We consider an ideal transformer in which the
primary has negligible resistance and all the flux in the core links both
primary and secondary windings Let f be the flux in each turn in the core
at time t due to current in the primary when a voltage vp is applied to it Then the induced emf or voltage es, in the secondary with Ns turns is
d
d
s
Ns
φt
ε
= −
(7 31)
The alternating flux f also induces an emf, called back emf in the
primary