22-mF capacitor as was used in the previously presented design,
the approximation sizing technique results in a prefluxing design
which stores 2.5 J of energy, roughly 43% more energy than
the previous design. Fig. 9 shows a comparison of the residual
flux levels achieved when applying several differently designed
prefluxing devices to the lab transformer’s primary winding.
These designs are differentiated by the capacitor size and the
initial capacitor voltage.
As a result of the increased energy in the device, it is evident
that the approximate sizing method resulted in a slightly larger
residual flux value. While less efficient than an exact measurement
of the magnetizing inductance, the approximation method
is a convenient way to design the prefluxing device if the magnetizing
inductance is unknown.
D. Initial Flux Level
A significant benefit of the prefluxing device technique is that
it eliminates the need to know the residual flux level of the transformer
after de-energization. Fig. 10 shows the curves from
three different prefluxing operations on the lab transformer. The
dashed trace was an application of the prefluxing device to the
lab transformer after demagnetizing it. The prefluxing device
was oriented to give a negative residual flux, which would represent
a worst-case initial flux level for a transformer that would
need to be prefluxed to a positive polarity.
The solid trace of Fig. 10 is another application of the prefluxing
device, but with the device connected to yield a positive
flux polarity. It is evident from the figure that the prefluxing device
easily sweeps the transformer’s flux polarity through the
negative region and establishes a residual flux level equal in
magnitude, but opposite in polarity, to the initial flux level.
The reason why the prefluxing device so easily switches the
polarity of the transformer’s flux is best explained by referring
back to Fig. 3, where the lightly shaded region represented
the energy required by the transformer when operating in the
linear region of its magnetization curve. This energy is much
less than what is required when the transformer operates in the
saturated region (darker shading), meaning that it is easy for the
prefluxing device to change the polarity of the transformer’s
residual flux. Thus, no matter what the initial flux level is prior
to prefluxing, the device will establish the desired flux polarity
prior to energization, removing the need for measurement
equipment.