In previous models, a transformer can dissipate poweronly by the
resistances of its windings (copper losses). Hovewer, two phenomenons
exist which induce losses too. The first one and the more well know is
currents inducted in the cores by the windings (Foucaut currents). A
cvore is indeed a conductor (we do not treat the case of HF ferrite
cores), the varying induction flux thru it wxill then induce current
cirulation, which can be easely seen to be belong to normal sections
of the core. A classical (and efficient!) solution is to construct the
core by stacking several iron foils : this procedure increases
dynamic resistance of core in the normal sections.
The second phenomenon yielding to losses ion core is linked to the
shape of hysteresis cycle. Magnetic energy in the core is :
where V is the volume of the core. It is clear that when a whole
cycle has been done, magnetic energy given to the core in the
ascending part of the induction curve is higher than
that recovered in the descending part : the difference is hysteresis
loss. When an alternating current is applied at the primary, a cycle
is done at each period, showing that hysteresis losses will increase
linearly with frequency (if other core characteristics are constant).