ABSTRACT
Inverters are used to create single or poly-phase AC voltages from a DC supply. The DC supply is typically created by rectification of AC voltage from the utility power grid with due consideration of harmonics and input power factor. In the class of poly-phase inverters, three-phase inverters are by far the largest group. A very large number of inverters are used for adjustable speed motor drives for motors ranging from fractional hp (horsepower) to several 100 hp. Considering that more than 50% of all the electricity generated in the United States is used to drive electric motors [25, Figure 8.3], the importance of this application cannot be overstated. The typical inverter for motor drives is a “hard-switched” voltage source inverter producing pulse-width modulated (PWM) signals with a sinusoidal fundamental [11]. For large motors, multilevel inverters [19,24] are used, which are described in more detail later. Recently, research has confirmed and explained several detrimental effects such as electrical breakdown and excessive mechanical wear on motor windings and bearings respectively, resulting from unfiltered PWM waveforms from voltage source inverters. To avoid these detrimental effects, especially in the case of long cable runs between the inverter and the motor, voltage filters on the inverter outputs [8,21] can be used. Multilevel inverters besides their larger voltage and power ratings also inherently avoid this problem.
