| Induction motors are widely used in industrial processes. The malfunction of a motor may not only lead to high repair costs, but also cause immense financial losses due to unexpected process downtime. Approximately 30-35% of motor failures are related to stator winding insulation [1-3]. Since thermal overload is one of the major root causes of stator winding insulation failure, an accurate and reliable monitoring of the stator winding temperature is crucial to increase the mean time to catastrophic motor breakdown, and to reduce the extraordinary financial losses due to unexpected process downtime. Because of the fast development and increased use of motor control devices, such as variable-frequency motor drives and reduced-voltage motor starters, the development of an improved thermal protection method that is applicable in combination with the motor control devices is highly desirable.;The objective of this research is to develop a non-intrusive, sensorless thermal protection scheme for induction motors fed by motor control devices under both in-service and de-energization conditions, using only motor terminal quantities, i.e. currents and voltages.;A comprehensive literature survey is presented to summarize the state of the art of stator winding temperature estimation techniques for in-service induction motors. These techniques are compared in terms of accuracy, implementation complexity, and practical feasibility. A novel thermal model reduction study is presented to further discuss the feasibility of the traditional type of thermal protection approaches. Based on the study of the existing techniques, the scope of this research is defined as: non-intrusive stator winding temperature estimation method for induction motors fed by motor control devices.;To provide a reliable thermal protection for induction motors fed by motor control devices, a dc signal-injection method is proposed for in-service induction motors fed by soft-starter and variable-frequency drives. The stator winding temperature can be monitored based on the estimated stator winding resistance using the dc model of induction motors. In addition, a cooling capability monitoring technique is proposed to monitor the cooling capability of induction motors and to warn the user for proactive inspection and maintenance in the case of cooling capability deterioration. The proposed cooling capability monitoring technique, combined with the proposed stator winding temperature monitoring technique, can provide a complete thermal protection for in-service induction motors fed by motor control devices. The feasibility of the proposed thermal protection scheme consisting of the stator winding temperature monitoring and the motor cooling capability monitoring is validated by experimental results.;In addition, the active stator temperature estimation concept is extended to the application on de-energized induction motors. DC signals can be injected into induction motors without inducing any output torque for de-energized motors using soft-starters, based on which the stator winding temperature can be estimated. Via stator temperature estimation, the required thermal recovery time for intermittently operated motors can be minimized, which can largely improve the usage of the overall motor system.;Aside from online thermal protection during a motor's normal operation, the thermal protection of de-energized motors is also essential to prolong a motor's lifetime. Moisture condensation is one of the major causes to motor degradation especially in high-humidity environments. To prevent moisture condensation, a non-intrusive motor heating technique is proposed by injecting currents into the motor stator winding using soft-starters. A motor's temperature can be kept above the ambient temperature due to the heat dissipation, so that the moisture condensation can be avoided.;To sum up, active stator winding temperature estimation techniques for induction motors under both operating and de-energization conditions are proposed in this dissertation for both thermal protection and optimizing the operation of a motor system. The importance of these proposed techniques lies in their non-intrusive nature: only the existing hardware in a motor control device is required for implementation; a motor's normal operation is not interrupted.;The conclusions, the recommendations for future research, as well as the major contributions of this research are presented at the end. |
