Technical requirements for insulation structure of drive motors for new energy vehicles

As we all know, the most feared things for any electrical equipment or machine are leakage and short circuit.

As the saying goes, "laymen watch the excitement, experts watch the doorway." The doorway of insulation structure and system has always been a key topic of continuous discussion and research within major OEMs.

Today we will take a look at the current application status of the main insulation materials used in new energy vehicle drive motors.

01Insulation system

The typical insulation solutions for current drive motors can generally be divided into primary insulation and secondary insulation, as shown in the figure below.

The main insulation is crucial to the safe operation of the motor, including electromagnetic wire insulation, slot insulation, phase insulation, slot wedge insulation, impregnation varnish, etc.

Secondary insulation mainly plays an auxiliary insulation role and provides mechanical support and protection for the coil, including insulating sleeves, binding ropes, busbar insulation and weld point coating.

02Corona resistant enameled wire

As drive motors rotate faster and faster and the power density of electric drive systems increases, the performance and quality requirements of enameled wires are bound to become more and more stringent.

At present, the main corona-resistant enameled wire used in electric vehicle drive motors is nanoparticle-modified H-grade (or above) corona-resistant enameled wire.

The paint film of this enameled wire has evolved from three layers in the early stage to two layers later. However, due to the short life of the three layers and relatively poor adhesion of the paint film, it gradually failed to keep up with the requirements.

In 2000, DuPont developed a two-coat corona-resistant enameled wire, with the base coating being a nanoparticle-modified polyesterimide corona-resistant coating and the top coating being a PAI coating.

Since the launch of the two-coat, it has been widely used in the field of drive motors. In recent years, with the rise of oil-cooled motors, PAI single-coat corona-resistant enameled wire has been increasingly used due to its resistance to ATF oil and high temperature.

As we all know, the choice of materials and technologies are constantly changing according to the technology of the application subject, and no material is a panacea.

With the rapid development of flat wire technology, higher slot fill rate and power density have led more and more OEMs to choose flat wire motors.

Traditional round conductor stator slot diagram

Rectangular conductor stator slot

However, the coating processability of the four "R" corners of corona-resistant enameled flat wire is poor, and pain points such as reduced corona resistance and unstable performance often occur in current applications.

Additionally, some recent commercial applications are using PEEK materials extruded onto magnet wire to replace solvent-based dip coatings.

03Insulation impregnation resin

The insulation treatment of motor stators mainly adopts vacuum impregnation resin (VI) and vacuum pressure impregnation resin (VPI). Generally speaking, the base resin is high-strength and high-heat-resistant modified polyester or polyester imide.

It is worth mentioning that the nanoparticle modification technology can improve the paint hanging efficiency, heat resistance and corona resistance by adding nano inorganic particles.

Speaking of the current situation, in recent years, resins that are suitable for new processes such as winding power heating curing, ultraviolet light curing, and rotary dripping have also come into people's view.

Among them, the electric heating process is a new method with high efficiency. It only takes a few minutes from impregnation to resin gelation, and the whole process can be completed in about an hour. In addition, the amount of paint can be accurately controlled, the filling performance is good, and no resin solidification waste residue is generated.

However, the main equipment for this electric heating process is still controlled by foreign companies in Germany and Italy, and the import price is relatively high. It has not yet been expanded on a large scale in the country.

04Soft composite materials

The current mainstream non-oil-cooled drive motor slot insulation, slot wedges, and phase insulation are mainly composed of two layers of polyaramid fiber paper (such as Nomex) and a layer of PI film to form a soft composite material.

This insulation material has H-class heat resistance and lower cost, so it is widely used. However, its corona resistance and ATF oil resistance are slightly poor.

With the popularity of oil-cooled motors, such soft materials that are not oil-resistant have begun to be unable to cope with the situation and are prone to stratification, thereby reducing the insulation performance.

This has led to a "vicious cycle": in order to improve oil resistance, single-layer thick polyaramid fiber paper with better oil resistance is used in motor development and design;

However, the electrical properties of this material are relatively poor. In order to improve the insulation performance, the material design thickness must be increased. Increasing the thickness will inevitably affect the power density, thereby increasing the manufacturing cost of the entire motor.

This is just an oil-cooled motor. If the currently popular 800V electrical system is added, the above problems will become more and more serious.