230 vac secondary voltage from transformer T2.
in the SOFT START position, the control rectifier
When relay K5 and K6 are energized, and timers M2
CR2 receives only half the normal ac voltage. The
resulting dc voltage applied to the control coils on T1
and M4 are as required, then 230 vac is applied to
is only half normal.
t h e coil of the contactor through a thermostatic
switch. The closed contacts of K8 then apply power to
(3) When the arc is struck, pilot relay K1 releases
and removes 24 vac from K2. Capacitor C2 begins to
transformer T1 and permits the arc to be established.
d. Pilot Relay K1.
discharge, and after a 2 second delay, relay K2 re-
leases. When K2 releases, the 24 vac is removed from
(1) When the contactor is energized and trans-
the coil of K3, causing relay K3 to release. This con-
former T1 receives power, pilot relay K1 becomes
nects the entire ac control winding across control
energized. This relay is energized by the de voltage
rectifier CR2 and results in a standard output.
from bridge rectifier CR3. The ac voltage applied to
f. Overload Relay K7. This relay protects the weld-
the bridge, and the resulting dc voltage applied to
ing machine against excessive current delivered by
K1, is dependent upon the output voltage at the
electrode, The open circuit voltage at the electrode
w i n d i n g of T1. When excessive current is being
is higher than the voltage while the arc is established.
drawn, K7 is energized. Normally closed contacts on
Initially, therefore, relay K1 is energized when power
is applied to T1, but drops out when the arc is struck
K7 open and remove the 24 vac output of T2 from
due to the lowered voltage across the bridge rectifier.
power relay K5. Relay K5 then becomes deenergized
(2) When K1 is initially energized, contacts (6-8
and removes power from contactor K8, shutting down
the welding machine output. Observe that when the
close and apply 230 vac to high frequency transform-
er T3, via the dummy plug and closed HIGH FRE-
contactor releases, the primary circuit of T1 is opened
and overload relay K7 is released. This reapplies 24
QUENCY switch S5. This results in a high frequency
vac to the POWER switch and contacts of power re-
output at the torch electrode. When the arc is struck
lay K5. Relay K5 is not energized, however, until the
K1 becomes deenergized and contacts 6-8 open, drop-
POWER switch is placed in the ON position.
ping out the high frequency circuit.
(3) Pilot relay K1 also has another function.
3-18. High Frequency Circuit
When power is applied to T1 and K1 becomes ener-
a. The high frequency circuit consists of high volt-
gized, contacts 1-3 close and apply 24 vac from the
CONTACTOR switch S2 to diode CR1, which ener-
citor C9 and tesla coil T4. When 230 vac is applied to
gizes relay K2 in the soft start circuit.
the primary of T3, a high voltage appears across the
e. Soft Start Relays K2, K3, and K4.
spark gap and R5, which are in series. The spark gap
(1) The soft start circuit employs three relays,
is adjusted to maintain an arc across the gap, which
K2, K3, and K4. When ac line-voltage is present at
results in a high frequency signal across the gap.
transformer T2, the 24 vac secondary voltage ener-
This signal is coupled by C9 to transformer T3, where
gizes soft start delay relay K2, through normally
it is imposed upon the welding current. The intensity
closed contacts of present relay K4. Soft start power
of the high frequency appearing at the output is con-
relay K3, becomes energized through closed contacts
trolled by R5.
of K2. Therefore, before the POWER switch is turned
b. When AC welding is being used, the high fre-
ON, K2 and K3 are energized and K4 is deenergized.
Note. Diode CRI conducts on each half cycle, charging
quency appears superimposed on the ac waveform.
capacitor C2 which holds K2 energized.
W h e n the ac waveform voltage momentarily be-
comes zero at each half cycle, the high frequency
When the POWER switch and CONTACTOR
switch are turned on and ac is applied to transformer
components is still present and helps to prevent the
arc from extinguishing.
Tl, an ac voltage appears across the ac control wind-
ing secondary. This quickly energizes soft start, pre-
3-19. Timer and High Frequency Drawer
set relay K4, which removes 24 vac from the coil of
u. Removal. Proceed as follows to remove the
K2. Relay K2 remains energized however, because
drawer for replacement.
pilot relay K1 now becomes energized and applies
(1) Disconnect ac power from the welding ma-
24 vac to K2 via contacts 1-3 on K1. The charged
capacitor, C2, across the coil of K2 develops the time
(2) With the key, unlock the front panel door
delay for the soft start. Relays K2 and K3 are now
and open it.
energized, with K2 being controlled by K1. During
(3) Disconnect the door-stop bar from the stop
this condition, the center tap on the ac control wind-
screw in the upper left corner and lower the front
ing of T1 is connected to terminal 1 of SOFT START/
STANDARD START switch S8. When the switch is