Stator current higher harmonics therefore have the same value regardless of the operating frequency. This is so due to the fact that DC voltage and hence rms values of all the voltage harmonics change proportionally to operating frequency. | ENGNG3070 Power Electronics Devices Circuits and Applications a f fn 25 50 Vk VDC T n k Isk Vk Zk Vk k X. X V5 n Vdc I 23 V V7 n Vdc I V Is5 V5 5a X ss Xr 23 A Is7 V7 7a Xas X A Stator current higher harmonics therefore have the same value regardless of the operating frequency. This is so due to the fact that DC voltage and hence rms values of all the voltage harmonics change proportionally to operating frequency. As leakage reactances are proportional to fundamental frequency as well ratio of phase voltage harmonic rms value to sum of leakage reactances for the given harmonic remains constant regardless of the fundamental frequency. PWM Operation of a Voltage Source Inverter In all the analysis so far it was assumed that the inverter operates in 180 degrees conduction mode and that variation of the inverter output voltage magnitude is achieved by rectifier control. Such a solution is nowadays rarely applied. Instead the inverter is operated in pulse width modulated PWM mode and is supplied from a diode bridge rectifier single-phase or three-phase so that input DC voltage of the inverter is constant. Variation of both inverter output voltage and output frequency is now achieved by the inverter due to operation in the PWM mode. The idea of PWM is explained using single-phase VSI circuit of Fig. . The most frequently utilised method of PWM is the so-called sinusoidal PWM technique in which a reference signal - a sine wave of desired amplitude and frequency - is compared with a triangular carrier wave of constant amplitude and frequency. The instants for turn-on and turnoff of semiconductors in Fig. are then determined with intersections of the reference signal and the carrier wave. Switches are turned on and off in pairs S1 and S2 are always together either on or off and similarly S3 and S4 are always together either on or off. The advantage of this approach is two-fold. First of all inverter now .
