Illustrative Modern Brush Applications
The heavens rejoice in motion, why should I Abjure my so much lov’d variety?
Elegies, John Donne
CONETNS
21.2.6 Altitude-Treated Brushes
21.3.2 Fractional Horsepower Motors
21.3.2.1 Wound Field/Permanent Magnet-Motor Characteristics
21.3.3 Automotive Brush Applications
21.3.5 Diesel Electric Locomotive Brushes
21.3.6 Aircraft and Space Brushes
A brush is an electrical conductor that acts as a sliding contact to carry current to and from a rotating surface, the most common forms of which are known as a commutator or slip ring. The brush may be anodic or positive, where the current flow is from the brush to the collector (electron flow from the collector to the brush), and cathodic or negative, where the current flow is from the collector to the brush (electron flow from the brush to the collector).
The commutator is an assembly of bars or segmental sections, insulated from each other, to which the coil ends of an armature winding are attached. Commutation is the process of current reversal in the armature coils that occurs when they are shorted by the brushes contacting the segments to which the coil ends are attached.
Ideal commutation occurs when the current reversal takes place at a constant rate from full current at the instant the brush contacts the commutator bar to full current in the reversed direction at the instant the brush leaves the commutator bar. This is viewed as linear or straight line commutation. However, to attain linear commutation, we must assume that the resistance between the brush and commutator bar varies inversely as the area of contact, that the resistance of the armature coil is so small as to be considered negligible, that the coils have negligible self or mutual inductance, and that the coil undergoing commutation is cutting no flux during commutation. Since these assumptions do not hold true in practical applications, the attainment of strictly linear commutation is rarely possible. The presence of these variables results in either over-commutation or under-commutation, as discussed below.
Briefly, there is always a self-induced or reactance voltage build-up in the coil undergoing commutation. The coil reactance will tend to delay or oppose current reversal. Therefore, under-commutation is obtained. The current has not fully reversed when the contact is broken between the brush and the commutator bar, which results in light arcing. If the counter-reactance voltage is too great, the result is over-commutation; complete reversal of current has taken place before separation of brush and commutator bar and the current exceeds the normal current for a very short time. There is less chance of visible arcing on over-commutation than with under-commutation. However, with over-commutation, there is a possibility of slightly higher contact temperatures since the current is greater than normal; for a short time.
On large industrial-size machines, one can measure the commutation zone by a test, which is often referred to as a “buckboost” curve, whereby the current in the interpole winding is changed to determine the commutation zone for a machine and a specific grade of material. On small machines, one can check commutation by changing the brushholder position and selecting the one that gives the least amount of sparking.
The slip ring is a conducting, rotating ring to which a winding or circuit is connected. Brushes under this condition act only as sliding electrical contacts. Therefore, the major technical concerns are the current-carrying capacity or temperature rise effects, the stability of the coefficient of friction or brush rideability, and the filming properties of the brush material as it relates to the preceding and to electrical noise.
Before beginning to discuss brush applications by motor type, it is desirable to briefly describe the general classes of material formulations that are in modern use for these applications.
The manufacture of electrographitic brush materials is an engineered process which is carried out with the technique, pride, and skill of expert craftsmen. Before beginning, representative samples of all raw materials are critically analyzed and approved before any lot of a specific material is accepted.
The lampblack, cokes, pitches, and tars, which are the major constituents for most electrographitic grades, are mixed, screened, and blended under controlled conditions to exacting specifications.
The “mix” is then molded in hydraulic presses into plates which are carefully checked by quality-control procedures before being baked to a typical temperature of 1000°C to convert the binder materials into amorphous carbon. After baking, the material is then converted into graphite by a special heat-treating process known as graphitization.
Graphitization is carried out in special furnaces designed to permit accurate control of temperatures of up to 3000°C. Engineers have developed systems of control which insure reasonably homogeneous material from lot to lot and from year to year.
The change from amorphous carbon to the graphite crystal structure takes place during the graphitization process. Electrographitic material is a form of carbon-approaching diamond in purity. As a brush material, the electrographitic grades of material generally have the best commutating characteristics and the lowest friction coefficients. However, in cases where very high current densities are required, or where high mechanical strength is a factor, it may be better to use another type of material.
The earliest carbon brushes were in the form of amorphous carbon or amorphous carbon plus natural graphite materials, There are many such grades active today although they have been greatly improved by modern manufacturing practices.
The raw materials are mixed and blended with the same care and under the same rigid process controls as those used for the electrographitic grades. They are molded into plates or pills which are baked to temperatures in the 1500–2800°F range to carbonize the binder. Carbon grades are stronger than the electrographitic materials and have a definite polishing action. There is a limit to the speeds at which they can be operated owing to their somewhat higher coefficients of friction and their current-carrying capacity is not as high as most electrographitic grades owing to their higher specific electrical resistivities. However, for applications where mechanical strength and adverse atmospheric conditions are a factor, this type of material can be very suitable.
Natural and artificial or synthetic graphites are used in the manufacture of graphite-type brushes. Natural graphite deposits of varying structural types and qualities are found in many locations throughout the world. The selected graphite is usually ground to a very fine powder, controlled in particle size and ash content, and mixed with a tar, pitch, or resin binder. The material is molded into plates or pills which are then cured by heating to a temperature sufficient to set the binder.
The graphite brush is characterized by a low coefficient of friction and a cleaning action owing to the ash content inherent in the natural and artificial graphite. Strict quality control of the graphite raw material helps to insure a consistent degree of cleaning action for a given material.
Resin-bonded brush materials are a special form of the graphite brush baked to a temperature in the range of 500°F. These materials are laminated in structure, and their characteristics are such that the resistance across the laminations is often from five to eight times the resistance taken parallel to the laminations. This characteristic is effective in reducing short-circuit currents in the face of the brush. For this reason, resin-bonded brushes are used on machines with high commutating voltage. The current-carrying capacity of these brush materials is limited, however, because of their high electrical resistivities.
Metal-graphite brushes are made from metal powders, natural and artificial graphite, and resins. The most commonly used metal is copper in percentages varying from 10% to 95% by weight although silver-graphite materials enjoy extensive use in speciality applications where their improved electrical performance is found to be cost effective. The low-metal-content brushes are generally mixed with a resin as a bonding agent. The medium-metal-content brushes may be bonded by either the sintering action of the metal or the use of a resin binder, depending on the specific grade. The high-metal-content brushes usually rely mainly on the sintering action of the metal for bonding.
Metal-graphite grades are susceptible to rapid wear when operated at absolute humidity levels below 1 grain per cubic foot or at a dewpoint of -10°C or lower. However, adjuvants which act as a film-forming agent can be added to the material to minimize the brush wear rate under these conditions (see Section 21.2.6 Altitude-Treated Brushes).
Metal-graphite materials are used where an exceptionally high current capacity is required and where the contact voltage drop must be kept low. In this regard, brushes of fine silver with graphite and other additives provide unusually low electrical noise levels, low and stable contact resistance, low friction, and high conductivity. As a result, silver-graphite brushes are suited for use on slip rings, commutators of low-voltage generators and motors, segmented rings, and many flat surfaces where the motion is reciprocating. Specific grades of this type of metal-graphite material have been developed for a wide variety of environmental conditions.
Silver-graphite brushes may be used against a variety of cooperating surfaces, although pure silver, coin silver, copper, bronze, and silver-plated copper and bronze are generally preferred. Radio interference noise levels—an increasingly important consideration in equipment design—show marked reductions in rotating equipment using silver-graphite brushes operating against coin silver slip rings.
21.2.6 Altitude-Treated Brushes
It is an interesting fact that in the absence of sufficient water vapor and/or hydrocarbon vapors, graphite can no longer function as a lubricant to provide the essential commutator film to enable the brush–collector system to function satisfactorily. In response to this problem, over the years a number of highly effective chemical compounds have been found to be effective in allowing a brush to function under low humidity conditions such as encountered in enclosed, dry atmosphere motor applications or at high altitudes where water vapor levels are very low. Typical examples of such materials include molybdenum disulfide and a number of halide salt formulations such as barium fluoride. Where it is known or found to be necessary for satisfactory brush performance, these materials can be incorporated by a variety of proprietary procedures into any of the material classes previously described.
No two pieces of motorized equipment are ever exactly alike. Close, perhaps, but never exactly the same. As a result of this condition, the grade of brush material used for the particular application must be tolerant of handling these minor variations.
Optimum brush life, commutation, and freedom from commutator wear, burning, and noise are obtained only after careful tests have been made under the actual operating conditions. During these tests, in addition to the evaluation of the brush material for life, a study should be made of the overall brush system such as brush position, spring force, holder stability, commutator conditions, etc. Should problems arise which are unfamiliar and require assistance, engineering guidance should be sought from a competent brush supplier for their analysis and evaluation of the situation.
Brushes for this class of motors are considered miniature since the cross-sectional area is less than 10.3 mm2 (0.16 in2). We could just as well include them with the following fractional horsepower class, but since they are so small in size and their applications are so varied, it is important that a few separate comments be made. Also, most miniature motors are operated at applied voltages of 24 V or less, while most fractional horsepower motors operate using applied voltages of 110 or 220 V.
Miniature motors range from the very expensive, well-designed flea-power instrument and control motors to the relatively cheap motors used in toys.
Control motors are well constructed because the applications on which they are used required high reliability. Many of the brushes are similar in design to the fractional horsepower brush except that they are miniature in size. The grade of brush material which is used depends on the motor application so that all types of materials are used including altitude-treated grades, copper-graphite materials, and silver-graphite materials.
Toy motor requirements consist mostly of life and performance at the lowest possible price. Therefore, it is necessary to look at all grades and, specifically, at those that lend themselves to production at a very low cost
The crimped connection where the brush is crimped to a leaf spring is often used for this class motor. The load current is usually small; thus, the spring carries the current in addition to applying pressure to the brush. The spring material is normally phosphor bronze or beryllium copper.
21.3.2 Fractional Horsepower Motors
The small, but powerful, “universal” motor has earned a popular place in commercial and industrial applications. These motors are used in such applications as home appliances, sweepers, blenders, fans, sewing machines, to name a few, and in all types of power tools for home and industry. The range of applications is continually expanding as well as are the variations in brush and motor requirements.
Fractional horsepower motors are primarily universal-type motors which use alternating current. The motors operate at speeds up to 30,000 r.p.m. with relatively simple speed control since the motors are sensitive to voltage and flux changes. Universal motors may be either wound field or permanent magnet-type.
21.3.2.1 Wound Field/Permanent Magnet-Motor Characteristics
In the fractional horsepower dc permanent magnet (PM) motor, the magnet replaces the field (stator) winding in most cases, leaving the armature essentially the same as before. There are, however, several important differences that arise from this change:
• High peak efficiency
• Lower circuit inductance
• Generally lower resistance (higher stall current) for the same performance
• Generally longer brush life
• The rotational alignment of the armature to achieve the lowest reluctance path, known as “cogging in the PM”
• Less energy wasted as heat loss, but the permanent magnets do not conduct away this waste heat as well as the wound field
The PM motor has almost completely replaced the shunt motor and has replaced most of the series motors in the sub-horsepower size primarily because of its simplicity, low cost, high efficiency, and longer life. The series-wound field motor is still extensively used where high stall torque, high no-load speed (e.g., reel winders, winches), speed multiplicity and temperature extremes (e.g., where demagnetization effects must be considered) are the primary design requirements.
Because of design and economic limitations, sparkless commutation is rarely achieved. Sparking may often be traced to a number of causes such as:
• Rough or uneven commutator surface
• Commutator eccentricity
• Flats, high or low commutator bars
• Improperly cured phenolic resin on molded commutators
• High mica insulation on built-up commutators
• Incorrect spring pressure
• Incorrect positioning or spacing of brushes
• Excessive reactance voltage
• Selection of an unsuitable brush grade
In general, universal motor brushes should have a low coefficient of friction and medium-to-high contact drop to suppress the short-circuit currents induced between adjacent bars during the commutation process. They should also be capable of carrying the required load current without excessive heating owing to I2 R power losses within the brush material.
The resin-bonded grades or high-resistance graphite grades normally give very good performance on this type of motor. However, for low-voltage, high-current applications, a low-metal content, metal-graphite material may be desirable.
21.3.3 Automotive Brush Applications
Automotive applications can be classified into three broad groups; namely (1) auxiliary motors; (2) alternators; and (3) starter motors.
There are many auxiliary motors in use in automobiles today, and the list is ever increasing. Some of the more recent high-volume applications are fuel pump motors and electric fans for engine cooling. The number of auxiliary motors per automobile also depends to a large extent on the various options desired by the buyer.
Automotive motor requirements cover a wide range from the high-torque, intermittent duty demands of seat adjustors and window lifts, to the high-speed, low-noise continuous-duty requirements of the blower or wiper motors. Very long brush life, often at high operating temperatures, is required for most continuous-duty applications such as air-conditioner and engine-cooling fan motors. With the addition of legally mandated automotive controls and fuel economy requirements, many of the automotive motors must operate in even more severe, high-temperature environments. However, for intermittent automotive applications, such as seat movers and antenna lifts, motor torque is an important factor and brush life is usually not a serious problem because of the short duty cycle. Therefore, these applications normally use a high-metal-content grade to obtain low contact drop and high torque.
For continuous duty applications, both brush life and efficiency are very important. Therefore, a compromise must ordinarily be made resulting in the use of a lower-metal-content brush. In addition to life and efficiency, there is also a problem, of audible and/or electrical brush noise which can usually be broken down into three categories:
• Audible or electrical noise that develops as the brushes ride over the commutator segments
• Audible magnetic noise
• Audible noise from bearings or washers
The brush formulation and its physical properties have a considerable influence on the damping of the audible noise generated by the commutator-brush-slot bar frequency. The frequency excited in this manner can be transmitted or amplified through its mounting. Thus, it is important that this factor also be kept as low as possible. Electrical noise which arises from commutation effects can also be minimized by proper engineering design and brush material selection to achieve the best possible commutation at an affordable price. Also, in addition to material selection, it is often recommended to use a small angle on the face of the brush to provide for quicker brush seating to enhance brush stability with attendant better commutation (assuming proper spring and holder design).
Audible magnetic noise caused by an imbalance in the magnetic circuitry or flux can usually be minimized by using skewed armature slots. Audible bearing noise is usually attributed to defective bearings or bearing misalignment during assembly. Audible washer noise can usually be eliminated or reduced by the use of a combination of steel and fiber washers.
The use of electronic circuitry in automobiles has increased the importance of damping the radio-frequency interference (RFI) generated within the auxiliary motor because of commutation effects. The RFI may be dampened or controlled by the use of capacitors, ferrite beads, choke coils, or some combination of both. This RFI factor has always been present, but its influence on other apparatus is much more detrimental today because of the much more extensive usage of electronic circuitry in modern vehicles. Therefore, it must be controlled.
Auxiliary motors operate directly from the battery which is charged by the alternator; therefore, the need for load current continues to increase as additional apparatus is added to the vehicle.
The efficiency of the permanent magnet motor is higher than the wound field motor since it requires no field excitation, thus the energy required per motor is less. This is a major reason why the permanent magnet auxiliary motor has almost completely replaced the wound field motors in these applications.
The conventional dc generator, a commutator-rectified generator, served for many years as the electrical power source for the automobile. However, its limitations resulted in its replacement by the diode-rectified generator, normally referred to as the alternator. Some of the advantages of the diode-rectified alternator over the dc generator are:
• The alternator has greater power at lower speed.
• The alternator is considerably lighter and smaller than the dc generator for the same power rating.
• The brushes in the alternator carry only excitation current to a rotating field and commutation is not involved. Thus, much longer life is normally achieved. The slip rings on which the brushes ride are usually made from copper, brass, or stainless steel.
• The overall service life of the alternator surpasses that of the dc generator with considerably higher reliability.
• The heavy current leads are in the stator, which does not rotate; and being near the outside surface, the heat generated owing to electrical resistance is more easily dissipated.
• The voltage regulator is much simpler since it does not require a current-limiting device nor a circuit breaker to prevent reverse current since the diodes conduct current in one direction only. Solid state rectifiers are very small in size and weight and, therefore, can be mounted inside the alternator.
The rating of alternators varies depending on the electrical output desired. Alternators are made in various sizes and are matched to the electrical output required for the vehicle on which they are used. Each manufacturer’s alternator differs slightly in appearance and brush configurations. However the electrical and mechanical principles are the same in all cases. Brush materials in common use for alternators are either the electrographitic type or the low-metal-content, metal–graphite type.
The invention of the electrical starter in the early 1920s was an important factor in the rapid growth and success of the automotive industry. The original concept invented by Kettering consisting of a high-torque motor used for short times at high speeds has remained unchanged in principle. However, the requirements of different applications have resulted in the development of a number of starter brush material types. The requirements of very high torque, high load current, and commutator speed for short periods of operation, in addition to the overspeed after the engine has started, must all be considered in the application of starter brush materials. Today, starter design trend is toward ever-decreasing engine cranking time under all weather conditions and environments. Starter grades generally contain moderate metal contents (50–70%) to give the low contact drop required and to carry high load currents with the graphite or lubricant content adjusted to achieve the required brush life.
Many starter motors require brushes with shunts that are quite large because of the heavy currents that must be carried. The voltage drop between the brush and the shunt must be kept low to keep electrical losses to a minimum. Since the cranking time is relatively short, normally a shunt is used at three to five times its normal current rating. The shunt connections are usually of the molded-in connection type. However, others can be used depending on the application.
Present automotive starter designs have changed from the wound field types used in the past to the almost universal use of permanent magnet field starters. Also, the small starter motors for outboard motors, lawn mowers, small garden tractors, and so forth are mostly permanent magnet starters. These differ in design, but they still have the same basic function and requirement. Many of these smaller permanent magnet starters use a faceplate commutator rather than a barrel commutator. Thus, a different brush configuration is used. These brushes are often wedge-shaped in design, conforming to the shape of the commutator bars.
The field of industrial brushes is very broad in application and involves grades of all types which must function over a wide range of conditions. Many of the problems encountered with industrial motors can be attributed to maintenance, atmospheric conditions, or operational procedures.
Very large generators and motors, such as those used for the generation of power, are well designed, and preliminary testing has been conducted pertaining to machine parameters and operation. This background data is very helpful in the selection or development of a grade that will give excellent performance. The maintenance of these units is such that the problems that occur are spotted immediately, and corrective action can often be taken before they can become serious.
Normally, electrographitic or carbon–graphite grades are used on these motors and generators. Exceptionally long brush life is obtained on many of these machines.
Medium-sized motors and generators generally use the same grades of material as mentioned above. These machines also operate over a wide range of conditions and are involved in a large variety of applications. The number of motors and generators in this type of service greatly outnumber the volume of the larger units; thus more application problems are likely to be encountered in practice with these units.
Problems on these machines often arise because of the speed and load conditions, and as a result of atmospheric conditions. Many machines operate at full load (typical brush current densities of 80–90 A in−2) and overload for short periods of time. Machine operation is often controlled by the workload and the nature of the application thus, the variation in its duty cycle can be extreme. Also, although it was more of a problem in the past before the principles of successful brush application were well known, light loading of brushes with current densities of 40 A in−2 (6.2 A cm−2) or less can also cause severe problems owing to the inability of the graphite brush material to properly maintain a satisfactory commutator film.
Machines may also be operated in contaminated atmospheres, many of which are chemical in nature. In these instances, overfilming may occur which results in threading or grooving of the collector and, therefore, rapid brush and commutator wear. Special grades of brush material have been developed which exhibit polishing action to prevent a heavy film build-up in these atmospheres, which will then allow for excellent performance and brush life.
Motor maintenance is very important. However, it is often the case that the smaller the motor, the less care that it receives.
Many motors operate under very high ambient temperatures and are subject to the aforementioned wide range of load conditions. Thus, it is very important that the mechanical or stability aspects of the brushholder be considered and the other electrical aspects during machine design.
The turbo-generator requires slip ring brushes. No commutation is involved; therefore, the main concern is selective brush wear when one polarity wears more than the other. Turbo-generator brushes should have low, stable contact drop and exhibit low friction. Normally, carbon-graphite type materials which contain additional added lubricants are used for this application.
21.3.5 Diesel Electric Locomotive Brushes
The motors used on the diesel electric locomotive and similar type applications are referred to as traction motors, which are a special class of the industrial motor. Because of the number of motors involved, reliability, and type of service, they are treated as a separate entity.
Traction motors have undergone many changes over the years with continual increases in horsepower rating for the same relative size. These changes have been accompanied by basic motor design modifications, new insulating materials and brush changes in the way of design and material formulations. The scope of operation ranges from heavy load, low speed and low voltage to light load, high speed, and high voltage, which results in motor operation at both under- and over-compensated commutation conditions.
Multiple-wafer brushes have been in service for a number of years to improve the motor commutating characteristics. However, there are still traction motors in service today where a one-piece or single-wafer brush is used, but most modern applications call for a split- or multiple-wafer brush. A resilient pad is added to the top of these brushes which acts as a shock absorber, with the pad adsorbing vibrations so that the brush will have more intimate contact with the commutator. Each wafer of a multiple-wafer brush acts independently of the others, resulting in improved commutation and commutator conditions, which leads to vastly extended motor operation between commutator resurfacing or overhaul.
Normally, traction motors use electrographitic grades of material which are resin impregnated to give added strength to withstand vibration and to give longer brush life.
The modern diesel locomotive also has an alternator, driven by a diesel engine, to generate power for the motors. The brushes for the alternator carry only excitation current. The same basic considerations hold as previously described for the automotive alternators. Brush mounting and stability are very important. Because of the presence of vibration, it is very important to study the mechanical stability of the system to obtain good overall performance.
An impregnated electrographitic or carbon–graphite material is normally used for alternator brushes.
21.3.6 Aircraft and Space Brushes
The carbon industry has been involved in supplying brushes for the aircraft and space industries ever since it was recognized that the normal type of brush materials used on generators and motors at sea-level conditions gave exceptionally short life when operated above 20,000 feet (6,000 m) in altitude. Once the problem was defined as lack of sufficient moisture to maintain proper filming on the commutators or slip rings, it was necessary to find the correct additive or adjuvant which could be put into the brush, either by impregnation or during mixing of the material, to compensate for this lack of moisture. The adjuvants used had to be capable of providing suitable performance at sea level where moisture was present and at altitude where the dry conditions existed. Initially, a lead iodide additive was used which was soon followed by barium fluoride. These treatments served well; however, in many applications it was necessary to seat-in or run-in the brushes to establish a good film before operation at altitude.
Continued research and development resulted in the introduction of quick-filming brushes eliminating the need for preseating of brushes. The adjuvant for these brush materials is usually molybdenum disulfide, which minimizes threading of commutators and slip rings and provides higher altitude reliability without sacrificing the other desirable brush characteristics. Today there are a large number of material grades available which serve a large variety of applications from aircraft, missiles, and space satellites to sea-level applications such as totally enclosed motors where low humidity conditions can exist.
These applications cover a broad range of operating conditions since they include alternators, starters, inverters, dynamometers, and snychronous motors. Typical materials used for the various applications include carbon–graphites, electrographites, and metal–graphites (copper and silver). The state of the art of brush design and use continues to evolve. New uses repeatedly arise where brushes are required for novel purposes.
Some relatively recent advances include development of special brushes (see Chapter 23) for use at extremely high current densities, carbon-fiber brushes, and brushes with wear detectors added for special applications.