Valvolare puro vecchia maniera

[pilovis]

Ultima cosa che dimenticavo sui trasformatori:

il fattore di forma e' il rapporto fra il valore efficace e il valore medio della grandezza in considerazione.

[marziom]

Bmax = Vmax x 10 alla ottava /fratto/ (Fattore di forma x frequenza di lavoro x numero di spire x area effettiva del nucleo ferromagnetico in cm2).

Da qui:
http://mysite.du.edu/~jcalvert/tech/transfor.htm

Two basic equations are used in transformer design. The first is essentially Faraday's law, E = dΦ/dt x 10-8 V, where Φ is maxwells (gauss times square centimeters). For transformers, this is written Bmax = √2 E 10-8 / 2πfNAK, where E = rms voltage in a winding, N = number of turns, f = frequency (Hz), A = area of core (cm2), K = stacking factor (the proportion of A occupied by iron). A sinusoidal variation in flux is assumed, which is a reasonable assumption when the core does not saturate, but by no means exact. Bmax is generally assumed as a design parameter, as well as a value for E/N in volts per turn, and the necessary A results. A typical value for a small transformer is E/N = 0.1 V/turn.

oppure da qua:
http://en.wikipedia.org/wiki/Transformer

Transformer universal EMF equation

If the flux in the core is purely sinusoidal, the relationship for either winding between its rms voltage Erms of the winding, and the supply frequency f, number of turns N, core cross-sectional area a and peak magnetic flux density B is given by the universal EMF equation:[36]



If the flux does not contain even harmonics the following equation can be used for half-cycle average voltage Eavg of any waveshape:

La formula che citi è una semplificazione valida in regime sinusoidale (no DC) ovvero senza fenomeni di saturazione del nucleo presenti.