A Brief Analysis of Powder Coating for Automotive Micro-Motor Rotors

Authors: Zhou Baowei, Zheng Taisan, Guangdong Provincial Institute of Mechanical Research

Abstract: Although powder electrostatic coating is a traditional technology with an early origin, its large-scale application in the field of rotor insulation for small automotive motors in China is still in its infancy. Currently, most foreign-invested small motor manufacturers in China have already adopted this technology. However, due to rising costs, only recently have some of China’s more established and larger automotive motor manufacturers begun gradually adopting this technology. Meanwhile, domestic specialized equipment manufacturers have started aligning their production technologies with German coating standards and have formulated corresponding enterprise standards of their own.

 

Keywords: rotor, insulation, powder electrostatic coating, CMC, FUSA

Keywords: rotor, insulation, electrostatic powder coating, CMK, FUSA

0. Introduction

In recent years, China’s automotive manufacturing industry has experienced rapid development. The number of vehicles in circulation has reached 130 million, meaning that, on average, one car is owned by every 10 people. According to the standard of a moderately prosperous life—where each household owns one passenger car—China’s current vehicle stock still needs to more than double, reaching at least 400 million vehicles. It is foreseeable that, as China continues to advance urbanization and the trend toward urbanization of the population becomes increasingly pronounced, coupled with the ongoing construction of new rural areas, demand for automobiles in both urban and rural regions will continue to grow. Moreover, with the rapid development of industries such as household appliances, motorcycle electrical systems, power tools, electric toys, and automotive electrical equipment, society’s demand for micro and special motors will keep rising, with ever-higher quality requirements and increasingly broad applications. In the production process of micro and special motors, insulating the end faces and slot openings of the motor rotor is an extremely critical task. As a result, insulation technologies have become increasingly mature and reliable. Major domestic and international manufacturers of small automotive motors—including Germany’s Bosch Automotive Components, Shanghai Bosh Auto Components Co., Ltd., Shanghai Valeo Automotive Electrical Systems Co., Ltd., and Tianjin Asmo Automotive Micro-Motor Co., Ltd.—have all adopted the new technology of epoxy powder coating, significantly enhancing the reliability and service life of small motor insulation.

1. Factors Affecting the Service Life of Motors

As is well known, the temperature rise (thermal aging) of electric motors is one of the major factors leading to a decline in motor insulation performance (aging of insulating materials) and loosening of insulation components. Therefore, when selecting an insulation structure, it is essential to consider not only the insulation properties but also the heat resistance and thermal conductivity of the insulation materials. Take electromagnetic wire as an example: as the temperature rises, the outer insulation layer softens, causing it to lose its shear strength. If, under high temperatures, this insulation is subjected to compression by other objects, it may undergo plastic deformation or even expose the conductor under external forces, ultimately resulting in a short circuit. Moreover, when the temperature remains above the insulation material’s rated heat resistance for an extended period, the insulation will degrade further, leading to excessive deterioration. This issue is particularly critical for miniature motors used in automobiles—such as window-regulator motors, power-steering motors, windshield-wiper motors, and seat-adjustment motors—where ventilation and heat dissipation conditions are relatively poor.

There are many reasons for motor heating. During normal operation, motors experience iron losses, copper losses, and mechanical losses, all of which eventually convert into heat energy and dissipate. The main sources of heat generation in motor insulation structures include: heat generated by current flowing through the windings; heat caused by dielectric losses; heat resulting from eddy-current losses and hysteresis losses induced by electromagnetic induction; heat generated by friction, vibration, noise, and poor contact due to deviations arising during mechanical assembly; and elevated base temperatures caused by inadequate ventilation. All these heat-generating factors can shorten the motor's service life and increase maintenance costs—especially in motors installed in specific locations within vehicles. Therefore, selecting insulation materials with excellent thermal resistance and employing advanced manufacturing processes is one effective approach to addressing these issues.

2. Insulating Materials and Processes

In the manufacturing of all types of electric motors, the selection of insulation materials, insulation structures, and insulation processes used in motor design not only affects the choice of electrical parameters, external dimensions, and overall structural layout of the motor, but also determines the reliability and service life of the motor during operation. The technical performance indicators of the insulation structure largely reflect the design and manufacturing standards of the motor. With advances in science and technology and increasing demands from operating environments, higher requirements are being placed on the reliability of motor insulation. Therefore, it is essential to develop and apply new insulation materials, adopt more rational insulation structures, implement advanced manufacturing processes, and employ scientifically sound insulation testing methods, so as to meet, to the greatest extent possible, the long-term requirements of motor electrical insulation systems under conditions of electrical stress, high temperatures, mechanical stresses, and various harsh and demanding operating conditions.

Due to their low operating voltage and harsh usage environments, micro-special motors used in automobiles are often employed in sealed, non-ventilated spaces. To reduce costs, manufacturers typically design these motors with lower power ratings—such as window-regulator motors and seat-motor systems, which operate on short-duty cycles. The power required to drive the load is often several times greater than the motor’s rated output power. This leads to excessively high instantaneous motor currents and rapid temperature rises within the motor housing, triggering a host of associated problems. Therefore, to extend service life and ensure quality and reliability, it is essential to adopt new manufacturing processes for both insulation materials and insulation techniques.

Thanks to the continuous efforts of researchers, insulation materials for small motors have undergone tremendous changes. The introduction of new materials has brought about a revolutionary transformation in the production processes of small motors. Today, as electrostatic coating technology becomes increasingly sophisticated, mainstream micro-motor manufacturers have begun to adopt epoxy powder on a large scale as an insulating material for motor rotors—products manufactured by companies such as AkzoNobel, 3M of the U.S., and Sumitomo Chemical. The advantages of this insulation material include stable dielectric properties, high-temperature resistance, ease of use, and the ability to cure all insulating surfaces of the rotor in one go through a special process.

3. Cost and Pros & Cons of Slotting Paper Process

Currently, many manufacturers of micro and special motors still use insulating paper as the insulation material for rotor winding slots. This material and its manufacturing process have been in use for quite a long time. In China, numerous small- and medium-sized motor manufacturers with relatively low production volumes continue to rely on this method. However, globally renowned companies such as Bosch Automotive Components, Shanghai Valeo, and Tianjin Asmo have gradually switched to powder-coating technology. The primary reason why small- and medium-sized motor manufacturers have not yet adopted the powder-coating process is that the material cost of the slot-paper method is lower, and the associated machinery is also comparatively inexpensive. Nevertheless, this insulation approach has several significant drawbacks: 1. Low slot-filling rate: This results in the motor’s actual output power falling short of its design specifications, and under heavy loads, the motor’s temperature rise can become excessively high. 2. High rate of defective products after winding: During the winding process, the deformation of the slot paper when it’s inserted into the slot can cause the electromagnetic wire to get wrapped around the outside of the insulation paper, rendering the entire rotor unusable. This type of defect is hidden—its true cost far exceeds several times the value of a single rotor before winding. 3. Lack of insulation protection on the rotor end faces, which reduces the insulation strength between the electromagnetic wire and the rotor body, creating potential insulation hazards in the motor. 4. Easy loosening of the windings.

4. Powder Coating Process for Motor Rotors

The new process of applying insulating powder to the rotor core slots of micro-motors for automotive applications has been developing rapidly due to its advantages such as large production volumes, high efficiency, and stable process performance. Powder electrostatic coating produces a pinhole-free finish with excellent edge coverage. The coating surface is smooth, the powder adhesion is strong, and the material exhibits good toughness. Its mechanical and electrical properties are comparable to those of impregnation varnish. The coating thickness can reach up to 0.381 millimeters—significantly thicker than coatings obtained by impregnation—and thus represents a highly desirable insulating material.

Some world-class enterprises, both domestically and internationally, are fully adopting this technology. Currently, several small-scale enterprises in China are also beginning to experiment with it. Powder electrostatic coating overcomes several drawbacks of the slot-paper process, such as the lack of insulation protection on the rotor end faces, the tendency for windings to become loose, low slot-filling rates, and the easy detachment of windings from the slot paper. Under the same rotor structure, an improved slot-filling rate can effectively boost the motor’s output power. Moreover, the integrated and one-step molding of rotor insulation eliminates the need for separate insulation treatment of the rotor end faces, as required under the conventional slot-paper process. This approach also significantly reduces the occurrence of defective products resulting from rotor winding operations. Furthermore, leveraging the inherent properties of the powder coating allows for a hot-fit assembly process of the rotor commutator. However, powder electrostatic coating does have certain limitations: electrostatic coating equipment is more expensive than the equipment used in the slot-paper process; it demands stringent environmental conditions, requiring a well-ventilated system. Additionally, some domestic manufacturers of electrostatic coating equipment suffer from poor sealing, unstable performance, and low powder utilization rates—factors that currently hinder the widespread adoption of electrostatic coating technology.

In the early stages, domestic micro-motor manufacturers used two types of powder electrostatic coating equipment. One type was low-end domestically produced equipment that dominated the market among small-scale motor manufacturers in China. This equipment featured outdated technology, unstable rotor coating quality, low powder utilization rates, and high labor intensity for workers. It could not be integrated into assembly lines, yet its initial cost was relatively low. The other type consisted mainly of imported equipment from overseas, widely adopted by several large domestic motor manufacturers. This equipment boasts advanced technology, a high degree of automation in assembly-line operations, stable performance, and excellent sealing capabilities. Its powder utilization rate exceeds 90%. However, the equipment is extremely expensive, and many domestic motor manufacturers, constrained by price considerations, have been unable to adopt it. This, in turn, has limited the development of powder coating technology in China.

To address this issue, Bosch Automotive Components Changsha Company sought to localize the equipment. After jointly developing the equipment with domestic manufacturers, they decided to import core components from abroad while producing other components domestically as substitutes. In the technical design, the equipment was divided into three operational zones: a coating zone, a cleaning zone, and a curing zone.

The coating zone (see Figure 1) is the process area where powder is applied to the workpiece. It consists of a fluidized bed, an electrostatic generator, and electrodes. The electrodes are made up of copper plates and discharge needles, with the electrodes connected to the cathode of the electrostatic generator. When the electrostatic generator is operating, the cathode generates a high static voltage ranging from 55 kV to 90 kV. At this point, a corona discharge phenomenon occurs in the air surrounding the discharge needles. Compressed air, which has been dried beforehand, is gradually introduced into the coating zone from the bottom of the fluidized bed and continuously flows past the discharge needles, thereby acquiring an electric charge. This charged air passes through a porous,透气 fluidizing plate located within the fluidized bed and enters the fluidization trough. As the charged gas comes into contact with the epoxy resin powder inside the fluidization trough, it transfers its electric charge to the powder, simultaneously lifting the powder and creating a boiling cloud of suspended particles. Meanwhile, the workpiece rotor is connected to the anode of the electrostatic generator, and the anode is also grounded. Under the influence of the electrostatic field, the charged epoxy resin powder moves upward toward the motor rotor and is adsorbed onto its surface. The electric charge carried by the powder is then returned to the electrostatic generator via the anode, thus completing the powder-coating process on the fluidized bed.

The cleaning area is equipped with a powder-scraping mechanism and a powder-blowing mechanism. As the lead screw rotates and drives the workpiece rotor into this area, the workpiece is lifted and rotated by the conveying mechanism. The inclined scraper of the powder-scraping mechanism removes any excess powder from surfaces of the workpiece’s outer diameter that do not require coating. Any residual powder remaining on the protective shaft sleeve is blown off by the air brush of the upper powder-blowing mechanism using compressed air, thereby completing the cleaning process for the workpiece rotor.

The curing zone uses high-frequency induction heating to solidify the powder coating on the rotor workpiece. High-frequency heating leverages the skin-effect thermal phenomenon, generating eddy currents on the surface of the workpiece and thereby rapidly heating its surface. The powder adsorbed onto the surface is continuously melted, leveled, and gelled, eventually solidifying to form a uniform, hardened insulating coating.

Rotor of the workpiece coated by the coating machine:

Automotive door and window motor rotor (outer diameter 24 mm), Automotive seat motor rotor (outer diameter 28 mm).

Automotive wiper motor rotor (outer diameter 40mm), automotive power steering motor rotor (outer diameter 54mm)

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After two years of extensive trial production and addressing a series of challenges—including ensuring the uniformity and stability of electrostatic powder application and enhancing the wear resistance of the rotor surface powder-scraping mechanism—this type of equipment now boasts a shorter production cycle, significantly lower prices, and consistently stable quality. Moreover, its utilization rate for epoxy powder exceeds that of comparable foreign equipment. Testing has shown that the equipment’s long-term capability index Cmk is ≥1.67, and the first-pass yield fusa is ≥98%. Currently, 80% of Bosch Automotive Components’ Changsha facility’s electrostatic coating equipment is supplied by domestic manufacturers. Ultimately, Bosch Automotive Components plans to replace all imported equipment in its domestic rotor production lines with domestically produced machinery and is preparing to export these systems to factories in Europe and the U.S.

5. Closing remarks:

The epoxy powder electrostatic coating process, utilizing a fluidized bed thermal melting technique, has been employed to provide electrostatic insulation for automotive micro-motor rotors and other electrical components in household appliances, replacing the conventional polyethylene film-based composite insulation process. This new method allows for a reduction in insulation layer thickness and increases the slot fill factor by approximately 10%, thereby creating favorable conditions for mass production of coated components. Moreover, this technology can also be applied to power electronic devices as well as chemical pipelines, containers, and valves. As technology continues to advance and the variety of powder coatings keeps expanding, coupled with ongoing improvements in fluidized bed thermal melting equipment and the integration of computerized intelligent control systems, this process will play an even greater role in industrial automation production lines.