Journal of ELECTRICAL ENGINEERING, VOL. 54, NO. 11-12, 2003, 287–292 FAULTS DETECTION OF INDUCTION MACHINES BY RADIAL FIELD MEASUREMENT Driss Belkhayat ** — Rapha¨ el Romary * — Mustapha El Adnani ** — Rodolphe Corton * — Jean Fran¸ cois Brudny * In this paper, the authors propose a diagnosis method based on the measurement of the radial magnetic field in the surrounding area of an induction machine. Considering a star coupled induction machine, the effect of broken rotor bar fault or a stator phase cutting fault on stator currents and radial field spectrum is analyzed as an illustration of this diagnosis method. When the saturation is taken into account, the study shows that the fault causes the circulation of a third harmonic current between the two fed phases. This harmonic current generates new spectrum lines in the air gap flux density linked to the slotting effect. These high frequency components are easily detectable by the radial field measurement. Keywords: Induction machine, diagnosis, radial magnetic field, slotting effect, flux density components 1 INTRODUCTION In the recent few years much work has been carried out to investigate the possible fault operations of electri- cal machines. These works have used the acquisition of an experimental data set. Monitoring and diagnostics of electrical machines is a very important topic which cor- responds to the actual industrial requests. The usual methods for both stator and rotor fault detection are based on vibrations analysis [1] or on the motor current signature analysis [2] that originated more than twenty years ago. For this purpose, these methods need dedicated and fixed equipment for each induction machine under test. In the mean time, a new non-invasive technique is proposed. This new method consists in using a voltage sensor based on an air coil antenna installed outside the machine. This antenna measures the voltage proportional to the emitted magnetic field. The technique using the measurement of the axial leak- age flux, by setting the antenna in the axis of the shaft, has been widely developed [3]. In this investigation the external coil is placed in a plant containing the axis of the machine (Fig. 1). The electric field intensity sensed by the antenna is so propor- tional to the radial magnetic field. Its spectrum analysis shows high frequency components (MHz) due to the fast front waves voltage when the machine is supplied by a PWM inverter [4], [5]. The mean frequency components (kHz) are due to the slotting effect [6] and appear, even if the machine is safe and supplied by the network [7]. The interference of these components by the fault field harmonics is used to diagnose the machine behaviour. This technique is very practical and can be imple- mented at low cost, except for very small motors, because the antenna can be removed from one machine to another without stopping work. 2 HEALTHY MACHINE 2.1 Magnetic field components This analytical study is based on the determination of the air gap flux density expression where the saturation is neglected. The principle of this determination has been presented in previous publications [8], [9]. Only the main results are presented in this paper. A three phase, p -pole pairs squirrel cage induction machine with single layer windings and all the conductors series connected is considered. Using an idealized slot model with a rectangular profile and taking into account the teeth, the general expression of the air gap permeance is p = P 0 + p teeth (1) where P 0 is the mean value of permeance. In a healthy running, this relationship can be written as [8] p = ∞  k s =−∞ ∞  k r =−∞ P k s k r cos((k s N s +k r N r )pα s -k r N r pθ) (2) where are ∗ Laboratoire Syst` emes Electrotechniques et Environnement, Universit´ e d’Artois, Facult´ e des Sciences Appliqu´ ees, Technoparc Futura, 62400 B´ ethune, France ∗∗ Facult´ e des Sciences et Techniques, D´ epartement de physique, Marrakech, Morocco, e-mail : belkhayat@fstg-marrakech.ac.ma ISSN 1335-3632 c  2003 FEI STU