E11 ), Theory of operation – Lincoln Electric INVERTEC POWER WAVE 450 SVM112-B User Manual

Page 60

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THEORY OF OPERATION

E-12

E-12

POWER WAVE 450

The term PULSE WIDTH MODULATION is
used to describe how much time is devoted to
conduction in the positive and negative por-
tions 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 FET is turned on and off for different dura-
tions during a cycle. The top drawing above
shows the minimum output signal possible
over a 50-microsecond time period.

The positive portion of the signal represents
one FET group

1

conducting for 1 microsec-

ond. The negative portion is the other FET
group

1

. The dwell time (off time) is 48 micro-

seconds (both FET 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 20 micro-
seconds each and allowing only 10 microsec-
onds 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.

PULSE WIDTH MODULATION

FIGURE E.11 – TYPICAL FET OUTPUTS

1

A FET group consists of the sets of FET modules grouped onto one switch board.

(+)

(+)

(–)

(–)

1

µ

sec

1

µ

sec

48

µ

sec

50

µ

sec

MINIMUM OUTPUT

MAXIMUM OUTPUT

20

µ

sec

10

µ

sec

20

µ

sec

50

µ

sec

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