As motors evolve towards higher energy efficiency, brushed motors are transitioning to brushless motors. The difference between brushless and brushed motors lies in the fact that brushless motors lack brushes and commutators. Brushes are a distinctive feature of brushed motors, but also a source of frequent criticism.
Motor brushes are sliding contacts used on the commutator or slip ring of a motor, located between the stationary and rotating parts of a motor or generator, to conduct and guide current. They are the core component of a brushed motor.
Types and advantages/disadvantages of motor brushes
Throughout the long history of brushed motor development, brushes have undergone numerous iterations. After long-term market and technological validation, most brushed motors now use carbon brushes, which are made primarily of graphite products and mixed with different proportions of metal powder to improve performance.
Early brushed motors used brushes made of copper wire. These brushes were very hard, and long-term use would cause severe wear on the brushes and commutator, eventually reducing the motor's efficiency and even causing other problems such as sparking. Moreover, this wear process was rapid, and the maintenance cost was not low, so pure metal brushes were phased out.
While carbon brushes do wear, the wear only occurs on the brushes themselves, and the degree of wear is relatively lower, so it won't damage the commutator. Furthermore, modern motor designs typically use fixed-position brush holder slots and compatible brush spring cable assemblies, making it easy to replace worn brushes.
Currently, carbon brushes are mainly classified into three types: natural graphite carbon brushes, electrochemical graphite carbon brushes, and metallic graphite carbon brushes. Natural graphite brushes have relatively low hardness but good lubrication and excellent current collection performance. They are mostly used in small and medium-sized DC motors with stable operation and moderate speed, and some can be used in the slip rings of high-speed steam turbine generators, with the S3 and S6 series being the most common.
Electrochemical graphite brushes have performance closer to that of natural graphite brushes, but with a slightly higher hardness. They also possess excellent commutation and self-lubricating properties. Generally, these brushes have a high resistivity and voltage drop, resulting in excellent wear performance and minimal wear on the commutator during use.
Due to their excellent commutation performance, electrochemical graphite brushes, represented by D374 and D479, are widely used in high-speed DC motors where commutation is difficult.
Metal graphite carbon brushes differ from the two types of brushes mentioned above. Their metallic properties are more prominent, resulting in better conductivity. Although they retain some of the frictional properties of graphite, their wear resistance is inferior to the two types of graphite brushes mentioned above. Of course, the reason for choosing metal graphite carbon brushes is their conductivity.
The difference in copper content among metal-graphite carbon brushes determines their resistivity and allowable current density. Metal-graphite carbon brushes with high copper content, such as J102, have extremely low resistivity and allow very high current density, which means that these brushes have strong overload capacity.
In general, metal graphite carbon brushes are suitable for low-voltage, high-current motors with high loads and low commutation requirements.
Considerations for brushed motors
Compared to brushless motors, brushed motors have a more mature manufacturing process, simpler structure, and are easier to control. In particular, they are also cheaper, so brushed motors are still chosen in many applications. Another important reason is that brushed motors have a large starting torque, allowing them to start quickly and reach rated speed, making them well-suited for applications that require generating large torque in a short time.
For these scenarios, the choice of brushes is crucial for the motor. Modern motors require brushes with low noise and no risk of sparking. Especially in high-temperature environments, excessive voltage drop at the commutation surface can easily generate sparks. In such cases, hard brushes or specially designed abrasive brushes should be used to reduce this risk.
Furthermore, there are requirements for motor wear. Although wear is unavoidable, the brushes should be used for as long as possible to minimize wear on the commutator or slip rings.
Furthermore, while it's impossible to achieve very high energy efficiency with brushed motors, current applications require that the electrical power loss and mechanical loss of the brushes be minimized, as motors are evolving towards higher energy efficiency.
summary
As motors evolve towards higher energy efficiency, many brushed motor applications are rapidly transitioning to brushless motors. For example, electric wrenches have largely become brushless, and applications in electric drills, high-voltage systems, and garden motors are also undergoing this transition. While moving towards brushless motors, brushed motors still retain some inherent advantages in certain applications, where the choice of brushes is crucial.
