Survey of Insulation Systems in Electrical Machines

Abstract

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. SECTION I. Introduction Due to the continued growth of renewable energys, the number of electrical machines used worldwide has significantly increased. Insulation system is a very critical component of electrical machines. Insulation breakdown can lead to failures, which eventually result unpredicted downtime and negative financial impacts and in some applications can be a safety hazard. For some specific industries where uninterruptible operation is required, the unpredicted downtime is unacceptable. The unpredicted downtime for an offshore oil plant would be $25,000/h [1]. The dielectric strength of electrical insulation materials has been gradually improved over the years. By introducing new materials in the past 20 years for instance, the dielectric strength of ground-wall insulation nearly doubled [2]. Electrical, mechanical, thermal, and ambient stresses can cause insulation degradation which consequently leads to insulation failure [3], [4]. Insulation failure causes short circuits in the stator winding and consequently high currents could pass through the defected stator winding [5]. A survey on 1141 induction motors with power ratings above 200 hp shows that around 30% of motor failures are due to insulation failures [6]. With advances in sensors, digital signal processing, diagnosis methods and test standards, insulation failures can be detected [7], [8]. Through online estimation of material degradation and lifetime in early stages, insulation systems of electric machines can be protected from further aging while unpredicted downtime could be avoided through scheduled maintenance. Even though insulation system is a passive component in an electrical machine which does not produce torque, insulation build/thickness (which represents the key thermal resistance in electrical machine)can have significant impact on the machine cooling and hence electrical loading and torque production. There have been several efforts to minimize insulation thickness for design compactness, low manufacturing cost and high efficiency [3]. The two main functions of insulation systems in electr...

An Overview of Modern Thermo-Conductive Materials for Heat Extraction in Electrical Machines

“…However, insulation materials are passive components of rotating machines as they do not contribute electromagnetic torque production, but they usually limit thermal class (i.e. maximum allowable temperature), manufacturing techniques and life expectancy [20,23]. Therefore, insulating materials are critical as their thermal properties are directly related to the thermal performance and reliability of electric machines.…”

“…However, thermoset and thermoplastic polymers which constitute the majority of insulation materials in rotating machines, mostly do not possess a high thermal conductivity and hence prevent good heat dissipation inside the machine components [23]. This causes accelerated aging, lower life expectancy, increased Joule losses and lower efficiency.…”

“…3. [23,28]. The majority of these insulating materials will be reviewed here in terms of their thermo-conductive aspects.…”

“…Several review papers with the main concern of cooling technologies, modern heat extraction systems and electrical insulation systems in electric motors have been reported in the literature [3,[14][15][16][17][18][19][20][21][22][23]. The structure of published review papers is based on the common cooling technologies such as passive cooling and forced cooling [6,21]; heat extraction methods with a brief overview of heat transfer by conduction, convection and radiation [6,20,22]; machine topology-specific cooling technologies such as thermal management in switched reluctance or axial machines [10,16]; and finally application-specific thermal review papers including automotive, traction and transportation.…”

“…Regarding review papers focusing on the thermal aspects, the main discussions include loss generation principles in electrical machines; analysis techniques including experiment informed finite element method (FEM) and computational fluid dynamics (CFD) investigations and cooling schemes/instruments such as natural and forced cooling with numerous application examples. Furthermore, another review paper concentrates on insulation systems in modern electrical machines with a principal objective in electrical aspects including dielectric properties, and partial discharge [23]. However, a comprehensive study of thermally conductive (i.e.…”