Motor Carbon Brush

Simo Electric Motor’s Original Carbon Brushes
The original carbon brushes of Xi’an Taifu Simo Electric Motors offer excellent lubricity, low friction coefficient, and good commutation performance. They are mainly suitable for the slip rings of large turbine generators, exciters, and hydroelectric generators, as well as DC motors with normal commutation.

Overview of Motor Carbon Brushes
A carbon brush is a device that transfers energy or signals between the stationary and rotating parts of a motor, generator, or other rotating machinery. Typically made from pure carbon with a binder, these brushes are generally shaped like small blocks and are fixed into a metal bracket, held against the rotating shaft by a spring. For example, after drilling for oil, instruments need to be lowered into the well for testing, and signals are transmitted from the rotating part (a cable drum wrapped in steel wire) to the surface instruments through the carbon brush. Carbon brushes resemble the erasers on pencils, with a wire protruding from the top. They come in various sizes and are widely used in many electrical devices as sliding contact components. The main materials used in carbon brushes include electrographite, impregnated graphite, and metal (copper, silver) graphite.

Function of Carbon Brushes
In a brushed motor, carbon brushes are pressed against the commutator’s surface. When the motor rotates, the brushes transmit electrical energy through the commutator to the coil. As they are mainly composed of carbon, they are referred to as carbon brushes and are subject to wear. Regular maintenance and replacement are necessary, along with the removal of accumulated carbon deposits.

Types of Carbon Brushes
Carbon brushes are categorized into resin-based, electro-based, and non-ferrous metal types, which are used in different motors depending on the material and manufacturing process. How to choose a high-performance carbon brush?

1. It should quickly form a uniform, appropriate, and stable oxide film on the surface of the commutator or slip ring.
2. The carbon brush should have a long service life without wearing down the commutator or slip ring.
3. It should offer good commutation and current collection performance, suppressing sparks within allowable limits and minimizing energy loss.
4. During operation, the brush should not overheat, should produce minimal noise, and should be reliably assembled without breaking.

Installation of Carbon Brushes

1. Carbon brushes should move freely up and down within the brush holder, with a gap between the brush and the holder’s inner wall of 0.1-0.3 mm to avoid oscillation. The lower edge of the brush holder should be about 2 mm away from the commutator’s surface.
2. The same type of carbon brush should ideally be used on the same motor, but for particularly challenging commutation in large or medium-sized motors, dual-type carbon brushes may be used, with the leading edge being lubricious and the trailing edge having strong spark suppression capabilities to improve performance.
3. The pressure applied to each carbon brush on the same motor should be uniform to prevent uneven current distribution, which could lead to overheating and sparking in individual brushes. The unit pressure should be selected according to the “Carbon Brush Technical Performance Table,” and should be appropriately increased for high-speed motors or those operating under vibration to ensure proper function. For example, the unit pressure for traction motor carbon brushes is 0.4-0.6 kgf/cm².

Inspection of Carbon Brushes

1. Visually, the brush should have proper chamfering, appropriate specifications, standard structure, and meet requirements for wire cross-section and length, without signs of loosening, detachment, damage, broken edges, or clips.
2. From a usage perspective, the signs of good carbon brush performance include:
   • Long service life without wearing down the commutator or slip ring.
   • Good commutation and current collection performance, with minimal spark production and energy loss.
   • Operation without overheating, minimal noise, and no breakage.
   • Ability to quickly form a uniform, appropriate, and stable oxide film on the commutator or slip ring surface.
3. Carbon brushes should be tested using instruments. From a technical standpoint, brushes should meet national standards. Instruments can measure performance parameters such as resistivity, Rockwell hardness, bulk density, current density, contact voltage drop, friction coefficient, 50-hour wear rate, permissible circumferential speed, unit pressure, and even metal content.
4. The same model and manufacturer’s carbon brushes should be used on the same motor. Brushes from different manufacturers or even different production batches can vary significantly in performance. To prevent unbalanced current distribution among parallel brushes that could affect motor operation, it is best to use carbon brushes of the same model, manufacturer, and production batch.
5. For motors with particularly challenging commutation, split braid carbon brushes may be used to improve commutation performance by increasing the internal short-circuit current within the brush. For motors with difficult unidirectional commutation, combination brushes may be used, with the leading edge featuring low-resistance graphite-based electrographite brushes for good lubricity and film-forming ability, and the trailing edge featuring high-resistance carbon black-based electrographite brushes for good commutation performance. Carbon fiber composite brushes may also be used on the trailing edge for their strong arc-extinguishing capabilities, achieving optimal motor operation.

Replacement of Carbon Brushes

1. When carbon brushes are worn to a certain extent, they should be replaced. It is best to replace all brushes at once; mixing old and new brushes can lead to uneven current distribution. For large units, shutting down to replace brushes can affect production, so it’s often recommended to replace 20% of the brushes at a time (i.e., 20% of each brush holder per motor), with intervals of 1-2 weeks, gradually replacing the remaining brushes to ensure continuous operation.
2. To ensure good contact between the carbon brushes and the commutator, new brushes should be arced on the motor. Fine glass sandpaper is placed between the brush and the commutator, and the brush is ground in the direction of motor rotation under normal spring pressure until the arc surface of the brush matches the commutator. The sandpaper should adhere closely to the commutator, and once the brush is arced, the sandpaper should be removed, dust blown off with compressed air, and the surface cleaned with a soft cloth. Diamond sandpaper should not be used to avoid embedding particles in the commutator slots, which could damage the brushes and commutator surface during motor operation. After arcing, the motor should run at 20-30% load for several hours to allow the brushes and commutator to bed in and form a uniform oxide film before gradually increasing the current to the rated load.

Features and Characteristics
The main function of carbon brushes is to conduct electricity while reducing friction with metal parts. Unlike metal-to-metal contact, where friction can increase and junctions may fuse together, carbon brushes do not cause these issues due to the different elemental nature of carbon and metal. Carbon brushes are mainly used in motors and come in various shapes, such as square or round. They are suitable for various types of motors, generators, and axle machines, providing good commutation performance and long service life. Carbon brushes are used on the commutator or slip rings of motors as sliding contacts for current conduction. They have good electrical conductivity, thermal conductivity, and lubrication properties, as well as sufficient mechanical strength and inherent spark suppression ability. Almost all motors use carbon brushes, which are essential components widely applied in various AC and DC generators, synchronous motors, DC battery motors, crane motor slip rings, and various welding machines.

Working Principle of Carbon Brushes

1. Transfer external current (excitation current) to the rotating rotor via the carbon brush (input current).
2. Discharge static electricity from the main shaft to the ground through the carbon brush (grounding brush) (output current).
3. Connect the main shaft (ground) to the protection device for rotor grounding protection and measurement of rotor positive and negative ground voltages.
4. Change the direction of current (in commutator motors, the carbon brush also serves as a commutator).

Apart from induction AC asynchronous motors, all other motors use carbon brushes wherever the rotor has a commutator ring. The principle of electricity generation is based on cutting the magnetic field lines with wires to induce current in the wire. Generators use a rotating magnetic field (the rotor) to cut the wires (the stator). To generate a magnetic field in the rotor, excitation current must be supplied to the rotor coil, and the carbon brush is used to deliver the excitation current from the exciter generator to the rotor coil.