New optimal PWM strategies for a VSI induction motor drive
The applications of robust squirrel-cage induction motors in variable speed inverter drive systems have increased considerably due to the availability of easily controlled semiconductor switching devices. One problem encountered in inverter drives is the non-sinusoidal nature of the supply voltage, which results in increased motor losses and harmful torque pulsations producing undesirable speed oscillations. The latter effects are negligible at high frequency operation, due to the damping effect of the rotor and load inertia. However, torque pulsations and speed ripple may be appreciable at low frequency, wore they may result in abnormal wear of gear-teeth or torsional shaft failure. Hence, in applications where constant or precise speed control is important, eg; machine tool, antenna positioning, traction drives etc., it is essential to establish a method for determining the magnitudes of these torque pulsations and speed ripple, as a first stage in minimizing or eliminating them. When a voltage source inverter is used in such applications, pulse width modulation (PWM) techniques are usually employed, whereby the quasi square waveshape is modulated so as to minimize or eliminate the low order harmonic voltage components and thereby reduce the torque pulsations. Recent investigations have shown that total elimination of low order components does not produce optimal efficiency or torque pulsations and speed ripple. minimization. This thesis describes new PWM strategies which does not rely on complete elimination of low order harmonics, but on controlling the magnitude and phase of these components to achieve a smooth rotor motion. Initially, a mathematical model for the inverter/induction motor drive was developed, based on numerical integration of the system differential equations. The changing topology of the inverter bridge was simulated using tensor techniques. Then an analytical method, based on harmonic equivalent circuit analysis was proposed for calculating the induction motor pulsating torque components under steady-state operating conditions, in terms of stator and rotor current harmonics. The accuracy of this method was verified by comparing its results with those obtained from the mathematical model developed earlier. This provided an extremely rapid, numerically stable and efficient means for evaluating harmonic current and torque components with balanced non-sinusoidal applied voltages. This method was then used to formulate the torque performance function necessary to determine the new optimal PWM switching strategies. Throughout the work, the predicted performance was extensively validated and supported by practical results obtained from an experimental rig specifically designed to drive the machine under different PWM techniques.