224
Electricity and Basic Electronics
of it and the secondary wrapped around the
other side. Some manufacturers insert layers
of insulation between the windings to reduce
the possibility of a short circuit.
Another popular kind of transformer is
the shell-form transformer, Figure 12-19. It is
constructed by wrapping the primary around
a cardboard core and then wrapping the
secondary over the primary. Insulating paper
separates each layer of windings from the
other, producing a transformer with a high
mutual inductance.
The E-shaped pieces of the laminated core
are inserted in the center of and around the
outsides of the windings. The direction of the
E-shaped and I-shaped pieces are alternated,
Figure 12-20. Bolts that are insulated from
all parts of the layers pull the stack together.
Insulating the bolts and constantly reversing
the E-shaped and I-shaped pieces further
reduces eddy currents in the laminations.
Laminated Cores
Transformer cores are made of thin strips
of metal that are laminated and stacked
together. If a solid core were used in a trans-
former, or if the thin strips of metal in a lami-
nated core were stacked one on top of the other
without being laminated, a lot of heat would
be produced. When heat is produced, energy
is lost, so this heat must be avoided.
Heat is created in the transformer when
eddy currents fl ow in the core because of
induced voltage, Figure 12-21. As the voltage
created by the magnetic fi eld increases,
the amount of heat produced by the eddy
currents in the transformer increases. Thus,
this means the greater the heat, the greater
the energy loss. This energy loss is called
eddy current loss.
Figure 12-19. This type of transformer is called
shell form.
Laminated
core
Shell-Form Transformer
Primary
windings
Secondary
windings
Figure 12–20. The core of a shell-form transformer consists of E-shaped
and I-shaped pieces stacked together.
E-shaped and
I-shaped pieces
are stacked together
I-shaped
piece
E-shaped
piece