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 3.2 TRANSFORMERS. Transformers are an essential part of any electrical system. They come
in various sizes and voltage ratings. Transformers are used for transforming power from one
voltage level to another.
3.2.1 Fundamental Transformer Principles. A transformer consists of a magnetic core built
up of silicon steel laminations with two sets of coils wound around the core. These coils are
called the primary and secondary windings. This combination may be used to derive a voltage
higher or lower than the voltage immediately available. The supply voltage is applied to the
primary winding, whether it is the higher or lower voltage winding. The other winding, to which
the load is connected, is termed the secondary winding. Since electromagnetic induction can
only take place when the magnetic flux is continually varying, transformers can only be used in
alternating current circuits.
3.2.1.1 Open-Circuit Characteristics. If an alternating electromotive force (emf) is applied
to the terminals of the primary winding of a transformer with the secondary winding
open-circuited (nothing connected between the secondary terminals), a very small current will
flow in the primary circuit. The exciting current has a magnetizing current component that
establishes the mutual magnetic flux that induces an emf in both primary and secondary
windings. Since the primary and secondary windings are wound on the same core, and the
magnetizing flux is common to the two windings, the voltage induced in a single turn of each
winding will be the same, and the induced voltages in the primary and secondary windings are
therefore in direct proportion to the number of turns in those windings. The exciting current also
has a core loss component that accounts for the power absorbed by the hysteresis and
eddy-current losses in the core.
3.2.1.2 Load Characteristics. The application of a load to the secondary side of the
transformer produces a considerable change in the internal phenomena. When the secondary
circuit is closed, a secondary current flows, the value of which is determined by the magnitude of
the secondary terminal voltage and the impedance of the load circuit. The secondary load current
produces in the core a load flux that is in phase with the secondary current, however, the
secondary load current is immediately balanced by a primary load current of such a value that the
primary and secondary load ampere-turns are equal. The secondary load flux is similarly
counteracted by a primary load flux which is in phase with the primary load balancing current,
Therefore, the core is left in its initial state of magnetization corresponding to the magnetizing
current component of the exciting current; this explains why the iron loss is independent of the
load. The total current in the primary circuit is the phasor sum of the primary load current and
the no-load exciting current.
Low core loss transformers, made of a new amorphous type alloy, are still in the prototype
development stage for most types of transformers. There are several test programs under way
with utilities in different parts of the country in which selected.groups of these new type
3-2
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