Practical considerations
Power capacity
As has already been observed,
transformers must be well designed in order to achieve acceptable power
coupling, tight voltage regulation, and low exciting current distortion.
Also, transformers must be designed to carry the expected values of
primary and secondary winding current without any trouble. This means
the winding conductors must be made of the proper gauge wire to avoid
any heating problems. An ideal transformer would have perfect coupling
(no leakage inductance), perfect voltage regulation, perfectly
sinusoidal exciting current, no hysteresis or eddy current losses, and
wire thick enough to handle any amount of current. Unfortunately, the
ideal transformer would have to be infinitely large and heavy to meet
these design goals. Thus, in the business of practical
transformer design, compromises must be made.
Additionally, winding conductor
insulation is a concern where high voltages are encountered, as they
often are in step-up and step-down power distribution transformers. Not
only do the windings have to be well insulated from the iron core, but
each winding has to be sufficiently insulated from the other in order to
maintain electrical isolation between windings.
Respecting these limitations,
transformers are rated for certain levels of primary and secondary
winding voltage and current, though the current rating is usually
derived from a volt-amp (VA) rating assigned to the transformer. For
example, take a step-down transformer with a primary voltage rating of
120 volts, a secondary voltage rating of 48 volts, and a VA rating of 1
kVA (1000 VA). The maximum winding currents can be determined as such:
Sometimes windings will bear current
ratings in amps, but this is typically seen on small transformers. Large
transformers are almost always rated in terms of winding voltage and VA
or kVA.
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