Pulse Width Modulation - Lincoln Electric INVERTEC V350-PRO SVM152-A Service Manual

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PULSE WIDTH
MODULATION
The term Pulse Width Modulation is used to describe
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 the
cycle. The top drawing in Figure E.9 shows the mini-
mum output signal possible over a 50-microsecond
time period.
The positive portion of the signal represents one IGBT
group 1 conducting for one microsecond. The negative
portion is the other IGBT group 1 . The dwell time (off
time) is 48 microseconds (both IGBT groups off). Since
only two microseconds of the 50-microsecond time
period is devoted to conducting, the output power is
minimized.
THEORY OF OPERATION
FIGURE E.9 — TYPICAL IGBT OUTPUTS.
sec
48
sec
50
MINIMUM OUTPUT
sec
24
sec
2
sec
50
MAXIMUM OUTPUT
MAXIMUM OUTPUT
By holding the gate signal on for 24 microseconds
each, and allowing only two 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 that is under the curve indi-
cates that more power is present.
1 An
V350-PRO
sec
24
IGBT group consists
modules feeding one
winding.
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sec
of two IGBT
transformer primary