Manual 0-5186 5-1
Main Circuit Description
MAIN CIRCUIT DESCRIpTION WELDSKILL 250, 350
SECTION 5:
MAIN CIRCUIT DESCRIPTION
5.01 Main Circuit Description
NOTE
This general description should be read in
conjunction with the respective WELDSKILL
Circuit Diagram (refer previous section).
In Single phase Power Sources, the standby switch
connects directly to the incoming supply. In Three
phase Power Sources, the standby switch connects to
two phases of the incoming supply.
The standby switch contacts are open when the machine
is placed in standby mode, thus isolating the cooling
fan and auxiliary transformer from the incoming mains
supply. There is no control voltage supply available
when the machine is placed in standby mode. Hence
no controls will function until the operator switches
the standby switch from the “standby” position to the
“on” position.
For protection of the wiring, there is on the WELDKSILL
250 and 350 a 3A fuse or circuit breaker in series with
the input to the standby switch
WARNING
In the standby position the machine is not
electrically isolated from the mains supply
voltage.
The main transformer consists of a primary coil which is
connected to the incoming mains via the main contactor.
CAUTION
Contactor KM does not electrically isolate
the main transformer from the mains supply
voltage when KM is de-energised.
Secondary output voltage from the main transformer is
controlled by the Coarse & Fine switch settings. These
switches determine which main transformer taps are
to be connected to the mains supply voltage. The main
contactor KM is controlled by the torch trigger switch
via the Control PCB.
The secondary side of the main transformer is connected
to a single phase bridge rectifier on the WELDSKILL 250
and to a three phase bridge rectifier on the WELDSKILL
350. The rectifier converts the AC voltage (from the
secondary of the main transformer) to DC voltage.
Output filter capacitors on the WELDSKILL 250 provide
voltage to the arc when the main transformer secondary
voltage drops below the arc voltage. Resistor R3
(15ohm 50W) provides a discharge path for the output
filter capacitors via a normally closed contact on the
main contactor KM.
An output inductor ensures smoothing of the welding
arc and reduces spatter.
A filter network consisting of a capacitor C1 and resistor
R1 provides high frequency protection for the rectifier.
A thermal overload device (thermostat) is each fixed to
the rectifier and transformer in series and connected
to the Control PCB. In normal operation, the normally
closed thermostats complete the circuit. When an over
temperature occurs, the affected thermostat opens and
breaks the circuit and thermal overload indicator LED
on the front panel illuminates. The trigger is disabled,
effectively disabling contactor KM, which controls
power to the main transformer and disables the gas
solenoid valve and motor drive.
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