John Fayia Bangura

Diagnosis and characterization of effects of broken bars and connectors in squirrel-cage induction motors by a time-stepping coupled finite element-state space modeling approach

In this paper, a comprehensive presentation of a time-stepping coupled finite element-state space modeling approach for diagnosis and characterization of effects of rotor (cage) broken bars and connectors in squirrel-cage induction motors is given. The model is used to compute/predict the characteristic frequency components which are indicative of rotor bar and connector breakages in the armature current waveforms and developed torque profiles, respectively. It was found that failures due to rotor connector breakages tend to degrade the developed torque of the machine considerably more than failures due to rotor bar breakages. Also, the effects and implications of rotor breakages on core loss distributions are quantified and described. Furthermore, this model has great potential in future applications to generate databases for use in overall noninvasive diagnostics of faults or troubleshooting in relation to quality assessments of the state of the motor and the overall drive.

Comparison between characterization and diagnosis of broken bars/end-ring connectors and airgap eccentricities of induction motors in ASDs using a coupled finite element-state space method

This paper describes how a rigorous and comprehensive time-stepping coupled finite element-state space (TSCPE-SS) modeling technique can be utilized in diagnostics and differentiation between induction motor rotor (cage) abnormalities of broken bars/connectors and airgap eccentricities. The model is used for the computation of time-domain performance characteristics, such as the stator phase current waveforms and developed torque profiles including these abnormalities. This is followed by analysis of the current waveforms and torque profiles using fast Fourier transform to obtain their corresponding frequency spectra. Comparison between the TSCFE-SS models simulation results, which correlate very well with theoretical results, clearly illustrate that rotor bar and/or end-ring connector breakages can be distinguished from static and dynamic airgap eccentricities. This paper also gives an interesting comparison between the effects and implications of these various rotor abnormalities on machine parameters and performance characteristics. Furthermore, the results indicate that frequency components reported earlier to be produced only by the combined effects of static and dynamic airgap eccentricity could be observed in case of either static or dynamic eccentricity. Finally, this paper demonstrates the possible opportunities that can be made use of in noninvasive detection of airgap eccentricities via TSCFE-SS and current signature techniques.

A time-stepping coupled finite element-state space model for induction motor drives. II. Machine performance computation and verification

In a companion paper, a time-stepping coupled finite element state-space algorithm for modeling of induction motor drives was developed. The model formulation and algorithm allows one to rigorously model the effects of space harmonics caused by magnetic circuit nonlinearities, topology and winding layouts, as well as their interaction with time harmonics caused by the power condition (inverter) operation. In this paper, the model is used in the computation of the parameters and prediction of performance characteristics of a 3-phase, 2 pole, 1.2-hp, 208 V squirrel-cage case-study induction motor. In addition, emphasis is laid on comparison between sinusoidal and inverter operations, with regard to effects on ohmic and core losses, as well as torque ripples. Furthermore, this includes harmonic breakdowns of selected inductance profiles and developed torque profiles. Numerical simulations were verified by comparison to test data with excellent correlation between both sets of results.

Simulation of inverter-fed induction motor drives with pulse-width modulation by a time-stepping coupled finite element-flux linkage-based state space model

In this paper, a detailed description of the simulation of inverter-fed induction motors in adjustable speed drives by a time-stepping flux linkage-based coupled finite element/state-space model for the computation of the drive performance is presented and verified experimentally. The flux linkage-based state-space model is iteratively coupled to a 2D time-stepping finite element model. The iterative nature of the coupling between these two models facilitates rigorous modeling of the comprehensive impact of inherent space harmonics due to geometries and motor magnetics and time harmonics resulting from the electronic switching of inverters, as well as effects of the synergistic interaction between these time and space harmonics. The state-space model includes no frame of reference transformation in so far as the flux linkages, currents or voltages. This implies that the state-space model directly couples the motor to its power electronic controller in one formulation and the natural variables (voltages and currents) are directly involved in the formulation and computation. Finally, results of motor drive performance simulations and corresponding test results for a six-switch inverter with 180/spl deg/ e duty cycle for each switch and a six-switch inverter with pulse-width modulation are given with excellent test result correlations, respectively.

Performance and torque-ripple characterization in induction motor adjustable-speed drives using time-stepping coupled finite-element state-space techniques

In this paper, the time-stepping coupled finite-element state-space (TSCFE-SS) model developed in an earlier companion paper is applied here for assessments of effects of machine geometry and magnetic circuit design modifications, and effects of pulsewidth modulation (PWM) carrier frequency on performance characteristics of induction motor drives. Namely, this has been accomplished through analysis of developed torque profile ripples and harmonic spectra of mid-air-gap radial flux density waveforms of the case-study motor. Furthermore, consequent effects of design modifications pertaining to geometry and/or magnetic circuit modifications and PWM carrier frequency on ohmic and iron core losses are investigated. The investigation has been performed on a case-study motor, which is a Y-connected single-layer three-phase two-pole 1.2-hp 208-V squirrel-cage induction motor with 24 stator slots and a cage with 34 rotor bars.

Effects of broken bars/end-ring connectors and airgap eccentricities on ohmic and core losses of induction motors in ASDs using a coupled finite element-state space method

In this paper, effects of rotor abnormalities such as broken squirrel-cage bars, broken cage connectors and airgap eccentricity on ohmic and core losses of induction motors are presented. In this investigation, a comprehensive time-stepping coupled finite element-state space (TSCFE-SS) model was fully utilized to compute the time-domain elemental flux density waveforms and various time-domain waveforms of motor winding currents useful for core loss and ohmic loss computations. Such investigation is feasible by use of the TSCFE-SS model due to its intrinsic nature and characteristics. The results obtained from the simulations of an example 1.2-HP induction motor clearly indicate that faults due to broken squirrel-cage bars/end-connectors can increase motor core losses in comparison to the normal case. The results also give the effect of saturation on the core loss distributions within the cross-section of the motor, and indicate the potential for possible excessive loss concentrations and consequent hot spots near zones of bar and connector breakages in the rotor. Effects of PWM inverter operation on motor losses are also studied.

Improvement of monitoring and diagnosis of broken bars/end-ring connectors and airgap eccentricities of squirrel-cage induction motors in ASDs using a time-stepping coupled finite element-state space technique

This paper discusses how a time-stepping coupled finite element-state space (TSCFE-SS) modeling technique can be utilized in diagnostic techniques to improve condition monitoring and diagnosis of induction motor rotor abnormalities of broken bars, broken connectors and airgap eccentricities. This investigation shows that application of the TSCFE-SS modeling technique can facilitate monitoring the characteristics of several frequency components as opposed to merely previous monitoring of sideband frequency components associated with the fundamental in motor phase current waveforms. The results presented in this paper clearly illustrate the possible potential use of the modeling technique for improvement of present diagnostic techniques in which diagnosis of broken bars, broken connectors and airgap eccentricities is restricted to identification of the sidebands associated with the first-order (fundamental) component in phase current waveforms. In addition, the results further demonstrate that database generation for various possible faults prior to their occurrences can be readily achieved using this modeling technique.

A comparison of phase space reconstruction and spectral coherence approaches for diagnostics of bar and end-ring connector breakage faults in polyphase induction motors using current waveforms

Two signal (waveform) analysis approaches are investigated in this paper for motor drive fault identification-one linear and the other nonlinear. Twenty-one different motor-drive operating conditions including healthy, 1 through 10 broken bars, and 1 through 10 broken end-ring connectors are investigated. Highly accurate numerical simulations of current waveforms for the various operating conditions are generated using the time stepping coupled finite element-state space method for a 208-V, 60-Hz, 2-pole, 1.2-hp, squirrel cage 3-phase induction motor. The linear signal analysis method is based on spectral coherence, whereas the nonlinear signal analysis method is based on stochastic models of reconstructed phase spaces. Conclusions resulting from the comparisons of these two methods are drawn.

Performance characterization and torque-ripple reduction in induction motor adjustable speed drives using time-stepping coupled finite element state space techniques

In this paper, the time-stepping coupled finite element state space (TSCFE-SS) model developed in the first of two companion papers is applied here for the assessment of effects of machine geometry and magnetic circuit design modifications and the effects of pulse-width modulation carrier frequency on the performance characteristics of induction motor drives. Namely, this has been accomplished through analysis of developed torque profile ripples and harmonic spectra of mid-airgap radial flux density waveforms of the case-study motor. Furthermore, consequent effects of design modifications pertaining to, geometry and/or magnetic circuit modifications, and pulse-width modulation carrier frequency on ohmic and iron core losses are investigated. The investigation has been performed on a case study motor which is a Y-connected, single-layer, 3-phase, 2-pole, 1.2-hp, 208 Volts squirrel-cage induction motor with 24 stator slots, and a cage with 34 rotor bars.