Rastko Fiser

Additional Cogging Torque Components in Permanent-Magnet Motors Due to Manufacturing Imperfections

This paper presents issues for reducing cogging torque of permanent-magnet motors. When proper methods are employed, it is theoretically possible to minimize its level or even to eliminate it. FFT analyses of cogging torque in mass-produced motors have shown additional harmonic components, which are in tight correlation with assembly tolerances and/or permanent-magnet imperfections. Finite element method has been used to study the sensitivity of different motor models relative to manufacturing tolerances and to determine rules for detecting imperfections in mass-production.

Direct current control-a new current regulation principle

In this paper, two novel methods for current regulation are proposed. Both methods follow the synchronized on-off principle. In the classical approach, transistors are switched depending on the sign of the current error, which in turn is sampled at equal time intervals. In the first method, the current vector at the end of the interval is predicted for two possible cases when either the active voltage vector pointing toward current error or the zero voltage vector is applied. The one producing the smaller current error at the end of the sampling interval is chosen, thus obtaining drastic reduction of the switching frequency. In the second method, the best fitting active voltage vector succeeds the zero vector during the same time interval. A simple algorithm is used to calculate the duty cycle thus gaining the smallest possible current error. The method is compared with the CRPWM. Both methods were simulated and tested on a laboratory model with passive load. In the last part of the paper, the behavior of the second method is tested for erroneously estimated load parameters. The two methods show very small degradation of performance even when a rather high parameter error is introduced.

Application of a finite element method to predict damaged induction motor performance

The paper presents the application of a finite element method for predicting the performance of induction motor having electric and magnetic asymmetry of the rotor cage due to some broken rotor bars. Quantities like magnetic vector potential, flux density, force components, rotor and stator currents, mutual and leakage inductance were determined very precisely. The paper takes into account the magnetic materials nonlinearity. The detailed insight in magnetic field distribution of a squirrel cage induction motor forms the basis for further evaluation of its operational behavior. Increasing anomaly in magnetic field distribution due to the increasing number of broken rotor bars results in a degradation of steady-state and dynamic performance of the induction motor and can be determined with computer simulation, eliminating expensive and time consuming laboratory tests. The obtained results are compared with measurements. Correct evaluation of faulty motor performance is a very significant part of condition monitoring and diagnostic procedure in modern supervision systems of electrical drives.

Detection of Broken Bars in Induction Motor through the Analysis of Voltage Modulation

The paper presents an application of a novel method for diagnostics of electric and magnetic asymmetry of rotor cage in induction motor due to broken rotor bars. Increasing anomaly in magnetic field distribution results in a degradation of steady-state and dynamic performance of induction motor. It can be determined through the analysis of the average duty cycle of the modulated voltage. Broken rotor bars would cause torque and speed ripple which is compensated by speed control algorithm. Consequently oscillation in duty cycle of the modulated stator voltage appears. This effect can be simply detected without additional hardware. Correct evaluation of faulty motor performance is very significant part of condition monitoring and diagnostic procedure in modern supervision systems of electrical drives

Detection of Broken Bars in Induction Motor Through the Analysis of Supply Voltage Modulation

This paper presents an application of a novel method for the diagnostics of electric and magnetic asymmetries of rotor cage in induction motor (IM) due to broken rotor bars. An increasing anomaly in magnetic field distribution results in degradation of steady-state and dynamic performance of an IM. This degradation can be determined through the analysis of the average duty cycle of the modulated supply voltage. Broken rotor bars would cause torque and speed ripple which is mitigated by an efficient speed-control algorithm. Consequently, specific oscillation in the duty cycle of the modulated stator voltage appears. This effect can be simply detected without additional hardware and therefore provides a correct evaluation of faulty motor performance, which is a very significant part of condition monitoring and diagnostic procedure in modern supervision systems for electrical drives.

Intensity of the native and additional harmonic components in cogging torque due to design parameters of permanent-magnet motors

High performance applications like power steering and robotics require motor drives that produce smooth torque with very low components of cogging torque. To fulfil such demands a variety of techniques are known to reduce native cogging torque components in permanent-magnet motors. However, motor designers usually do not consider that assembly tolerances in mass-production give rise to additional cogging torque components, which are not present in the case of a perfect motor. A finite element method (FEM) and fast Fourier transformation (FFT) were used to analyse the sensitivity of several motor simulation models with regard to manufacturing assembly tolerances and material imperfections. The ascertainment that PM motor designs with higher values of the sensitivity parameter F are much more responsive to the phenomenon of additional harmonic components in cogging torque than the ones with lower values of parameter F is an important fact for the producers of PM motors, which must be seriously considered in the process of motor design optimization.

Modeling, analysis and detection of rotor field winding faults in synchronous generators

The paper presents an approach to modeling of shorted turns in rotor field winding of synchronous generator using finite element method. It enables detailed analysis of magnetic field at several operating conditions under healthy and faulty states which are difficult or even impossible to carry out by available measurement methods in industrial environment. Modeling of field winding faults are performed for both typical generator designs - turbo and hydro, and analysis reveals some differences, which are significant for practical use in fault detection procedures. It is confirmed that an extensive analysis should be performed to assure accurate healthy/faulty state predictions, since the level of diagnostic signal is considerably influenced by a combination of many machine and operating parameters. The scheme of developed on-line diagnostic system with its hardware and software concept is also presented and discussed.

Phenomena of additional cogging torque components influenced by stator lamination stacking methods in PM motors

Purpose – The purpose of this paper is to estimate and evaluate how cogging torque in permanent magnet (PM) motor designs is sensitive to the number of applied interlocks in stator back‐iron, which is a standard method for stator lamination stacking.Design/methodology/approach – The PM motors exhibit inherent cogging torque, which creates torque ripple and prevents smooth rotation of the rotor resulting in undesirable vibration and noise. While cogging torque minimization is necessary to improve PM motor performance, several FEM models have been developed to study and present data demonstrating sensitivity of the cogging torque to the applied interlocks. A procedure that would predict and evaluate cogging torque components relative to chosen number and positions of interlocks was proposed.Findings – On the basis of theoretical considerations, which were verified by numerous performed simulations using different FEM models, it was found out and proved that interlocks in the stator back‐iron cause the pheno...

Simplified model of induction machine with electrical rotor asymmetry

In this paper, a simplified two-axis model for induction machines with electrical rotor faults is derived. The model is intended for analysis of the impact of rotor fault on control loop with inverter fed induction machines, therefore detailed modeling of internal states of the machine is not required. The model is derived from unified theory of electrical machines, where rotor parameters are asymmetrical. The assumptions made during the development of the model are confirmed by experimental measurements. Finally, the comparison between proposed simplified model and detailed induction machine model is performed. The comparison shows very good agreement between simplified and detailed induction machine model, thus making the proposed model suitable for simulations of rotor faults.

Prediction of cogging torque level in PM motors due to assembly tolerances in mass‐production

Purpose – The purpose of this study is to estimate the sensitivity of cogging torque in permanent magnet (PM) motor designs due to PM assembly tolerance and/or PM imperfections and to evaluate how such faults can be reliably detected in simulated and measured cogging torque signals.Design/methodology/approach – PM motors exhibit inherent cogging torque, which creates torque ripple and prevents smooth rotation of the rotor, resulting in undesirable vibration and noise. While cogging torque minimization is necessary to improve PM motor performance, several FEM models have been developed to study and present data demonstrating the sensitivity of cogging torque to PM assemblies and/or PM imperfections. Some procedures that would predict and evaluate cogging torque components relative to measured PM positions on assembled PM motors were proposed.Findings – On the basis of numerous performed simulations using different FEM models and experimental results on rotors from mass‐production, it was found and proved t...