Pulse Width Modulation - Lincoln Electric PRO-CUT 55 SVM140-A Service Manual

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PULSE WIDTH MODULATION

The term PULSE WIDTH MODULATION describes
how much time is devoted to conduction in the posi-
tive and negative portions of the cycle. Changing the
pulse width is known as MODULATION. Pulse Width
Modulation (PWM) 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 below shows the minimum
output signal possible over a 50-microsecond time
period.
1 An IGBT group consists of two IGBT modules feeding one transformer primary winding.
sec
THEORY OF OPERATION
FIGURE E.8 – TYPICAL IGBT OUTPUTS
48
50
MINIMUM OUTPUT
sec
22
sec
3
50
MAXIMUM OUTPUT
PRO-CUT 55
The positive portion of the signal represents one IGBT
group
1 conducting for 1 microsecond. The negative
portion is the other IGBT group
time) is 48 microseconds (both IGBT groups off).
Since only 2 microseconds of the 50-microsecond
time period is devoted to conducting, the output
power is minimized.
MAXIMUM OUTPUT
By holding the gate signals on for 22 microseconds
each and allowing only 3 microseconds of dwell time
(off time) during the 50-microsecond cycle, the output
is maximized. The darkened area under the top curve
can be compared to the area under the bottom curve.
The more dark area under the curve, the more power
is present.
sec
sec
sec
22
sec
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1 . The dwell time (off
sec