A carbon brush of dc motor

A carbon brush, also called a brush, is widely used as a sliding contact in many DC motors. Carbon brushes in the product application materials are mainly graphite, soaked graphite, metal (including copper, silver) graphite. A carbon brush is a device for transmitting energy or signals between a stationary part and a rotating part of an electric motor or a generator or other rotating machine. It is generally made of pure carbon plus a coagulating agent. The outer shape is generally a block and is stuck on a metal support. A spring presses it tightly on the shaft. When the motor rotates, electric energy is sent to the coil through the commutator. Since the main component is carbon, called a carbon brush, it is easy to wear. Maintenance should be regularly maintained and cleaned up.

Category 1

▪ Metal graphite

▪ Natural graphite

▪ Electrochemical graphite

Metal graphite

The main materials of this type of carbon brush are electrolytic copper and graphite. According to the needs of the use of silver powder (very expensive for precision instruments), aluminum powder, lead powder, and other metals, these carbon brushes also contain binders and binders. Such carbon brushes have both the frictional properties of graphite and the high conductivity of metals, and are therefore suitable for low-voltage motors (such as automotive starter motors) that do not require high loads and commutation. Its peripheral speed does not exceed 30 m/s.

Natural graphite

Natural graphite is the main raw material for such carbon brushes. The binder is made of asphalt or resin, baked or sintered at 1000 degrees. This kind of motor has good lubrication performance and current collecting performance. Most of them are used for smooth running of small and medium-sized DC motors and high-speed turbine generator collector rings.

Electrochemical graphite

The main components are carbon black, coke, graphite, and other carbon powder materials, which are transformed into microcrystalline artificial graphite after high temperature treatment at 2500 degrees. Such carbon brushes have excellent commutation and self-lubricating properties, and are widely used in various types of AC and DC motors to not only have a long life but also have little wear on the commutator. [1] 

From the point of view of use, the signs of good brush performance mainly include the following situations:

The good performance of carbon brushes should be:

1) The brush has a long service life and does not abrade the commutator or slip ring.

2) When the carbon brush is running, it is hot, with low noise, reliable assembly and no damage.

3, carbon brush wear to a certain extent to replace the new carbon brush, carbon brush is best to replace all at once, if the old and new mixed, there may be uneven current distribution. For large units, stopping the replacement of the carbon brush will inevitably affect the production. We can choose not to stop the machine. We usually recommend that the customer's practice is to replace 20% of the carbon brushes (ie, 20% of each brush rod of each motor) each time. The interval time is 1-2 weeks, and the rest of the carbon brushes are gradually replaced after being worn to ensure the normal continuous operation of the unit.

4, in the same motor, in principle, should use the same type of carbon brush, but for individual reversing is particularly difficult for large and medium-sized motor, can use twin-carbon brush, the sliding edge of the use of good lubrication performance, sliding out the use of suppression A carbon brush with a high sparking ability, so that the operation of the carbon brush is improved.

5. The unit pressure exerted on each carbon brush of the same motor should be uniform in order to avoid uneven current distribution, resulting in overheating and sparking of individual carbon brushes. The unit pressure of the carbon brush should be selected according to the “carbon brush technical performance table”. For the motor with higher rotation speed or the motor working under the vibration condition, the unit pressure should be appropriately increased to ensure normal operation. For example, the unit pressure of the carbon brush of the tractor motor is 0.4-0.6kgf/cm2.

6, in order to make the carbon brush and the commutator good contact, the new carbon brush should be carried out by the degree of curvature, the arc is usually carried out on the DC motor. Place a fine glass sandpaper between the carbon brushes and the commutator. Grind the carbon brushes in the direction of rotation of the motor under normal spring pressure. The sandpaper should be adhered to the commutator as far as possible until the arcs of the carbon brushes coincide and then removed. Sand paper, blow dust with compressed air, then wipe clean with a soft cloth. Grinding carbon brushes should not use emery paper, to prevent emery particles embedded in the commutator slot, in the motor running, scratch the surface of the brush and commutator. After grinding, the motor first runs 20-30% at the load for several hours, allowing the brush and commutator to run in and establish a uniform oxide film. Then gradually increase the current to the rated load.

Usually the unit pressure of the carbon brush is too high, it is the abrasion of the carbon brush to aggravate, the unit pressure is too small, the contact is unstable, it is easy to appear the mechanical spark.

The choice of the carbon brush model plays a key role in the stability of the entire motor. [3]

Troubleshooting edit

1. In order to ensure the normal operation of the DC motor, it is very important to choose the correct brush type, because it is selected when the brush is manufactured.         from: www.abettermotor.com