The Basics of Transformers

The Basics of Transformers
By raising and lowering voltage levels, transformers
make power systems versatile
By R. Fehr, P.E., Engineering Consultant
hen alternating
current (AC)
prevailed over
direct current
(DC) in the late
1800s, one of
the deciding factors was the need for voltage levels to be raised and lowered
throughout the power system to make the
system efficient and safe. There is no
simple way of changing DC voltage levels. Because of its time-varying magnetic
field, the AC system allows the use of
the transformer to change voltage levels
as needed.
How transformers work. A basic
transformer consists of two sets of coils
or windings. Each set of windings is simply an inductor. AC voltage is applied to
one of the windings, called the primary
winding. The other winding, called the
secondary winding, is positioned in close
proximity to the primary winding, but is
electrically isolated from it.
The alternating current that flows
through the primary winding establishes
a time-varying magnetic flux, some of
which links to the secondary winding
and induces a voltage across it. The magnitude of this voltage is proportional to
the ratio of the number of turns on the
primary winding to the number of turns
on the secondary winding. This is known
as the “turns ratio.”
To maximize flux linkage with the secondary circuit, an iron core is often used
to provide a low-reluctance path for the
magnetic flux. The polarity of the windings describes the direction in which the
coils were wound onto the core. Polarity
determines whether the flux produced by
one winding is additive or subtractive
with respect to the flux produced by an-
58 EC&M August 2003
Shown is a typical single-phase two-winding power transformer.
other winding. A basic two-winding
transformer is shown in the Figure above.
Three-phase transformers. A basic
3-phase transformer consists of three sets
of primary windings, one for each phase,
and three sets of secondary windings
wound on the same iron core. Separate
single-phase transformers can be used
and externally interconnected to yield the
same results as a 3-phase unit.
The primary windings are connected
in one of several ways. The two most
common configurations are the delta, in
which the polarity end of one winding is
connected to the non-polarity end of the
next, and the wye, in which all three nonpolarity (or polarity) ends are connected
together. The secondary windings are
connected similarly. This means that a 3phase transformer can have its primary
and secondary windings connected the
same (delta-delta or wye-wye), or differently (delta-wye or wye-delta). It’s important to remember that the secondary voltage waveforms are in phase with the pri-
mary waveforms when the primary and
secondary windings are connected the
same way. This condition is called “no
phase shift.” But when the primary and
secondary windings are connected differently, the secondary voltage waveforms
will differ from the corresponding primary voltage waveforms by 30 electrical
degrees. This is called a 30° phase shift.
When two transformers are connected in
parallel, their phase shifts must be identical; if not, a short circuit will occur when
the transformers are energized.
Autotransformers. An autotransformer is a transformer with an electrical connection between the primary and
secondary windings. Autotransformers
have considerably more MVA capacity
per pound of core iron and winding conductor than standard power transformers, but are limited to small turns rations—ideally 2:1.
Although the designs of different
transformers vary extensively, their basic
operation remains the same.