For an ideal transformer, the
magnetizing current is assumed to be negligible. For a real
transformer, some magnetizing current must flow when voltage is
applied to the winding in order to establish a flux in the core.

The voltage induced in the winding by
the flux restrains the magnetizing current. It was shown earlier that
the magnetizing current is not really sinusoidal, but contains many
odd harmonics in addition to the fundamental frequency.

If we neglect the harmonics and
concentrate on the fundamental frequency, the magnetizing current in
the winding lags the applied voltage by 90°. In a two-winding
transformer, this is equivalent to placing a reactance Xm, called the
magnetizing reactance, in parallel with the transformer terminals.

The peak value of the magnetizing
current is determined from the B-H curve of the core, which we have
seen is very nonlinear. Therefore, the magnetizing reactance is not a
constant but is voltage dependent; however, if the peak flux density
is kept well below the point of saturation, Xm can be approximated by
a constant reactance in most engineering calculations.

It is generally desirable to maximize
Xm in order to minimize the magnetizing current. We saw earlier that
inductance is inversely proportional to the reluctance of the core
along the flux path and the reluctance of an air gap is several
thousand times the reluctance of the same distance through the steel.

Therefore, even tiny air gaps in the
flux path can drastically increase the core’s reluctance and
decrease Xm. A proper core design must therefore eliminate all air
gaps in the flux path.

Since any flux that is diverted must
flow between the laminations through their surfaces, it is vital that
these surfaces lie perfectly flat against each other. All ripples or
waves must be eliminated by compressing the core laminations together
tightly.

This also points out why the oxide
layers on the lamination surfaces must be extremely thin: since these
layers have essentially the same permeability as air and since the
flux that is diverted from the air gaps must then travel through
these oxide layers, the core’s reluctance would greatly increase if
these layers were not kept extremely thin.

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