The brush is usually called carbon brush in electric tools, which is a part of the single-phase universal motor. In the single-phase universal motor, in addition to connecting the armature and the external circuit, it also plays the role of current commutation. The brush and the commutator constitute the weak and important link of the single-phase universal motor. There is not only mechanical wear and mechanical vibration between the brushes and the commutator, but also serious sparks will be generated when used improperly, which will greatly reduce the life of the commutator and affect the normal operation of the motor. Therefore, reasonable selection of brush material, size and spring pressure will play a very important role in improving the commutation performance of the single-phase universal motor and prolonging its service life.

The selection of brushes is mainly determined by the temperature rise of the brushes and the circumferential speed of the commutator. The temperature rise of the brush is related to the density of the brush and the contact pressure drop of the commutator, the mechanical loss and the thermal conductivity of the brush. If the peripheral linear speed is too high, the brushes and commutators will be heated, the sparks will increase, and the wear of the brushes and commutators will be aggravated.

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

1）Long life and no wear on the commutator, no scratches, unevenness, burning, wire drawing, etc.

2）It has good commutation performance, so that the spark is suppressed within the allowable range, and the energy loss is small;

3）When the brush is running, it is not overheated, has low noise, is no damaged, no changed color, not burn;

4）During operation, a uniform, moderate and stable oxide film can be quickly formed on the surface of the commutator.

1. The performance of the brush

（1）Classification

     Brushes are generally classified according to their blank material composition and different processing methods

   1) Carbon-graphite brushes

Natural graphite brush: This type of brush has high contact voltage drop, good rectification performance, lower current collecting performance than electrochemical graphite brush, good lubrication performance,

It is used for synchronous machine slip rings and AC winding motors with high linear speed.

Resin-bonded graphite brush: This type of brush is characterized by high resistance, high contact voltage drop, good commutation performance, anti-oxidation and anti-wear performance.

It is ideal, but the power consumption is large, and it is mostly used in AC commutator motors.

   2) Electrochemical graphite brushes

Graphite-based brush (soft brush): It is characterized by low friction coefficient, good lubricating performance, current collecting performance, thermal stability and anti-oxidation performance is Good used for large synchronous generators with high linear speed and rolling electric motors with large instantaneous impact loads machines, and small and medium-sized DC motors.

Coke-based brushes (medium-hard brushes): It is characterized by large contact voltage drop, good ability to form oxide films, good commutation performance, and certain current collecting performance. It is used for rolling steel motors with large impact loads, etc. And general DC motors with voltages higher than 220V.

Carbon ink-based brushes (hard brushes): These brushes belong to the high-resistance brushes of the electrochemical graphite brushes. It is characterized by large contact resistance of the brush and good commutation performance. It is used for DC motors with difficult commutation, but the structure is loose and the friction coefficient is large. This kind of brush should preferably be soaked before leaving the factory.

   3）Metal graphite brushes

It is composed of metal and graphite, and uses the characteristics of good electrical conductivity and good lubricity of graphite to adjust the content of metal and graphite to change the performance of the brush. It is characterized by low contact voltage drop, resistivity coefficient and electrical loss. This brush is mainly used for low-voltage high-current motors, low-voltage AC winding motors and automotive motors.

In advance, natural graphite brushes and electrochemical graphite brushes have larger resistivity and brush voltage drop, are more wear-resistant, and allow higher linear speeds (up to 50~70m/s). Metal graphite brushes resistivity and voltage drop The brush pressure drop is small, the wear resistance is poor, and the allowable linear speed is low, about 15~35m/s. The material of the brush for power tools is mainly electrochemical graphite.

（2）Performance

The main items of brush technical performance include resistivity, hardness, contact voltage drop on a pair of brushes, friction coefficient, 50h wear, etc. Resistivity is a physical quantity to measure electrical conductivity. At 230V, the resistance coefficient of the brush can be larger, and the resistance coefficient of the 120V brush should be smaller. The motor current of the 120V motor with the same power is larger than that of the 230V motor. If the brush with the same resistivity is selected, the brush of the 120V motor is easier to use. Overheating, the temperature of the brush holder may be too poor.

The contact voltage drop across a pair of brushes is the potential difference between the current flowing into the brushes, through the commutator, and back to the brushes.

When the brush and the commutator are in contact with each other and move under the action of external force, the tangential resistance generated on the contact surface is called frictional force. The ratio of the friction force to the spring pressure is the friction coefficient between the brush and the commutator.

50h wear value: under the specified experimental conditions, under the specified current density and specified unit pressure, when the linear speed of the commutator is 15m/s, the wear amount of the brush for 50h.

2.Structure of the brush

According to the installation direction of the brush on the commutator or the collector ring, the graphite brush device is divided into three types: radial type, backward-inclined type and forward-inclined type

Figure1  Three structural forms of carbon brush

In the commonly used radial structures, the springs are also pressurized differently. As shown in Figure 2, (a) is the coil spring of the socket wire, (b) is the helical compression spring, and (c) is the tension spring. These three spring compression methods all make the spring pressure act directly on the brush; ( d) It is a lever pressure plate type, and its structural characteristic type spring pressure does not directly press on the brush.

 Figure2  Radial brush spring pressure mode