Overview
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A transformer is an electrical device that converts the voltage level of
alternating-current power using electromagnetic induction.
Although the secondary voltage is required to be constant regardless of
the load, it varies with amount of the load and the power factor. To ensure
the access to constant voltage, the size of the voltage variation is one
of the important output characteristics of the transformer.
Maintaining the balanced state is also the critical issue, since the imbalanced
voltage and current of each phase may cause the trouble to the device as
well as the temperature rise.
This note presents the use of magnetic field analysis to evaluate the changes
in the secondary voltage caused by the load variation of a low frequency
transformer. |
Load Variation of the Secondary Voltage
| Fig. 1 shows the change in voltage caused by the load variation. The ideal
secondary voltage of the transformer used for the analysis is 14.1 V. The
secondary current increases as the load resistance decreases. Accordingly,
the secondary voltage falls because the drop of the voltage increases due
to the resistance of the secondary coil and the leakage reactance. |
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Induced voltage in the secondary coil
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Fig. 2 shows the vector plots for the induced voltage in the secondary
coil, when a load resistance is A and B. The amplitude is shown by the
vector length, and the phase is shown by the slope of the vector.
The load resistance A is balanced because the amplitude is almost equal
and the phase difference is almost even with 120 degrees. The load resistance
B is unbalanced because the amplitude is different, and the phase difference
is not even with 120 degrees. |
Flux Density - Intensity Distribution
| Fig. 3 shows the flux density - intensity distribution of the transformer
with the load resistance A and B. For the load resistance A , the flux
density distribution is almost symmetric. The flux density for the load
resistance B is asymmetric because the current in each phase is unbalanced.
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