Gojko Joksimović

The detection of inter-turn short circuits in the stator windings of operating motors

This paper develops a winding-function-based method for modeling polyphase cage induction motors with inter-turn short circuits in the machine stator winding. Analytical consideration which sheds light on some components of the stator current spectra of both healthy and faulty machines is developed. It is shown that, as a result of the nature of the cage rotor, no new frequency components of the line current spectra can appear as a consequence of the fault. Only a rise in some of the frequency components which already exist in the line current spectra of a healthy machine can be observed. An experimental setup comprising a 3 kW delta-connected motor loaded by a generator was used to validate this approach. The experimental results obtained clearly validate the analytical and simulation results.

Dynamic simulation of dynamic eccentricity in induction machines-winding function approach

The paper describes a method for the dynamic simulation of dynamic rotor eccentricity in squirrel cage rotor induction machines. The method is based on a winding function approach, which allows for all harmonics of magnetomotive force to be taken into account. It is demonstrated how this complex dynamic regime can be modeled using the mutual inductance curves of symmetrical machine using proper scaling techniques. Experimental results demonstrate the effectiveness of the proposed technique and validate the theoretical analysis.

Stator-Current Spectrum Signature of Healthy Cage Rotor Induction Machines

Before applying current-signature-analysis-based monitoring methods, it is necessary to thoroughly analyze the existence of the various harmonics on healthy machines. As such an analysis is only done in very few papers, the objective of this paper is to make a clear and rigorous characterization and classification of the harmonics present in a healthy cage rotor induction motor spectrum as a starting point for diagnosis. Magnetomotive force space harmonics, slot permeance harmonics, and saturation of main magnetic flux path through the virtual air-gap permeance variation are taken into analytical consideration. General rules are introduced giving a connection between the number of stator slots, rotor bars, and pole pairs and the existence of rotor slot harmonics as well as saturation-related harmonics in the current spectrum. For certain combinations of stator and rotor slots, saturation-related harmonics are shown to be most prominent in motors with a pole pair number of two or more. A comparison of predicted and measured current harmonics is given for several motors with different numbers of pole pairs, stator slots, and rotor bars.

Harmonic Signatures of Static Eccentricities in the Stator Voltages and in the Rotor Current of No-Load Salient-Pole Synchronous Generators

This paper shows that a static eccentricity makes rise a double fundamental frequency ripple in the rotor current of salient-pole synchronous machines. This ripple leads, under conditions ruled by the stator windings, to precise signatures in the no-load voltage spectrum. Both rotor current ripple and voltage harmonics can be used for diagnosis. The fault-related voltage harmonics are theoretically previewed in this paper through analysis of the windings. Simulations performed by using the winding function approach confirm the theoretical predictions. A four-pole 15-kVA generator was used for experiments, featuring an innovative flange with exact and easy regulation of mixed-type eccentricities. Rotor current monitoring has advantages of single-sensor measure and accentuate fault sensitivity. Experiments also showed an additional rotor-rotation frequency ripple in the rotor current, in case of mixed-type fault.

Cage Rotor MMF: Winding Function Approach

An analytical expression for the cage rotor magnetomotive force (MMF) is derived using the winding function approach. This approach enables the analysis of stator current spectra in the cage rotor induction machine. It is demonstrated that in stator current spectra one can observe only harmonic components at high frequencies, known as the rotor slot harmonics. The position of these components in stator current spectra depends on the number of rotor bars, number of poles of the induction machine, and slip (i.e., on the actual speed of rotor).An analytical expression for the cage rotor magnetomotive force (MMF) is derived using the winding function ap- proach. This approach enables the analysis of stator current spectra in the cage rotor induction machine. It is demonstrated that in stator cur- rent spectra one can observe only harmonic components at high fre- quencies, known as the rotor slot harmonics. The position of these components in stator current spectra depends on the number of rotor bars, number of poles of the induction machine, and slip (i.e., on the ac- tual speed of rotor). monitoring of cage rotor induction machines is current signature analy- sis. This method is based on the monitoring of the stator current spectra in real time. The appearance of stator current components or increase of magnitude of some components at characteristic frequencies indicates a fault condition in a machine. These characteristic frequencies for common faults in cage rotor machines such as broken rotor bar(s), static or dynamic eccentricity, etc., are widely known in the literature (1). On the other hand, the rotor slot harmonics, whose position in the stator current spectra depends on actual rotor speed, have found appli- cation in modern speed detection techniques (2). In this letter an analytical equation is derived for the cage rotor MMF by using the winding function approach, (3). In addition, the stator current spectrum in the cage rotor induction machine is analyzed in relation to the nature of cage rotor MMF. MMF of Symmetrical Three-Phase Stator Winding: The equation for this MMF is known to be (4)

A method to detect missing magnetic slot wedges in AC machines without disassembling

In high voltage induction machines the stator slots usually are wide opened to facilitate the assembling of the stator winding coils. Thus the magnetically effective air gap and higher order harmonics are rising, the power factor is decreasing. To compensate this negative effect magnetic stator slot wedges are frequently applied. During operation these slot wedges can get loose and eventually fall out totally. Currently a detection of fallen out slot wedges is only possible by time consuming partially disassembling the machine and optical inspection. Simple and reliable testing methods can thus increase the reliability and reduce costs due to unnecessary disassembling of the machine. For such testing methods high frequency or transient electrical properties of an electrical machine suit very well as the base. When high frequency or transient voltage signals are applied to the terminals of the machine the resulting current response contains information about the machines magnetic state. Therein superposed are the magnetic material properties, several inherent asymmetries such as spatial saturation or slotting, as well as fault induced asymmetries. This paper introduces a new signal processing chain to detect and isolate the fault induced asymmetries caused by fallen out stator slot wedges. The chain consists of data capturing by collecting current response values due to voltage pulses and following Fast Fourier transformations. Measurements for several slot wedge fault cases are presented. The measured and calculated results show the high sensitivity and reliability of the proposed method.

Fault-tolerant control of a blade-pitch wind turbine with inverter-fed generator

A fact is that wind energy is both green and expensive energy. In order to increase its economic competence, wind turbine faults should be reduced and prevented. In wind turbines faults most commonly occur in the gearbox and in the generator system components like power converter or generator electromechanical parts. This paper proposes a fault-tolerant control strategy for variable-speed variable-pitch wind turbines in case of identified and characterized generator electromechanical faults like broken rotor bar or winding inter-turn fault. In particular we propose an upgrade of the torque control loop with flux-angle-based torque modulation. Usage of pitch controller in the low wind speed region is also proposed to intentionally reduce power capture in order to avoid or to postpone the system fault development. Presented fault-tolerant control techniques are developed considering their easy implementation and installation in available control systems of existing wind turbines to extend their life cycle and energy production. Simulation results for the case of 700 kW direct-drive wind turbine and the identified stator winding fault are presented.

State and parameter estimation for field-oriented control of induction machine based on unscented Kalman filter

Modern electric machines are required to have the best possible dynamic performances. In induction machines this is achieved by control strategies that are applied with respect to the flux in the air gap and therefore they require precise information on flux position. This paper proposes an observer with autotuning capability that uses the unscented Kalman filter algorithm for providing on-line estimation of states and parameters of the fundamental wave model of the machine. The algorithm uses power converter reference values of stator voltages, measured stator currents and rotor speed as inputs. Such observer provides accurate estimates of flux position and fundamental stator currents required for e.g. field-oriented control, taking into account machine parameters variability. Design procedure of the observer is presented and both simulation and experimental results are provided.

Stator line current spectrum content of a healthy cage rotor induction machine

The paper analyzes the stator current spectrum of a healthy squirrel cage induction machine. The knowledge of stator current spectrum of the healthy cage rotor induction machine is a starting point for diagnosis of different faulty regimes using a noninvasive diagnostic technique known as motor current signature analysis. Magnetomotive force (MMF) space harmonics, slot permeance harmonics as well as saturation of main magnetic flux path through the virtual air gap permeance variation are taken into analytical consideration. Air gap flux density waves were obtained by multiplying the corresponding MMF waves with air gap permeance waves. General rules which give the connection between the number of rotor bars and the existence of rotor slot harmonics in the stator current spectrum are derived, too. Their appearance as well as magnitude depends on the corresponding air gap flux density wave pole pair number. Predicted spectral components of the stator current are experimentally verified on two laboratory motors with different number of rotor bars.

Fault-tolerant Control of a Wind Turbine with a Squirrel-cage Induction Generator and Rotor Bar Defects

Wind turbines are usually installed on remote locations and in order to increase their economic competence malfunctions should be reduced and prevented. Faults of wind turbine generator electromechanical parts are common and very expensive. This paper proposes a fault-tolerant control strategy for variable-speed variable-pitch wind turbines in case of identified and characterised squirrel-cage generator rotor bar defect. An upgrade of the torque control loop with flux-angle-based torque modulation is proposed. In order to avoid or to postpone generator cage defects, usage of pitch controller in the low wind speed region is introduced. Presented fault-tolerant control strategy is developed taking into account its modular implementation and installation in available control systems of existing wind turbines to extend their life cycle and energy production. Practical implementation aspects such as estimation of variables used in control and estimate errors are considered and respected in operation, as well as fault-induced asymmetries. Simulation results for the case of a megawatt class wind turbine and the identified rotor bar fault are presented.