to charge the capacitor. Plugging these quantities into (4) reveals
that a capacitance of roughly 3.9 mF is needed to properly flux
the transformer.With the desired capacitance and initial voltage
level, the total energy ideally stored in the capacitor is approximately
1.75 J. The operation of Fig. 5 actually used a 4-mF capacitor,
the closest capacitance to the desired amount that was
available in the lab.
A detail to note in Fig. 5 is that the peak level of the current
during the device operation is approximately 55 A, which
is the same as the peak magnetizing current drawn by the transformer
in steady-state, open-circuit operation (Fig. 4), demonstrating
that the Volt-Second design method does an excellent
job of fluxing the transformer under conditions similar to the
rated transformer operation.
It is worthwhile to mention that the transformer inductance
used in the preceding equations is not a constant value, as the
slope of the curves in Fig. 3 demonstrated. If the prefluxing
operation forces the transformer flux further past the knee point
then what occurs under rated operation, is reduced in value
and the resonant frequency increases, decreasing the volt-seconds
delivered to the transformer.
This behavior is of no concern, as will be shown in part D, because
it is only exhibited when the transformer’s residual flux
is already established in the desired polarity prior to prefluxing.
The prefluxing device delivers fewer volt-seconds in this case
because it does not need to move the flux in the transformer as
far as designed in the volt-second method. In addition, as Fig. 3
shows, whether the transformer is operating under rated conditions
or saturated conditions, the maximum residual flux is
the same. In either case, the application of the prefluxing device
will result in the transformer being fluxed to approximately the
same value, but in the case of saturated operation, more of the
device energy is dissipated as heat.
2) Primary Winding Application: The prefluxing results
presented up until this point have involved the tertiary winding
of the transformer. However, the device can be applied to any
winding on the transformer and actually performs best on the
highest voltage transformer winding, where the magnetizing
current is the lowest, resulting in reduced losses in the device
and transformer. Fig. 7 plots the curve measured when
applying the prefluxing device to the 240-V primary winding of
the lab transformer after having demagnetized the transformer.
Applying the prefluxing device to the primary winding
requires referring the transformer’s magnetizing inductance to
that side of the transformer. Multiplying the tertiary inductance
by the turns ratio squared (Table I) results in a magnetizing
inductance of 230 mH. For the prefluxing operation shown in
Fig. 7, a power-supply voltage of 12.5 V was used to charge
the capacitor. Plugging the values into (4) yields a required
capacitance of 22.5 mF to properly flux the transformer.
An important point to note is that the energy stored in the capacitor
for this new prefluxing design is the same as that for the
previously mentioned design used for the tertiary winding; both
designs store 1.75 J. The same amount of energy is required to
flux the transformer regardless of the winding the device is applied
to. This characteristic comes from the fact that under ideal
circumstances, the flux required to support transformer action is
the same regardless of the transformer winding that is excited,
to charge the capacitor. Plugging these quantities into (4) reveals
that a capacitance of roughly 3.9 mF is needed to properly flux
the transformer.With the desired capacitance and initial voltage
level, the total energy ideally stored in the capacitor is approximately
1.75 J. The operation of Fig. 5 actually used a 4-mF capacitor,
the closest capacitance to the desired amount that was
available in the lab.
A detail to note in Fig. 5 is that the peak level of the current
during the device operation is approximately 55 A, which
is the same as the peak magnetizing current drawn by the transformer
in steady-state, open-circuit operation (Fig. 4), demonstrating
that the Volt-Second design method does an excellent
job of fluxing the transformer under conditions similar to the
rated transformer operation.
It is worthwhile to mention that the transformer inductance
used in the preceding equations is not a constant value, as the
slope of the curves in Fig. 3 demonstrated. If the prefluxing
operation forces the transformer flux further past the knee point
then what occurs under rated operation, is reduced in value
and the resonant frequency increases, decreasing the volt-seconds
delivered to the transformer.
This behavior is of no concern, as will be shown in part D, because
it is only exhibited when the transformer’s residual flux
is already established in the desired polarity prior to prefluxing.
The prefluxing device delivers fewer volt-seconds in this case
because it does not need to move the flux in the transformer as
far as designed in the volt-second method. In addition, as Fig. 3
shows, whether the transformer is operating under rated conditions
or saturated conditions, the maximum residual flux is
the same. In either case, the application of the prefluxing device
will result in the transformer being fluxed to approximately the
same value, but in the case of saturated operation, more of the
device energy is dissipated as heat.
2) Primary Winding Application: The prefluxing results
presented up until this point have involved the tertiary winding
of the transformer. However, the device can be applied to any
winding on the transformer and actually performs best on the
highest voltage transformer winding, where the magnetizing
current is the lowest, resulting in reduced losses in the device
and transformer. Fig. 7 plots the curve measured when
applying the prefluxing device to the 240-V primary winding of
the lab transformer after having demagnetized the transformer.
Applying the prefluxing device to the primary winding
requires referring the transformer’s magnetizing inductance to
that side of the transformer. Multiplying the tertiary inductance
by the turns ratio squared (Table I) results in a magnetizing
inductance of 230 mH. For the prefluxing operation shown in
Fig. 7, a power-supply voltage of 12.5 V was used to charge
the capacitor. Plugging the values into (4) yields a required
capacitance of 22.5 mF to properly flux the transformer.
An important point to note is that the energy stored in the capacitor
for this new prefluxing design is the same as that for the
previously mentioned design used for the tertiary winding; both
designs store 1.75 J. The same amount of energy is required to
flux the transformer regardless of the winding the device is applied
to. This characteristic comes from the fact that under ideal
circumstances, the flux required to support transformer action is
the same regardless of the transformer winding that is excited,
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