Pulse wave modulation , electric

AC electric drives require more sophisticated converters when they are supplied with DC sources, because electric machines requires periodic voltage and current waves with a variable frequency depending on the load requirements. In Fig. 5.8, the scheme of an example of three-phase induction motor driven by a pulse-width-modulated inverter is reported. In this scheme a three-phase bridge connection with six power modules is shown to form the so-called inverter. Each power module can be composed by a number of power switches connected in parallel to carry higher currents. Across each power switch (IGBT) a parallel diode is connected to provide a return path for the phase current when the power module is switched off. 

Typical electro-optical responses of the suspensions of spherical particles (CS81), recorded by the scattered light intensity are presented in Fig. 2. They are detected for the crystal state of the systems and for different intensities and frequencies of the applied sine-wave electric pulses. The low-frequency responses are modulated they follow the field frequency at sufficiently low field intensity and exhibit a double frequency modulation at higher field intensity. Two different time scales are involved in the decay of the responses (10 " and 1 s), which can be both exponential and oscillatory. At higher field intensity or frequency the effects cannot be distinguished by the responses of anisotropic colloids. 

The laser pulses typically used in an OKE experiment are characterized by an optical frequency (700 nm), a duration longer than 10-20 fs and they are normally transform-limited (i.e., with no time modulation in phase, apart from the optical frequency). The time dependence of the electric held generated by these laser pulses can be safely described as a plane wave in the approximation of the slow parameter variation, see [40]. Hence, using the complex notation we can write the expression of a linearly polarized pulse, as follows... 

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