Low-voltage Machines Research Articles

Comparison of concentrated winding topologies considering transient voltages in the winding system of inverter-driven low-voltage machines

Comparison of concentrated winding topologies considering transient voltages in the winding system of inverter-driven low-voltage machines

Towards a Holistic Approach to Inverter-Fed Machine Design: FEM-Based PDIV Prediction of Complete Windings

The Schumann inception criterion was used to predict the partial discharge inception voltage of insulation models representing both wire wound and form wound (hairpin) low voltage machines. It was found that the fitting parameter <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> (the logarithm of the critical number of electrons that define the transition from Townsend to streamer discharge) is almost independent of material parameters when dealing with a given type of insulation (e.g., turn-to-turn using round conductors or flat conductors). These numbers can be fed to software for finite element analysis to provide a worst-case estimate of the partial discharge inception voltage.

Partial discharge diagnostics on inverter-fed drives of electric vehicles

In this paper, the possibilities of partial discharge measurement at square-wave voltages are discussed. The focus is on low-voltage machines, such as those used in electric vehicles. An insight into common measurement methods is given. In this work, high-frequency current transformers and near-field probes are used as sensors. The generation of a realistic steep-edged test voltage according to IEC/TS 61934 is also discussed and demonstrated on an inverter with silicon carbide semiconductors.Experiments are conducted to compare the sensors used. A direct comparison between cable-based sensors (HFCT) and antennas (near-field probes) is provided. Resonance points to open motor windings according to IEC/TS 60034-27‑5 and their significance for partial discharge measurement are also investigated. The results show that the high-frequency current transformers and the ball probe are able to detect PD even at steep voltage edges. Excitation with a fast voltage edge results in non-negligible overshoots in the overall system due to resonances. It can be shown that the partial discharges occur during these overvoltages.

Low Complexity Non-Linear Spectral Features and Wear State Models for Remaining Useful Life Estimation of Bearings

Improving the reliability and performance of electric and rotating machines is crucial to many industrial applications. This will lead to improved robustness, efficiency, and eco-sustainability, as well as mitigate significant health and safety concerns regarding sudden catastrophic failure modes. Bearing degradation is the most significant cause of machine failure and has been reported to cause up to 75% of low-voltage machine failures. This paper introduces a low complexity machine learning (ML) approach to estimate the remaining useful life (RUL) of rolling element bearings using real vibration signals. This work explores different ML recipes using novel feature engineering coupled with various k-Nearest Neighbour (k-NN), and Support Vector Machines (SVM) kernel and weighting functions in order to optimise this RUL approach. Original non-linear wear state models and feature sets are investigated, the latter are derived from Short-time Fourier Transform (STFT) and Hilbert Marginal Spectrum (HMS). These feature sets incorporate one-third octave band filtering for low complexity multivariate feature subspace compression. Our proposed ML algorithm stage has employed two robust supervised ML approaches: weighted k-NN and SVM. Real vibration data were drawn from the Pronostia platform to test and validate this prognostic monitoring approach. The results clearly demonstrate the effectiveness of this approach, with classification accuracy results of up to 82.8% achieved. This work contributes to the field by introducing a robust and computationally inexpensive method for accurate monitoring of machine health using low-cost vibration-based sensing.

Inverter-Embedded Partial Discharge Testing for Reliability Enhancement of Stator Winding Insulation in Low Voltage Machines

Partial discharge (PD) due to high dv/dt pulsewidth modulation has been identified as one of the leading causes of insulation failure in low voltage (LV) variable frequency drive (VFD) fed motors with random wound stator windings. With the recent increase in electrical stress with faster risetime wide bandgap power semiconductor devices and higher dc link voltage for higher efficiency and power density of the system, PD-induced insulation failures are expected to increase. Since LV motor insulation is not resistant to PD, an increasing number of VFD motors are being qualified for PD-free operation. However, the PD inception voltage of qualified motors decreases with insulation aging making LV motors susceptible to failure. In this article, an offline standstill test method is proposed for automated inverter-embedded PD testing under impulse excitation. The main concept is to perform the impulse PD test using the VFD at increased voltage levels for providing advanced warning of PD activity for preventing forced outage of VFD motors. Options for application of increased impulse test voltage and automated detection of PD for implementing the proposed concept are presented. Testing on an 800 V VFD driven motor shows that the proposed method can provide automated impulse PD testing whenever the motor is at standstill to warn the end user of impending PD activity that can lead to failure.

The Insulation Resilience of Inverter-Fed Low Voltage Traction Machines: Review, Challenges, and Opportunities

The use of wide bandgap (WBG) semiconductor devices, which enable higher switching slew rates, and the increase in DC link voltage, which provides system-wide advantages, exposes the winding system of traction machines to enhanced electrical stress. The resulting nonlinear voltage distribution along the motor winding favors partial discharges (PD), which in low voltage (LV) machines causes excessive damage to the insulation system, and premature failure can occur. A simple solution by increasing the enamel thickness of the wires leads on the one hand to a lower copper fill factor. On the other hand, this measure is not necessarily accompanied by an increase in electrical resilience in the case of pulsed voltage. It is therefore essential to understand the phenomenon of partial discharges, which is composed of a large number of processes and mechanisms, to be able to make an estimation of aging. The ultimate goal is to derive a lifetime model that links the dynamic load collectives – ideally in conjunction with the environmental stress influences – of a traction application to a usage-dependent and realistic prediction of the residual lifetime. To this end, this paper provides an overview of the current state of science and technology in this interdisciplinary topic by describing, with reference to high voltage (HV) technology/engineering, the design of the insulation system, the discharge physics, the degradation mechanisms, the statistical effects to be considered, and the partial discharge measurement methods.

FREQUENCY CHARACTERISTICS OF ELECTRICAL MACHINES WITH A MESH WINDING DURING HEAT-AND-MOISTURE AGING

&#x0D; &#x0D; &#x0D; &#x0D; Operation of an electric drive with damages in power electric circuit of the motor stator results in asymmetry&#x0D; &#x0D; &#x0D; &#x0D; &#x0D; of the motor phase current charge, increase of heating losses in certain phases, occurrence of variable components of electromagnetic torque and consumed power. An electric motor stator winding consists of a number of stator bars and overhang connections. Due to the complicated winding structure and the steel core, the attenuation and distortion of a pulse transmitted through the winding are complicated, and frequency-dependent. A low voltage impulse method and digital analysis techniques to determine the frequency characteristics of the winding are described. The frequency characteristics of electric motor stator windings are discussed in some detail. The analysis presented in this chapter could be applied to other rotating machines such as low voltage motors. An experiment of damping of electric motor wilding was conducted. Changes in frequency characteristics after the cycle are shown. In this article an analysis of the frequency characteristics of low-voltage electric machines with mush-wound windings, taking into account the processes of successive destruction of the insulation structure by the influence of heat-wet cycles. It is shown that the frequency characteristics can be generalized by the parameter of the state of isolation in the conditions of periodic monitoring of the quality of insulation during regulatory audits. It is proved that the frequency characteristics taken in idle and short-circuit modes have diagnostic features of the level of insulation destruction during humidification, which consist of shifting the characteristics of extremes into the region of lower frequencies, as well as reducing the area under the curve between the minimum and maximum extrema of the characteristic.&#x0D; &#x0D; &#x0D; &#x0D;

Phase to ground insulation in low voltage machines: lifetime evaluation under enhanced thermal stress

Phase to ground insulation in low voltage machines: lifetime evaluation under enhanced thermal stress

Modeling of the Partial Discharge Process Between the Winding and the Stator of Low Voltage Machines for Traction Applications

The application of wide-band-gap-semiconductors in the power-electric circuit of future traction drives leads to an increased stress on the insulation system. This can cause partial discharge, which leads to a rapid breakdown of the insulation and must, therefore, be avoided. To enhance the durability of the insulation system and to fill up the air gaps between the enameled wires, impregnation material is applied. This leads to an increased partial discharge inception voltage between neighboring conductors as the discharge process only occurs in the remaining air enclosures. Due to the change of the electric field distribution, which is caused by the impregnation material, also the threshold voltage for partial discharge between the stator and the winding changes. While so-called twisted pairs are commonly applied to study the partial discharge behavior of the inter-turn insulation isolated from other effects, in this article a specimen that considers the phase-to-ground insulation is proposed.

Uncertainty Sources in the Estimation of the Partial Discharge Inception Voltage in Turn-to-Turn Insulation Systems

Partial discharges (PD) are one of the main causes of premature failure in low-voltage motors driven by variable-speed drives. The use of these control systems are being quite extended due to new applications, such as the more electric aircraft (MEA) or hybrid and electric vehicles, and this has pushed research towards appropriate designs of electric motors to avoid, as much as possible, the presence of PD within their windings. This article presents a model to predict the partial discharge inception voltage (PDIV) in the insulation of low-voltage machines. A value for the secondary ionization coefficient based on a statistical study is also proposed. The deviations of the model are also studied by obtaining the uncertainty of the value of that coeficient and the predicted values of the PDIV for a set of wires. This uncertainty will be compared with other error sources such as generator harmonics and humidity. Finally, the tests are done for different temperatures extend the model applicability.

Effect of repetitive impulsive voltage duty cycle on partial discharge features and insulation endurance of enameled wires for inverter-fed low voltage machines

Dealing with electrical rotating machines fed by voltage converters, understanding the effect of impulsive voltage parameters on partial discharge features is of great importance to select appropriate impulsive voltage to perform partial discharge inception voltage measurements. Similarly, the dependence of insulation endurance on voltage parameters might help to develop qualification tests providing more accurate evaluation of insulation performance. The effect of voltage duty cycle on partial discharge characteristics and endurance of inverter-fed motor insulation were investigated through a large number of tests performed on single-contact crossed enameled wires under bipolar repetitive impulse wave voltages. The partial discharge pattern, magnitude, repetition rate show that short impulsive voltage duration (i.e. small duty cycles), can induce asymmetric PD patterns and reduce the probability of PD inception at the falling flanks of impulsive voltages. Moreover, the endurance tests indicate that the duty cycle of the repetitive square wave voltage can affect the insulation endurance significantly. Repetitive square wave voltages with small duty cycles tend to overestimate endurance. Accordingly, when performing partial discharge inception voltage and endurance tests on insulation of inverter-fed motors, the influence of duty cycle should be carefully considered.

Impact of Number of Poles on the Steady-State Performance of Induction Motors

A simplified analysis of influence of pole pairs on the steady-state behavior of induction motors is presented. The analysis makes simplifying assumptions to express relationship between key steady-state performance parameters of 2-pole reference motor compared to its multiple pole counterparts. The analysis is performed using considerations of fixed geometrical dimensions on one hand and fixed output power on the other hand. Available manufacturer catalog data of medium voltage (MV) induction machines from one manufacturer are used to evaluate the soundness of the analysis. Also test and catalog data analyses of commercially manufactured low-voltage (LV) machines from different manufacturers are evaluated to determine extent to which they conform to the theoretical considerations of the analyses. The preliminary findings indicate more agreement with the MV motors than the LV motors.

Regression methods for improved lifespan modeling of low voltage machine insulation

This paper deals with the modeling of insulation material lifespan in a partial discharge regime under certain accelerated electrical stresses (voltage, frequency and temperature). An original model, relating the logarithm of the insulation lifespan, the logarithm of the electrical stress and an exponential form of the temperature, is considered. An estimation of the model parameters is performed using three methods: the design of experiments (DoE) method, the response surface method (RSM) and the multiple linear regression (MLR) method. The estimation is obtained on learning sets determined according to each method specification. The performance, in terms of estimation, of each of the three methods is evaluated on a test set composed of additional experiments. For economic reasons and flexibility, the learning and test sets are composed of experiments carried out on twisted pairs of wires covered by an insulator varnish. The ability of the DoE and the RSM methods to organize and to limit the number of experiments is confirmed. The MLR method, however, shows more flexibility with regard to the studied configurations. Thus, it offers an efficient solution when organization is not required or not possible. Moreover, the flexibility of MLR allows specific ranges for the factors to be explored. A local analysis of the estimation performance shows that very short and long lifespans cannot be simultaneously represented by the same model.

Current Sharing Analysis of Parallel Strands in Low-Voltage High-Speed Machines

The electrical frequency in high-speed machines leads to more pronounced parasitic effects such as skin and proximity effects. In low-voltage machines, a low turn count is needed, requiring individual strands to be connected in parallel and further enhancing these adverse effects. This results in an uneven distribution of the current across the strands and circulating currents. This paper presents a simple yet flexible method to estimate this distribution, based on finite-element simulations and basic circuit equations. The effects of strand displacements or twisting can be accounted for in the actual machine geometry using a 3-D finite-element method, without requiring excessive computer memory or time. Experiments are performed to confirm the modeling method. Straight and twisted windings are compared, clearly showing the effectiveness of twisting the strands.

Electrical Aging of the Insulation of Low-Voltage Machines: Model Definition and Test With the Design of Experiments

The aim of this paper is to present a method for modeling the life span of insulation materials in a partial discharge regime. Based on the design of experiments, it has many advantages: It reduces the number of time-consuming experiments, increases the accuracy of the results, and allows life span modeling under various stress conditions, including coupling effects between the factors. Accelerated aging tests are carried out to determine the life span of these materials. The resulting model presents an original relationship between the logarithm of the insulation life span and that of electrically applied stress and an exponential form of the temperature. Results show that the most influential factors can be identified according to their effects on the insulation life span. Moreover, the life span model validity is tested either with additional points which have not been used for modeling or through statistical tests. Finally, it is shown that fractional plans are not suitable to reduce the number of experiments. This application of the experimental design is best used during the initial phase, before the final drive has been built and any online diagnosis.

Online Surge Testing Applied to an Induction Machine With Emulated Insulation Breakdown

A breakdown of the electrical insulation system can cause catastrophic failure of the electrical machine and bring large process downtime losses. Preventive maintenance and online monitoring are some of the methods to improve reliability and reduce unscheduled downtime. The offline surge test is the most commonly used offline test to assess the condition of the turn insulation. There is no online monitoring method that is applicable to low-voltage machines and has the same capabilities as the surge test. This paper shows how to conduct a surge test online on 460-V/230-V rated induction machines that are connected in the low-voltage configuration. The required hardware, test procedures, and evaluation of the results are described. Instead of using a bolted short to emulate the insulation breakdown, a new approach is used for the experimental validation. The arcing is emulated either by using an insulation sample that breaks down at a certain test voltage or by using a circuit that emulates the breakdown.

A New Concept for Online Surge Testing for the Detection of Winding Insulation Deterioration in Low-Voltage Induction Machines

A breakdown of the electrical insulation system causes catastrophic failure of the electrical machine and brings large process downtime losses. Preventive maintenance and online monitoring are some of the methods to improve the reliability and to reduce unscheduled downtime. One of the main reasons for the failure of the machine is the breakdown of the stator turn insulation. The offline surge test is the most commonly used offline test to assess the condition of the turn insulation. There is no online monitoring method that is applicable to low-voltage machines and has the same capabilities as the surge test. This paper introduces new concepts to implement an online surge test. The possibilities and limitations of the online surge test are presented, as well as the simulation and experimental results, to validate the concepts.

Robust diagnostics of stator insulation based on high frequency resonances measurements

The stator insulation breakdown is a major cause of AC machine failures. Ground insulation defaults are easily detected by classical systems based on leakage current measurements, however the turn-to-turn insulation degradations are more difficult to detect. For large machines, on-line methods, based on partial discharge detection and analysis, give good results but they cannot be used for low-voltage machines fed by adjustable speed drives (ASD). Previously, it has been shown by some of the authors that it was possible to estimate the aging of an AC machine winding thanks to HF measurements of current or magnetic field. In this paper, it is proposed to exploit conjointly all these different estimations to obtain a more robust and reliable diagnostic. The merging of the different estimations being realized through the belief functions framework, this approach is tested on real measurements.

A Survey on Testing and Monitoring Methods for Stator Insulation Systems of Low-Voltage Induction Machines Focusing on Turn Insulation Problems

A breakdown of the electrical insulation system causes catastrophic failure of the electrical machine and brings large process downtime losses. To determine the conditions of the stator insulation system of motor drive systems, various testing and monitoring methods have been developed. This paper presents an in-depth literature review of testing and monitoring methods, categorizing them into online and offline methods, each of which is further grouped into specific areas according to their physical nature. The main focus of this paper is on testing and monitoring techniques that diagnose the condition of the turn-to-turn insulation of low-voltage machines, which is a rapidly expanding area for both research and product development efforts. In order to give a compact overview, the results are summarized in two tables. In addition to monitoring methods on turn-to-turn insulation, some of the most common methods to assess the stator's phase-to-ground and phase-to-phase insulation conditions are included in the tables as well.

A New Method for AC Machine Turn Insulation Diagnostic Based on High Frequency Resonances

The stator insulation breakdown is a major cause of ac machine failures. Ground insulation defaults are easily detected by classical systems based on leakage current measurements, however the turn-to-turn insulation degradations are more difficult to detect. For large machines, on-line methods, based on partial discharge detection and analysis, give good results but they cannot be used for low-voltage machines fed by adjustable speed drives (ASD). This paper presents a new monitoring system able to detect slight variations of high frequency resonances in the winding of a working machine fed by an industrial inverter. The new system works in association with statistical data on accelerated aging of the magnet wire used to build the machine.