Electronics > Beginners
Homemade Transformer
gregariz:
I have no problem with you experimenting with components. You have had plenty of warnings so you should know the risks. I wound a transformer years ago, although it was a high voltage secondary transformer that I used in a Valve/Tube amplifier. I did this because at the time it was extremely expensive to buy a 350v CT transformer
So what I did was this;
I bought an existing transformer because you will not be able to get the silicon steel E-core laminations you need. Yes eddy losses are significant and the laminations have a lacquered or plastic coating to insulate them from each other. You may contact a transformer rewind business in your local area and ask them but I never did that. I bought the following transformer
http://dicksmith.com.au/product/M1991/general-purpose-ac-ac-transformer-12-30v-2-amps.jsp?bmForm=selImgPathSubFrom&bmFormID=j2YzadP&bmUID=j2YzadQ&bmIsForm=true&bmPrevTemplate=%2Fproduct%2FM1991%2Fgeneral-purpose-ac-ac-transformer-12-30v-2-amps.jsp&bmEditable=selImgPathHdn&bmHidden=selImgPathHdn&selImgPathHdn=%2Fmedia%2FDSAU%2FProduct%2F300x300%2FM1991_0_med_v1_m56577569835038708.JPG&bmSubmit=selImgPathSub&selImgPathSub=Submit+Query&bmHidden=PRODUCT%3C%3Eprd_id&PRODUCT%3C%3Eprd_id=845524441781404&bmHidden=FOLDER%3C%3Efolder_id&FOLDER%3C%3Efolder_id=2534374302025589&bmFields=bmFormID%2CbmHidden%2CbmIsForm%2CbmUID%2CbmForm%2CbmEditable%2CbmPrevTemplate%2CbmSubmit&bmHash=2267c7187747727594f6a7e1cc5d32018fba73cd
You will notice that the secondary and primary windings are seperated in the center by a plastic insulator. The windings are not wound on top of each other but rather side by side. This means you need not touch the primary winding.
I then removed the steel shroud and carefully tapped the first lamination out. Doing this a few times will loosen the laminations enoug h to get to the bobbin. I did not touch the primary winding as I said. I didnt touch it because they use non flamable insulating material that also is not readily available - but maybe you can find it. I then unwound the secondary and counted the number of turns. Using the specified secondary voltage and knowing the secondary turns I then calculated the turns ratio or turns per volts. Then knowing your desired secondary voltage I calculated the new number of secondary turns and rewound the secondary with these turns. I taped it up and then carfully tapped the laminations back in being careful not to scratch the laminations and reinstalled the shroud. Bingo
IIRC if you dont know what you are doing then you should be careful or buy a book (jaycar sells a small coil transformer design book) or go talk you a motor/transformer rewinder in your city (probably best)
mishimaBeef:
Thanks for the info, do you have the link for the book? I searched through twelve pages of results after searching "books" on the website http://www.jaycar.com.au/index.asp and found nothing about transformer design.
Also is there a reason they are using non-flammable insulation for the primary? I guess to prevent fires from developing due to arcs in the transformer? If that's the case, what is the cause of the arc? Insulator breakdown?
Seems like including a large safety factor for the insulation system would probably increase safety (although suffer from larger flux leakage losses). Does that sound like a reasonable precaution for safer implementation?
vk6zgo:
--- Quote from: mishimaBeef on July 06, 2011, 12:48:05 am ---I don't understand what all the fuss is about. Fusing the primary is a good precaution isn't it? Also, isn't the household wiring protected by a 15 A circuit breaker and then fuses downstream from that?
Why is wiring the secondary so much less trouble than wiring the primary exactly?
Part of the reason I'm doing this project is to gain insight into the transformer as a component and all the physical relationships that play a role in its operation.
To the comment about making my own resistor and capacitor, I do intend to at some point for the same reasons as this project. Am I going to build every capacitor and resistor I ever use? No. But I do wish to gain deep insight into the electromagnetic principles at play in our everyday devices.
--- End quote ---
I am more than a little worried about your University if you are in the 3rd year & they still haven't addressed the reasons for using laminations in transformers.
Basically,a solid iron core looks like a shorted turn,& will have massive currents induced in it.
Due to transformer action,the primary current will also be very high,& will vapourise your fuse in an instant!
The previous posters said why they suggest you stick to rewinding of the secondary.-------Special insulation,wire type,etc.
I suggest you consult your lecturer before you go any further .He/she will be able to show you where to look for the required information.
VK6ZGO
gregariz:
--- Quote from: mishimaBeef on July 06, 2011, 01:52:25 am ---Thanks for the info, do you have the link for the book? I searched through twelve pages of results after searching "books" on the website http://www.jaycar.com.au/index.asp and found nothing about transformer design.
Also is there a reason they are using non-flammable insulation for the primary? I guess to prevent fires from developing due to arcs in the transformer? If that's the case, what is the cause of the arc? Insulator breakdown?
Seems like including a large safety factor for the insulation system would probably increase safety (although suffer from larger flux leakage losses). Does that sound like a reasonable precaution for safer implementation?
--- End quote ---
Heat is due to copper and core losses. You should have noticed when you use a Wallwart/plug pack that they get hot. Since you have listed yourself as a student, and I am not an expert, you perhaps really should talk to one of your power engineering lecturers about this. You will find that students kill themselves every now and then because they do not know what they are doing. When I went through one of our graduates killed himself within a few months of graduation because he didn't have the commonsense as to where not to stick his fingers. Mains will kill you that is why you've had so many warnings. Almost everything connected to the mains is significantly overdesigned to cope with varying usage conditions.
mishimaBeef:
Thanks. Yeah I am about to take electromagnetics II and power devices this year so I will probably get some more insight into the subject. It's not the best university around but I won't make excuses as I feel learning and understanding is the student's responsibility, not the university's. It's actually amazing how many students I see managing their way through with so little knowledge. I guess people learn to play the test-taking game. But I'm getting off topic here ;).
I was thinking about the eddy current losses and I guess I need to investigate further why the losses are so much higher for a solid core. It seems to me that the eddy currents are set up to oppose the flux that is forced through the core's cross section. But isn't the magnitude the same in any case? In other words, don't the eddy currents need to generate the same amount of opposing flux?
Yes you make a good point about the inrush current blowing the fuse instantly. I have not considered this as of yet. Do you know how transformer designs protect against this?
---
Also I should probably mention that I am in no rush to jump into connecting to the mains but that is something I would like to try at some point (after much design and analysis of the physics involved).
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