Insulation materials in electric machines
Insulation material plays key role in the construction of electric machines. It is essential for the functionality of the machine by providing dielectric insulation of live parts. Moreover, the long term stability of the insulation material is crucial for long lifetime of the machine. In this blog post we write few lines on insulation materials in rotating electrical machines.
The insulation materials can be classified according several categories. One criteria is certainly the maximum admissible temperature during operation. Insulation can be main or auxiliary.
Properties of insulation materials
Insulation material provides several functions. Thus, good insulation material shall perform well in multiple categories. Most relevant properties are:
A) Thermal
- Temperature class
- Coefficient of expansion
B) Electrical
- Dielectric strength
- Dissipation factor
- Partial discharge resistance
C) Mechanical
- Tensile strength
- Flexural strength
- Compressive strength
D) Water absorption
e.g. very good resistance to moisture
E) Chemical resistance
e.g. material swells when subjected to solvents and lubricants such as oil
F) Radiation resistance
e.g. material has good radiation resistance
A material that excels in one category but performs poor in another category may not be the most suitable insulation material after all. Instead, a material is preferred that checks most of the boxes.
Insulation class and temperature limit
Thermal properties of the material are important. The electric machines produce heat losses as part of the electro-mechanical conversion. Material used for insulation shall withstand the service temperature without any degradation or loss of life. It shall also have good thermal conductivity so that the heat load can be evacuated from the source. Improvement of thermal conductivity has machine-uprating potential and is therefore attractive for the upgrades of existing installed base.
Insulation systems are classified in (thermal) insulation classes defining the maximum working temperature and temperature rise for a given ambient temperature. Standardized insulation classes are shown in Table 1.
Dielectric strength
When the voltage between electrodes situated in insulation exceeds certain threshold, sudden phenomena happen that lead either to breakthrough or flashover. In the insulator there is a full discharge linked with a voltage drop on the electrodes. Breakthrough is a full discharge in solid insulation material. Flashover is a full discharge in gaseous or liquid insulation material.
Breakthrough is characterized by sudden large current (providing that the source has enough power), bright light trace between electrodes, high temperature and mechanical effects such as fragmentation in solid insulation and pressure wave in liquid insulation. Solid insulation materials often lose their insulation capability after a breakthrough. Liquid insulation materials often regenerate similar to gases, i.e. their dielectric strength increases again. Solid and liquid insulation decomposes because of breakthrough, especially organic insulation material.
Dielectric strength is the ratio of voltage causing breakthrough or flashover and minimum thickness of the insulation material between electrodes. Thus, the unit is usually kV/mm or kV/cm.
Factors influencing dielectric strength
Dielectric strength is not a constant. Instead, it is influenced by several factors. Moisture always reduces dielectric strength. Increase of temperature up to 100°C mostly causes just marginal reduction of dielectric strength. Mechanical pressure may significantly increase the dielectric strength.
Dielectric strength for DC voltage is higher than AC voltage (referring to RMS value). Thicker insulation has typically lower dielectric strength than thin insulation layer.
Largest impact on dielectric strength has the duration of electric stress. In practice, dielectric strength is differentiated according to the duration:
- Impulse strength – tested by impulse wave of a defined shape
- Short time strength – tested by rapid increase of voltage until the breakthrough
- Strength when exposed to continuous stress
The factor of time is considered in voltage versus time dielectric strength.
Structure of insulation
There is not just one insulation. Instead, we differentiate two to three insulation sub-categories: Groundwall insulation and conductor insulation. The latter one can be further divided into turn and strand insulation so that we obtain following:
- Groundwall insulation
- Turn insulation
- Strand insulation
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Stator winding insulation systems in use
Most insulation systems are based on various mixtures and types of epoxy resin and mica paper. This section presents some of the commercially available stator winding insulation systems used nowadays or in recent times.
Conventional form-wound stator winding is practically limited to approx. xx kV. At higher voltages there is an excessive partial discharge activity and need for very long end windings. Despite all the advancements in insulation technology, it has not been possible to make a mica paper and epoxy grounwall completely void free in order to eliminate PD above xx kV.
Conclusion
Electrical insulation plays an important role in design and construction of electric machines. The systems are continuously developed and improved. This blog post presented several aspects of insulation systems including their properties, temperature class, dielectric strength, structure of insulation or commercially available insulation systems developed by leading machine manufacturers. As usual, full text article is available for our premium subscribers.
References
[1] Greg C. Stone, Ian Culbert, Edward A. Boulter, Hussein Dhirani, Electrical Insulation for Rotating Machines: Design, evaluation, aging, testing, and repair, John Wiley & Sons, New Jersey, USA, ISBN 978-1-118-05706-3
[2] ABB Motors and Generators, https://new.abb.com/motors-generators
