Theory of operation, Switch board & main transformer – Lincoln Electric INVERTEC SVM158-A User Manual

Page 45

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SWITCH BOARD &
MAIN TRANSFORMER

There is one switch board in the Invertec V350-Pro.
This board incorporates two pairs of input capacitors,
two insulated gate bipolar transistor (IGBT) switching
circuits, a fan motor drive circuit, and a voltage/fre-
quency capacitor feedback circuit. The two capaci-
tors in a pair are always in series with each other.
When the reconnect switch is in the lower voltage
position the capacitor pairs are in parallel . Thus two
series capacitors in parallel with two series capacitors.
When the reconnect switch is in the high voltage posi-
tion the two capacitor pairs are in series. Thus four
capacitors in series. This is required to accommodate
the higher input voltages.

When the input capacitors are fully charged they act
as power supplies for the IGBT switching circuits.
When welding output is required the Insulated Gate
Bipolar Transistors switch the DC power from the input
capacitors, "on and off" thus supplying a pulsed DC
current to the main transformer primary windings. See
IGBT Operation Discussion and Diagrams in this
section. Each IGBT switching circuit feeds current to
a separate, oppositely wound primary winding in the

main transformer. The reverse directions of current
flow through the main transformer primaries and the
offset timing of the IGBT switching circuits induce an
AC square wave output signal at the secondary of the
main transformer. The two current transformers (CT)
located on the switch board monitor these primary
currents. If the primary currents become abnormally
high the control board will shut off the IGBTs, thus dis-
abling the machine output. The DC current flow
through each primary winding is clamped back to
each respective input capacitor when the IGBTs are
turned off. This is needed due to the inductance of the
transformer primary winding. The firing of the two
switch boards occurs during halves of a 50 microsec-
ond interval, creating a constant 20 KHZ output. In
some low open circuit Tig modes the firing frequency
is reduced to 5KHZ.

The Invertec V350-Pro has a F.A.N. fan as needed cir-
cuit. The fan operates when the welding output ter-
minals are energized or when a thermal over tempera-
ture condition exists. Once the fan is activated it will
remain on for a minimum of five minutes. The fan dri-
ver circuit is housed on the switch board but it is acti-
vated from a control board signal.

THEORY OF OPERATION

E-3

V350-PRO