Lincoln Electric POWER MIG SVM165-A - Manual (Page 45)

CHOPPER TECHNOLOGY
FUNDAMENTALS
The new era of welding machines such as the Power
MIG 350MP employ a technology whereby a DC
source is turned on and off (chopped up) at high speed,
then smoothed through an inductor to control an arc.
Hence the name “Chopper.” The biggest advantage of
chopper technology is the high-speed control of the
arc, similar to the inverter machines. A block diagram
for this is as follows:
THEORY OF OPERATION
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DC RECTIFIER
AND FILTER SOLID STATE
SWITCH INDUCTOR
AND DIODE
ARC
ARC
CONTROL
SINGLE-PHASE
INPUT POWER
+
_
80VDC DIODE
SWITCH
INDUCTOR
CURRENT WITH SWITCH OPEN
ARC
CURRENT WITH SWITCH CLOSED
In this system, the engine drives a three-phase alternator, which generates power that is rectified and filtered to
produce about 80VDC. The current is applied through a solid state switch to an inductor. By turning the switch
on and off, current in the inductor and the arc can be controlled. The above diagram depicts the current flow in
the system when the switch is open and closed:
When the switch is closed, current is applied through the inductor to the arc. When the switch opens, current
stored in the inductor sustains flow in the arc and through the diode. The repetition rate of switch closure is 20Khz,
which allows ultra-fast control of the arc. By varying the ratio of on time versus off time of the switch (Duty Cycle),
the current applied to the arc is controlled. This is the basis for Chopper Technology: Controlling the switch in such
a way as to produce superior welding.
Figure E.10