Lincoln Electric POWER WAVE C300 Service Manual page 17

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PULSE WIDTH MODULATION
The term PULSE WIDTH MODULATION (PWM) is
used to describe how much time is devoted to conduc-
tion in the positive and negative portions of the cycle.
Changing the pulse width is known as MODULATION.
Pulse Width Modulation is the varying of the pulse
width over the allowed range of a cycle to affect the
output of the machine.
MINIMUM OUTPUT
By controlling the duration of the gate signal, the IGBT
is turned on and off for different durations during a
cycle. The top drawing shows the minimum output sig-
nal possible over a 16.6-microsecond time period. The
shaded portion of the signal represents one IGBT
group1, conducting for 1 microsecond. The negative
portion is the other IGBT group. The dwell time (off
time) is 14.6 microseconds (both IGBT groups off).
Since only 2 microseconds of the 16.6-microsecond
time period are devoted to conducting, the output
power is minimized.
THEORY OF OPERATION
FIGURE E.7 - IGBT OPERATION
MINIMUM OUTPUT
sec
7.3
sec
16.6
MAXIMUM OUTPUT
sec
7.3
sec
1
sec
16.6
POWER WAVE® C300
sec
1
sec
7.3
MAXIMUM OUTPUT
By holding the gate signals on for 7.3 microseconds
each and allowing only 2 microseconds of dwell or off
time (one microsecond during each half cycle) during
the 16.6 microsecond cycle, the output is maximized.
The darkened area under the minimum output curve
can be compared to the area under the maximum out-
put curve. The more darkened area, the more power is
present.
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