Transformer winding - Page 1
Forum section: electronics / beginners

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 19, 2017, 02:33:02 pm
content: 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.

author: MrAl
author_rank: Super Contributor
timestamp: March 20, 2017, 03:07:11 pm
content: 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.

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 20, 2017, 03:26:46 pm
content: 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 ?Sent from my iPhone using Tapatalk

author: james_s
author_rank: Super Contributor
timestamp: March 20, 2017, 03:48:36 pm
content: 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.

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 20, 2017, 03:52:04 pm
content: 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 copperMagnetic wire on the primary.Sent from my iPhone using Tapatalk

author: james_s
author_rank: Super Contributor
timestamp: March 20, 2017, 04:55:00 pm
content: 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.

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 20, 2017, 05:08:27 pm
content: 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 mm2as that is unit used to determine the area for the core.Thanks in advance.

author: james_s
author_rank: Super Contributor
timestamp: March 20, 2017, 06:11:16 pm
content: 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.

author: MrAl
author_rank: Super Contributor
timestamp: March 20, 2017, 06:55:23 pm
content: Quote from: anishkgt on March 20, 2017, 03:26:46 pmHi,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)whereBmax 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.

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 20, 2017, 07:33:12 pm
content: Thanks James and MrAI.Ok that's a mouth full of math . 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 ? ToMeasure 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.Sent from my iPhone using Tapatalk

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 20, 2017, 07:36:52 pm
content: My other transformer has 3 turns and the open voltage is 3vSent from my iPhone using Tapatalk

author: james_s
author_rank: Super Contributor
timestamp: March 20, 2017, 07:56:23 pm
content: 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.

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 20, 2017, 08:13:05 pm
content: Quoteyea it sure has. So i foundhttp://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

author: tautech
author_rank: Super Contributor
timestamp: March 20, 2017, 09:06:39 pm
content: This site is handy:http://ludens.cl/Electron/trafos/trafos.html

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 20, 2017, 09:49:37 pm
content: 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 ?

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 20, 2017, 10:33:24 pm
content: One last question would increasing the primary copper size to 16AWG increase current at the secondary ?Sent from my iPhone using Tapatalk

author: james_s
author_rank: Super Contributor
timestamp: March 20, 2017, 11:13:42 pm
content: 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.

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 20, 2017, 11:18:55 pm
content: 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.Sent from my iPhone using Tapatalk

author: james_s
author_rank: Super Contributor
timestamp: March 21, 2017, 12:12:02 am
content: 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.

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 21, 2017, 12:15:54 am
content: Will be using it for Spot Welding. Secondary would be at 3v open circuit and 1200amps on short. Max time would 500ms.Sent from my iPhone using Tapatalk

author: james_s
author_rank: Super Contributor
timestamp: March 21, 2017, 12:18:09 am
content: 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.

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 21, 2017, 06:28:44 am
content: 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 ?Sent from my iPhone using Tapatalk

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2017, 07:19:48 am
content: 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

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 21, 2017, 07:43:25 am
content: Quote from: T3sl4co1l on March 21, 2017, 07:19:48 amSo 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 ?Sent from my iPhone using Tapatalk

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2017, 11:15:31 am
content: 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

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 21, 2017, 11:27:11 am
content: as per the link in this postQuote from: tautech on March 20, 2017, 09:06:39 pmits 697 turns. So would that have any affect on the secondary current when i use a 2 AWG flexible copper wire ? On a normal MOT i'd get 1100amps at the secondary. would i get comparatively more or less with the difference in the Gauge at the primary.

author: MrAl
author_rank: Super Contributor
timestamp: March 21, 2017, 07:26:27 pm
content: Hi,Probably dont want to waste your time with aluminum.The other thing to consider next is the window area. The idea is to fill up the window as much as possible so that you get the full area used up. The reason is that the power rating will be based on the window being full with copper which is 1 turn with copper that fills up the whole window. This is because of the resistance of copper vs the cross section of the copper. That's of course if you want to make full use of the core. IF not, then you can go with almost anything. Note that 22 gauge wire will only result in maybe three layers, which is barely using any of the core, but if the secondary takes up the rest of the room then your optimized. If not, then you can increase the wire size of the primary and secondary.

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 21, 2017, 08:33:36 pm
content: Thanks AI.Will not be using Aluminium for sure. Now about the size of the wire for the primary. What is the difference with the gauge of 22 and 16. isn't 16 more space taking as that is more thicker ? Moreover i will be filling the remaining window with the secondary with 2 or 3 Gauge flexible copper wire whichever is available here. probably 3 or 4 turns if possible to get the max current out of it.Question is if am drawing that much current at the secondary would that heat up the primary with a thin wire like 22 AWG wire ? or would the sweet spot be 16AWG ?

author: james_s
author_rank: Super Contributor
timestamp: March 21, 2017, 09:36:45 pm
content: #16 is thicker and will take up more of the window, it will also result in lower resistive losses and less sag under load. How much is something you will have to calculate, nobody can give you a definitive answer without measuring the core and making all these calculations themselves.

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 21, 2017, 09:48:16 pm
content: Thanks James Will stick to the 16AWG then. the calculations is something i can't figure out like the flux and stuffs. Well i will just go with the excel sheet. So would the flux density be 1 ? how is that calculated ?

author: MagicSmoker
author_rank: Super Contributor
timestamp: March 21, 2017, 10:06:49 pm
content: Quote from: anishkgt on March 21, 2017, 08:33:36 pmIn a typical transformer the usable window area is split evenly between the primary and secondary windings (note that only 40-50% of the actual window area can be used).The design process to follow:1. Calculate the number of primary turns required given the applied voltage, effective core area (Ae) and peak flux density swing. For typical silicon steel mains frequency transformers a flux density swing of 1-1.2 Tesla (or 10000-12000 Gauss) is commonly used. The formula for calculating turns, assuming sine wave excitation at 50Hz, cm² for area and a flux density swing of 10000G is:N = VRMS/ 0.0222 * AeEarlier on you measured the center area and it came out to around 25.4cm², so that means a 230VAC/50Hz primary on this transformer will need 408 turns.This seems reasonable, as a typical 50/60Hz transformer of this size will need around 2 turns per volt.

author: anishkgt
author_rank: Frequent Contributor
timestamp: March 21, 2017, 10:10:47 pm
content: Thanks magicSmoker. Appreciate it.So that would be 480 turns of 16AWG and the transformer is switched on for about 500ms max and mostly 200 - 300ms. So would 25 AWG be ok or i stick with 16AWG like James had suggested.What does Ae stand for ? how is the peak density flux calculated ?

author: MagicSmoker
author_rank: Super Contributor
timestamp: March 21, 2017, 10:29:11 pm
content: Quote from: anishkgt on March 21, 2017, 10:10:47 pmYou keep asking this question but we can't answer it because we don't know the window area of the transformer! A good rule of thumb is that 40-60% of the total window area can be occupied by copper and this area is split evenly between the primary and secondary windings (with perhaps a bit more allotted to the primary, since it also has to make up for any losses). So to determine what gauge wire to use you need to first measure the window area, then assign 20-25% of that area to the primary and another 20-25% to the secondary. Here is a handy wire chart I found with my google machine:http://diyaudioprojects.com/Technical/American-Wire-Gauge/#16 AWG has an area of 1.31mm² so 408 turns would require 535mm² (or 5.35cm²). You need a total window area of at least 21.4cm² to use #16 for the primary, then.

author: MrAl
author_rank: Super Contributor
timestamp: March 23, 2017, 08:07:13 pm
content: Quote from: anishkgt on March 21, 2017, 08:33:36 pmHi,Yes #16 will take up more room than #22, but if that was the only consideration then why not use #34 wire as that takes up less room than #22.The whole thing is based on how much room the primary takes plus the secondary, and because there is a relationship between the primary turns and secondary turns for both voltage and current, this creates a situation for optimization of primary and secondary wire sizes. Once you pick the primary wire size you know the secondary wire size because you know the primary voltage, secondary voltage, turns ratio, and thus you know the secondary current.For example, if you use a wire size for the primary that has cross sectional area 1 unit and the current is 1 amp and voltage 10 volts, then if the turns ratio is such that the secondary is 1 volt then the second current is 10 amps. This means you know the wire size for the secondary as it must have a cross sectional area of 10 times that of the primary, which would then be 10 units.What this means is that we know the primary size and secondary wire size, and we then proceed to see how it fits on the core in the window area. If it fits perfectly then we've optimized the design for the given input voltage. That gives us the maximum current output on the secondary. If it is too large we have no choice but to lower the primary wire size which then makes the secondary wire size smaller too. If it is too small then we know we might be able to increase the primary wire size (and thus the secondary wire size also) but there is a catch and that is if we get too close to the max going up even one wire size may go over the limit of the window area so we have to back up and call the first try the optimum.So to recap, once you choose the primary wire you know the secondary wire size, and then you can try to figure out if it all fits in the window, and this will result in three possible cases:1. It fits perfectly.2. It goes over the window area limit.3. It does not use up the whole window area.Depending on the case you may or may not try to increase or decrease the primary (and thus secondary) wire size.Because of skin depth you also dont want to use too big of a secondary wire size. 1/4 inch diameter is probably a good limit. If you need heavier wire, use two turns wound bifilar.Quick example:primary 100v, secondary 10v, which means the turns ratio is 1/10.If we pick the area of the wire size for the primary of 1 unit the secondary wire size has area 10 units.If the required number of turns on the primary is 200, then the required secondary turns is 20. We see if that fits in the window area and if not, increase both wire sizes or decrease both wire sizes and see if it is a good fit after that.This can all be put into a formula but i am not sure if you would be comfortable with that.

author: james_s
author_rank: Super Contributor
timestamp: March 23, 2017, 09:58:42 pm
content: You don't always have to increase both the primary and secondary wire size. If you have space to increase the size of either one, you will reduce the resistive losses in that winding which will improve the overall efficiency of the transformer and reduce the temperature rise.

author: MrAl
author_rank: Super Contributor
timestamp: March 24, 2017, 11:24:43 am
content: Quote from: james_s on March 23, 2017, 09:58:42 pmHi,In the raw theory where we are working with a continuously variable wire size the optimum is to change both wire sizes. This is of course before the actual construction starts. Under this kind of theory increasing one wire size reduces resistance but that's all. Increasing both reduces the resistance more and also increases the current.Of course because in practice we have to work with integer wire sizes there may be times when if we did increase one the other would not increase that much and so we would not have to change that wire size, but in general they both change to get to the optimum, and why not get to the optimum. If we dont want the optimum then anything is possible. The optimum wire sizes leads to the best possible design for a given transformer window area.Interesting is if we have wire size with area A on the primary and wire size with area B on the secondary, since the optimum wire size is based on the area A it will be N*A where N is the turns ratio. Since the number of secondary turns is p/N where p is the number of primary turns and N*A the wire area, the total secondary area is p/N*N*A=p*A, which is the same as the primary total area. This of course means that the secondary winding should take up the same area in the window as the primary winding, in a perfect world. Since the winding goes on layer by layer, the optimum size wire for the primary winding is the size that leads to a winding build height of 1/2 of the window width. That's for the ideal case.

author: MagicSmoker
author_rank: Super Contributor
timestamp: March 24, 2017, 12:14:59 pm
content: Quote from: anishkgt on March 21, 2017, 10:10:47 pmForgot to answer this - Ae is the accepted/standard terminology for the area of the center pole (or the cross-sectional area of a toroid) in a transformer or inductor (Ae means "Area, effective"). Note that sometimes the center pole area - usually abbreviated as Ac - will also be specified, particularly for E-shape cores, and Ac will likely be slightly higher in value than Ae; in such cases you use Ae to determine the number of turns, but Ac to determine core losses (if core volume, V or Ve or Vc, is not given).The usual procedure for designing a transformer is to set the peak flux density ahead of time based on either the saturation flux density or the loss factor (usually in mW/mm³). If the operating frequency is low enough for a given core material then the design will be saturation limited, whereas at high frequencies* core losses start to dominate and flux density will need to be reduced to keep those losses in check.Most grades of silicon steel appropriate for transformers will have a saturation flux density around 1.5 Tesla (15000 Gauss), so if you aren't core loss limited then 1.2T is a relatively safe value. Note that the number of turns is inversely proportional to peak flux density and core area, and directly proportional to applied voltage, so for a given core area and applied voltage, increasing the turns decreases the flux density which in turn decreases the core losses (but more turns means higher winding losses). Similarly, a larger core area will need less turns per applied volt for a given flux density, and it is for this reason alone that larger cores can handle more power (this often comes as a surprise to those new to transformer design).Normally you want to split the primary into two halves of equal number of turns and sandwich the secondary between them to minimize proximity effect losses and leakage inductance, but in this particular application the proximity losses are irrelevant and more leakage inductance is a good thing (it will help limit short circuit current), so wind the primary first, then wrap it with yellow mylar (polyester) tape (e.g. - 3M #56) and wind the secondary on top.* - what is "high" frequency very much depends on the core material and lamination thickness, if applicable. 400Hz would be considered a high frequency for 0.25mm thick silicon steel lamination, while 25kHz would still be considered a low frequency for most power ferrites (e.g. - Ferroxcube 3F3, Magnetics Inc. R, etc.).

author: MrAl
author_rank: Super Contributor
timestamp: March 28, 2017, 09:25:54 am
content: Hi again,I forgot to mention something that is very useful also.When dealing with wire sizes like 22 gauge AWG wire, it helps to remember that if you go DOWN in wire gauge number three times then the current capability is doubled. So for example if we were using 22 gauge at 1 amp and we wanted to know the wire size for 2 amps, just go DOWN three wire sizes, which brings us to number 19 gauge.Looking at a wire size table, we see that #22 is good for about 1 amp with 600 circular mils per amp current density, and #19 is good for twice that, 2 amps at 600 cm/amp.Note that the diameter does not go up by 2 because the area changes in two dimensions, so the diameter goes up by the square root of 2 which is about 1.4142 .Also remember that you can only do this so many times before it becomes inaccurate, so it's not a good idea to start with wire gauge 36 and try to come up with a wire size that works at 30 amps.