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Thursday, October 7, 2010

DRIVER OF DC MOTOR

Introduction

The carbon brush has been a critical component in variable speed rotating equipment for over 100 years. The trend towards reduced maintenance has resulted in the elimination of many well established procedures related to carbon brushes. Today new technology is attempting to replace this component and permanently eliminate these maintenance costs. However, when subjected to the proper operating conditions carbon brushes do yield excellent life and perform an amazing function.
The purpose of this presentation is to review the basics in the composition and application of carbon brushes and to address the specific issues related to elevators. Through the development of greater awareness and a more thorough understanding of the proper function of a carbon brush, it is hoped that we can work together to solve the unique problems on elevator applications.
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Carbon Brush

A carbon brush is an electrical component which makes contact between a stationary and a moving electrical circuit. A carbon brush performs both an electrical and a mechanical function within a system; it is a conductor of current in an electrical circuit and it is subjected to mechanical forces as it makes physical contact with a surface in motion.
One end of a brush consists of a carbon/graphite composition for contact with a moving surface. This portion of the brush is unique in that it is adequately conductive to perform electrically and has lubricating characteristics for satisfactory mechanical performance at the moving contact surface. The other end consists of a terminal or cap to make a stationary electrical contact. A copper wire usually connects the carbon/graphite portion with the terminal for maximum conductivity.
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Carbon Brush Grades

There is no standard within the carbon industry for the assignment of grade numbers. Each manufacturer has its own formulas and numbers for the series of materials in its program. All grades are processed similarly by grinding and mixing raw materials such as petroleum coke, lampblack, natural graphite, and/or metal powders with a binder such as pitch or resin. The material is then molded into plates and baked in large ovens for an extended period to cure the binders. There may be further processing including graphitization and/or impregnation to enhance strength and friction characteristics.
The many different grades in use today are derived through variation in raw materials, molding pressures, temperature and duration of the baking process, and after-treatments. However, all grades fall within the 4 categories of Carbon Graphite, Electrographite, Graphite, and Metal Graphite.
Carbon Graphites are relatively high in friction for use at slow speeds, low current densities, and medium to low voltages. These grades were developed early in the history of motors and generators and therefore are found most often on older equipment, particularly with flush mica commutators.
Electrographites are the most common grades used on modern equipment with good performance at high voltages high current densities, and high speeds. There is a wide range of characteristics within this category. Most electrographite grades handle overloads very well.
Graphites are for use in special applications requiring the low friction characteristics of these materials. When brushes must operate at very low current densities or very high peripheral speed a graphite grade should be used. Natural graphites can be abrasive due to its ash content.
Metal Graphites have material contents of 15-95% copper, copper alloys, or silver. The added conductivity and lower voltage drop of the metals allows metal graphite brushes to perform well at very high current densities and low voltages.
Each grade has been designed to perform under certain operating conditions of amps, volts, and speed. The carbon brush manufacturer is best qualified to select the proper grade for an application as it takes many years of experience to develop expertise in this area. Although there are many grades with similar characteristics, brush materials should not be mixed on the same unit as it can result in severe current imbalances and selective action. Also, it is virtually impossible to determine equivalence of materials by visual inspection. Materials may have identical appearance and very different characteristics.
In particular, the hardness or softness of a grade is not a major factor in brush and commutator wear. The hardness factor will only give indication of the readability of the brush on the contact surface. Resistivity, density, strength, and filming rate are far more important factors in determining performance.
In general, over the years grades have been given much more credit or blame for brush performance than is deserved.
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Carbon Brush Wear

The carbon brush is a critical component of a complex electromechanical system. As the weak point in the system it is highly susceptible to wear particularly when subjected to conditions beyond it s operating parameters. This brush wear can only be mechanical due to friction or electrical due to the imperfect moving electrical contact.
Mechanical wear occurs as the friction at the contact surface causes abrasion of particles from the brush ace. If there is proper brush function, the coefficient of friction will be kept to a minimum through the formation of a thin low friction layer on the contact surface called a film. an ideal film will be 50 Angstroms thick or .0000002 inches and consist of only microscopic graphite particles, water droplets, and the metal oxide of the contact surface. A film is formed by an electro-chemical reaction involving current, the graphite in the brush, humidity in the air, and the metal of the contact surface. The proper film will minimize the mechanical wear, while avoiding a significant increase in contact resistance, which would inhibit the passage of current.
The film condition is a primary indicator or symptom of the operating condition of a unit. slot bar marking, bar edge burning, copper drag, grooving, streaking, and threading are all film conditions indicating possible poor performance and the need for adjustments. A consistent brownish colored film indicates satisfactory operation and should result in good life of the commutator and the brushes. (See Commutator Condition Guide)
Electrical wear occurs as current is conducted across the high resistance of the imperfect electrical contact, which results in arcing. This wear can be kept to a minimum by maintaining the best possible electrical contact.
The commutator or ring surface must be as smooth and round as possible so that continuous contact can be maintained. Flat spots, high and low bars, loose bars, high mica, and vibration will all cause interruption of the electrical contact and destructive arcing.
The brush design can also improve the electrical contact and reduce electrical wear. A concave eases the seating process and gives a new brush greater contact area. Multiflex brushes increase the cross resistance of the brush thereby reducing damage from circulating currents. This style also increases the effective electrical contact by multiplying the number of contact surfaces. The Red Top feature also improves brush contact by absorbing shock and vibration.
The function of a brushholder is to support the brush in the proper position and provide the force for electrical contact. Many manufacturers have neglected the importance of having adequate spring pressure resulting in this being the most common area of deficiency in carbon brush performance.
When attention is given to the formation of a proper film and to the factors effecting a good electrical contact, brush and commutator or ring wear can be kept to very low levels.

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