PWM is used in inverters to supply power to the ac motors more commonly in the industry now. In PWM, a sawtooth wave is used to modulate the chops as shown in Figure 4.8.

FIGURE 4.8 The principle of pulse-width modulation (PWM). (From Arrillaga, J., Bradley, J.D., and Bodger, P. (1985), Power System Harmonics, John Wiley, Chichester, U.K. With permission.)

The sawtooth wave has a frequency that is a multiple of three times the sine-wave frequency, allowing symmetrical three-phase voltages to be generated from a three phase sine-wave set and one sawtooth waveform. The PWM control signal is generated by feeding the sawtooth carrier signal and sine-wave modulating signal to a comparator circuit. In this simple form, this method controls line-to-line voltage from zero to full voltage by increasing the magnitude of the sawtooth, with little regard to the harmonics generated.

More efficient PWM techniques have been developed to control the fundamental and harmonic voltages simultaneously. Using suitable switching times with five on/off actions per cycle, one can eliminate both fifth and seventh harmonics together.

Generally, at any fundamental switching frequency, each chop per half-cycle of the inverter phase voltage waveform can eliminate one harmonic of the waveform or reduce a group of harmonic amplitudes. Assuming that there are m chops per half-cycle, one chop can be utilized to control the fundamental voltage and the other (m - 1) to reduce the other specified low-order harmonics or to minimize power losses caused by a specified range of harmonics within the motors.

However, it must be noted that, as the total rms harmonic voltage cannot change, the portion of the rms voltage that was provided by the elimination of harmonics will be spread over the remaining harmonic magnitudes. Hence, the motor designer must take this fact into account.

Also, the integrating filter characteristic of the motor will reduce some of these higher-order-current harmonics. Because of the number of switchings, the usable fundamental rating and the converter efficiency are reduced as GTOs require significant energy (resulting in increased thyristor losses and heating effects) for each switching operation.

In contrast, devices such as IGBTs (Integrated state Bipolar Transistors) and IGCTs (Integrated State Commutated Thyrestors) require much lower switching energy and are better suited to the use of PWM techniques. However, with the use of chain circuits that require reduced switching frequency, that is, once per two cycles of frequency, high-power inverters can be constructed using GTOs.

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