SECTION 1 -INTRODUCTION TO BRUSHES AND CARBON GRADES
Question 1:7 What General Groups Do Brush Grades Fall Into?
1. Hard Carbon
Hard Carbon, one of the earliest brush types, is mechanically strong and hard resulting in a high coefficient of friction which give them a pronounced cleaning action. It is made up from coke materials and pitch or resin binders.
Hard carbon brushes often include a small amount of graphite to improve the brush lubricating property, hence, carbon graphite class.
The use of hard carbon is restricted to low speed and low current density but some of this group are used for flush Mica commutators, others for collectors and some carbon contacts.
2. Graphite
Brushes made from graphite are soft and possess a high lubricating property and good electrical conductivity.
Brushes manufactured from natural graphite bonded with resin or pitch are recognisable by the silver metallic appearance and distinctive greasy feeling when touched.
Graphite is suitable for machines running at high speeds and moderate current density.
Some residue remains in the brush material after processing and this provides the increased friction essential for satisfactory current collection at high collector speeds. This residue may sometimes result in more commutator wear than with brushes of the Electrographite class. However, natural graphite brushes have long life and operational stability for particular applications.
There is also artificial or synthetic graphites which do not have the ash and impurities or the flaky structure of natural graphite. These are widely used in mechanical carbon seals and bearings. These artificial graphites form the bulk of the graphite materials available.
3. Electrographite
Electrographite brushes are produced by an additional process of passing electrical current through the hard carbon material. The high temperature produced converts the material into a micro-crystalline form of graphite. This type of brush possesses the strength of carbon and the lubricating property of graphite.
The high thermal conductivity enables electrographite brushes to withstand overloads of up to four to five times the normal rating for short periods.
The removal of impurities by heat treatment gives the brush a low commutator wear rate. Electrographite brushes have a slightly porous balck appearance (which permits impregnation of various types), and are hard and smooth when touched. They are suitable for a wide speed range at moderate current density and are capable of withstanding overload. These are the most common types of brushes and are used extensively throughout the industry.
4. Metal-Graphite
These brushes are produced by mixing graphite with a metal, usually copper but silver or bronze can also e used. The metal-graphite brush has a very low electrical resistance, which allows for high current carrying capacity.
The metal-graphite brushes is distinguished by the copper colouring and its heavy weight in proportion to its physical size.
Copper-graphite brushes are commonly used for low voltage, high current, motors and generators, slipring brushes and current collectors.
5.Metal-impregnated Graphite
The grades in this class have its porous structure impregnated with a metal. The metal is present in a continuous phase, almost as veins, and not as discrete particles as in metal-graphite mixtures.
The essential character of the base material is relatively unaffected by the comparatively small proportion of metal. However, the increase in mechanical strength given by this metal reinforcement, plus an increased thermal and electrical conductivity, has substantially extended the uses for electrographitic based carbon. The grades are also used for brushes on short-time, high current duty, machines such as engine starters and current collectors (for example pantographs for trains and trams).
Battery electric vehicle motors are another application where this type of material performs very well.
Carbon bearings with white metal, antimony and other impregnations are used in mechanical applications.
6. Resin Bonded
The grades in this class are based on graphite, but have a resin bond. They are sometimes referred to as "Bakalite Class". This type of bond produces a high electrical resistance which endows the grades with extremely good communtating ability. Their current carrying capacity is however limited and peak overloads would be limited to a maximum of approximately 20% normal rating.
Question 1:8 Are Slipring And Commutator Brushes Different?
The information has been drawn from experience of Morganite Taiwan Limited Application Specialists combined with a wide range of published information from major carbon supplier and motor manufacture. However, the material can not be downloaded, copied or used without written permission.
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