Comprehensive loss optimization of induction motor drives

Extensive use of power electronics-controlled induction motor drives over the past few decades has enabled the development of loss minimization control algorithms. With the technological advancements in power semiconductor switching devices such as insulated gate bipolar transistors and gate commutated thyristors, induction motor drives are increasingly used in applications, ranging from automotive traction to more-electric aircraft, which have widely varying speed, torque and power requirements. Advances in control technology have enabled the development of various sophisticated controllers for motor drives aimed at performance enhancement. Substantial energy savings may be obtained when drive controllers are optimized for loss reduction under varying operating conditions. This dissertation addresses loss optimization opportunities in induction motor drives from system perspectives. First, a constrained loss optimization method is developed. Past work on loss minimization has focused on specific drive components such as the machine stator and rotor windings, inverter and dc-link. Component-level loss minimization, however, will not guarantee minimum total loss in the drive system. So, a system-level loss minimization method is proposed using a comprehensive loss model, to achieve true minimum total loss. Next, a lossless damping controller is proposed to suppress undesirable resonant oscillations in the machine voltages and currents due to the use of LC filters between the inverter and motor terminals. Passive damping methods employing physical resistors to suppress these oscillations, contribute to additional losses. Lossless active damping methods with virtual resistors have been explored in the literature. Conventionally, this resistance value is fixed, based on empirical rules, and left unchanged for all operating conditions. Choosing incorrect resistance values for the damping controller can result in degraded system behavior. A small-signal transfer function approach based on operating conditions and dynamic adjustment of the virtual resistance, is developed for the damping controller. The controller is designed to allow a flexible differential damping approach. Finally, power electronics loss reduction is investigated in a voltage source inverter (VSI)-based induction motor drive. It is known that low drive speeds will result in poor bus utilization and increased power electronics loss for higher link voltages. Losses can be reduced by dynamically varying the dc link voltage according to operating conditions. In addition to reducing losses, varying the link voltage also reduces the switched voltage magnitude across the inverter switches, potentially increasing inverter reliability. In the proposed method, the link voltage is varied using a front-end dc-dc buck converter according to a loss minimization algorithm. The effect of additional loss from the front-end converter on the total loss is also studied. Benefits of the proposed methods are verified by simulations and experiments