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.