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Master of Science Thesis in Electric Power Engineering - page 24 / 114

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THREE PHASE CONTROLLED RECTIFIERS

2.4 Pulse Width Modulation

The Pulse Width Modulation used in our system, a sinusoidal PWM will be used.

2.4.1 Sinusoidal PWM

Sinusoidal modulation is based on a triangular carrier signal. The idea is to compare three sinusoidal reference voltages Ua*, Ub* and Uc* to this triangular wave. By comparison, the logical signals Sa, Sb and Sc, which define switching instants of power transistor, are generated. Operation with constant carrier signal concentrate voltage harmonics around switching frequency and multiple of switching frequency.

Figure 2.11 : Sinusoidal PWM

Vdc/2

Vdc/2

Figure 2.12 : Sinusoidal PWM basic waveforms

 Maximum sinusoidal reference voltage Using a sinusoidal PWM, the maximum reference value is Vdc/2.

Figure 2.13 : Maximum sinusoidal reference voltage (sinusoidal PWM) [23]

 Maximum slope The slope of the triangular wave should be higher than the slope of the reference voltage. We can write

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