Locating rotor broken bars in induction motors using finite element method

Number of broken bars and varying load have been so far proposed in the literature in the process analysis of induction motors under broken rotor bars. In this paper, it is shown that there is the third factor which affects the diagnosis of the broken bars fault. This deterministic factor is the location of the broken bars which is determined precisely here. It is also shown that distribution of the broken bars over different poles of the motor reduces the amplitude of the harmonic components due to the fault. In this paper, the stator current frequency spectrum of a faulted induction motor is obtained for all cases in which four broken bars are distributed over poles of the motor. It is shown that the amplitudes of harmonics (1 +/- 2s)f(s) are a suitable index for locating the rotor bars breakage. In this paper, the torque frequency spectrum of a faulted induction motor has been obtained for various cases of the bars breakage location and shown that the bars breakage location influences the amplitudes of harmonic components 2sf(s) in the torque frequency spectrum. Time stepping finite element method (TSFEM) is used to model an induction motors with rotor broken bars. In this modeling, geometrical and physical characteristics of all parts of the motor, spatial distribution of stator windings, slots on both sides of the air gap and non-linear characteristic of the core materials are included. Meanwhile the current of the broken bar is taken to be non-zero, instead resistance of the broken bar is considered large enough. In fact, this is the real case, because there is an inter-bar current within the broken bar induction motor. Since there is noise, unbalanced magnetic pull and arc in the broken bar induction motor, acceleration of the faulty motor up to the steady-state is considered here. It is indicated that the location of the rotor bars has significant effect upon the torque of the faulty motor and when the broken bars concentrate over one pole of the motor, the torque of faulty motor oscillates more. (C) 2008 Elsevier Ltd. All rights reserved.