Of the electric coal products, the most widely used are electro-brushes, which are most often called simply brushes.
Currently used electric carbon brushes are divided into four main groups:
- graphite,
- carbon-graphite,
- metal-graphite,
- electrographed.
Graphite brushes (grades 611M, G1, G3, high-resistance, etc.) are made from natural graphite without use (soft grades) and with the use of binders. The graphite brushes obtained without a binder are not sintered (fired) after pressing.
Carbon brushes are soft and cause little noise during operation. They can be used at circumferential speeds of the collector or rings from 12 to 40 m / s, and natural graphite up to 70 m / s. The specific electrical resistance of granite brushes ranges from 10 to 45 ohm · mm2 / m, and for high-resistance ones from 200 to 1000 ohm · mm2 / m.
High resistance graphite brushes are used in high voltage machines. The rest of the grades of graphite brushes are mainly used in DC machines of small and medium power. Natural-graphite brushes are also used in high-speed turbine generators with steel rings.
The permissible current density for all graphite brushes (except for high-resistance ones) is 7 ÷ 12 A / cm2, for high-resistance 4 ÷ 5 and / cm2. Specific pressure: 200 - 250 g / cm2.
Carbon-granite brushes (grades T2, T6, etc.) are made of graphite with the introduction of other carbonaceous materials (soot, coke) and binders (resins, pitches). After pressing the initial mixture, brushes or their blocks are sintered at a temperature of 1000 - 1200 ° C. The resulting brushes are copper plated in an electrolytic bath.
Carbon-graphite brushes have increased hardness and mechanical strength. They have some abrasiveness, that is, they can themselves clean oxide films on collectors and rings prone to contamination (traction motors, etc.). These brushes can be used at circumferential speeds of the collector or rings from 10 to 12 m / s. The specific electrical resistance of the brushes is 40 ÷ 60 ohm · mm2 / m, and the permissible current density is 6 - 8 a / cm2. Specific pressure 200 - 250 g / cm2.
Carbon-graphite brushes of medium hardness are used in generators and electric motors of small and medium power. Brushes with increased hardness are used in electric machines with jolting load.
Metal-graphite brushes (grades M-1, M-Z, M-6, MG, M20, MGS5, etc.) are made of graphite and copper powders. In some of them, powders of lead (MGS5), tin and silver are also added. In brushes with a high permissible current density, the copper content reaches 80 - 90%. This achieves a reduction in the resistivity of the brushes and a low voltage drop.
This group of brushes has a low specific resistance of 1 ÷ 6 ohm mm2 / m and even 0.03 ÷ 0.25 ohm mm2 / m. In brushes of this group with a low copper content (less than 50%), the specific electrical resistance reaches values of 6 ÷ 12 ohm · mm2 / m (M-3). The permissible current density for brushes of this group is in the range from 12 to 20 A / cm2. Specific pressure: 180 ÷ 230g / cm2.
Metal-graphite brushes are used at peripheral speeds from 20 to 35 m / s in automobile and aircraft generators and electric motors, as well as in synchronous machines and in traction machines with reduced voltage (mine electric locomotives).
Electrographite brushes (brands EG-2a, EG4, EG8, EG14, etc.) are made from powders of graphite and other carbonaceous materials (coke, soot) with the addition of binders. After pressing and firing (sintering), the products go to electric furnaces for graphitization. The graphitization process takes place at a temperature of 2500-2600 ° C. This converts the carbon of the starting materials into graphite.
Electrographite brushes tolerate jerky load changes well and can operate at high speeds of 40 ÷ 50 m / s.
The specific electrical resistance of the brushes of this group is 9 ÷ 70 ohm · mm2 / m. They are applied:
- in electric machines of medium and high power;
- in machines with varying load and severe conditions of current switching;
- in traction motors;
- in high-speed electric machines with steel rings (EG-74).
The group of electrographitized brushes has the largest field of application in electrical engineering.
"Electrical Materials Science" N. Drozdov
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