At a time when motors are moving toward high energy efficiency, brushed motors are shifting to brushless motors. The difference between brushless and brushed motors is that brushless motors do not have brushes or commutators. Brushes, a highly characteristic and often criticized device of brushed motors.
Motor brushes are used between the fixed and rotating parts of a rotating machine such as a motor or generator, and are used in the commutator or slip ring of the motor as a sliding contact for exporting and importing electric current, and they are the core component of a brush motor.
Types of motor brushes and their advantages and disadvantages
In the long history of brush motor development, brushes have gone through a long iteration, in the long-term market verification and technical verification, the vast majority of brush motors are now used in carbon brushes, graphite products as the main material, which is mixed with different ratios of metal powder to improve performance.
Early brush motors, will be made of copper wire brushes, this brush hardness, long-term use will produce serious wear on the brush and commutator, over time will make the motor efficiency is reduced, and even produce sparks and other hidden problems. And this loss process is very fast, the cost of maintenance is not low, pure metal brushes are therefore eliminated.
Although carbon brushes also wear out, the wear only occurs on the brushes, and the degree of wear is relatively low, and will not damage the commutator. Moreover, today's motor designs often use fixed-position brush holder slots and combined brush spring cable assemblies that fit into the slots, and worn brushes can be easily replaced.
The main types of carbon brushes available today are natural graphite carbon brushes, electrochemical graphite carbon brushes, and metallic graphite carbon brushes. Natural graphite brushes are low in hardness, but have better lubrication and excellent current collection performance. Most of them are used in small and medium-sized DC motors with smooth operation and moderate speed, and some of them can be used in the collector ring of high-speed turbine generators, mostly in S3 and S6 series.
Electrochemical graphite brushes are closer to natural graphite brushes in terms of performance and relatively higher hardness than natural graphite brushes, with excellent commutativity and self-lubricating performance. Generally, the brush resistance coefficient and brush voltage drop are large, and the wear performance is excellent in use, and the wear on the commutator is very small.
Because of its extremely excellent commutation performance, the electrochemical graphite brushes represented by D374 and D479 have a large number of applications in high-speed DC motors with difficult commutation.
Metal graphite carbon brushes, different from the above two brushes, its metal characteristics are more prominent, so better performance in electrical conductivity, although taking into account part of the friction characteristics of graphite, but not as good as the above two types of graphite brushes in wear resistance, of course, the choice of metal graphite carbon brushes is naturally for its electrical conductivity characteristics.
Metal graphite carbon brushes in the copper content of the difference, determines the coefficient of resistance and the size of the permissible current density. The high metal content of J102 and other metal graphite carbon brushes have a very small resistance coefficient and allow a very high current density to pass, which means that these brushes have a high overload capacity.
Overall, metal graphite carbon brushes are suitable for low voltage, high current motors with high loads and low commutation requirements.
Brushed motor considerations for brushes
Compared with brushless motors, brushed motors are relatively mature in manufacturing process, simple in structure, and relatively easier to realize in control, especially it is also lower in cost, and brushed motors are still chosen for many scenarios. Another important reason is that brushed motors have a large starting torque during startup, which can quickly start and reach the rated speed, which is very suitable for the application scenario of generating a large torque in a short period of time.
For this part of the scenario, the choice of brushes is very important for the motor. Nowadays, the first requirement of the motor is that the brushes should have low noise and should not have the potential danger of sparking. Especially in some high temperature scenarios, the contact voltage drop on the commutated surface is too large and sparks are easily generated. In this case, hard brushes or special abrasive brushes should be used to reduce the hidden danger.
Further, it is the motor's requirement for wear and tear. Although wear and tear is unavoidable, it is important to use brushes for as long as possible, with little wear and tear on the commutator or collector ring.
In addition, although it is not possible to make brush motors to achieve high energy efficiency, the current application also needs brush electrical power loss and mechanical loss as small as possible, after all, the motor to the development of high energy efficiency.
Summary
In the motor to the development of high energy efficiency, many brush motor applications are accelerating to the brushless motor transformation, such as electric wrench applications have basically realized the brushless, electric drills, high pressure and garden motor applications are also accelerating the conversion process. In the development of brushless now, brushed motors in some applications still retain some of its inherent advantages, these applications on the choice of brushes is crucial.
