In professions such as mining, metallurgy, and cement plants, the collector ring and electric brush of wound rotor asynchronous motors are used to connect the starting resistor and short-circuit the rotor winding after the micro motor is started. The selection of collector rings and brushes is crucial for wound rotor micro motors in order to reduce wear and heat generation during the entire service life of the collector rings and brushes.
1. Collecting ring structure Collecting ring structure is divided into: all plastic die-casting collecting ring (suitable for small and micro motors) according to its application properties; Prefabricated collector ring (medium-sized micro motor); Screw assembled collector ring (for medium and low-speed large micro motors); Hot sleeve collector ring (suitable for high-speed micro motors).
The selection of materials for the 2 collector rings is based on their conductive function, the necessary wear resistance and stable sliding contact characteristics when sliding with electric brushes, as well as the requirements for mechanical strength. Slip rings can be made of brass, bronze, low carbon steel, or medium carbon steel, and the slip rings for high-speed micro motors are made of high-strength alloy steel (such as 35SMin). For corrosive environments, stainless steel slip rings can be used, but the sliding characteristics of stainless steel are unstable and not suitable for combination with electric brushes, which usually cause electric brushes to jump and result in excessive temperature rise or abnormal wear. Therefore, it is advisable not to use this material as much as possible.
The diameter and width of the metal ring on the collector ring are related to factors such as the size of the current passing through it, the number of brushes, the size of the brushes, the heat dissipation area, and the circumferential velocity that the brushes can withstand. The distance between metal rings should be determined based on the insulation distance required by the winding voltage and the space required for components such as brush holders and brushes in the device. When the circumferential velocity of the metal ring surface is 3070m/s, the gas film on the surface of the metal ring will cause the electric brush to float, resulting in poor contact between the electric brush and the metal ring, uneven current distribution, and the appearance of sparks and electric erosion, accelerating the wear of the metal ring and the heating of the electric brush. Machining spiral grooves on the metal ring surface can prevent this situation from occurring. When the current passing through the collector ring exceeds 1000A, the metal ring should be equipped with air vents or fans to enhance its cooling.
There are many types of brush materials to choose from, which can be roughly divided into two categories: carbon brushes and metal brushes. Due to the high current density promised by the metal graphite brush, it can meet the needs of low and medium speed micro motors. For high-speed micro motors (such as 2p micro motors), natural graphite brushes with good sliding and stability characteristics should be selected.
When selecting electric brushes, it is necessary to summarize and consider the skill requirements and characteristics of electric brushes for micro motors. The skill characteristics of electric brushes include touch characteristics and physical and chemical properties. The factors that affect the touch characteristics of electric brushes mainly include circumferential velocity, current density, unit pressure applied to the brush, and surrounding medium conditions. The touch characteristics of electric brushes mainly include touch voltage drop and conflict coefficient.
Touch voltage drop refers to the voltage drop caused by the current passing through the brush, touch point film, and collector ring. The touch voltage drop of each type of electric brush has its limit value. The touch voltage drop of metal graphite brushes is lower than that of graphite brushes and electrochemical graphite brushes. If the limit value of touch voltage drop is exceeded, the electrical loss of sliding touch points will be too large and cause overheating.
For the collector ring, it is more suitable to use brushes with lower touch voltage.
Conflict is an important factor that needs to be considered for the normal operation of electric brushes, and it is one of the causes of heat generation. The composition and function of the thin film on the surface of the collector ring have a significant impact on the conflict situation, and it is also influenced by the raw materials of the electric brush and collector ring, the condition of the touch surface, and the operating conditions. The conflict situation is usually measured by the conflict coefficient of the electric brush. The more brushes there are, the higher the conflict coefficient and circumferential velocity, and the greater the conflict loss. From the perspective of mechanical losses, the demand conflict coefficient for brushes used in high-speed micro motors is the smallest.
Intense conflicts can cause electric brushes to oscillate, emit noise, and cause unstable touch during operation. When severe, it can cause the electric brush to shatter.
The physical and chemical properties of electric brushes mainly include resistivity, hardness, and ash impurities. The application range of electric brushes can be roughly determined based on the size of the resistivity. Electric brushes with high resistivity are suitable for micro motors with difficult commutation; Medium resistivity electric brushes are suitable for general DC micro motors; Electric brushes with low resistivity are suitable for low-voltage micro motors. The hardness and resistivity of electric brushes can be summarized to reflect the general condition of the quality and functional application of electric brushes.
The high resistivity and low hardness indicate that the density of the electric brush is low and it is prone to significant wear. Electric brushes contain a small amount of extremely fine ash, which can improve their wear resistance. However, if the ash impurities contain a small amount of hard abrasive particles, such as iron carbide and silicon carbide (i.e. diamond sand), it will cut grooves on the surface of the collector ring, causing great damage to the micro motor.
Based on the characteristics of the collector ring and the electric brush, our company has conducted matching experiments using different types of electric brushes and collector rings. The results show that selecting the 04523B type electric brush to match with tin bronze or silicon brass slip rings is more suitable. This electric brush is a metal graphite brush made of a mixture of copper powder and graphite powder. It has low electrical resistivity, high linear velocity, and a large allowable current density. It has good sliding characteristics, wear resistance, and other advantages. To achieve the best match between the selected brush and slip ring, the brush needs to be completely ground to ensure that the arc surface of the brush matches the outer surface of the collector ring. The touch area should reach 80. In 2005, our company produced YR560-41000kW micro motor for Puyang Steel Plant. During the user's test run, the micro motor ran for half an hour, and the temperature of the brush and collector ring increased sharply. The brush braid of the electric brush changed color and could not be used. After analysis, it was found that the electric brush used did not meet the planned requirements. The slip ring was made of tin bronze and should be equipped with a 04523B type electric brush. However, the type of electric brush used during on-site practice was HCTD247, a pure graphite electric brush, which could not meet the high current requirements. After replacing it with a 04523B type electric brush, the micro motor operated normally.
5 Conclusion: How to choose the collector ring and brush for communication motors? Currently, there is no mature experience at home and abroad. To achieve the best match between the collector ring and brush, further theoretical research and practical assignments are needed.