Gorgon53:-
Don't worry about it. I have since obtained a copy of the relevant parts of IEC 60317, in particular IEC-60317-0-1-2013 which gives wire physical dimensions.
1. You are indeed reading IEC 60317 (Table 1 on page 10 of IEC-60317-0-1-2013 is the relevant table) wrong.
It does not give values for maximum enamel thickness. it does give the tolerance on copper diameter (eg 0.250 +,- 0.004 mm) minimum enamel thickness (eg for 0.25 nom wire, grade 1 enamel 0.017 mm), and conductor overall maximum diameter (eg for 0.25 mm grade 1 wire, 0.281 mm) You cannot subtract (0.25 - 0.004) from 0.281 to get 0.035 mm and call that the maximum possible enamel thickness. That simply IS NOT how the standard should be read.
IEC-60317, for 0.25 mm Grade 1 wires specifies a minimum enamel thickness of 0.017 mm. Your figure of 6.5 um is garbage.
IEC 60317 for 0.25 mm Grade 1 wire specifies a maximum overall diameter of 0.281 mm. That DOES NOT imply that it can be 0.281 mm when the copper diameter is only 0.246 mm. You need an allowance for conductor out of round (specified elsewhere in the standard) as well.
2. You cannot construct a 2 layer test winding, measure the self capacitance, and use that as a basis for linear scaling to the final transformer for several reasons. For a start, interlayer capacitance varies with the interlayer diameter - outer layers have a greater diameter. Secondly, the self capacitance seen at the terminals increases as the 4th power of frequency, and this increase is highest for a 2 layer winding and this increase decreases roughly exponentially with the number of layers.
Thirdly and most importantly, the scaling, which is non-linear, depends on the proportions of enamel thickness and enamel permittivity, as I stated before.
3. I have since devised a successful way to wind together 7 wires such as one central wire is surrounded by 6 other wires, with a moderate helical winding of consistent pitch. I get a consistent value of capacitance between the central wire and the 6 wires twisted around it and connected together. I devised a machine that resembles a tiny rope making machine, made out of cardboard and hobby glue. For calculation convenience I make the "ropes" 1 metre long - capacitance is in the range 300 to 450 pF which can be accurately measured. Machine tested for wires 0.25 to 2.0 mm gauge so far. Larger gauges give lower capacitance as expected (electric flux concentrates where wires touch. Obviously the capacitance measured is precisely 6 times the capacitance between any 2 adjacent turns scaled for length PROVIDED the transformer is wound hexagonal close packed with perfect packing (an ideal not possible in practice).
This is much quicker and cheaper than making a test transformer, even one with only 2 layers and no core. The 6 outer wires touch each other and the inner wire and all variables that occur in a transformer (variation in packing, winding inductance, effect of intra-layer wrap, etc) are eliminated, giving a much better basis for calculation taking into account these variables, all of which are known or easily estimated.
Now, all I need is to find a way to test enamel thickness and I have everything I need.
4. You seem to think that manufacturers would allow enamel thickness to vary 1:2.7. That is utterly ridiculous! You obviously have not worked in a production environment. If thickness varies 1:2.7 then the quantity of enamel purchased must vary 1:2.7. I can assure you, having been a production engineer myself, that such an extreme variation in purchase cost would trigger a company accountant and CEO to come down on the production engineer lick a ton of hot bricks. They would want him to improve his process so to stay close to the mandated minimum all the time - that is how companies stay competitive.
Don't worry about it. I have since obtained a copy of the relevant parts of IEC 60317, in particular IEC-60317-0-1-2013 which gives wire physical dimensions.
1. You are indeed reading IEC 60317 (Table 1 on page 10 of IEC-60317-0-1-2013 is the relevant table) wrong.
It does not give values for maximum enamel thickness. it does give the tolerance on copper diameter (eg 0.250 +,- 0.004 mm) minimum enamel thickness (eg for 0.25 nom wire, grade 1 enamel 0.017 mm), and conductor overall maximum diameter (eg for 0.25 mm grade 1 wire, 0.281 mm) You cannot subtract (0.25 - 0.004) from 0.281 to get 0.035 mm and call that the maximum possible enamel thickness. That simply IS NOT how the standard should be read.
IEC-60317, for 0.25 mm Grade 1 wires specifies a minimum enamel thickness of 0.017 mm. Your figure of 6.5 um is garbage.
IEC 60317 for 0.25 mm Grade 1 wire specifies a maximum overall diameter of 0.281 mm. That DOES NOT imply that it can be 0.281 mm when the copper diameter is only 0.246 mm. You need an allowance for conductor out of round (specified elsewhere in the standard) as well.
2. You cannot construct a 2 layer test winding, measure the self capacitance, and use that as a basis for linear scaling to the final transformer for several reasons. For a start, interlayer capacitance varies with the interlayer diameter - outer layers have a greater diameter. Secondly, the self capacitance seen at the terminals increases as the 4th power of frequency, and this increase is highest for a 2 layer winding and this increase decreases roughly exponentially with the number of layers.
Thirdly and most importantly, the scaling, which is non-linear, depends on the proportions of enamel thickness and enamel permittivity, as I stated before.
3. I have since devised a successful way to wind together 7 wires such as one central wire is surrounded by 6 other wires, with a moderate helical winding of consistent pitch. I get a consistent value of capacitance between the central wire and the 6 wires twisted around it and connected together. I devised a machine that resembles a tiny rope making machine, made out of cardboard and hobby glue. For calculation convenience I make the "ropes" 1 metre long - capacitance is in the range 300 to 450 pF which can be accurately measured. Machine tested for wires 0.25 to 2.0 mm gauge so far. Larger gauges give lower capacitance as expected (electric flux concentrates where wires touch. Obviously the capacitance measured is precisely 6 times the capacitance between any 2 adjacent turns scaled for length PROVIDED the transformer is wound hexagonal close packed with perfect packing (an ideal not possible in practice).
This is much quicker and cheaper than making a test transformer, even one with only 2 layers and no core. The 6 outer wires touch each other and the inner wire and all variables that occur in a transformer (variation in packing, winding inductance, effect of intra-layer wrap, etc) are eliminated, giving a much better basis for calculation taking into account these variables, all of which are known or easily estimated.
Now, all I need is to find a way to test enamel thickness and I have everything I need.
4. You seem to think that manufacturers would allow enamel thickness to vary 1:2.7. That is utterly ridiculous! You obviously have not worked in a production environment. If thickness varies 1:2.7 then the quantity of enamel purchased must vary 1:2.7. I can assure you, having been a production engineer myself, that such an extreme variation in purchase cost would trigger a company accountant and CEO to come down on the production engineer lick a ton of hot bricks. They would want him to improve his process so to stay close to the mandated minimum all the time - that is how companies stay competitive.