Lincoln Electric SVM151-A service manual Pulse Width Modulation, 8E-8

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E-8E-8

THEORY OF OPERATION

FIGURE E.7 – TYPICAL IGBT OUTPUTS

MINIMUM OUTPUT

2 sec				(Dwell or Off Time)
		48	sec
	50		sec

MAXIMUM OUTPUT

(Dwell or Off Time)

48sec

￿

50sec

PULSE WIDTH MODULATION

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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.

The positive portion of the signal represents one IGBT group conducting for 2 microsecond. The dwell time (off time) is 48 microseconds. Since only 2 microseconds of the 50-microsecond time period is devoted to conduct- ing, 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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MULTI-WELD 350