Weird Science

The Transformer and the Burning Coil

Polish ver­sion is here

Trans­for­mer

What is a trans­for­mer? Let’s explore the mea­ning of this word from an ety­mo­lo­gi­cal per­spec­tive. At first glance, it is asso­cia­ted with trans­for­ma­tion, mea­ning a kind of change. A trans­for­mer is the­re­fore an elec­tri­cal device desi­gned to change vol­tage or cur­rent levels in alter­na­ting cur­rent (AC) cir­cu­its.

A trans­for­mer ope­ra­tes by trans­fer­ring the elec­tri­cal energy of alter­na­ting cur­rent thro­ugh induc­tion from one elec­tri­cal cir­cuit to ano­ther, main­ta­i­ning the ori­gi­nal fre­qu­ency. Typi­cally, the vol­tage is chan­ged simul­ta­ne­o­u­sly. The only excep­tion is an iso­la­tion trans­for­mer, which does not alter the vol­tage.

A trans­for­mer con­si­sts of two or more coils (cal­led win­dings) wound aro­und a com­mon magne­tic core, usu­ally made of fer­ro­ma­gne­tic mate­rial. Both cir­cu­its are usu­ally galva­ni­cally iso­la­ted. This means that there is no elec­tri­cal con­nec­tion between the win­dings, and energy is tran­s­mit­ted exc­lu­si­vely thro­ugh the magne­tic field con­cen­tra­ted in the core. The struc­ture of a sim­ple trans­for­mer is illu­stra­ted in the dia­gram below:

Ilustracja

The prin­ci­ple of trans­for­mer ope­ra­tion is very sim­ple. The pri­mary win­ding is con­nec­ted to an AC power source. This cau­ses alter­na­ting cur­rent to flow thro­ugh it, cre­a­ting a chan­ging magne­tic field. The alter­na­ting magne­tic field, con­duc­ted thro­ugh the trans­for­mer core, pas­ses thro­ugh the secon­dary win­ding. The chan­ging magne­tic flux in the secon­dary coil indu­ces elec­tro­ma­gne­tic induc­tion, gene­ra­ting alter­na­ting elec­tro­mo­tive force (EMF).

There is a direct rela­tion­ship between the num­ber of turns in the pri­mary and secon­dary win­dings and the vol­ta­ges on both sides. If we denote the vol­ta­ges as U and the num­ber of turns as n, where index 1 refers to the pri­mary side and index 2 to the secon­dary side, the equ­a­tion is as fol­lows:

U1/U2 = n1/n2

If n1 = n2, the secon­dary vol­tage is equal to the pri­mary vol­tage (U1 = U2). Such a trans­for­mer is cal­led an iso­la­tion trans­for­mer. In all other cases, the secon­dary vol­tage dif­fers from the pri­mary vol­tage: it is higher if n2 n1 or lower if n1 n2. Thus, a trans­for­mer allows for the trans­for­ma­tion of alter­na­ting cur­rent vol­tage levels.

What about the cur­rent values on both sides of the trans­for­mer? Let’s recall the prin­ci­ple of energy con­se­rva­tion. As we know, energy can­not be cre­a­ted from nothing (the pro­blem of the infa­mous per­pe­tual motion machine) nor disap­pear without a trace. Apply­ing this to elec­tric cur­rent, we can say that the power on both sides of the trans­for­mer must be equal:

P1 = P2

The power con­su­med on the pri­mary side must be equal to the power deli­ve­red on the secon­dary side. We are not con­si­de­ring los­ses that occur in real sys­tems, such as those cau­sed by eddy cur­rents in the core or sur­ro­un­ding metal objects. Remem­be­ring that elec­tric power is defi­ned by the for­mula P = I * U, we can easily derive the fol­lo­wing equ­a­tion:

I2/I1 = n1/n2

We can see that the rela­tion­ship for cur­rent is the inverse of that for vol­tage. This means that the trans­for­mer is not a per­pe­tual motion machine: while it can incre­ase vol­tage, this will simul­ta­ne­o­u­sly reduce cur­rent, and vice versa.

Also, remem­ber that the flow of elec­tric cur­rent thro­ugh a con­duc­tor cau­ses it to gene­rate heat. The gre­a­ter the cur­rent, the more the con­duc­tor heats up.

Demon­stra­tion!

War­ning: This demon­stra­tion using a ZVS cir­cuit is safer than others because no high vol­tage is invo­lved. Howe­ver, the wire loop heats up inten­sely, so cau­tion is requ­i­red. Per­form the demon­stra­tion on a non-flam­ma­ble sur­face, away from flam­ma­ble mate­rials. The author assu­mes no respon­si­bi­lity for any damage or injury that may occur. You pro­ceed at your own risk!

Let’s try to apply this know­ledge in prac­tice. To achieve the gre­a­test hea­ting effect on the secon­dary side, we need to gene­rate a high cur­rent in this win­ding. Using the for­mu­las above, we find that the secon­dary win­ding must have fewer turns than the pri­mary win­din­g—in this case, the out­put vol­tage will be lower, but the cur­rent will be higher. By redu­cing the num­ber of turns in the secon­dary win­ding, we even­tu­ally reach a point where it con­si­sts of just a sin­gle loop:

Ilustracja

In this case, the out­put vol­tage will be lower than the input vol­tage by a fac­tor equal to the num­ber of turns in the pri­mary win­ding (and the cur­rent will be higher by the same fac­tor). A good source of alter­na­ting cur­rent is the ZVS gene­ra­tor built accor­ding to the descrip­tion on my web­site. In this setup, the high-vol­tage win­ding is remo­ved from the fer­rite core and repla­ced with a ring made of rela­ti­vely thick wire. When the conver­ter is powe­red on, the wire will heat up to a red-hot state due to the high cur­rent indu­ced in it. At higher power levels, even steel wire with a dia­me­ter of 0.5 mm (0.02 inches) will melt, which requ­i­res a very high tem­pe­ra­ture. The ring sho­uld not fit tigh­tly aro­und the core, as the core could crack from the heat. Below is my video demon­stra­ting the expe­ri­ment. The ring was made of steel wire, which quic­kly hea­ted to white-hot and mel­ted:

The secon­dary cir­cuit bre­aks when the wire melts, stop­ping fur­ther hea­ting.

Enjoy and have fun expe­ri­men­ting! :)

Fur­ther rea­dings:

Marek Ples

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