Lincoln Electric RANGER 305 D Service Manual page 56

RANGER 305D THEORY OF OPERATION
CHOPPER TECHNOLOGY
FUND MENT LS
The new era of welding machines such as the Ranger 305D 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:
3 PH SE DC RECTIFIER SOLID ST TE INDUCTOR
RC
LTERN TOR ND FILTER SWITCH ND DIODE
RC
CONTROL
In this system, the engine drives a three-phase alternator, which
generates power that is rectified and filtered to produce about 90
VDC. 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 following diagram depicts the current
flow in the system when the switch is open and closed:
INDUCTOR
SWITCH
RC
90 VDC
DIODE
CURRENT WITH SWITCH OPEN
CURRENT WITH SWITCH 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.
E 7