Let N1 = number of turns in primary,
N2 = number of turns in secondary,
ɸm = maximum flux in the core, Wb.
= Bm × A, [where Bm is the maximum flux density in the core and A is the core area], and
f = frequency of a.c. input, Hz.
Refer Fig. 18. Since the flux increases from its zero value to maximum value ɸm in one quarter of the cycle i.e. in T/4 or 1/4f second (T being time-period of the cycle),
Average rate of change of flux