Detection of rotor slot and other eccentricity related harmonics in a three phase induction motor with different rotor cages

Detection of rotor slot and other eccentricity related harmonics in the lin e current of a three phase induction motor is important both from the viewp oint of sensorless speed estimation as well as eccentricity related fault d etection. However, it is now clear that not all three phase induction motor s are capable of generating such harmonics in the line current. Recent rese arch has shown that the presence of these harmonics is primarily dependent on the number of rotor slots and the number of fundamental pole pairs of th e machine. While the number of fundamental pole pairs of a three phase indu ction motor usually is within one to four (higher pole pairs are generally avoided due to increased magnetizing current), the number of rotor slots ca n vary widely. The present paper investigates this phenomenon further and o btains a hitherto nebulous theoretical basis for the experimentally verifie d results. Detailed coupled magnetic circuit simulation results are present ed for a four pole, three phase induction motor with 44, 43, and 42 rotor s lots under healthy, static, dynamic and mixed eccentricity conditions. The simulation is flexible enough to accommodate other pole numbers also. These simulations are helpful in quantifying the predicted harmonics under diffe rent combinations of load, pole pair numbers, rotor slots and eccentricity conditions, thus making the problem easier for drive designers or diagnosti c tools' developers. Data from three different induction machines, namely, a 4 pole, 44 bar, 3 HP, a 4 pole, 28 bar, 3 HP, and a 2 pole, 39 bar, 100 H P motor have been used to verify the results experimentally. The simulation and the experimental results clearly validate the theoretical findings put forward in this paper.