Study On PWM Induction-motor VVVF Speed Regulation System

Since the high quality of the speed regulation, the PWM induction-motor variable- frequency speed regulation systems are getting more widely used. Based on the application of the VVVF power supply, the reduction of common-mode EMI in PWM inverter and the self-excitation of the induction-motor caused by LC filters are studied in this paper.The source and propagation path of CM EMI in three-phase inverters are investigated. The conclusion is that CM EMI is due to the dv dt charging and discharging the parasitic capacitors of the power devices. The equivalent circuit of CM EMI is presented in this paper.An approach for designing PWM strategies is studied, which is able to reduce CM currents by limiting the amount of variations of the CM voltage. Theoretically ,the dv / dt of the two changing phases and the CM currents can be cancelled out automatically, since the variations of the voltage of two of the three phases are on opposite direction, and the third one remains constant when the voters change. Due to the dead band and inevitable deviation in signal transmission characteristics of devices, transmission delay of each gating signal can be different, so the CM current cannot be cancelled out perfectly. To address this, a simple solution is to fine-tune the gating signals before they are transmitted. The main drawback of such a switching strategy is that, the maximum linear modulation is reduced to 0.67. Then a new approach based on the former approach is studied, which increase the maximum linear modulation to 0.77. The experimental results on a TMS320F240 on a VVVF power supply are also given to verify the validity of the method.In some area of the speed regulation, LC filters are added to the output of the VVVF power supply to the requirement of high quality of output wave. Self-excitation of induction motors caused by output LC filters of variable voltage variable frequency (VVVF) power supplies is harmful both to the motor and the power supply. Self-excitation-suppression technique based on proper control strategies of the VVVF power supply is investigated in this paper. Based on theoretical analysis and experimental study of the self-excitation phenomenon, the impact of the magnitude and the frequency of the output voltage of the power supply before closing the gating pulses on the self-excitation voltage are discussed, and two approaches, named the"frequency-down"and"voltage-down", are proposed and verified by experiments.