windings, or a single secondary with several tap
an ac generator supplies a reactive load, however, the
c o n n e c t i o n s . These transformers have a low
current flowing through the armature conductors
volt-ampere capacity and are less efficient than large
produces an armature magnetomotive force (mmf) that
constant-potential power transformers. Most
influences the terminal voltage by changing the
power-supply transformers for electronic equipment are
magnitude of the field flux across the air gap. When the
designed to operate at a frequency of 50 to 60 Hz.
load is inductive, the armature mmf opposes the dc field
Aircraft power-supply transformers are designed for a
and weakens it, thus lowering the terminal voltage.
frequency of 400 Hz. The higher frequencies permit a
When a leading current flows in the armature, the dc
saving in size and weight of transformers and associated
field is aided by the armature mmf, and the flux across
equipment.
the air gap is increased, thus increasing the terminal
voltage.
CONSTRUCTION
TRANSFORMERS
The typical trzansformer has two windings insulated
A transformer is a device that has no moving parts
electrically from each other. These windings are wound
and that transfers energy from one circuit to another by
on a common magnetic core made of laminated sheet
electromagnetic induction. The energy is always
steel. The principal parts of a transformer and their
transferred without a change in frequency, but usually
functions areas follows:
with changes in voltage and current. A step-up
transformer receives electrical energy atone voltage and
Function
Piece
delivers it at a higher voltage, Conversely, a step-down
Provides a path for the magnetic
transformer receives energy at one voltage and delivers
Core
it at a lower voltage. Transformers require little care and
lines of flux
maintenance because of their simple, rugged, and
Receives the energy from the ac
Primary winding
durable construction. The efficiency of transformed is
source
high. Because of this, transformers are responsible for
the more extensive use of alternating current than direct
Receives energy from the
Secondary winding
current. The conventional constant-potential
primary winding and delivers it
transformer is designed to operate with the primary
to the load
connected across a constant-potential source and to
provide a secondary voltage that is substantially
Protects the above components
Enclosure
constant from no load to full load.
from dirt, moisture, and
Various types of small, single-phase transformers
mechanical damage
are used in electrical equipment. In many installations,
transformers are used on switchboards to step down the
When a transformer is used to step up the voltage,
voltage for indicating lights. Low-voltage transformers
the low-voltage winding is the primary. Conversely,
are included in some motor control panels to supply
when a transformer is used to step down the voltage, the
control circuits or to operate overload relays,
high-voltage winding is the primary. The primary is
Instrument transformers include potential, or
always connected to the source of the power; the
voltage, transformers and current transformers.
secondary is always connected to the load It is common
Instrument transformers are commonly used with ac
practice to refer to the windings as the primary and
instruments when high voltages or large currents are to
secondary rather than the high-voltage and low-voltage
be measured.
windings.
Electronic circuits and devices employ many types
There are two principal types of transformer
of transformers to provide the necessary voltages for
construction--the core type and the shell type (fig. 3-31,
proper electron-tube operation, interstage coupling,
views A and B). The cores are built of thin stampings
signal amplification, and so forth. The physical
of silicon steel. Eddy currents, generated in the core by
construction of these transformers differs widely.
the alternating flux as it cuts through the iron, are
minimized by using thin laminations and by insulating
Power-supply transformers, used in electronic
adjacent laminations with insulating varnish.
circuits, are single-phase, constant-potential
Hysteresis losses, caused by the friction developed
transformers with either one or more secondary
3-24
