(A) new overmodulation method for induction motor drive systems

Abstract

The full range control of the PWM inverter is addressed. The Space Vector pulse width modulation is used as the industry standard method because of its outstanding characteristics. It is used for the applications adjustable speed AC drives, UPS and FACTS. However, inverter nonlinearities, as well as the modulation method, limit the operating range of the Voltage Source Inverters. Low voltage operation of inverter is limited by the inverter nonlinearities such as inverter deadtime, semiconductor turn-on voltage drop and pulse dropping. These nonlinear effects can be counterbalanced by the bipolar mode operation of the PWM inverter. The characteristics of the overmodulations are analyzed in detail using modulation index relation and inverter switching number. In particular, the inverter switching number is the new performance measure introduced in this thesis. It can measure and analysis the overmodulation characteristic more exactly than the modulation index relation. It is shown the equality between the discontinuous modulation by Depenbrock and overmodulation method by Krah in modulation index relation. The equality of the methods enables the freedom of implementations with analog circuitry or digital software. A new discontinuous modulation method based on the space vector control is proposed and analysed. The proposed technique employs a pulse dropping method and is designed in the time domain. It features a very wide modulation range while maintaining the required waveform qualities and switching numbers in the overmodulation region. Since the modulation method and modulation index equation are simple, the proposed technique can be easily implemented by software and applicable to the overmodulation region in AC motor drives. The performance indices are discussed and the experiments have been performed.