wire enamel actually is, as 2.5-2.6. Hardly helpful or confidence inspiring. There is at least 2 different chemistries used for wire enamel. I have wire of three different colours - I don't know what that means in respect to permittivity. Several different thickness are used. It is desirable to use Chinese supplied wire due to low cost and fast shipping, but God knows what the properties are. I want to measure and know, if I can.

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#### basreflex

one way to measure is to soak a known length of wire of known diameter in salt water and measure the capacitance, assuming the wire enamel is isolating perfectly. use a metallic beaker for ease of contacting the salt water. That should give you the dielectric constant if you know the enamel layer thickness, and wire thickness.

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#### Keit

That will measure the capacitance of the enamel over the full circumference of the wire, but will not help me one bit. I don't know either the thickness or the permittivity. If I knew one, getting the other is easy. Capacitance is a function of both thickness and permittivity - how do I determine one without knowing the other?
As i said, I need both, so I can calculate taking into account the degree of electric field concentration where turns touch each other.
How do i measure enamel thickness, given it is quite a bit smaller than the manufacturing tolerance of the copper wire itself?
In any case, the capacitance measured by the soak-in-liquid method will be huge compared to the capacitance of wires touching each other

#### edgarsls

Relative Permeability is what you're looking for. Not Permittivity. Just search for "relative permeability of 'x'"... Looking for that I came across these, which might be useful: https://www.researchgate.net/profil...ical-Properties-of-Enamel-Wire-Insulation.pdf
https://www.researchgate.net/public...r-High-Frequency-High-Voltage-Application.pdf

The insulation thickness should be on the datasheet for the wire you have.
The insulation thickness isn't the whole story since you wont get 100% stacking factor.

#### Elvee

The problem is complicated by two factors: one is that adjacent turns have a very small fraction of the total voltage between them, reducing the apparent capacitance by a factor of 1/r², r being the ratio of the total voltage to the inter-turn voltage and the other is more subtle: it is akin to a transmission line effect. When a signal is sent from one end of the wire to other, it doesn't arrive instantly, and the delay results in a virtual capacitance that can be extremely significant, sometimes larger than the physical capacitance for low-capacitance types of windings.
All of that is very complicated to estimate, which is why a test winding is the best option

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#### bucks bunny

one way to measure is to soak a known length of wire of known diameter in salt water and measure the capacitance, assuming the wire enamel is isolating perfectly. use a metallic beaker for ease of contacting the salt water. That should give you the dielectric constant if you know the enamel layer thickness, and wire thickness.
On the other hand it enables you to estimate the capacitance /1m of your unknown wire.
But without using a winding machine with controlled wiring tension and using wire of unknown origin -
you can not expect to achieve predictable results.

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#### tomchr

Paid Member
If you want controlled impedance it's usually best to shop at reputable vendors who specify such things. At least that's the case for PCB dielectrics and I would imagine it's even more true for wire enamel.

When you shop on ePay or AliBlahBlah you get what you get and you may not get the same next time you order. That's the cost of cheap.

Assuming you don't want to buy another spool of wire, your best bet is to measure the relative permittivity of the enamel on the wire you have and go from there.

Tom

#### JMFahey

Osvaldo, that's obviously true, but I need better than a vague idea. It is very costly to design a transformer and make it up to make it up, only to find that the self capacitance is twice or whatever that you thought it would be.
The capacitance measured is a function of the enamel thickness, enamel permittivity, wire diameter, and the permittivity of the air or transformer impregnation varnish surrounding the wire. It is surprisingly hard to twist two wires one metre long in a consistent way that matches the neat near zero spacing achievable on a coil winding machine, which has controlled wire tension.
Inter-tune and interlayer capacitance is the air or varnish capacitance in series with two lots of enamel, plus in series with any interlayer wrap. Thus In a transformer, each turn is not next to one turn, it is surrounded by turns on all sides. This means the ratio of the enamel capacitance to the other contributors is completely different to just two wires twisted together. Electric flux tends to concentrate where the wires touch, as enamel permittivity is much higher than air. You need to know the enamel thickness to sort it out.
I have been experimenting with ways to wrap 6 wires around a central wire, so that I can measure the capacitance between all six outer wires and the central wire. This is more simply translatable to the conditions in a transformer. But it is not an easy thing to do.
1) it is more than a vague idea, it is the capacitance per meter, go figure.
More accurate than the simulation you want to run because it is an actual measurement.

2) so the transformer is important?
Then buy from a reputable supplier.
No ifs or buts.

#### edgarsls

On the other hand it enables you to estimate the capacitance /1m of your unknown wire.
But without using a winding machine with controlled wiring tension and using wire of unknown origin -
you can not expect to achieve predictable results.
yes you can.
The limiting factor wont be the enamel thickness but the stacking and packing factors, you you can easily control in the winding process.

#### Tony Salsich

Much depends on not just the wire insulation but the winding method and the inter-winding insulation (tape or paper). If one has access to software that allows finite element modelling, then there is a chance of predicting the parasitics with some accuracy. The more pragmatic approach is to make a prototype and measure the transformer self resonance and inductances. See Ray Ridley's website for design guidlines: https://ridleyengineering.com/design-center-ridley-engineering.html
Dr. Ridley recommends measuring the self-resonance of the primary with secondary shorted over a wide frequency range as a way to 'fingerprint' the construction (check up on the manufacturer)

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#### JMFahey

.All of that is very complicated to estimate, which is why a test winding is the best option
+1000

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#### JMFahey

The more pragmatic approach is to make a prototype and measure the transformer self resonance and inductances
That.

#### wrenchone

What is customarily used in transformer construction is a nylon/polyurethane combination that is solder-strippable so that connections can easily be terminated. The combo goes by many names, Solderon, Solldereze, etc. A major wire manufacturer like Belden or Alpha will likely have dielectric data on that particular insulation film. It has been asserted that winding technique is more important than the characteristics of the film in determining the stray intrawinding capacitance.
This is true - if you go for the "easy" technique of winding layers back and forth rather than using "Z" winding, where one brings back the finish of the winding back to the start of the next layer and winds the next layer on top of that trailing finish. With "Z' winding, you can realize lower intrawinding capacitance than the usual technique of winding back and forth. It's somewhat of a PITA, as you are interrupting the normal flow of an automatic winding machine to get that cross-the winding termination, but it results in lower capacitance.

#### edgarsls

-Because most of the dielectric gap is filled by air not enamel.

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#### bucks bunny

That is the point