The core loss (no-load loss) of a power
transformer may be obtained from an empirical design curve of watts
per pound of core steel (Fig. below). Such curves are established by
plotting data obtained from transformers of similar construction.
The basic loss level is determined by
the grade of core steel used and is further influenced by the number
and type of joints employed in construction of the core. Figure 10-1
applies for 9-mil-thick M 3-grade steel in a single-phase core with
45” mitered joints.
Loss for the same grade of steel in a
3-phase core would usually be 5% to 10% higher. Exciting current for
a power transformer may be established from a similar empirical curve
of exciting volt-amperes per pound of core steel.
The steel grade and core construction
are the same as for Fig. 10-1. The exciting current characteristic is
influenced primarily by the number, type, and quality of the core
joints, and only secondarily by the grade of steel.
Because of the more complex joints in
the 3-phase core, the exciting volt-amperes will be approximately 50%
higher than for the single-phase core. The exciting current of a
transformer contains many harmonic components because of the greatly
varying permeability of the steel.
For most purposes, it is satisfactory
to neglect the harmonics and assume a sinusoidal exciting current of
the same effective value. This current may be regarded as composed of
a core-loss component in phase with the induced voltage (90DEG ahead
of the flux) and a magnetizing component in phase with the flux.
Sometimes it is necessary to consider
the harmonics of exciting current to avoid inductive interference
with communication circuits. The harmonic content of the exciting
current increases as the peak flux density is increased.
Performance can be predicted by
comparison with test data from previous designs using similar core
steel and similar construction. The largest harmonic component of the
exciting current is the third.
Higher-order harmonics are
progressively smaller. For balanced 3-phase transformer banks, the
third harmonic components
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