Optimum base/gate drive strategy of a PWM inverter and precision estimation of AC output voltage and DC link current

Development of the thyristor static inverters has opened a new era of solid state speed control of highly reliable and robust ac machines, both induction or synchronous type. These drives are now in close competition with the conventional dc motor drives and are replacing them because of their higher speed, accuracy and reliability, less maintenance, and economic considerations. Various sophisticated control methods such as Pulse Width Modulation (PWM) techniques have been developed which achieve considerable reduction in harmonic heating and torque pulsation problems. It can be observed that a lot of unnecessary switching occurs in a PWM inverter using sophisticated control techniques, when the load current flows through "bypass" or "freewheeling diodes" connected across the power devices. In this thesis an optimum control strategy for a square wave as well as PWM voltage-fed inverter drive is presented so as to mimimize the device stress, base drive power, and size of the cooling equipment. Actual implementation is done for the three-phase transistorised PWM inverter. A method of precision estimation of ac output voltage and dc link current is also developed with the information of actual stator phase currents, actual dc link voltage, conduction drop characteristics of the power devices, and accurate PWM base drive signal pattern of the power devices. The resulting advantages include less stressing of the power devices, saving of power in the base drive circuit, less heat generation in power devices, reduced size of the heat dissipating equipment and of the inverter system, and possibility of accurate estimation of ac output voltage and dc link current of the inverter. Finally, a discussion of the results and the salient features and advantages of the proposed scheme are described.