Grainger CEME

Optimizing Variable DC Link Voltage for an Induction Motor Drive under Dynamic Conditions

PhD student Srikanthan Sridharan with adviser P. Krein

Induction motor drives are conventionally operated with a fixed inverter dc-link bus voltage. However, low-drive speeds will result in a low modulation index and poor dc bus utilization of the inverter. Aperiodic current controllers, such as hysteresis controllers, exhibit increased switching frequencies and losses for higher dc-link voltages. These high switching frequencies can be avoided by dynamically varying the dc-link voltage according to drive operating conditions. In addition to reducing inverter switching losses, varying the dc-link voltage also reduces the switched voltage magnitude across inverter semiconductor switches, potentially increasing inverter reliability. In this work, the link voltage is varied by using a front-end dc-dc buck converter. An algorithm is developed to derive a lower limit constraint on the optimal link voltage command. The optimal voltage command is derived using a loss minimization technique that uses flux and rotor speed commands to the drive. The effect of additional loss from the front-end converter on the total loss is also investigated. Simulation results of optimization and experimental implementation are shown in Figs. 1 and 2, respectively. Studies indicate that the loss reduction is significant at low speeds. This implies considerable energy savings in applications subject to wide speed variations. The loss minimization approach has been demonstrated on a field-oriented-control based induction motor drive. This research is supported by the Grainger Center for Electric Machinery and Electromechanics.

Fig. 1. (a) Optimal dc link voltage command

(b) Comparison of losses in fixed and variable dc-link systems.

Fig. 2. Experimental implementation of variable dc-link voltage scheme