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Filter Design for Estimating Parameters of Induction Motors With Time-Varying Loads

Periodically time-varying loads such as reciprocating compressors produce torque and speed oscillations in grid-connected induction motors. In each compression cycle, the torque and speed oscillations manifest themselves through periodic pulsations in the induction motor's stator currents. In resistance-based rotor temperature tracking and overheating monitoring, low- or bandpass filters are frequently used to estimate induction motor electrical parameters from stator currents that contain periodic pulsations. Design of such filters relies on comprehensive knowledge of the load-related periodic pulsations in the stator currents. In this paper, a narrow-band angle modulation concept is first formulated in the framework of complex vector analysis to explain the periodic pulsations in the stator current. Carson's rule is then introduced to establish an empirical relationship between the stator current's modulating frequency and the appropriate bandwidth for low- or bandpass filters. The proposed filter design rules are experimentally validated, and explanations are provided using the phase angle between the motor's complex stator current and voltage vectors.