2082 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 59, NO. 5, MAY 2012 Study of Rotor Faults in Induction Motors Using External Magnetic Field Analysis Andrian Ceban, Remus Pusca, and Raphaël Romary, Member, IEEE Abstract—This paper presents a new signature for detection of rotor faults in induction motors, such as eccentricity and broken rotor bars, that uses the external magnetic field analysis. The proposed method is based on the variations of axial flux density in the presence of these faults. The low frequency part of the magnetic field spectrum is particularly analyzed. The analysis is realized through a machine modeling based on permeance circuit under eccentricity fault and also by machine modeling based on coupled magnetic circuit theory under broken rotor bars fault. Analytical relations which describe the machine operation under broken bars fault highlight the influence of speed variation to modify the low frequency components of the external magnetic field. The theoretical results have been validated by experimental measurements. In particular, an inverse stator cage induction machine have been used to measure the bar currents under healthy and faulty cases. Index Terms—Broken rotor bar, eccentricity, fault diagnosis, field analysis, induction motors, low frequency, magnetic flux density, magnetic spectral analysis. NOMENCLATURE Symbol Explanation α Position of an air-gap point in a stator reference frame, m. β Position of the minimum air-gap thickness. δ Value of the rotor off-centering. C e_tot Total torque, N · m. C e_srp Mean torque due to the interaction of the stator and the rotor fields in the clockwise direction, N · m. C e_srn Torque corresponding to pulsation 2sω, N · m. C r Load torque, N · m. e m Average value of the air-gap thickness, m. e sensor Emf provided by coil sensor, V. ε 1 Mmf between the stator and the rotor along the air- gap inside the coil. ε 2 Mmf between the stator and the rotor along the air- gap outside the coil. f Frequency, Hz. Φ A Axial flux, Wb. Φ ′ A Flux generated only by the end coil (axial-radial). Manuscript received December 10, 2010; revised March 24, 2011 and June 20, 2011; accepted July 8, 2011. Date of publication July 29, 2011; date of current version February 3, 2012. The authors are with the University Lille Nord de France, 59000 Lille, France, and also with the Laboratory of Electrical Systems and Environ- ment (LSEE), Artois University, 62400 Béthune, France (e-mail: apceban@ gmail.com; puscaremus@hotmail.com; raphael.romary@univ-artois.fr). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2011.2163285 Φ e1 , Φ e2 Air-gap fluxes, Wb. Φ ext Linked flux, Wb. Φ AT Total axial flux, Wb. ϕ B s Phase relative to the stator phase current of the stator flux density. ϕ B rp Phase relative to the rotor flux density in the clock- wise direction. ϕ B rn Phase relative to the rotor flux density in anticlock- wise direction. [I ] Current vector, A. I s Stator rms current, A. I rn Amplitude of negative-sequence current system, A. I rp Amplitude of positive-sequence current system, A. i e End ring current, A. i bn Current crossing the bar n of the rotor cage, A. i rn Current crossing the loop n, A. i s Stator instantaneous current, A. J Total moment of inertia, kg · m 2 . k r Coefficient that depends on the rotor cage constitution. k s Coefficient that depends on the stator winding arrangement. L Active length of the magnetic circuit (of a core), m. [L] Global inductance matrix, H. L b Rotor bar leakage inductance, H. L e Rotor end ring leakage inductance, H. L mr Magnetizing inductance for each rotor loop, H. L ms Inductance for each stator coil, H. [L s ] Stator inductance matrix, H. L sf Leakage inductance, H. L s1 Total inductance of the stator coil 1, H. M rr Mutual inductance between two rotor loops, H. M s Mutual inductance between the stator phases, H. M sr Mutual inductance between stator coils and rotor loops, H. μ 0 Magnetic permeability of the air. n Number of rotor bars or rotor loops. n t Number of turns per pole pair and per phase. n c Number of coil sensor turns. n ′ Number of elementary coil turns. Ω r Angular velocity, rad/s. Ω 0 Mean value of Ω r , rad/s. Ω s Synchronous speed, rad/s. ω Angular frequency, rad/s. P e1 ,P e2 Air-gap permeances. p Number of pole pairs. R Average radius of the air-gap, m. [R] Global resistance matrix, Ω. 0278-0046/$26.00 © 2011 IEEE