TRANSFORMER calculations and equations in here

[eevblogfan]

hey

because you can salvage some UPS transformers and audio amps's transformers and sometimes re-wind them , it's important to know few equations , Hence I've opened this little thread which I hope to be on sticky up there

so what do you need to know about transformer ?

firs here's a link which covers planty os the metirial needed : 

so the power rating ( in Watts ) is going like : the area ^2 ( Ie . you're transformer's deminitiond are : 3CM with and 5 CM deep - hence 3*5 is = 15 , then we'll do 15^2 which is = 225W

so we now have got the power rating of the core . now we need to know what is the winding per VOLT ( how many winding are needed in order to say wind 240V primary )

so we then take the constant 42 and divide it by the area SQRT of the core ( which we've just calculated to be 15 )  Ie 42/15=2.8 , thus 2.8winding per Volt ,

so we need 240*2.8= 672 winding on primary side

and the same applies to the secondary , say we need 20V out (note1)* we will then multiply20*2.8 which is = 56 winding on secondary side ,

note1*= you should add around 2V ( rule of thumb) to the needed voltage in order to cover for the losse within the diode bridge and filtering losses , ( stand to be corrected plus please give me more input in here if needed ) 

now . VPP , VP and VRMS

VP(Volt Peak) is the voltage's highest point measured between the GND and the V+

VPP( Volt Peak to Peak) is the addition of both the VP from the minus and the plus waveforms ( Ie Vp+ plus VP- equals VPP

VRMS ( V  Ruth Mean Squear ) is the ruth mean squear of the voltage , this is the equivalent  power been dissipated on purly ressistive load on DC ,

in order to get the VRMS we first meassure the VP ( NOTE : VP and not to be confused with VPP ) and multiplying it by 0.707

say we meassured ( on our min/max via our multimeter ) and the max voltage ( Ie VP ) is 17Vp , thus 17*0.707= 12.019V ( round it to 12V ) so 12V is our'e VR.M.S ( good for the ones who doesn't have True R.M.S multimeter )


now I don't know . ussualy transformers goes tight with P.S's hence Should I add some tips I've learnt by cross referencing with HP/agilent's supplies ( and one I've got by my own ) ?

I'd like you to correct me where's needed plus I'll add much more by listening to the comments ,

thank you in advance and happy transformer time anyone !

[eevblogfan]

any one ? 

[ddavidebor]

no, i'm not here.

[eevblogfan]

not too sure I got your point 

[Psi]

Answering your post would require a pretty long and in-depth reply.
We're all waiting for someone else to do it.

[eevblogfan]

hey

LOL , but I thought you have nothing else to do but replaying us the guys with the questions 

LOL , yup I am kinda expecting someone wise to replay buy obviously only if he got the time and neves needed to that task

Cheers !

[Balaur]

Hey, that takes me back to the time when I was winding the transformers by myself. I've ridged a calculator (1+ and then =, =, =) to keep the count.

Your approach and calculations are OK.

Just a few quick remarks:

- I was normally using the constant 50, instead of 42. It's a bit more conservative and it was justified by the bad quality of the cores that I was using back then it. My transformers ran a bit warm.

- Is not "2.8V per winding", but rather "2.8 windings per Volt". Your further calculations are correct.

[eevblogfan]

hey Balaur

thank you - you understand the the 2.8V per winding it's miss understanding LOL ><"

P.S : Fixed it ! thank you for the needed correction

[darko31]

Ok, I'll bite and give a go at that extensive reply.

First off all, this should go in Beginners section.

Second thing that eevblogfan missed is basic transformer equations

V1/V2=N1/N2=I2/I1=n

V1- voltage primary winding
V2 - voltage secondary winding
N1 - number of turns of primary winding
N2 - number of turns of secondary winding
I1 - current in primary winding
I2 - current in secondary winding
n - ratio of everything.

Third, transformer power is defined in VA (volt amperes) because transformer converts AC sine wave and if you use AC you have to be aware of different types of power. In this case we need to know what is apparent power (VA) as this power is different than real power.

More info on that http://en.wikipedia.org/wiki/AC_power#Real.2C_reactive.2C_and_apparent_power

Also this type of thing really needs to be learned from hard and cold books, and tried out on some model transformers.

I'm sure I've missed something, but this is summed up.

For beginners, only thing that should be important are next things

for what voltage transformer is designed for the primary winding
for what voltage transformer is designed for the secondary winding
what's transformers VA rating, you can consider it as W
and voltage at secondary can vary a lot depending of load (for rated 12V secondary voltage, it can jump from 15V and go down a lot if it's shorted) that's why voltage regulator are for.

 

[eevblogfan]

I do know what VA is - But in order to measure it you need special equipment arn't you ?

measuring the RMS AC current on sec side  and simultaneously measuring the VRMS and all of that under full load ( I found 90% Voltage RMS Drop from 0% load to full 100% load   )

I ment the article to be accessible to beginners who are not necessary have got true RMS DMM

Via min/max option you can calculate the true RMS value ( VP*0.707 = true RMS ! )

[darko31]

VA is not that important, we can consider it as W, for regular application.

Another important thing to remember for a beginner, I forgot to mention, regarding transformers is that

U1/U2=I2/I1

in other words the current is reverse proportional to voltage, so primary voltage is higher and primary current is lower and secondary voltage is lower but secondary current is higher.

[eevblogfan]

yup, good point !

BTW : I indeed forgotten the Voltage proportion . But that doesn't matter when winding your own winding's

BTW : when salvaging transformer , always check the primary ( mains side ) windings , Ie , say you have got 10 square c'm core . so 42/10 = 4.2 windings per volt ,  so on 220V mains you'd like to wind 240V ( there are some places in which the voltage can accidently claim up to 250 or near , usually the one's who live near a huge several MWts transformer and he's not fully loaded )

and then you'll unwind the secondary and wind one winding instead , then plug the transformer into mains and measure the RMS voltage on the single winding ( you may prefer to wind 10 turns for better absolute votgae accuracy )  then simultaneously ( can be difficult for the ones who doesn't have got 2 meters nor true RMS one's )  measure the mains voltage ( ***BE CAREFUL HERE ! ***)

then assuming you got say 0.2V ( or 2V on the 10 turns method )  then you divide the mains Voltage by the 1 turn's voltage ( if you wound 10 turns simply divide the voltage been measured by the number of turns you wound ) you then divide the mains voltage by the 0.2V and then you get = 1100 ---> this 1100 is the number of turns you have got on the primary side ,

now assuming that is less then the turns needed YOU SHOULD ADD MORE in order to run your transformer cooler and efficiently'er ( good example is MOT transformers -_- )

example : say your core area is 19Cm^2 , then we apply the constant 42 and divide it by the area ( 42/19=2.21 turns per volt ) so  then . we do ---> 240*2.21= 530.4 ( round it to 530 or 531 , doesn't that matter )   so assuming you measured 520 turns ( via the method described above in my post ) THEn you should wind at least 10~11 more windings in order to do it properly ,

**ONLY THEN **you are good to go and start with the secondary winding(s)

good 500W or so source for transformer(S) is MOT ( Micro-wave Oven Transformer)

you can get old for free or may pay 5$ for used one or such , note the price difference !

don't remember the prices for 500+ W transformers , but they cost more then 100$ not talking about shipping or so ,

here you pay tops 5$ for the microwave and then all you need is to buy chiefly the correct sized AWG enameled copper wire ( can see on ebay ) and then you have got extremely cheaply 500W++ transformer for only few bucks ( in case of heavy copper etc , tens on dolars , no more then that , comparing to 100$+ it's saving big time ! )

anyway hope I did helped some guys out there and I do hope that via the help of some of you who replay here have included anything relating .

thank you in advance !

[crispus]

Nice subject.

Dummy question: How to determine turns/volt ratio? Why that constant, 42?

Basically I need to calculate the impedance for the primary in order to get the desired power (S = Z * I^2). For 240VA (1A) for instance, Z will be 240 Ohms.
How many windings I will need in order to get that?

[sorin]

the transformer is more complex that you think.
the power rating of transformer is proportional with the frequency.
UPS transformer are designed to work with ~400Hz, but can work with 50Hz, with reduced power rating.
I have a material with simply calculation, but is in Italian
http://digilander.libero.it/giunchifabrizio/_private/Manuale%20Trasfo.pdf

[crispus]

Thanks for the material. I don't know italian, I will try to translate with google.

So the primary part cannot be approximated with an inductor. It seems that mutual inductance really matters 

I thougt that Z= XL = 240 / (2 * pi * f * L)  ~ 0.76H which seems to be very big

[sorin]

So the primary part cannot be approximated with an inductor. It seems that mutual inductance really matters 

are you kidding, the principle of operation on transformer is based on mutual inductance.
try to short the secondary and measure again the inductance, now you should have a better idea ...

[eevblogfan]

hey

do you mean that the core is cipable of handling 400Khz ?

if so . that mean's I can use that as SMPS trasformer ? 0_0

and if so indeed ---> can I re-wind  100VA transformer ( at 50 HZ )  and use it at say 10+Khz and ummmm 400W or so ?

thank you in advance and sorry for asking stupid questions , but that the best part - mistaking !

[mariush]

I actually had to design a transformer when I was at university, as a final grade for a semester for a class. What a pain in the ass that was... The courses didn't have all the mathematical formulas, or were too confusing... and what I could get from the university were only books from the '60-'70s that had parametric data for wires and metal sheets made in Russia that were no longer available anywhere (so I couldn't make the actual transformer in real world with that info)

In the end I made it but was lucky enough to be able to borrow a couple of books from the technical library that belonged to a factory that made relays and transformers and got the whole theory and formulas from those books.

But back then I wasn't smart enough to know where to search online for books (yes, illegal but whatever, 30-50 year old books had enough time to make money), nowadays it should be easy to find stuff online. Also, the university libraries (and some larger libraries) should have some books about transformers.

[sorin]

hey

do you mean that the core is cipable of handling 400Khz ?

if so . that mean's I can use that as SMPS trasformer ? 0_0

and if so indeed ---> can I re-wind  100VA transformer ( at 50 HZ )  and use it at say 10+Khz and ummmm 400W or so ?

thank you in advance and sorry for asking stupid questions , but that the best part - mistaking !

sorry I meant 400Hz

[eevblogfan]

 lols , half a MHz didn't sound right for me .... but i trusted you 

anyhow , 400Hz woun't  be noisy ?

[G7PSK]

This is a good source of information for transformer design.

http://ludens.cl/Electron/Magnet.html

http://ludens.cl/Electron/trafos/trafos.html

[eevblogfan]

hey

thank you all !

thank you for the great inputs and good links

thank to you I'l have a hard time reading  on that fascinating topic ! 

Cheers !