©
COPYRIGHT 1999 THE ESAB GROUP, INC. LESSON
I, PART B 1.8.5.2
The transformer in a welding machine
performs much the same as a large power plant
transformer. The primary voltage coming into the machine is too high for
safe welding.
Therefore, it is stepped down to a useable voltage. This is best illustrated
with an explanation
of how a single transformer works. 1.8.5.3
In the preceding paragraphs, we have
found than an electrical current can be induced
into a conductor when that conductor is moved through a magnetic field to
produce alternating current. If
this alternating current is passed through a conductor, a
pulsating magnetic field will surround
the exterior of that conductor, that is the magnetic field
will build in intensity through the first 90 electrical degrees, or the first
cycle. From that point,
the magnetic field will decay during the next quarter cycle until the voltage
or current reaches
zero at 180 electrical degrees. Immediately, the current direction reverses
and the magnetic
field will begin to build again until it reaches a maximum at 270 electrical degrees
in the cycle. From that point
the current and the magnetic field again begin to decay until
they reach zero at 360 electrical degrees,
where the cycle begins again. 1.8.5.4
If that conductor is wound around a
material with high magnetic permeability (magnetic
permeability is the ability to accept large amounts of magnetic lines of force)
such as steel, the magnetic field permeates
that core. See Figure
14. This conductor is called the primary coil, and if
voltage is applied to one of its terminals
and the circuit is completed,
current will flow. When a second coil is wound around
that same steel core, the energy that is stored in this
fluctuating magnetic field in the core is induced into this
secondary coil. 1.8.5.5
It is the build-up and collapse of
this magnetic field
that excite the electrons in the secondary coil of the transformer.
This causes an electrical current of the same frequency as the primary coil
to flow when the
secondary circuit is completed by striking the welding arc. Remember that
all transformers operate only on alternating
current. 1.8.5.6
A simplified version of a welding transformer
is schematically shown in Figure 15. This
welder would operate on 230 volts input power and the primary winding has 230
turns of wire
on the core. We need 80 volts for initiating the arc in the secondary or
welding circuit,
thus we have 80 turns of wire in the secondary winding of the core. Before
the arc is struck,
the voltage between the electrode and the work piece is 80 volts. Remember
that no current
(amperage) flows until the welding circuit is completed by striking the arc.
FIGURE 14 STEEL
CORE PRIMARY
COIL SECONDARY
COIL 80
V 80
TURNS 460
V 460
TURNS BASIC
TRANSFORMER
