Introduction to the advantages and disadvantages of the brush ...

Introduction to the advantages and disadvantages of the brush process of electric slip ring

Advantages and disadvantages of brush technology for conductive slip ring

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Slip ring is a rotating connection componentof electric power, signal and other media composed of rotating (rotor) and stationary (stator)device.Electriccurrent and signal are connected and transmitted through brushes. Therefore, the performance of the brush determines the performance quality of the entire slip ring. Different types of slip rings require different materials and types of brushes, and each has its own characteristics.

Alloy brush wire

The brush wire is generally made of alloy materials, and the surface electroplating process is carried out according to different application requirements. Mainly used in slip rings with low power and small size. It is characterized by easy manufacture, space saving and relatively low cost. However, there are also shortcomings such as easy deformation and poor transmission stability.

Carbon graphite brush

Carbon graphite brush a rectangular contact point, it has a larger contact surface with the conductive ring, which is suitable for high-current electric slip rings. it is made of carbon graphite sometimes with metals included to boost current capacity. It has high conductivity, good oxidation resistance and stability. The disadvantage is that different graphite materials and manufacturing processes have large differences in conductivity, large volume, and high manufacturing costs.

Polyfilament brushes

Polyfilament is also made of precious metal and partially wraps the slip-ring drum, includes multiple contacts per channel. Due to the large and large contact surface, higher current can be transmitted, which makes them suitable for transmitting sensitive analog signals or data at high rates for real-time control. The disadvantage is that the cost is high, the transmission volatility is large and difficult to control, and there is a potential risk of short circuit due to the deformation of the brush wire.

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Rolling ring brush 

The rolling ring brush is similar to the ball principle in the ball bearing, and uses the rolling between the flexible ring and the ring to connect and transmit the power signal. It is characterized by low noise, reduced dust caused by friction, higher speed, lower rolling friction and longer life. However, they also have disadvantages such as higher cost, higher requirements for manufacturing process precision, and higher difficulty in repairing once worn.

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Types of Carbon

There are five main groups which differ in use of raw material and manufacturing process:

1. Hard carbon brushes

The most important raw materials are amorphous carbons like retort coke and petroleum coke, bonded with pitch or resin, the raw materials are pressed and kilned at temperatures up to 1200º C.

The kilning process carbonises the binder and a relatively hard carbon is produced with much better polishing qualities than electrographite grades. Therefore, hard carbon brushes are mostly used on small machines with flush mica which require strongly polishing brushes. The permissible electrical and thermal load for hard carbon brushes is rather low and their elasticity limits their use to machines with peripheral speeds of up to 20 m/sec.

2. Carbon-graphite brushes

Electrographite, the most important raw material, and the binder pitch or resin are pressed and heat-treated similarly to hard carbon. The principle field of application for carbon graphite brushes is, therefore, intermediate between those of electrographitic and hard carbon brushes. Their polishing properties are better; their electrical carrying capacity is lower than in the case of electrographitic brushes. Carbon graphite brushes are also characterized by outstanding commutating abilities as a result of their high contact drop. They are used on machines where electrographitic brushes cannot be used due to their low polishing ability, and where hard carbon brushes are unsuitable because of their lower commutating properties. Carbon graphite brushes are mainly used on F.H.P. motors with mica undercut and on small thyristor fed DC machines.

3. Resin-bonded brushes

In principle, the raw material is graphite, bonded with resin, which is pressed and heat treated in a special process. The advantage of special graphite brushes is their high contact drop and high internal resistance. They also have good oxidation resistance. These properties are very valuable for machines with high commutating requirements. It is to its disadvantage that the high contact resistance causes high electrical loss due to circulating current. This might lead to destruction of the resin bond. Therefore, the main field of application for special graphite brushes covers all machines with high commutating requirements, but with relatively low brush current. These are three-phase commutator motors and small machines with mica undercut. Due to their low elasticity, special graphite brushes are normally used only up to 35 m/sec.

4. Electrographite brushes

The raw materials petroleum coke or retort coke are subjected to processes similar to those described under 1 to 3 above and then further heat-treated at temperatures up to 2500ºC. This converts the carbon material into a micro- crystalline form of graphite, called electrographite, a material having good electrical and thermal conductivity. Its elasticity and refractoriness are improved and its polishing properties are generally lower than hard carbon or graphite brushes. Depending on the composition of the basic materials, brushes having widely variable characteristics can be produced. Electrographite brushes, therefore, cover a wide range from the robust traction brush to very elastic grades having outstanding commutating abilities. Because of all these characteristics, electrographite grades are found in all classes of service, though they are mainly used on large commutator machines with high electrical, thermal and mechanical requirements.

5. Metal graphite brushes

Raw materials are metal graphite mixtures which are processed on a powdered metal basis. The metal most frequently used is electrolytic copper. For certain applications also silver, tin, lead and other metal powders are used. The behaviour of metal graphite brushes is intermediate between graphite brushes and metal brushes. Depending on the proportion of graphite and metal, either the good frictional qualities of graphite or the high conductivity of metal predominates. Contact and internal resistance are relatively small. From this point of view, this group is the opposite of the special graphite class. Consequently, the main field of application cover all types of machines with high brush loads, with or without high commutating requirements, such as slip rings and low voltage machines. Owing to their low elasticity, metal graphite brushes can only be used at peripheral speeds up to approximately 30 m/sec.

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