Chopper Technology Fundamentals; Power Mig - Lincoln Electric POWER MIG SVM160-B Service Manual

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CHOPPER TECHNOLOGY
FUNDAMENTALS
The new era of welding machines such as the Power
MIG 300 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:
SINGLE-PHASE
INPUT POWER
+
80VDC
_
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.
THEORY OF OPERATION
FIGURE E.10
SOLID STATE
DC RECTIFIER
SWITCH
AND FILTER
CONTROL
DIODE
SWITCH
CURRENT WITH SWITCH CLOSED

POWER MIG 300

INDUCTOR
AND DIODE
ARC
ARC
CURRENT WITH SWITCH OPEN
INDUCTOR
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ARC