Voltage Source SPWM Inverter

During the positive cycle, S1 and S2 are switched by the high frequency pulse train shown in Figure (next slide). During the negative cycle, the pulse train switches S3 and S4. The load inductance integrates the generated pulse train and produces a sinusoidal voltage (V_ac) and current wave, as shown in the next Figure. The width of each pulse is varied in proportion to the amplitude of a sine wave. A typical PWM waveform is shown in the previous slide. The switches in this converter are controlled by gate pulses. The gate signal contains several pulses distributed along the half-cycle. The control circuit produces the gate pulse train by generation of a triangular carrier wave and a sinusoidal reference signal. The two signals are compared, and when the carrier wave is larger than the reference signal, the gate signal is positive. When the carrier wave is smaller than the reference signal, the gate signal is zero. This results in a gate pulse with variable width.

Figure: Single-phase voltage source converter.

Figure: Gate pulse input signal, and ac voltage and current outputs of a pulse width modulation (PWM) converter.

(a) Triangular carrier wave and sinusoidal reference signal

(b) Variable-width gate pulse signal

Figure: Pulse width modulation (PWM) signals.

Figure (a) shows the carrier wave and reference sine wave; (b) depicts the resulting gate signal with variable width pulses. It has to be noted that several other methods are used for generation of PWM signals as discussed earlier.

The frequency of the reference sine wave determines the frequency of the generated ac voltage. The amplitude of the ac voltage can be regulated by the variation of the reference signal amplitude. The amplitude of the fundamental component of the ac voltage is:

The modulation index (m or M) is the ratio of the peak-to-peak ac voltage (2Vac) to the dc voltage.

Freewheeling diode

The inverter interrupts the current several times each cycle. The interruption of an inductive current would generate unacceptably high overvoltage. This over voltage generation is eliminated by providing freewheeling diodes connected in parallel with the switches.

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