66
3
3-7 Welder circuit applications
3-7-1 Arc welders
MCCBs installed in arc welder circuits should not inadvertently
trip due to the massive inrush current generated at ignition.
Inadvertent tripping often occurs when inrush current instantly
trips the overcurrent tripping element in the MCCB. Since
the transient inrush current in arc welders is 8 to 9 times the
primary current, an MCCB that can handle at least ten times
the rated primary current without tripping should be selected
for this kind of application.
3-7-2 Resistance welders
(1) Characteristics specific to resistance welder
circuits
Resistance welders are characterized by intermittent operation
with short switching intervals and also by switching in the
primary circuit of the welder transformer. Consequently, the
following points must be considered when selecting an MCCB:
(a) Thermal equivalent current
The current that flows through the welding circuit is repetitive
with short periods as shown in Fig. 3-22. Since the MCCB
operation or the temperature rise in the wire is determined
by a thermal equivalent current, the current flowing during
intermittent operation must be converted to a thermally
equivalent continuous current.
(i) Thermal equivalent current Ia during period t (seconds)
Assuming that the current flowing time for resistance welding
by the current I
L
[A] is t
L
(seconds) per point, and that
resistance welding is conducted at one point per t (seconds),
then the on-load factor
D
of the welder can be stated in an
equation as:
Period
Current flowing time
D
=
=
t
t
L
In this current flowing state, the amount of heat W generated
by the total circuit resistance R per t (seconds) can be
represented as
W = (I
L
)
2
·R·t
L
(joule)
If this value is taken as the average amount of heat generated
per
t (seconds), then the equation derives as follows.
t
W
= (I
L
)
2
r
R
r
t
t
L
= (I
L
)
2
r
R
r
D
=
R(I
L
D
)
2
This means that the generated heat is equal to the amount
of heat that would be generated upon continuous flow of the
current
I
L
D
(A)
. Hence, the thermal equivalent current Ia at
period t (seconds) can be stated as
I
a
= I
L
D
(A)
(ii) Thermal equivalent current IB at period T (seconds)
In Fig. 3-22, the thermal equivalent current IB at period T
L
(seconds) is similar to that at period t (seconds). At period T
(seconds), however, the thermal equivalent current IB can be
represented as:
I
B
= I
L
E
(A)
where,
E
= n
x
t
L
/T
n = T
L
/t
Fig. 3-22 Typical intermittent operation
t
L
I
L
t
T
L
T
(b) Transient inrush current caused by switching
transformer primary circuit
For resistance welders load switching is carried out in the
primary circuit of the welder transformer. Consequently, a high
transient inrush current may flow when the circuit is closed, as
mentioned under “Selecting an MCCB for transformer primary
circuit” (See page 63).
Whether or not inrush current flows depends on the type of
switching control system used in resistance welders because
inrush current is generated by the closed circuit phase or
by residual magnetic flux in the transformer core. Switching
is controlled using synchronous, semi-synchronous, or
asynchronous systems.
Inrush current does not occur with synchronous control
systems because they can control the current flow start phase
and they can reverse the start polarity by the time the current
flow ends.
Semi-synchronous control systems can control the current
flow start phase, but cannot necessarily reverse the start
polarity by the time the current flow ends. Inrush current may
therefore occur here due to biased excitation of the core, but
this is generally not a problem because these systems can
adequately control the making phase.
Most semi-synchronous control systems today use thyristors
for main current switching. With the anti-surge current
capability of the thyristor as well, these systems take the half
cycle at the start of the closed circuit phase and insert it just
past the voltage phase
S
/2 to prevent inrush current.
Asynchronous control systems use a magnetic contactor
for main current switching. Here, the closed circuit phase
generates massive inrush current as high as 20 times the
steady state current. This is why newer welders now use either
synchronous or semi-synchronous control systems.
Selection and application
3-7 Welder circuit applications