Random Pulse Width Modulation Technology for Control and Protection of Asynchronous Motor Ding Hongbing, Shen Bing, Li Hongjiang (Department of Electrical Engineering, Naval University of Engineering, Wuhan 430033, Hubei, China) Randomly changed the variable coefficient k proportional to the zero vector V. to achieve random pulse width modulation, The modulation method is used in a variable frequency speed control system. Random pulse width modulation is implemented by the DSP controller. In the paper, uiU6 is six non-zero vectors, whose length is /23Udc and the phases are mutually different by 60 degrees. They divide the whole space into 6 sectors. The basic principle of voltage space vector PWM modulation is to approximate a given voltage vector U' using several switching voltage vectors. If U' is located in the interval 1 between Ui and U2, and the angle with Ui is 0, Ui, U2 The action time Ti can be calculated as shown in Table 1. Table 1 Space Vector Pulse Width Modulation Time Sector Taking the first sector as an example, in the conventional voltage space vector modulation PWM, the two zero-vector action times are usually equal, that is, the principle voltage space of 3 random space vector pulse width modulation. The three-phase PWM waveforms produced by the vector modulation method are shown in the figure. In the figure, zero vectors are added at both ends and in the middle of a switching cycle. PWM inverter voltage control is achieved by controlling the duty cycle of the switching device. The duty cycle is independent of the on-time during the switching cycle. However, the change in conduction position affects the spectral distribution of the output voltage. If the on-time is changed in a random manner under the condition that the on-time is not changed, the inverter output voltage can obtain a wide and uniform continuous spectrum with the fundamental wave component unchanged, thereby effectively Inhibits harmonic components with large amplitudes. This paper presents a novel random SVPWM method based on voltage space vector modulation. In the voltage space vector modulation, it is equivalent to the duty cycle of the regular sampling and determines the action time of the non-zero vector of the output voltage only, irrespective of the time of action of the voltage vectors Ui, U2, if not in each switching cycle. Using the two zero vectors U and U7 to have the same time of action, but instead randomly changing the action time of a zero vector in each switching cycle, random voltage modulation is achieved by randomly varying the variable coefficient k proportional to the zero vector V. This random change of the zero-vector position will result in a random variation of non-zero vector positions, thus enabling random SVPWM. DSP-based random SVPWM implementation and spectrum analysis Random SVPWM based on voltage space vectors, using the DSP Event Manager can be easily achieve. The space vector base is used to generate the three-phase PWM signals of A, B, and C, and the drive signals of the six power switches are generated through the dead-time circuit setting. Random signals can be generated using the congruential method. When 16-bit words are used, their equations can be taken as +1. The current random number variable coefficient k can be calculated based on the random number. The equation is two X and therefore zero vector. The action time is To 2 instead of the zero vector. The sequence and time of the action are the same as those of the space vector modulation described above. The random voltage space vector modulation is used in the variable frequency control system, and 12 gives the experiment waveform of the 2.2 kW experimental prototype with half load. Its switching frequency is 5kHz fundamental frequency 50Hz. In order to evaluate the merits of the proposed random pulse width modulation method, the performance of conventional SVPWM and random SVPWM is compared and analyzed. Conventional SVPWM Line Voltage Power Spectrum 0 Sampling Frequency/Hz Random SVPWM Line Voltage Power Spectrum -20 Sampling Frequency/Hz Random SVPWM DC-Side Current Power Spectrum and Line Voltage Power Spectrum Representing Conventional SVPWM and Random SVPWM, respectively The current power spectrum of the two modulation methods can be seen from the figure, the peak value of the harmonics of the random SVPWM voltage is reduced by 8dB, the current harmonics are also significantly improved; and the display of the random SVPWM can reduce the current harmonics on the DC side, so this method Can reduce the DC side filter capacitor; sampling frequency / Hz 0 random SVPWM motor noise power spectrum and 0 are two kinds of motor noise power spectrum From the figure can also be seen that the motor noise can be significantly improved. Experiments show that the random SVWPM method is very effective. The stochastic PWM method based on voltage space vector modulation proposed in this paper is achieved by randomly changing the coefficient ko (ie, randomly changing the action time of the zero vector) while keeping the sampling frequency constant. This method proves that it can significantly reduce noise and disperse voltage and current harmonics through experiments. The random PWM can be easily implemented using DSP.
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