Impact of PWM Voltage Waveforms in High-Speed Drives: A Survey on High-Frequency Motor Models and Partial Discharge Phenomenon

The insulation system’s dielectric of the electric motor is very often subjected to severe electrical stress generated by the high dv/dt seen at the machine’s terminals. The electrical stress and high reflected wave transient overvoltage are even more evident in case of high-speed machines fed by high-frequency (HF) converters featuring very fast wide-bandgap devices. They are promoting the occurrence of partial discharges and consequently accelerate ageing. As this is serious issue and the main cause of the drive failure, it is important to analyse and characterise the surges at the motor terminals. Several HF models of motors have been proposed in the literature for this purpose. This article presents a survey on HF motor models, which is crucial in understanding and studying the most critical parameter identification and overvoltage mitigation techniques. Moreover, it offers a comparison of the models’ main features as well as a comparison with the experimental voltage waveform at motor terminals. A general overview of the partial discharge (PD) phenomenon is also provided, as it is favoured by HF operation and together with HF motor modelling provides key insights to the insulation ageing issue. In particular, an analysis of the effects of PWM waveform affecting insulation is given, as well as useful methods for developing strategies for the inspection and maintenance of winding insulation

Accelerated Aging Study of Machine Winding Insulation under AC and High Frequency Pulse Voltage Application

It is common practice to perform accelerated aging with 60 Hz ac to determine the lifetime characteristics of insulation used in the machine. Comparable breakdown measurements are performed at different voltage levels and temperatures for the polyimide insulated machine winding under normal operating conditions of 60 Hz ac. The result shows that the time to failure can be represented by the inverse power law and the Arrhenius equation with respect to test voltage and temperature respectively. However, the use of power electronic devices causes harmonics, and spikes that electrically degrade the machine winding insulation. Applied high frequency pulse voltages can be used to study the impact of electrical degradation of the machine winding insulation that exists in electrical machines. Evaluation of change in dielectric strength, partial discharge and breakdown voltage is monitored versus aging caused by high frequency pulse voltage at 90% of operating temperature

Accelerated Aging Study of Machine Winding Insulation under AC and High Frequency Pulse Voltage Application

It is common practice to perform accelerated aging with 60 Hz ac to determine the lifetime characteristics of insulation used in the machine. Comparable breakdown measurements are performed at different voltage levels and temperatures for the polyimide insulated machine winding under normal operating conditions of 60 Hz ac. The result shows that the time to failure can be represented by the inverse power law and the Arrhenius equation with respect to test voltage and temperature respectively. However, the use of power electronic devices causes harmonics, and spikes that electrically degrade the machine winding insulation. Applied high frequency pulse voltages can be used to study the impact of electrical degradation of the machine winding insulation that exists in electrical machines. Evaluation of change in dielectric strength, partial discharge and breakdown voltage is monitored versus aging caused by high frequency pulse voltage at 90% of operating temperature

Survey of Insulation Systems in Electrical Machines

Insulating materials and insulation systems design have been gaining more attentions as more electrical machines tend to operate in harsher environments for various applications. Harsh environments include high temperature, humidity, erosion, low air pressure, etc. This paper discusses recent advances in insulation systems for electrical machines. Insulation tests as well as test standards that have been used to evaluate insulation systems and detect insulation failures will be discussed. Insulating materials used for a wide range of industrial applications such as wind turbine generators, aerospace hybrid/electric powertrain, and hydro generators have been summarized. For the emerging high-altitude, highvoltage aerospace applications, partial discharge and its impact on insulation systems will be discussed. Finally, polymer nanocomposite materials with excellent thermal conductivity and dielectric strength are highlighted as an outlook

Non-intrusive partial discharges investigations on aeronautic motors

The present paper reports on a study of non-electrically intrusive partial discharge investigations on an aeronautic motor. Relevancy, robustness and repeatability of partial discharge testing procedures, both on insulating materials characterization and on operating aeronautic equipment are essential to ensure reliability of the aircraft systems. The aim of this paper is to be the very first step of defining such procedures and the associated test equipments. To do so, the paper will start by providing an understanding of partial discharge phenomena and will review typical more electrical aircrafts architectures and their key characteristics causing partial discharge risk to increase. The impact of harness length, high performance power electronics and increase of voltage level on insulation system is demonstrated. Then, an analysis is conducted from motor design point of view to find out which test procedures are relevant to each insulation system performance both at atmospheric pressure and reduced pressure. The study helps to realize the benefits of using multiple test samples and procedures to assess insulation system performance levels through the design process of the motor. It is shown that a good agreement is find out between the stator and representative samples. This paper will also review the ability of the non-intrusive test method used to detect partial discharge with a full scope of voltage signal from AC to impulse voltage and under different pressure. The paper is concluded with analysis of results and thoughts about future work regarding motor design taking into account partial discharge risk and evolution of proposed procedures

Failure Mechanism and Property Modification of Insulation Material Used in Inverter-Fed Motors

The insulation system of inverter-fed motors is subjected to repetitive impulse voltages, which are generated by pulse width modulation (PWM) converters, and is expected to withstand partial discharge (PD) activity during service. PD-induced dielectric breakdown becomes one of the most important reasons for premature failures of motor insulation. Therefore, the dielectric properties of insulation material used in inverter-fed motors need to be improved to meet the requirement for rapid development of applications. Many approaches have been proposed to improve dielectric properties of insulation material used in inverter-fed motors. In this chapter, we will discuss the failure mechanism, modification approaches, and their effects on dielectric properties of insulation material of inverter-fed motors

Multistress characterization of fault mechanisms in aerospace electric actuators

The concept behind the More Electric Aircraft (MEA) is the progressive electrification of on-board actuators and services. It is a way to reduce or eliminate the dependence on hydraulic, mechanical and the bleed air/pneumatic systems and pursue efficiency, reliability and maintainability. This paper presents a specialised test rig whose main objective is to assess insulation lifespan modelling under various stress conditions, especially investigating the interaction between ageing factors. The test set-up is able to reproduce a multitude of environmental and operational conditions at which electric drives and motors, used in aerospace applications, are subjected. It is thus possible to tailor the test cycle in order to mimic the working cycle of an electrical motor during real operation in aircraft application. The developed test-rig is aimed at projecting the technology readiness to higher levels of maturity, in the context of electrical motors and drives for aerospace applications. Its other objective is to validate and support the development of a comprehensive insulation degradation model

The effect of voltage distortion on ageing acceleration of insulation systems under partial discharge activity

The features of harmonic distortion which may affect significantly the reliability of typical ac-power network equipment, such as low-voltage self-healing capacitors used for reactive power and harmonic compensation are investigated. Moreover, the effect of high-frequency pulse-like voltage generated by adjustable speed drives (ASD) on electrical machine insulation is also investigated, resorting to life tests carried out on different insulating materials of the standard and "corona resistant" type, at electrical field levels able to incept partial discharges (PD)