information theory. MBD is well suited for system level faults as well as component faults [4]. In MBD, models interpret signals and locate faults. Parameter tuning is for extracting health information from signals. Fault locations are isolated from parameter values. Health condition assessment via Shannon’s theorem of information and signals [5] provides reliable diagnostics because it is based upon the principles that all telecommunication industry stands on. Figure 1 illustrates the framework of induction motor fault diagnosis as an example of the general model based diagnostic/prognostic approach. For other systems, such as pump, gear, turbine, and etc., sensors and model are changed, but the rest of approach is same. Prognosis of future health is performed by extracting parameter values and simulating performance. Fig. 1. Model-based diagnostics framework with health assessment unit This article illustrates model-based diagnostics of induction motors. A detailed induction motor model is key to successful diagnostics. The detailed bond graph model and dynamic equations will be discussed in the next section. The principles of parameter tuning will be explained. Continuous-discrete extended Kalman Filter (EKF) is employed for parameter estimation. Shannon’s information theory and channel capacity analogy for machine diagnostics will be illustrated for fault assessment. Sensor selection will be discussed in terms of observability analysis. Fault isolation is explained based on the simulation results. Finally, the general procedures of MBD are re-summarized. 2 Induction Motor Models A detailed induction motor model is essential for successful diagnostics. Kim and Bryant developed a bond graph model [6,7] of induction motors. The model is reconstructed into the vector bond graph as shown Figure 2. The model is a universal model of induction motors with emphasis on correspondence to the physical details. Real induction motors may vary in size, shape, and parameters. But, the physics is the same and the proposed bond graph model is still valid. Fig. 2. Vector bond graph model of 3-phase induction motors 440
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