What are motor brushes？
1. Motor Brush
Motor brush is used in the commutator or slip ring of the motor, as a sliding contact body to export the introduction of current. Its role is mainly to metal friction while conducting electricity, widely used in a variety of AC and DC generators, synchronous motors, battery DC motors, crane motor collector ring, various types of welding machines and so on.
The brushes used in the early machinery are generally made of multi-strand copper wire. The brushes made of copper wire with high hardness will scratch the smooth commutator piece and groove on it, therefore, it is necessary to treat the commutator surface frequently. And as copper brushes wear, brush dust and debris can wedge between commutator blades, shorting them out and reducing the efficiency of the equipment. Using copper brushes, therefore, is metal-on-metal friction, high loss, and high maintenance costs.
While carbon brushes, generally made of pure carbon plus solidifying agent, lubrication performance and good current collection performance, and carbon brush and metal friction, there is loss is carbon brush, more uniform wear than copper brushes, and soft carbon on the commutator blade damage is much smaller, the generation of sparks is also less, in the process of use, replace the carbon brush can be, metal does not require maintenance, low maintenance costs and convenient.
Therefore, it has developed that modern rotary motors with commutators almost exclusively use carbon brushes, part of which may be mixed with copper powder to improve conductivity. Metal copper brushes are still used in toy or very small motors, as well as on some motors that run only intermittently, such as car starter motors.
Electric motors and generators suffer from a phenomenon called “armature reaction”, one effect of which is to change the ideal position of the current reversal through the winding as the load changes. Early machines had brushes mounted on a ring with a handle. During operation, the position of the brush ring needed to be adjusted to adjust the reversal in order to minimize sparking from the brushes. This process was called “shaking the brush”.
With the development of technology, “high resistance brushes” were invented, i.e. brushes made of a mixture of copper powder and carbon. These brushes can automate the commutation process and minimize sparking from the brushes. Although these brushes are described as high resistance brushes, the exact value of the resistance of these brushes depends on the size and function of the machine. In addition, high resistance brushes are not constructed like brushes, but in the form of carbon blocks with curved surfaces to match the shape of the commutator.
The size of a high-resistance carbon brush is much wider than the insulation segment it spans (and may often span two insulation segments on larger machines). Therefore, when the commutator segment passes under the brushes, the current flowing to it is smoother than if the pure copper brush contact were to break abruptly. Similarly, the section in contact with the brushes has a similar slope rise in current. Thus, although the current through the brushes is more or less constant, the instantaneous current through both commutator pieces is proportional to the relative area in contact with the brushes.
The ratio of copper to carbon can be varied for specific purposes. Brushes with higher copper content perform better at very low voltages and high currents, while brushes with higher carbon content perform better at high voltages and low currents.
Springs are often used with brushes to maintain constant contact with the commutator. As the brushes and commutator wear, the spring steadily pushes the brushes down toward the commutator. Eventually, the brushes wear small enough and thin enough that stable contact is no longer possible or no longer securely held in the brush holder, at which point the brushes need to be replaced.
Flexible power cables are often connected directly to the brushes, but because the current flowing through the support springs causes heat, this can lead to metal tempering and loss of spring tension.
When a commutator motor or generator uses more power than a single brush can conduct, multiple brush holder assemblies are mounted in parallel on the face of the very large commutator. This parallel mounting distributes the current evenly across all brushes, allowing removal of bad brushes and replacement with new ones even if the machine continues to rotate at full power and load.
High power, high current commutation devices are not as common today because alternator designs are less complex, allowing low current, high voltage rotating field coils to power high current fixed position stator coils. This allows the use of very small individual brushes in the alternator design. In this case, the rotating contacts are continuous rings, called slip rings, that do not switch.
Modern devices using carbon brushes are often designed to be maintenance free, requiring no adjustment throughout the life of the device, using a fixed position brush holder slot and a combination brush spring cable assembly that fits into the slot. Pull out worn brushes and insert new brushes.
2. Brush contact angle
The brush contact angle is how the brushes contact the commutator. Different brush types make contact with the commutator in different ways. Since the hardness of copper brushes is the same as the hardness of the commutator, the rotor cannot rotate backwards against the end of the copper brushes without the copper penetrating deep into the commutator and causing serious damage.
Therefore, the strip/stacked copper brushes make tangential contact with the commutator only, while the copper mesh and wire brushes use an angled contact angle to contact their edges through the commutator tabs that can only rotate in one direction. The softness of the carbon brush allows it to make direct radial end contact with the commutator without damaging the fan, allowing easy reversal of rotor direction without reorienting the brush holder to run in the opposite direction.
Although never reversed, common appliance motors using wound rotors, commutators and brushes have radial contact brushes. In the case of reactive brush holders, the carbon brushes may be tilted in the opposite direction from the commutator so that the commutator tends to push against the carbon to achieve solid contact.
3, Commutation plane
The commutation plane is the contact point where the brushes touch the commutator. In order to conduct enough current to or from the commutator, the brush contact area is not a thin line, but a rectangular patch across the section. Typically, the brushes are wide enough to span 2.5 commutator segments. This means that two adjacent segments are electrically connected by the brushes as they contact each other.