Overvoltage and surge protection of electric devices is an indispensable part of the system protection. Metal Oxide Varistors (MOVs) are the most common components used for surge protection. However, MOVs are known to degrade overtime when they experience severe discharges. Understanding the degradation of the MOVs and their health monitoring is one of the industry’s interests. One of the areas that there are difficulties in MOV application is in inverters and drives that use PWM techniques. For instance, Variable Frequency Drives (VFDs) are becoming more and more widespread for motor speed control and energy saving purposes. However, when the connecting cable between the inverter and the motor is longer than a given length, high frequency overvoltages appear on motor terminals due to the voltage reflection phenomenon. These overvoltages can damage the motor and the cable insulation, and at the same time, cause installed MOVs to conduct high currents and fail. In this research, we investigate the MOV’s behavior and degradation. Also, we study VFD systems, characteristics of the overvoltages and possible solutions. Furthermore, this research tries to develop a new method of applying MOVs in PWM motor drives to mitigate the overvoltages. Finally, guidelines will be developed for MOV application in VFD systems.
- Professor Bradley Lehman (Advisor)
- Professor Ali Abur
- Professor Mahshid Amirabadi
- Dr. Peng Li
- Professor Dorin Neacsu