Pulse Width Modulation - Lincoln Electric Ranger 305D 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
cycle. Changing the pulse width is known as MODU-
LATION. 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 shows the minimum output sig-
nal possible over a 50-microsecond time period.
THEORY OF OPERATION
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FIGURE E.6 – IGBT OPERATION
RANGER 305D
The positive portion of the signal represents one IGBT
group conducting for 2 microseconds. The dwell time
(off time) is 48 microseconds. Since only 2 microsec-
onds of the 50-microsecond time period is devoted to
conducting, the output power is minimized.
MAXIMUM OUTPUT:
By holding the gate signals on for 48 microseconds and
allowing only 2 microseconds of dwell time (off time)
during the 50-microsecond cycle, the output is maxi-
mized. The darkened area under the top curve can be
compared to the area under the bottom curve. The
more darkened area under the curve, the more power
is present.
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