Transformer winding

[anishkgt]

A few questions about the insulation part of a transformer.  Is it kypton or Mylar tape that I should go for or both one over the other ? What would be thickness for the tape to be used and how many turns of it. On googling i could see several types of magnet wire. I am just rewinding an existing microwave transformers primary winding which happened to be damaged during the process of removing the secondary.


Thanks in advance.

[MrAl]

Hi,

Back in the 80's we used Mylar.  Dont remember the thickness though.  It will depend on what the difference is between adjacent winding turns (the winding just under the top one).  If the voltage is low there then thinner can be used, if higher voltage, then more than one layer of tape.  This can be calculated from known the insulator properties of whatever material you use for the tape.  The temperature rating will also depend on this.

[anishkgt]

Appreciate your response. Thank you. I will just need to rewind the primary of microwave oven transformer which I had damaged while removing the secondary. The existing secondary was aluminum 17AWG. I was thinking of going with copper of the same gauge. The previous had 2cm air gap on the primary from the core. How important is that ?


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[james_s]

I used mylar tape when I wound some transformers, these were iron core mains frequency types. The gap is probably not nearly as important assuming you are replacing the HV secondary with a much lower voltage secondary. You will want to remove the magnetic shunts too, assuming you don't want current limiting.

[anishkgt]

Thanks James. I will be using 2 gauge thick soft wire with 3 turns in the secondary. So I would need that air gap I guess. I last thing how many turns would I need for 17AWG copper
Magnetic wire on the primary.


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[james_s]

Actually I started thinking about it and I think the tape I used was polyester, it was slightly stretchy so it made a really nice clean fit.

Primary turns depends on the core, there are ways to calculate it based on the cross sectional area, you can assume ordinary transformer iron as the material. It's much easier to count the number of turns you took off though.

[anishkgt]

Would it be ok to wind the new 17 AWG (haven't bought it yet) wire to the same turns as the previous one that was aluminum. Would it matter, like would it be dangerous if the winding's are higher or lower ? Would increasing the thickness of the wire increase current at the secondary ? My mains is 240. I am using it get the most current out of it.

Secondly i know the dimension of the core (being the center part of the 'E' piece) but how do i convert it to mm² as that is unit used to determine the area for the core.

Thanks in advance.

[james_s]

You multiply the mm of one dimension with the mm of the other dimension, and that gives you the square mm, just like finding the square area of any other unit, imagine looking at the center leg of the core from the end.

Copper has a lower resistance than aluminum so you can get away with a smaller gauge. The turn count on the primary is somewhat important, if you have too few turns you will saturate the core and these transformers are already cost engineered to use as little material as possible so there is really no margin to reduce the primary turns lower than what it came with, but you can go a bit higher if space allows. You can calculate the resistive losses of the winding based on the length of wire and the RMS current you expect to flow through it and that is useful to calculate the wire gauge. Transformer design is not trivial, there are a lot of factors to consider and everything is a compromise.

[MrAl]

Appreciate your response. Thank you. I will just need to rewind the primary of microwave oven transformer which I had damaged while removing the secondary. The existing secondary was aluminum 17AWG. I was thinking of going with copper of the same gauge. The previous had 2cm air gap on the primary from the core. How important is that ?


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Hi,

You're welcome :-)

I guess you mean the distance between the primary winding and secondary winding or do you mean the distance between the primary coil and the metal of the core itself?
If this is the distance between primary coil and secondary coil then that distance has two main effects.  The first is isolation between the primary and secondary wires to keep them physically apart and that makes the transformer safer for humans to use.  The second is the leakage inductance, which aids in filtering the DC when using a rectifier on the secondary but also could act to reduce voltage somewhat, so it's a tradeoff.

The formula for the minimum number of turns comes from this transformer equation which is easy to remember:
Bmax=E*1e8/(4.44*FAN)
where
Bmax is the max flux density in Gauss,
E is the AC rms voltage,
F is the frequency in Hertz,
A is the core area in square centimeters,
N is the number of turns.

Since this is the actual B and core material can only go so high and there must be some slack, we might go with Bmax=15000 Gauss rather than the 20000 Gauss that the laminations are probably good up to.  We might even go less than that, like 10000 Gauss but of course the lower we go the more turns we need.
A is in square centimeters and the conversion factor for inches to cm is 2.54^2 so we end up with this formula:

Bmax=1.55e7*E/(4.44*FAN)

and here A is in square inches.  Lumping the constants, we end up with:
Bmax=3.5e6*E/(FAN)

which is the simplest form.

At 60Hz this comes to:
Bmax=5.8e4*E/(AN)

and at 50Hz this comes to:
Bmax=7e4*E/(AN)

So here we have the formula with A in square inches and at 50Hz.
The design goal should be Bmax=15000 tops.

[anishkgt]

Thanks James and MrAI.

Ok that's a mouth full of math [emoji51]. Soi've attached some pictures of the actual transformer. Now with math 72.14 x 82.14 2,544. Really !! that much mm2.

About the formula for turns. How is flux determined ? Other variables are 240vac 50Hz and with gauge, since it was aluminum I guess copper would be ok with 16 AWG or smaller ? To
Measure resistance of the wire,is it the resistance of the whole and that wouldn't be practical to measure 50mts of magnetic wire.

When I said gap I meant the core to the primary did not really consider between primary and secondary. Will not be rectifying for D.C so that would omitted. 

Sorry if am a bit pain in A**. Just don't want to throw away the core coz I had damaged the primary winding and the same time learning something.






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[anishkgt]

My other transformer has 3 turns and the open voltage is 3v


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[james_s]

Yes engineering has a lot of math, that's just how it works. Fortunately in this case the math is not anything exotic, just plug in the numbers and take it one step at a time.

You don't have to measure the whole piece of magnet wire, just look up the resistance per foot/meter/whatever of that gauge copper wire and calculate it for the whole piece. From the number of turns you can calculate the length of wire by measuring around the circumference of the winding window.

[anishkgt]

Yes engineering has a lot of math

yea it sure has . So i found http://www.giangrandi.ch/electronics/trafo/trafo.shtmlonline calculator and that says 346 turns on the primary. Wow that's a lot of turns now would it possible with the 16 Gauge wire ?

The last calculator on the linked page shows a field for current, is that the current in the primary of  the transformer during operation when the transformer is switched on ?


Thanks

[tautech]

This site is handy:
http://ludens.cl/Electron/trafos/trafos.html

[anishkgt]

Thanks Tauatech ! So my final results as per the calculation from the link above is attached. Would it ok to purchase the 16AWG wire and start the work ?

[anishkgt]

One last question would increasing the primary copper size to 16AWG  increase current at the secondary ?


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[james_s]

The gauge of the primary wire will determine resistive losses and resulting temperature rise and that is going to determine the VA you can draw from the transformer. Whether it increases the current you can draw from the secondary depends on what the limiting factor is.

[anishkgt]

So what would be sweet spot. 22 AWG ? So if I understand this, the heat loss, resistance depends on the primary winding gauge and the current draw in the secondary depends on the width of the cemetery core and the gauge of the secondary winding, correct ? With that said 22AWG would be a little less thereby creating more heat in the primary but would help adding more turns on the secondary thereby increasing current draw.


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[james_s]

You're going to have to calculate it. What are you wanting to use this transformer for? What voltage do you want it to deliver and how many amps to you intend to draw? What is the required duty cycle?

Once you know the RMS primary current you can use Ohms law to calculate the resistive power dissipation and from that you can calculate the temperature rise.

[anishkgt]

Will be using it for Spot Welding. Secondary would be at 3v open circuit and 1200amps on short. Max time would 500ms.


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[james_s]

That's going to be a low duty cycle then so temperature rise is not likely to be much of an issue. The best would be to approximate the original primary winding. If it was aluminum and you are using copper, you can probably go down a size from the original wire.

[anishkgt]

Yea initially I did plan to go that way but then since I am rewinding though I'd clear some doubts and make the most out of the transformer. So the only concern was the temperature rise and since that would not be an issue here I was thinking of getting it done like that and that would give me more room for the secondary. Would that be ok ?


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[T3sl4co1l]

Use slightly more turns (like 20% more) than the original had.  You'll probably need to use smaller wire, which will reduce your VA capacity as well.  That core is probably good for ~500 VA, continuous duty.

Tape: Mylar (polyester) and Kapton (polyimide) are both fine.  The latter is good to 200C (silicone adhesive), the former to 130C or whatever.  Either way, it's good stuff, well worth going out of your way to purchase -- don't try and hack it with craft store substitutes.

You may also find it worthwhile to use fishpaper, aka vulcanized paper.  This is stiffer than tape, so can be used to insulate layers of heavy wire, without it crumpling up, leaving an uneven surface.  It's also thicker, so you get tons of insulation voltage handling without using much material.

Tim

[anishkgt]

Use slightly more turns (like 20% more) than the original had.  You'll probably need to use smaller wire, which will reduce your VA capacity as well.  That core is probably good for ~500 VA, continuous duty.

So I was thinking of 22AWG with 200-300 turns. It had 17AWG aluminum before. What would be the difference during operation with the change in gauge ?



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[T3sl4co1l]

Hmm, about half the current rating.  Well, not quite, because aluminum.  Enough for a few amperes, which is basically what to expect (~500VA).

360 turns would give Bmax = 1.2T, which is a pretty normal value for steel.  Maybe 300 turns would be okay too (1.44T).  200 turns would be way too few (giving 2.16T), even for special core material.

Tim