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THEORY OF OPERATION
E-8 E-8
INVERTEC® V450-PRO
PULSE WIDTH
MODULATION
The term Pulse Width Modulation is used to describe
how much time is devoted to conduction in the positive
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.7 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.
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 IGBT group consists of two IGBT modules feeding one trans-
former primary winding.
FIGURE E.7 - TYPICAL IGBT OUTPUTS
(+)
1 μsec
48 μsec
50 μsec
1 μsec
(-)
(+)
(-)
2 μsec
24 μsec
50 μsec
24 μsec
MINIMUM OUTPUT
MAXIMUM OUTPUT
(+)
1 μsec
48 μsec
50 μsec
1 μsec
(-)
(+)
(-)
2 μsec
24 μsec
50 μsec
24 μsec
MINIMUM OUTPUT
MAXIMUM OUTPUT
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