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How to Select Carbon Brushes for Motors and Generators

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How to Select Carbon Brushes for Motors and Generators How to Select Carbon Brushes for Motors 1 and Generators ® have use as contacts and as a base for metal impregnated Contents grades. Page Electrographitic Brushes Grade Selection . Brush Grade Families 1 Electrographitic brushes are baked at temperatures in Grade Characteristic Definitions . .3 excess of 2400°C that changes the material physically to a more graphitic structure. Apparent density, strength, Special Brush Types hardness, and resistivity can be closely controlled through Laminated Brushes . .5 raw material composition and processing to achieve Fluted Brushes . .5 . superior commutating ability while providing long life. Pre-Radius Brushes . .6 . The high processing temperature volatizes impurities which Rubber Hardtop Brushes . .6 6 makes electrographitic brushes generally free from Surface Cleaning Brushes . abrasive ash. Therefore, commutators must have undercut Surface Rounding Brushes .. 6. mica since very little mechanical wear results. Electrographitic materials in general are fairly porous which permits treatment with various organic resins or inorganic Engineered Brush Grades materials. The treatments increase strength and lubricating Electrographitic . 6. ability which generally increases brush life significantly at Graphite . .9 high operating temperatures and at lower humidity. 10 Metal Graphite . Treatments can also permit electrographitic materials to operate satisfactorily in a variety of contaminated atmospheric environments. Brush Request Form . .11 . 12 Friction characteristics with electrographitic materials can Grade Matrix . be controlled through raw material combinations before graphitization and also by treating the finished product with organic resins. Brush face temperature is a primary influence in determining the coefficient of friction as shown by the typical curve of coefficient of friction vs. temp chart. Brush Grades Families Typical Coefficient of Friction vs Temperature Brush grades are usually classified according to the 0.40 manufacturing processes and the types of carbons and other ingredients used. The four main brush 0.30 grade families are: 0.20 Carbon Graphites Electrographites 0.10 Graphites CoefficientofFriction Metal Graphites 80 100 120 140 160 180 Brush Temperature - Degrees C Carbon Graphite Brushes Figure 1 Carbon graphite brushes made their entrance early in the brush industry. They are high-strength materials with a pronounced cleaning action. Carbon graphite brushes are Electrographitic brushes are generally capable of generally limited to lower current densities 45 amps/in² continuous operation at 80 amps/in² (12.4 amps/cm2) and 2 (7 amps/cm ) and are used on older, slower speed surface speed of 7000 feet/minute (35.6 m/sec). machines that reach maximum surface speeds of Intermittent operation at higher values is not uncommon. approximately 4000 feet/minute (20.3 m/sec). The high They are widely used in the industrial, transportation, friction generated with this type of material also makes it mining and aerospace industries on both AC and DC unattractive for present day use on commutators, but does machinery. 2 1 Grade Characteristic Definitions Graphite Brushes The characteristics of the most widely used brush grades Graphite brushes are composed of natural or artificial are shown in the tables on pages 6 - 10. The following graphite bonded with resin or pitch to form a layered brush definitions and explanations will help you to interpret these material. Natural graphite usually contains ash which gives tables. the brushes an abrasive or cleaning action. Artificial graphite generally does not contain ash nor does it have the Specific Resistance flaky structure of natural graphite. Unless otherwise specified, specific resistance (or Graphite brushes are characterized by their controlled resistivity) in ohm-inches (micro ohm-meters) is equal to the filming and excellent riding qualities on both commutators resistance that a specific volume of brush material offers to and slip rings at brush current densities 45 to 65 amps/in² the passage of current. Specific resistance is measured in (7 to 10 amps/cm2). They are not capable of sustained the length direction of the brush, since resistance in the operation at higher current densities like electrographitic direction of width or thickness may be considerably materials; however, higher surface speeds are often different. Specific resistance is calculated from permitted with some type of graphite materials. measurements on a test sample as follows: The fast filming properties of graphite brushes is very R = (E x W x T) / (I x L) (English) beneficial in protecting the commutator or slip ring during operation in contaminated atmospheres such as those seen R = (E x W x T x 10-3) / (I x L) (Metric) in paper mills. Their low porosity and higher density are Where R = the specific resistance in ohm-inches valuable in reducing commutator threading often (micro ohm-meters) encountered in contaminated environments. E = voltage drop over length “L” Metal Graphite Brushes I = amps of current passed through the sample Metal graphite brushes are generally made from natural W = width of sample in inches (millimeters) graphite and fine metal powders. Copper is the most common metallic constituent, but silver, tin, lead and other T = thickness on sample in inches (millimeters) metals are sometimes used. L = that portion of the length, in inches (millimeters), over which the voltage Metal graphites are ideal for a variety of applications drop “E” is measured because of their low resistivity. Metal graphites are used on commutators of plating generators where low voltage and high brush current densities are encountered. They operate on rings of wound rotor induction motors where high brush current densities are also common. Metal graphites are used for grounding brushes because of their low contact Apparent Density drop. For a brush material, the apparent density (also known as The following table describes some general applications bulk density) is equivalent to its weight in grams divided by where metal graphite brushes are used. its volume in cubic centimeters. Density must be considered Metal jointly with other brush characteristics in estimating brush Content Application quality. (Percent) Forklift and battery truck motors rated 24 – 72 Abrasiveness volts Battery charging and welding generators The ability of the brush to prevent excessive build up of film 50% or Less rated 24 – 72 volts usually caused by corrosive or oily atmospheres is called the abrasiveness or “polishing action”. The abrasiveness of Slip rings at brush current densities 100 amps/in2 (15.5 amps/cm2) or less a brush may be influenced by its hardness, grain structure Plating generators rated 6 – 24 volts and ash content. The brushes are classified according to 65 to 85% abrasiveness as follows: “Low” indicates very little Slip rings at brush current densities less than abrasiveness (commonly referred to as “non-abrasive” by 125 amps/in2 (19.4 amps/cm2) the trade), “Medium” indicates some polishing action, “High” DC machines rated less than 6 volts. indicates pronounced polishing action which is usually 75% and Slip rings at brush current densities 150 obtained by using a material with high ash content or by the Above amps/in2 (23.3 amps/cm2) or less addition of a polishing agent. Grounding brushes 2 3 Contact Drop Maximum Speed Contact voltage drop for the brushes listed in this brochure The highest peripheral or surface speed in feet per minute are approximate values only and represent the total voltage (meters per second) recommended for the collector or drop (positive plus negative) obtained on a copper ring at commutator on which the brush is to ride is referred to as 50 amps/in2 (7.75 amps/cm2) while rotating at 2500 f/m the maximum speed. The maximum speed depends not (12.7 m/sec). only upon the characteristics of the brush material, but also upon the spring force, current density, type of brush holder, They are classified as follows: brush angle, condition of the ring or commutator, atmospheric conditions, etc. Consequently, the maximum Contact Drop Volts speed, conventionally listed as a brush characteristic, is Very High 1.7 and above only an approximation. High 1.2 to 1.7 Medium 0.6 to 1.2 Friction Coefficients Low Below 0.6 Brush friction is influenced by many variables including Current Carrying Capacity brush temperature, spring force, current, atmospheric conditions, mechanical conditions, ring or commutator The actual current carrying capacity of a brush is widely materials, surface films, speed and other factors. The slip influenced by operating conditions such as type of ring/commutator surface even under favorable conditions is ventilation, continuous or intermittent duty, speed and other continually undergoing changes caused by oxidation, factors. The brush grade current ratings are conservative, abrasion and moisture. some allowance having been made for overloads. Brushes have been run at currents considerably above those listed Friction chatter, noise and associated brush wear can be in the grades table. Metal graphites, for example, have accelerated with certain brush holder configurations. This is been operated at 180 amps/in² (27.9 amps/cm2) on certain especially true under conditions of light loads and lower high current generators. Electrographitic brushes have brush temperatures when the coefficient of friction is been operated at 100 amps/in² (15.5 amps/cm2) on similar relatively high (refer to Fig. 1). Specific brush grades are
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