!" # $ %& % Toomas Vaimann 1) , Ants Kallaste 2) , Aleksander Kilk 3) Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia, http://www.ttu.ee 1) tel: +372 620 3800, email: toomas.vaimann@ttu.ee, 2) tel: +372 620 3800, email: ants.kallaste@ttu.ee 3) tel: +372 620 3800, email: kilk@cc.ttu.ee ’%’ Induction motor, Rotor faults, Diagnostics, Stator current, Broken rotor bars, FFT, Clarke transforma6 tion, Current vector amplitude oscillation, Wavelet analysis, Finite element method ( Induction motors are critical for many industrial processes because they are cost effective and robust in the sense of performance. They are also critical components in many commercially available equipment and industrial proc6 esses [1]. In developed countries today there are more than 3 kW of electric motors per person and most of it is from induction motors [2]. Furthermore, induction motors are often used in critical duty drives where sudden failures can cause high safety risks and unnecessary economic expenses. Differ6 ent failures can occur in electrical drives and one of the most common faults is the breaking of the rotor bars. The majority of all stator and rotor faults are caused by a combination of various stresses, which can be thermal, elec6 tromagnetic, residual, dynamic, mechanical or environ6 mental [3]. Broken rotor bars are not only one of the most common faults but also one of the most uncomfortable ones [4]. The worst case of such fault is when the broken rotor bars are situated closely one after another. This case is also most probable in practice as broken rotor bars are usually not de6 tected at early stage. As the resistance of the broken bar is very high in comparison with the healthy ones, currents start to distribute unproportionally in the rotor cage. Parts of the rotor currents, which are unable to flow in broken bars, are flowing in adjacent bars. This increases the rms value of currents in the bars next to the broken ones [4]. As the healthy bars next to the broken one have too high current density, they will start to overheat, so these bars are under severe thermal stress. Those bars start cracking and breaking and when attention to the problem is not paid, the fault will continue to cascade, until the rotor cage is destroyed. Broken rotors bars can lead to vibration problems, but more likely, in severe cases, the bar pounds out of the slot and makes contact with the stator core or winding [3]. The biggest problem of those faults is that it is often not worth or possible to repair the rotor. However all of this can be avoided, when the motor is supervised by an appropriate condition monitoring or diagnostic system. ’ #) The biggest advantage that sensorless diagnostic of induc6 tion motors provides is that testing and diagnostics can be done without any disturbances to the motor’s normal working cycle. It means that in case of motors which are used in criti6 cal duty drives or perform on high risk conditions, no addi6 tional changes have to be made in order to perform the tests. There are different possibilities to perform sensorless di6 agnostics on induction motors. One of the ways is to measure stator currents of the motor and analysing the measured sig6 nals. Some diagnostic methods for performing this analysis are described in this paper. * + +" , -++. Fourier analysis is very useful for many applications where the signals are stationary, as in diagnostic faults of electrical machines [5]. Its purpose is to monitor a single6phase stator current. This is accomplished by removing the 50 Hz excitation com6 ponent through low6pass filtering and sampling the resulting signal. Single6phase current is sensed by a current trans6 former and sent to a 50 Hz notch filter where the fundamental component is reduced. Analog signal is then amplified and low6pass filtered. Filtering removes the undesirable high6fre6 quency components that produce aliasing of the sampled signal while the amplification maximizes the use of the ana6 log6to6digital converter input range. Analog6to6digital con6 verter samples the filtered current signal at a predetermined sampling rate that is an integer multiple of 50 Hz. This is