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Frequently asked questions - FAQ

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1 Frequently asked questions - FAQ

2 Brush manufacturing, design and application Theory Brush holders
Content General Brush manufacturing, design and application Theory Brush holders Maintenance Common problems Frequently asked questions

3 1. General 1. Why are they called brushes ? 2. Why carbon ?
3. How is carbon made ? 4. What additives are used ? 5. What general groups of brush grades do exist ? 6. Are slip ring and commutator brushes different ? 7 .General principle for grade selection ? 8. Is there any shelf live for carbon brushes ? 9. Are any safety measures necessary while cleaning motors? Frequently asked questions

4 2. Brush manufacturing, design and application
1. How are brushes made ? 2. Which connections cable / brush do exist ? 3. Which information is needed for a brush supplier ? 4. How may I select the right grade ? 5. Why do brushes have top or bottom angles ? 6 .What is the purpose of a rubber pad on the top ? 7. Why do some brushes have saw cuts in the face ? 8. Why do some brushes have grooves in the side or internal face ? 9. How is the cable size and number determined ? 10. Are there any alarm devices to indicate short brushes ? Frequently asked questions

5 3. Theory and characteristics
1. What is commutation ? 13. Why do some slip rings have spiral grooves ? 2. What role does resistivity play in a brush ? 14. What is field weakening ? 3. How important is Hardness ? 15. What is the neutral zone ? 4. What is contact drop or voltage drop 16. Which limiting values of the leak resistance should be kept ? 5. How does the voltage drop influence commutation ? 17. How does the controller influence brush performance ? 6. What is current density ? 18. How does the brush design influence brush performance ? 7. How is current density calculated ? 19. What are the main factors influencing commutation ? 8. What is the reason for low load problems ? 9. What causes brush wear ? 10. What is reasonable brush life ? 11. What causes the commutator or slip—ring film ? 12. What is a black band ? Frequently asked questions

6 4. Brush holders 1. What types of brush holders are there ?
2. What is the pressure curve of a brush holder ? 3. Do brush holders have a corrosion protection ? 4. What is the correct spring force ? 5. How is the spring force measured and adjusted? 6. Are there any standards for brush holders ? 7, How important is brush holder spacing ? 8. What is axial staggering ? 9. What is circumferential staggering ? 10. How much clearance should a brush have ina holder ? Frequently asked questions

7 5. Maintenance 1. Which regular machine checks should be done ?
13. Can brushes be washed in solvent ? 2. What are suitable parameters to indicate motor performance ? 14. What atmosphere contaminants affect brushes ? 3. At what length should a brush be replaced ? 15. What happens if the machine is subject to vibration ? 4. Do I need to bed brushes in ? 16. How to measure collector temperature ? 5. How are brushes bedded in ? 17. What has to be done if the motor has to stored for a long period ? 6. What should be done if changing grades ? 18. Why is surface roughness so important ? 7. What happens if grades are mixed ? 8. How can I set the neutral position ? 9. Can commutators and slip-rings be ground in the machine ? 10. When and how should commutators be turned ? 11. Is undercutting necessary ? 12. How does oil influence commutators and brushes ? Frequently asked questions

8 6. Common problems 1. Why do shunts fall out of brushes ?
11. What is out-of-roundness ? 2. What causes commutator grooving and threading ? 12. What are reasons for copper drag ? 3. Why do commutators get flat spots ? 4. What causes brushes to get stuck in the holder ? 5. Why do brushes sometimes wear differently in the same machine ? 6. Why do some brushes sometimes have overheated flexibles ? 7. What causes brushes to wear and dust excessively ? 8. Why do some commutators show regular light and dark patterns on the segments ? 9. What causes differential wear of slip rings ? 10. What causes selective action ? Frequently asked questions

9 Chapter 1 General Frequently asked questions

10 1. Why are they called brushes ?
The inventors of rotating electrical machines were faced with the requirement to transfer current from a stationary position to a rotating object. They initially solved this problem with bundles of copper wire assembled like a paint brush rubbing against the rotating current collector. The term "Brush" correctly described the item they used but because it had high friction and wear it wasn't long before the bristle brush was replaced and carbon blocks were used as a much better alternative. The name "Brush" however has remained to the present days Approx.1870 copper brushes 1885 – first patent for carbon for sliding contacts Frequently asked questions

11 1.2. Why carbon ? Carbon has some unique properties which makes it the preferable material for electrical sliding contacts Good electrical and thermal conductivity Low shear strength of graphite crystal Low friction coefficient Low modulus of elasticity Retains moderate strength at high temperature No melting point, passing from solid to vapour at 3500°C No welding between carbon and counter material Wide band of phys. Characteristics by means of - Raw materials Process - Design Frequently asked questions

12 1.3. How is carbon made ? The basic raw materials of coke, graphite, carbon black or lamp black are combined with a variety of special additives which have been formulated in various ways. A binder is added and this mixture is then baked. In the case of electro-graphite grades a further process of passing electric current through the blocks or by inductive methods changes the crystalline structure of the material. In the first case the process is called “electro graphitisation“ or Acheson-graphiti-sation. The porous structure of the materials enables various after-treatments to modify the materials properties. The total process can take up to 6 months to complete. Frequently asked questions

13 1.4 What additives are used
All manufactured carbon is porous, to some degree, making later treatments possible. Special operating conditions like low humidity, bad ambient conditions etc. sometimes require the introduction of additives into the brush material to counteract any adverse effects of such conditions and to help in the control of commutator patina or skin formation.  Additives like paraffin, resin, special oils and inorganic additives can improve the performance and brush life under certain circumstances. . Frequently asked questions

14 1.5. How many brush grades are there?
There are literally thousands of different grades. Each manufacturer has its own series. Published lists are usually of the most common grades but others are developed according to the needs of industry and applications. Most manufactures have grades which will perform equivalent duties but subtle differences can make one particular grade perform better for a specific application.  Frequently asked questions

15 1.6. What general groups of brush grades do exist ?
1. Hard Carbon – Carbon Graphite From the point of view of material characteristics, this material is between carbon (hard carbon) and electro-graphite. 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. It thus has a certain abrasive or polishing capability, but this is slight enough that flush commutator insulation cannot be abraded. Its main field of application is in universal motors with undercut inter-segment insulation. In the industrial carbon brush fields, the material is only used in special cases, where for example, electrographite brushes do not have a sufficient cleaning capacity, but hard carbon cannot be used, e. g. because of excessive friction. Frequently asked questions

16 1.6. What general groups of brush grades do exist ?
2. Graphite Depending on the raw material used, this group contains a greater or lesser proportion of very finely distributed inorganic impurities, which give the natural graphite a certain abrasive property as well as good frictional performance. On the one hand, this makes the material suitable for operation on steel rings at high running speeds, while on the other, it can be used in the form of so-calIed cleaning brushes as supplementary equipment, for example to remove slight burn marks or to counteract excessive film formation. Because of the particular structure of natural graphite, the material feels extremely soft and smooth. Natural graphite brushes can be loaded continuously up to 10 A/cm², but will also withstand short-time current peaks up to 20 A/cm² Frequently asked questions

17 1.6. What general groups of brush grades do exist ?
3. Electrographite Electrographite is the material with the widest field of application and therefore the most widely used material for carbon brushes. Electrographite is used, within certain limits, both on commutators and on slip-rings. Because of its high purity, electrographite protects the material on which it runs and, because of its crystal structure, it has very good frictional properties. Depending on the material structure and the operating conditions, the coefficients of friction normally are in the range µ = 0.1 to Some electro-graphites can be used up to relatively high peripheral speeds of 50 to 60 m/s, and in special cases, when special grades are used, up to 80 m/s. It is always possible, by varying the raw materials and the production process, to lay particular emphasis on individual properties and, for example, to produce materials with good current distributing capability and high overload capability, carbon brushes with high strength for severe mechanical stresses and brushes with high commutation capabilities. Depending on the field of application and the cooling conditions, the nominal current capacity of electrographite brushes lies between 12 and 16 A/cm² ( A/in²). Depending on the duration and type of material, peak loadings up to 60 A/cm² (387 A/in²) are possible. Frequently asked questions

18 1.6. What general groups of carbon brushes do exist ?
4. Metal-Graphite These materials are composed of graphite and metal powders, preferably copper, and thus have a relatively high electrical conductivity. Depending on the proportion of meta1 and the structure, the specific e1ectrica1 resistance of meta1-graphite is in the region of O.1 to 10 µΩm. This results in low contact resistance and voltage drop. Its hardness is relatively low. The graphite incorporated in the material gives the good friction properties which are necessary for satisfactory operation. With a high proportion of metal, metal-graphite brushes have a noticeably greater mass than metal-free carbon brushes, so that it sometimes becomes necessary to provide a greater contact pressure for these grades. The maximum permissible peripheral speed lies in the range of 30m/s. Depending an the metal content, current loadings up to 25 A/cm² (161 A/in²) are possible in continuous operation. The main field of application for metal-graphite brushes is in low-voltage machines with high current densities and commutation conditions which are not too extreme, and on slip-rings with high brush current densities Frequently asked questions

19 1.6. What general groups of brush grades do exist ?
5.Metal-impregnated Graphite The grades in this class have its porous structure impregnated with a metal. 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 electro-graphite based carbon. The grades are used for brushes on high current duty machines such as low voltage motors for battery driven vehicles and current collectors (for example pantographs for trains and trams).  Carbon bearings with white metal, antimony and other impregnations are used in mechanical applications. 6. Resin Bonded  Graphite As a result of the resin bonding, this materia1 has a relatively high intrinsic resistivity (of the order of100 to 350 µΩm) and also a high ratio of transverse to longitudinal resistance. The latter is due to the laminar structure of the graphite used. In conjunction with a high contact voltage, the material is therefore capable of greatly attenuating short circuit currents between segments bridged by the carbon brushes. It is therefore particularly suitable for three-phase commutator machines. But due to the improved load capacity at the present day, carbon brushes of this kind have also proved themselves increasingly on small and some medium sized D.C. machines and are used in relatively large quantities. Through the resin bonding, however, the lead capacity and especially the overload capacity is still low in comparison with electrographite brushes. Specific continuous current densities of A/ cm² (51-64A/in²) should not be exceeded for long periods. On D.C. machines, short-term peak values up to 12 A/cm² (77A/in²) are permissible. Frequently asked questions

20 1.6. Are slip ring and commutator brushes different ?
Slip-ring and commutator brushes can be distinguished in their application. Slip-ring brushes have only to transfer the current to a ring. They generally have the wider dimension tangential to the shaft with the appropriate number of brushes per ring based on the size and grade of carbon necessary to carry the required current. Low resistance electrographite brushes can be used for lower currents and metal graphite grades (metal content up to 90% metal) are used for higher currents. Brushes with 50%-75% metal content are the most common brushes used on slip-rings of induction machines. The construction of a slip ring brush will usually a solid block brush with the cable number and size to carry the relatively high currents. Commutator brushes generally have their widest dimension axially along the length of the commutator segments with the number of brushes per arm according to the required current and type of carbon used. Commutator brushes can be of the copper graphite type for less than 48 volt DC supply because of the high current involved. However, by far the bulk of commutator brushes will be made of electrographite with medium to high resistance depending on the load, the application and machine design. The construction of commutator brushes can be varied from a block brush to the most complex multi-wafer type with grades and other features from a large range of possibilities.  Frequently asked questions

21 1.7. General principle for grade selection ?
For practical reasons the answer is limited to slip ring drives, DC Machines. The OEM of the machine will generally select a grade appropriate for the design of the machine assuming it will operate at full load. However this is often not correct for the actual load. To avoid problems a brush specialist should be contacted with all relevant information like name plate data and actual load data. 1. Slip rings Low resistance electrographite grades are used for current density less than 10A/cm² (65 A/in²) and effective cooling conditions. Grades from the metal graphite class with 50%-75% metal are used for current densities up to 15A/cm² (97A/in²). Very high current welding jigs etc. require metal graphite up to 90% metal. 2 Low Voltage DC Motor (Up to 48V) Battery powered vehicles, starter motors etc. use metal graphite brushes with a percentage of 25%-75% metal. Generally the higher the voltage, the less metal percentage is required depending on the brush configuration. 3.General Industrial DC Motors Voltages from 350 to 500V will require electrographite brushes of medium to high resistance. Lower load can permit the use of higher resistance brushes. As a general rule, the brush grade with the lowest resistance which will achieve minimum arcing but still generate enough heat to permit good film formation, should be used. Frequently asked questions

22 1.8. Is there any shelf live for carbon brushes ?
Electro-graphite grades are manufactured at a temperature of 3000°C and are more or less “dead material”. Some additives may lose some of their effectiveness. However, this should not greatly affect the general operation, though the benefit of the additive may be reduced or lost. Metal graphite brushes or the cooper flexible connection may be subject to corrosion if stored in an unsuitable atmosphere. If corrosion is present, a possibility may be to order brushes with tinned shunts which will assist in protecting the leads against corrosion if stored for long periods Frequently asked questions

23 1.9. Are any safety measures necessary while cleaning motors ?
Carbon itself is not toxic but appropriate breathing protection is recommended when cleaning electrical machines from carbon dust, particularly with metal graphite brushes. Persons with existing respiratory conditions may experience irritation from breathing high concentrations of dust. The material safety data sheets (MSDS) are available for full details. Frequently asked questions

24 Chapter 2 Brush manufacturing
Frequently asked questions

25 2.1. How are brushes made ? Some metal graphite and resin bonded brushes for automotive applications and FHP motors are pressed to size and come out as a fully formed brush complete with the flexible connection lead. The bulk of brushes however are cut from blocks of raw material. Dimensions and the features of the brush are produced in manufacturing plants specialized for that purpose. The flexible connection (shunts or pigtails) are connected to the brush body by tamping or rivetting.   Frequently asked questions

26 2.2. How is the connection fixed ?
1 Tamped Connections The tamped connection is mechanically strong and of low electrical resistivity. A copper wire is placed in a hole and fine powder is compacted around the flexible with special machines, which guarantee a virtually solid bond between the brush body and the copper lead. The top of the tamped connection is sealed to prevent corrosion. This is the preferred and most effective method of wire connection for normal applications. 2 Riveted Connections This type of connection is also widely used. It is mainly applicable when brush proportions are not suitable for the tamped version or for soft carbon grades. The flexible lead is looped around a copper rivet fitted into a prepared recess and the rivet flared over to hold pressure between the wire and the carbon surface. The same rivet can be used to secure a metal top when fitted. However, this dual use of the rivet is not recommended. Frequently asked questions

27 2.3. What information is needed by a brush manufacturer ?
In order to recommend a suitable brush grade the carbon brush manufacturer need the following data: OEM of the motor - Power kW/HP - Voltage V - Nominal current A - Actual current A - Peripheral speed m/s - Number of Poles - Brushes / Pole - Brush dimensions Application In case of problems additional information is necessary. Description of the problem Present brush grade Actual number of brushes Frequently asked questions

28 2.4. How may I select the right grade?
There is no general rule for grade selection, but it requires a lot of experience. Much of the knowledge of a brush supplier on brush design, construction and operation is the result of correlation of reports on the behavior of brushes in service received from customers and from field engineers in all parts of the world. Results from the laboratories of the brush suppliers support grade selection. In order to make the right choice some basic information like OEM of the machine, application, actual load data and grade presently in use are helpful (see here…) Grade selection is always a compromise, since there are no “super grades” existing. Different parameters, sometimes mutually contradictory, have to be taken into consideration. Some trials, patience and time can be required to find the best grade for a particular application. Frequently asked questions

29 2.5. Why do brushes have top or bottom angles ?
The most common purpose of a top bevel is to promote side thrust on one brush face and thereby give stability of location on the brush holder.   A bevel in the contact face is applicable to brushes running in a trailing or reaction position relative to the collector. The values for bevel angles are important to the mechanical relationship of brush and holder. The reasons and supposed benefits of "trailing" holders and "reaction" holders and the appropriate angles of setting have been the subject of much differing opinion by machine designers.   Frequently asked questions

30 2.6. What is the purpose of a rubber pad on the top?
Rubbers and rubber/plastic composites can act as a damper in case of vibrations. They are known in a bonded and a loose design. Even in the case of block brushes the rubber/fibre pad absorbs some of the vibrations in the brush like a shock absorber which gives better brush face contact. The electrical insulation of brush top from the holder pressure device is a supplementary advantage. If loose dampers are used, care has to be taken that the pressure device of the brush holder is fitted correctly on the brush top. Otherwise it might happen, that the brush top hangs itself up on the upper end of the brush box.   Frequently asked questions

31 2.7. Why do some brushes have saw cuts in the face ?
These saw cut, which generally is only applied to slip ring brushes, serves two functions.  It collects and expels dust to the side. The cut interrupts the air cushion in the contact face, which develops at high speeds and might lift individual brushes from the collector. Thus the cut avoids the so called “aero-planning” of brushes and guarantees more uniform current distribution. Frequently asked questions

32 2.8. Why do some brushes have grooves in the side or internal face ?
These are grooves which minimize the risk of carbon dust building up on the brush faces which can result in the brush sticking in the holder. The grooves tend to clear the dust assisted by air movement through the grooves. Dust grooves are most commonly applied to slip ring brushes where the copper dust is more prone to building up inside the holders. DC motor brushes can also have grooves on the outside or inside faces. Low voltage forklift motor and traction motor brushes often have diagonal grooves across the outer faces. Frequently asked questions

33 2.9. How is the cable size and number determined ?
1 Flexible Length The length of the shunt is measured from the top of the brush to the centre of the terminal.  The cable must be long enough to allow full travel of the brush to its shortest position in the brush holder. If the cables are excessively long there is the possibility that they could foul in rotating parts, particularly in the case of Schrage type motors where the leads could catch in the moving gears. Another problem with long flexibles can be that in motors with high velocity cooling air the turbulence may cause some leads to move excessively and to get damaged by this movement. Placing a plastic band or a metal clip approximately midway along the length provides additional support. In some traction applications the copper wired can incorporate some steel strands to strengthen it. If the flexibles are relatively small in diameter placing a single insulation over two leads also gives additional support and keeps the leads tidy. : Frequently asked questions

34 2.9. How is the cable size and number determined ?
2 Flexible Diameter  The cable diameter is generally selected in relation to the maximum current that the particular brush can handle considering its dimension and the type of material used. For high current carrying metal graphite brushes the cables would be much larger than for an electrographite brush of the same dimension. Another consideration of the shunt diameter is potential short term overload. The brush material can stand overload to varying degrees, however the lead may be the limiting factor, particularly with high starting currents that can occur with, for example, traction motors or electric forklift motors. In these cases the shunt is best dimensioned to the largest cable practical to fit in the brush, taking into consideration how the flexibility of the lead my affect the free movement of the brush up and down in the brush holder. 3 Flexible Insulation Though wire insulation prevents the cable shorting to earth or other live components nearby, it is better not to fit it as a standard option, if it is not necessary. This is because it adds to the cost and decreases the radiation and dissipation of heat arising in the shunt. Frequently asked questions

35 2.10. Are there any alarm devices to indicate short brushes ?
There are some type of brush holders which have so called micro-switches built into the holder which can be combined with external circuits to indicate individual brushes or rows of brushes which require attention. The switches can be incorporated into the protective control system and be sued to raise an alarm or shut the machine down. Detectors can also be embedded within each brush which, combined with appropriate external circuits, can positively indicate brushes of critical length. If every brush has to be monitored the amount of cables in the machine is a major disadvantage of this method. Adequate short circuit protection must be used where these indicating circuits are directly connected to the brush holders as armature voltages and high prospective fault currents are present. Power tool brushes can have lift-off devices fitted into the brush which consist of a spring loaded pin which releases when the brush wears to the predetermined position. Frequently asked questions

36 Chapter 3 Theory Frequently asked questions

37 3.1. What is commutation ? The closed-circuit armature winding of a commutator machine must be regarded, in conjunction with the carbon brushes, as being built up from individua1 branches. The transition from one armature branch to the other takes place in each case at the point where the winding current is fed in or out through the commutator segments. As the armature rotates, the current in a coil of the armature winding must change its direction, when it changes from one armature branch to the next. This change of direction is called current reversal or commutation Welsch & Partner, scientific media Frequently asked questions

38 3.1. What is commutation ? Switching-off Switching-on 2 IA ½ VK - IA ½
Switching-off Switching-on Frequently asked questions

39 3.1. What is commutation ? When the coil is powered a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn toward the right The armature continues to Rotate Frequently asked questions

40 3.1. What is commutation ? When the armature becomes horizontally aligned the commutator reveres the direction of current through the coil, reversing the magnetic field. The process then repeats. Frequently asked questions

41 3.2. What role does resistivity play in a brush ?
Obviously the resistance of the material differs from one type to another depending on its ingredients and its production process, especially the final temperature treatment. Although carbon is unique among the non-metals in being a fairly good conductor of electricity, it is a poor conductor when compared with metals. The low resistivity electrographite materials are of the order of 10 µΩm. The corresponding figure for copper is µΩm. Thus the resisitivity of electrographite material is over 450 times that of copper while hard carbon grades can be ten times higher than this factor. Even so, the energy loss arising from this resistance of the brush material is only 10% of that which is dissipated at the brush contact surface due to contact-resistance and friction. Contact resistance, coefficient of friction and thermal conductivity of the brush have a much greater influence than the resistivity of the material. The measurement of resistivity is however a useful quality control test in production. Frequently asked questions

42 3.3. How important is brush hardness?
The Rockwell Hardness is well proven as hardness measurement of carbon ceramic materials due to its accuracy and repeatability. Brinell-Hardness and Shore-Hardness are used by some of our competitors, but in our opinion less suitable due to the worse repeatability. It is particularly valid for the hardness: There is no correlation of hardness and wear. The wear rate of carbon brushes or other carbon contacts like carbon strips is determined by various surrounding effects and the electrical and mechanical stability of the contact points in the contact surface of brush and strip, This stability cannot be described with the macroscopic value hardness. The most impressive example is a "soft" carbon brush grade , which gives a much better performance e.g. on large mill or fast running machines than "hard" brush grades. Cleaning and polishing action is also not determined by the hardness of a material. The ingredients of the material do have a much bigger influence. Commutator attack is not triggered by "hard" materials, but by the ambient conditions, uneven current distribution or electrical overload. The same is valid for the catenary wear caused by carbon strips. The catenary wear is particularly triggered by abrasive mineral ingredients as impressively. Appropriate information can be given on request. Frequently asked questions

43 3.4. What is contact drop or voltage drop ?
When contact is made between a carbon brush and a collector, an electrically conductive contact is made only at a limited number of points. This small number of contact points causes a reduction of the cross sectional area of the brush, causing an increase. In the electrical resistance. This resistance is called the constriction resistance. This, together with the resistance of the patina, forms the contact resistance, to which must also be added the resistance of the brush itself plus pigtail and terminal. The sum of al1 these resistances is cal1ed the contact resistance of a carbon sliding contact The voltage drop due to the contact resistance of two carbon brushes series-connected across a short circuited commutator or slip-ring is called the contact voltage. (In accordance with IEC Publication 276, voltage drop for two brushes in series) This is an important quantity for the user of carbon brushes since it inf1uences commutation and ohmic losses. The voltage drop is made up of the component voltage drops shown. The contact voltage drop ∆U3 of an electrographite brush grade of average resistivity amounts to about % of the tota1 voltage drop ∆u. The percentage is still higher n the case of metal-carbon grades. It is therefore permissible to use the contact voltage as a measure of the contact conditions in the practical application and evaluation of the quality of a carbon brush U3 U U2 U1 Frequently asked questions

44 3.5. How does the voltage drop influence commutation ?
In the case of dynamic current loading such as exists with commutation between carbon brushes and segments, the individua1 contact points have insufficient time to adapt themselves to the current densities which are constantly changing at relatively high frequency. The trend of the voltage drop which forms the criterion for the current reversal is therefore determined by the maximum current density which occurs. If the coil resistance and the inductance of the commutation circuit are predominant in a machine, the influence of the voltage drop becomes rather less important. The factor of contact stability with high current densities, such as occur during commutation at the edges of the brushes, becomes more important. This is also the reason why relatively low-resistance materials without a particularly wide commutation band but with good contact stability ( coke-based materials) have better performance on large machines than high-resistance materials which are less capable of withstanding surge loadings (carbon-black based materials). Things are somewhat different, f the commutation is not only influenced as a result of inductances, hut additional induced voltages are present in the commutation circuit. This is more or less the case with D. C. supplies with a relatively high harmonic content in the supply voltage. Here it sometimes becomes necessary to use high-resistance material, right up to resin-bonded graphite or even sandwich carbon brushes, with a high voltage drop in order to reduce the transverse currents caused by the induced vo1tages. If a machine shows mechanical difficu1ties, the commutation characteristic of a carbon brush as such can only be influenced to a slight degree, in order to improve the commutation, by the use of a different material. The greatest successes are obtained here by changing the design of the carbon brushes, e. g., with twin brushes etc. Frequently asked questions

45 3.5. What is current density
Current density is the value of the current passing through a particular brush in relation to its contact area and is expressed as Amps per cm² or Amps per inch². The actual current a brush can carry is widely influenced by operating conditions such as type of ventilation, continuous or intermittent duty, speed and other factors. The published data sheet ratings for electrographite brushes are generally conservative, some allowance having been made for short term overloads above those listed in the published data. The current carrying capacity of a brush depends ultimately on the operating temperature. On well-ventilated machines having small brushes with larger surface area in proportion to their volume and where brushes cover only a small percentage of the commutator or ring surface, conventional current densities for electrographite grades can often be doubled without seriously jeopardising their performance. On the other hand, increasing the current density without making provisions for maintaining a suitable low brush temperature may reduce the brush life dramatically. In practice low current density in a machine caused by running a machine below full rated load is potentially more damaging than a moderate overload. For good operating temperature and performance as a general rule, the actual operating current density should be not lower than 60% of the published rated current density. Frequently asked questions

46 3.7. How is current density calculated ?
DC Motors I = Current [A] N = Number of Carbon Brushes t = Tangential Dimension [cm] a = Axial Dimension [cm} 1000 A – 6 pole 5 cb’s each, i.e. 30 brushes, i.e. calculation with 15 brushes x 32 x 50 mm³ 1000A - 4 pole 5 ea. Tandem Brushes , i.e. 20 brushes, calculation with 10 brushes 12,5 x 32 x 50mm³ Tandem Brushes i.e. total dimension - t – is 25mm Frequently asked questions

47 3.7. How is current density calculated?
Slip ring drives I = Current [A] N = Number of Carbon Brushes t = Tangential Dimension [cm] a = Axial Dimension [cm} Asynchronous-Slip-Ring Drive 500A - 3 rings 5 cb’s, i.e. calculation with 5 brushes – 40 x 20 x 40 mm³ Turbogenerator 1000A - 2 rings with 10 cb’s each , i.e. calculation with 10 brushes – 32 x32 x64mm³ Frequently asked questions

48 3.8. What is the reason for low load problems and what are remedies ?
Standard conditions Low load Carbon brush Collector Contact point Patina Carbon brush Collector Contact point Patina Carbon brush Collector Plasma The film consists of Graphite and copper oxides copper oxide is a semiconductor High el. resistance at low temperatures current goes via some frit bridges only Frequently asked questions

49 3.8. What is the reason for low load problems and what are remedies ?
Carbon brush Collector Mechanical breakage of copper particles out of the surface Hard copper particle in the contact surface grooving Remedial action reduce number of brushes track wise use preheated cooling air reduce cooling air (ask OEM first) use a low load resistant brush grade Frequently asked questions

50 Probably error in measurement Possibly collector wear
3.9. What causes brush wear ? There are many variables which influence brush wear, but individual influencing factors can not be calculated nor evaluated separately. It is only possible to specify approximate values for brush wear. Peripheral speed Brush wear is normally specified in mm/1000h for industrial drives or mm/10.000km for traction machines. The wear is determined by the distance travelled by the brush on the surface of the collector External effects Probably error in measurement Possibly collector wear Frequently asked questions

51 3.9. What causes brush wear ? Electrical load
A film consisting mainly of metal oxides and graphite from the carbon brushes builds up on the surface of the collector. The motor current is transferred through this skin via metallic bridges formed by the process of “fritting”. Under low-load conditions and at low rotor surface temperature only a small number of “fritting” bridges develop with corresponding high, localized current densities. This causes copper particles to melt and deposit themselves in the brush contact surface. This phenomenon causes grooves and ridges on the collector surface. Sharp edges of these frit points increase the mechanical brush wear. Possible remedial actions are; Reduce the number of brushes track wise. Reduce the rate of cooling-air flow The commutator temperature should not drop below 60°C or exceed 90°C Use a low load resistant brush grade Frequently asked questions

52 3.9. What causes brushes to wear ?
Ambient influences Silicones It is generally prohibited to use silicone in the cooling air for motors with brushes ! The silicone components form an insulating skin on the collector surface. These components are degraded into silicon oxide (SiO2) by brush sparking, which increases the brush wear rapidly. The only feasible remedy in this case is to avoid the use of silicones. It must be ensured that air ducts are not sealed with materials which contain silicone. For suitable material please contact our field engineers. Oil, grease A similar effect can be observed in the presence of oil. By sparking underneath the brush the oil is cracked and transformed to hard abrasive particles. So excessive creasing of the bearings etc. should be avoided. Sulfur, ammonina, chlorine, oil Sulfur in the cooling air causes metal sulfides to form on the collector surface. The surface colour of the collector can then range from blue/black to grey. Ammonia and chlorine can turn the surface patina into a quasi insulator. Sparking will come up and grooves will develop on te collector surface and the brushes start to wear faster and very unevenly. Frequently asked questions

53 3.9. What causes brush wear ? Humidity
If the humidity is too low (< 3g water per m³ air), friction increases and causes chattering, which might destroy the brushes. As a remedial action specially impregnated brushes should be used. If the humidity is too high (>25g water per m³ air) the patina on the collector surface becomes too thick, resulting in too few fritting bridges and high localized current densities. Tracks then begin to form on the collector surface. As a remedy a different brush grade can be used. g/m³air Absolute Humidity wear ideal acceptable unacceptable Frequently asked questions

54 3.9. What causes brush wear ? Supply source
Current fluctuations caused for example by a poor or incorrectly set controller (dynamic response of speed controller is too high), result in rapid rates of current change in the machine and therefore to brush sparking. Spots develop at regular intervals around the commutator, depending on the armature winding design. Also flats in pole pitch can develop. The actual values at the controller should be checked by means of an oscilloscope. The speed controller should be optimized if necessary. Of course the operating guides of the supplier should be followed. Out-of-round commutator Out-of-round commutators cause the carbon brush to move in the brush box, to lift off the commutator and therefore to brush sparking. A distinction made is made between non-circularity, flats and high bars. The maximal permissible value is 80µm. Most important is the commutator profile, i.e. long wave or short wave out-of-roundness. The permissible out-of-roundness limits depend on the machine size. The difference in height between adjacent commutator bars must not exceed 2µm. If excessive deviations are detected, the cause must be identified and the commutator overhauled. Frequently asked questions

55 3.9. What causes brush wear ? Brush pressure
Electrical contact is made by means of pressing the carbon brushes on the rotating collector (contact pressure in cN/cm²). Contact pressure should therefore be regarded as an important factor together with the requirements for trouble-free current transference and low carbon brush wear. If the contact pressure is too low, it may, in combination with out of-round collectors and vibration, cause contact separation, resulting in brush sparking and arcing. This causes increased brush wear. If the pressure is too high, mechanical wear predominates. The values of pressure recommended by brush suppliers are the result of many years experience of normal operating conditions. The recommendation depends on the application, the brush grade in use and the overall conditions. Guide line values are given here… Frequently asked questions

56 3.9. What causes brushes to wear ?
Foreign bodies Foreign bodies like dust, cement etc. in the cooling air cause increased brush wear and grooving on the collector surface. Particles between brush and brush box will also damage the brush box, Remedial measures are Filter the cooling air Supply cooling air from alternative (clean) source Change cooling air flow direction Use brushes with dust grooves Frequently asked questions

57 3.10. What is reasonable brush life ?
The wear rate of carbon brushes depends on many parameters Electrical load Speed State of the collector Ambient conditions etc. Due to the multiplicity of influences it is difficult or even virtually impossible to give firm information about the wear rate to be expected in individual cases. Dependent on loading, operating conditions and carbon brush material, the wear rate for stationary machines lies normally in the region of 2 – 7 mm/1000 hrs. An available wear length of 20 mm, for example, gives a brush life of between 2,900 and 10,000 hours. In traction application the wear rate is usually given in terms of wear in mm per 1,000 km. Normal wear is regarded as being in the region of 0.2 to 0.35 mm per 1000 km. Uneven brush wear should only be objected to if there are large differences in length after a long running time. Smaller differences in length, e. g. 10 % should be considered as normal. . Frequently asked questions

58 3.10. What is reasonable brush life ?
Brush wear depends mainly on the distance the brush travels on the collector. An empirical formula for industrial application is: Brush wear (mm/1000h) = d = diameter (mm) n = speed rpm Frequently asked questions

59 3.11. What causes the commutator or slip-ring film ?
The patina is a complex composition. The colour on the surface of the commutator, which is called the ''patina'‘, '‘film'' or "skin'' is mainly copper oxide which forms on the commutator surface by a combination of temperature, oxygen, copper, graphite and other free particles. This oxide layer is very thin, approximately 100 times thinner than a human hair. Even though a good skin is in fact harder than the copper commutator material, it can be easily penetrated or damaged and is really quite complex. It is changing all the time with some things building it up while others are destroying it. Since correct brush operation depends on it, this patina or skin must be treated with extreme care and respect during maintenance. Every care should be taken to protect a good patina. Carbon Brush Face Charts indicate good commutator conditions however in practice many commutators have operated for many years with a less than ideal appearance. Provided the brush and commutator wear is within normal limits then action to try and achieve the ideal appearance may be pointless. Water Brush Metal oxides Graphite Frequently asked questions

60 3.12. What is a black band ? To assess the commutation capability of carbon brush materia1s and designs, black-band curves are generally used. For these curves, on machines with commutating poles, the commutating pole f1ux is varied by means of an additiona1 voltage applied to the commutating pole winding, until sparking occurs at the brushes. The measurement is carried out with various armature currents. The occurrence of sparking at the brushes is observed both when the commutating pole f1ux is strengthened (boosting current) and when it is weakened (bucking current). The limit curves of the boosting or bucking current, with which sparking occurs at the brushes as a function of the load current, are called black-band curves and the area between them is ca1led the black band. The wider the black band, the more reliable is the commutation process. Frequently asked questions

61 3.13. Why do some slip-rings have spiral grooves ?
Grooves have been used on slip-rings since a long time. At high peripheral speed so called air cushions are formed under a brush contact surface. This causes unstable contact between brush and ring and differential brush wear, which could lead to burned shunts etc. The grooves break up these cushions. Frequently asked questions

62 3.14. What is field weakening ?
DC motors can be designed for operation above their base speed. To accomplish this, the drive will run the motor up to full rated speed ( base speed) using full armature voltage and full field current. Then, to obtain greater speeds, the drive will keep the armature voltage constant but reduce the field current thereby achieving higher speeds. This area of operation is often referred to as the: • Constant HP area • Extended Speed Range • Field Weakened zone As the speed increases, available torque is reduced therefore, delivered horsepower remains the same. Motors that are designed with this capability are known as Field Range Motors and will typically have 2 speeds and 2 Field Currents stamped on the nameplate. See the example below. Motor Field Current 4.2 / 2.5 amps Motor Rated Speed 1750 / 2100 RPM The motor will deliver full Torque and Horsepower only at Full Field, Full Armature Voltage and Full Armature amps. Frequently asked questions

63 3.14. What is field weakening ?
The plot illustrates what happens to the field current, armature voltage and the motor speed once the drive crosses over into the Field Weakening Region Frequently asked questions

64 3.15. What is the neutral position ?
In a DC motor, commutation is the process of periodically reversing the current flowing in individual armature coils in order to maintain unidirectional torque as the armature coils move under alternate field poles. The commutator must reverse current through armature coils which left the influence of one field pole and are approaching the influence of an alternate field pole. The motor brush then contacts more than one commutator segment and an armature loop is momentarily shorted. If the short has a difference of potential across it's ends, severe sparking can occur between the brush and the commutator. The commutator then can burn and pit and brush life is reduced. It is thus necessary to insure that voltage is not induced in the commutator loop at the time of the momentary short. If the short occurs when the active conductors in the armature loop are moving in parallel to the field, magnetic lines of force will not be cut and voltage will not be induced in the armature loop. This vertical axis occupied by the shorted armature loop is the geometric neutral axis. In theory, this is where black commutation takes place. But life is not that simple! Due to the self induced e.m.f. and changes in load, the situation is somewhat more involved and beyond the scope of this article. In the end however, electrical neutral must be properly set to assure good commutation and good brush life. Frequently asked questions

65 3.16. Which limiting values of the leak resistance have to be kept ?
The insulation resistance of windings can deteriorate while the machine is running as a result of ambient and operating conditions. The critical insulation value at a winding temperature of 25 °C must be calculated by multiplying the rated voltage Urated (kV) with the critical resistivity (MΩ/kV), e.g. critical resistance for Urated = 690 V: 0,69 kV x 0,5 MΩ/kV = 0,345 MΩ If the measured value is close to the critical value, the insulation resistance should be regularly checked thereafter or the winding should be cleaned. After cleaned windings have been dried, it is important to remember that the insulation resistance is lower when the winding is warm. Insulation resistance can be accurately measured only when the winding is allowed to cool down to room temperature (approximately 20 to 30 °C). Measuring voltage: 500 V – (at least 100 V ) at a winding temperature of 25 °C    The minimum insulation resistance of new, cleaned or repaired windings must be > 10  MΩ. Frequently asked questions

66 3.17. How does the controller influence brush performance ?
Depending on the circuit and the modulation degree, supply units may deliver a D.C. voltage with a greater or lesser harmonic content, which impairs the commutation properties of the machine and can even cause vibration forces in the motors in extreme cases. The control circuits have short response times which can result in high rates of current rise and high surge stresses in the motors. We must distinguish between electrical and mechanical stresses. Electrical stress There is permanent stress due to the proportion of harmonics, differing according to the circuit and the operating conditions, in the D.C. voltage. There are also high rates of rise of current and surge stresses, which only occur occasionally during control or regulation processes. Harmonics in the D.C. voltage and eddy currents in the magnetic circuits make current reversal difficult. These increased stresses are counteracted by appropriate motor design ( e. g. lamination of the magnetic circuit etc.). With a high ripple or even discontinuous operation (in sma1l motors on simple equipments), however, considerable residual voltages still remain and must be dealt with by the carbon brushes. For this purpose, high-resistance carbon brush materials (including sandwich brushes) or even resin-bonded materials are used for preference. High surge stresses are more likely to occur with large machines. With such a surge load, in many cases a phase shift occurs between the commutating field and the armature field, resulting in severe sparking at the brushes. Since a steep rise in current is also often associated with a current overload, the commutation difficulties increase. Carbon brush materials which make good contact and are capable of withstanding surge loads are a suitable remedy. It is generally acknowledged that there is no universally applicable carbon brush for the field of D. C. machines fed from controlled rectifiers. High resistance, medium resistance or low-resistance materials must be selected depending on the type of stresses. Frequently asked questions

67 3.17. How does the controller ínfluence brush performance ?
Mechanical stress In principle, these stresses are initiated by the same factors which have already been mentioned. High ripple levels or discontinuous-current operation can cause continuous vibrations, resulting from magnetic forces. These have an unfavourable effect on the mechanical contact between the carbon brushes and the commutator. Electrical surge loadings also lead to mechanical stresses on the sliding contact of the carbon brushes, as a result of magnetic forces on the mechanica1 parts. With rapid changes of speed, the position of the carbon brushes in the holders can change, even to the extent of tilting during reversal. In all the cases mentioned above, the contact between the carbon brushes and the commutator can be improved if, for example, and provided the brush dimensions permit, split brushes are used rubber pads are provided on the brush head or the brush pressure is somewhat increased. Frequently asked questions

68 3.18. How does the brush design influence brush performance?
Block brush That is the most simple brush design, suitable for many applications. Twin and tripple brush In order to improve the contact conditions, a single block brush is divided into two or three brush wafers. Each of these wafers has its own power connection. It is important that the brush wafers are pressed uniformly on to the surface of the rotor. This is best achieved by means of a rubber or laminate plate laid or glued on to the top surface. Apart from giving uniform pressure distribution, the plate also ensures that the brush sections can move independently for a short distance in a radial direction – which means that the carbon brush can contact the commutator independently from its ovality. Where a commutator is out of true, the fact that the brush is divided also results in lower acceleration respectively inertia forces, so that the contact points on the commutator are subject to lower mechanical stress. Twin and triple carbon brushes have a larger number of contact points between the running surface of the carbon brush and the commutator, with the result that the local current density compared to a block carbon brush is lowered. At the same time this is associated with an extension of the commutation time so that the current reversal stresses are reduced. Block Brush Twin Brush Tripple Brush Frequently asked questions

69 3.18. How does the brush design influence brush performance?
Split brush This type is a special form of the twin brush. Both brush wafers have their upper surface inclined towards the middle of the brush. The two brush sections are spread apart – seen from the brush head side – so that the clearance between the carbon brush and the brush box is reduced, or even closed up entirely. With machines susceptible to oscillation the increased friction between the carbon brush and the wall of the holder (damping) gives better contacting with the commutator. Split Brush Sandwich brush Where commutation is difficult, so-called sandwich brushes may be used in order to avoid any difficulties that may arise, such as excessive sparking, scorching of the bars, heavy wear etc. Two wafers are bonded together by means of an insulating adhesive. As a result the cross resistance in the commutation circuit is increased, thus improving the commutation. The current supply to this kind of brush is normally arranged that every brush section has its own individual cable. With dual section brushes one of these is tamped into the area of the adhesive layer so that both carbon sections are contacted at the same time. Sandwich Brush Insulating layer Frequently asked questions

70 3.19. What are the main factors influencing commutation ?
“Commutation” is a quite often stressed word. The way it is used to describe the commutating capability of brush grades sometimes reads as though commutation is added to carbon material like pepper and salt to cooking. The course of the commutation process is mainly determined by the Following variables and properties: From the viewpoint of the machine: The inductance of the commutation coil Additional induced voltages in this coil The Ohmic resistance of this coil Concentricity and surface quality of the commutator and any vibrations of the brush holders From the viewpoint of the carbon brushes The electrical contact resistance between the carbon brush and the segment The so-called energy capacity of the current-carrying contact points of the carbon material, i.e. the capability of the contact points to carry given current densities without thermal destruction, according to the material grade The mechanical running performance of the carbon brushes. Uneven running of the brushes decreases the commutation time and reduces the number of contact points. The mechanical performance is determined by the friction coefficient, elasticity, mass and internal damping of the brush material. Frequently asked questions

71 Chapter 4 Brush holders Design an adjustment
Frequently asked questions

72 4.1. What types of brush holders are there ?
Flanschbürstenhalter Taschen-Bürstenhalter Schenkel-Bürstenhalter Tubular holder Flange brush holder Leg type holder Köcher-Büstenhalter Single brush holder Doppel-Klemm-Bürstenhalter Tandem-Bürstenhalter Bürstenbrücken Steck-Bürstenhalter / -elemente Double clamp brush holder Tandem holder Brush rocker Plug in holder Frequently asked questions

73 4.2 What is the pressure curve of a brush holder ?
A spring of a brush holder does not give constant force during brush wear. Depending on the spring type – see the next slides - the run of the pressure with the brush length is linear or like a curve. Those curves are measured by the brush holder manufacturer during QC inspection. Frequently asked questions

74 4.2 What is the pressure curve of a brush holder ?
Flat Spiral spring: simple - durable - no joints - wear resistant - economically Pressure Characteristic constantly dropping pressure (variation about 20-30%) Extension Spring mostly used for industrial holders better vibration resistance than Constant Coil Spring Pressure Characteristic: usually crescent-shaped curve. (variation about 10-15%) Frequently asked questions

75 4.2. What is the pressure curve of a brush holder ?
Constant Coil Spring directly and indirectly working possible with and without isolating role or carriage construction small tangential space requirement usage of brushes with max. length Pressure Characteristic: almost same pressure over the entire brush length (variation about %) Frequently asked questions

76 4.3.Do brush holders have a corrosion protection ?
For all components normally materials are used, which are already to a considerable grade corrosion resistant: sheet brass and casting, generally high-grade steel with joint and bearing bolts as well as screws, zinc plated steel for other parts For improved acid protection all brass parts can be nickel plated or chromium-plated and all steel parts can be made from stainless steel (A2 or A4) Frequently asked questions

77 4.4. What is the correct spring force?
Electrical contact is made by means of pressing the carbon brushes on the rotating collector (contact pressure in cN/cm²). Contact pressure should therefore be regarded as an important factor together with the requirements for trouble-free current transference and low carbon brush wear. If the contact pressure is too low, it may, in combination with out-of- round collectors and vibration, cause contact separation, resulting in current transfer by sparking and arcing. This causes increased brush wear. If the pressure is too high, mechanical wear predominates. The Figure shows the fundamental trend of the curves of wear. Frequently asked questions

78 4.4. What is the correct spring force?
Stationery commutator machines – Data in cN/cm² (PSI) All data in cN/cm² Frequently asked questions

79 4.4. What is the correct spring force?
Slip ring drives – Data in cN/cm² (PSI) Frequently asked questions

80 4.5. How is the spring force measured and adjusted ?
pressure sensor brush brush holder During QC inspection a computer controlled pressure sensor passes through the entire wear length of the brush and creates a diagram of the spring force. Frequently asked questions

81 4.5. How is the spring force measured and adjusted ?
New measurement with sensors Old measu-rement Frequently asked questions

82 4.6. Are there any standards for brush holders ?
The dimensions of the carbon brushes are standardized according to DIN Also single standards for brushes in different holder designs exist All usual holder types with connection dimensions and accessories are defined in DIN to DIN in single standards The dimensions and tolerances of brushes and brush holders are defined in IEC 136. The measurement of the brush pressure takes place according to DIN Frequently asked questions

83 4.7. How important is brush holder spacing ?
Max 2 mm A large gap between brush holder and commutator is disadvantageous as there would be a high turning moment caused by the friction force pivoted on the bottom edge of the brush box and the brush is not guided properly by the brush box. Frequently asked questions

84 4.8. What is axial staggering ?
In order that the copper wear resulting from the brush friction may be uniform on the whole surface of a commutator, it would be necessary that the covering rate of the brushes be the same on all parts during the full rotation. This is a theoretical condition and, in fact, it is not workable. However, there is a quite satisfying solution which is to stagger the brushes laterally and by pairs, with a distance of a/2, according to the figure: By this method, the intervals between the brushes, for each pair of lines, are covered systematically by the brush pairs of the following and preceding lines. On the other hand, in placing in the same track two brushes successively positive and negative, in order that each track may be covered by an equal number of brushes for each polarity, a frequent cause of apparition of stripes on the commutators is suppressed. This ordering of brushes on the commutator surface is called staggering or axial or lateral staggering. But this operation, easy to make, can be undertaken only on new commutators or renovated by the usual ways of grinding (lathe, grinding wheels, abrasive stone, etc) Frequently asked questions

85 4.9. What is circumferential staggering?
Circumferential brush staggering on a commutator is a method for improving the commutating ability of a machine, mainly when on overload. Circumferential brush stagger increases the commutator arc covered by the brushes by staggering tangentially one or more brushes on each arm. There are two types of staggering: Asymetrical according to the neutral line (b) for unidirectional machines with staggering in the direction of rotation; hence the name "advanced brushes" sometimes given to the staggered brushes. Symetrical according to the neutral line (d) for reversing machines. These two arrangements can be obtained by "all or nothing" as shown on b and c, or by progressive stagger as shown on d. By increasing in this way the number of bars covered by the brushes, the commutating time is increased and the speed of current reversal decreased in the commutated coil. The com-mutating difficulties of the machine are accordingly reduced. The effect of circumferential stagger is especially sensitive when: the number of commutator bars is great, the speed of the machine is high, the thickness of the brush is small ('t' dimension), i.e. the number of bars covered by a brush is smaller. Frequently asked questions

86 4.10. How much clearance should a brush have in a holder ?
Nominal values Brush tolerance on t and a dimensions Min/Max clearance on t and a dimensions 2 -0,03 / -0,09 0,044 / 0,144 2,5 3,2 -0,03 / - 0,09 0,050/0,158 4 -0,03 / - 0,11 0,050/0,178 5 6,3 0,055 / 0,193 8 10 12,5 -0,04 / -0,13 0,072 / 0,232 16 20 -0,04 / - 0,13 0,080 / 0,254 25 32 -0,05 / - 0,15 0,100 / 0,300 40 50 64 0,110 / 0,330 80 Unless otherwise specified, brushes are machined with the tolerances given in the table in conformity with the recommendation IEC Metal free brush grades Frequently asked questions

87 4.10. How much clearance should a brush have in a holder ?
Nominal values Brush tolerance on t and a dimensions Min/Max clearance on t and a dimensions 2 -0,06 / -0,12 0,074/ 0,174 2,5 3,2 -0,07 / - 0,15 0,090/0,218 4 5 6,3 -0,08 / - 0,17 0,105 / 0,253 8 10 12,5 -0,15 / -0,26 0,182 / 0,362 16 20 -0,16 / - 0,29 0,200 / 0,414 25 32 -0,17 / - 0,33 0,220 / 0,480 40 50 0,230 / 0,490 64 -0,19 / - 0,29 0,250 / 0,560 80 0,260 / 0,570 Metal brush grades Frequently asked questions

88 Brush and machine maintenance
Chapter 5 Brush and machine maintenance Frequently asked questions

89 5.1. Which regular machine checks should be done ?
Preventive maintenance is an important tool to prevent brush problems. The rules for preventive maintenance vary necessarily according to the type and size of machines, the particular conditions of service, the length of the running and rest periods The following points should be part of routine preventive maintenance: Watch for vibrations and noises which may appear during slow speed running and which may affect either the frame, the bearings, or even the brushes. By means of a location spot traced on the shaft of the machine check that the rotor does not stop in a preferential position As soon as the machine stops, measure by any appropriate means the temperature of the commutator and/or each of the rings. Thoroughly blow out the rotor and stator with dry compressed air from both ends of the machine in order to be effective, the blowing should remove all the dust to the outside of the machine Blow out and clean the filters with a solvent and dry. Wipe all insulators with a dry rag. Brush between the commutator bars with a fibre glass or nylon brush. Check for concentricity of the commutator (or ring) by applying a comparator to the top of a brush. Insulation resistance of windings to be measured four times. When warm, when cold, before and after blowing. Measure with a spring balance the pressure applied by the brush holder pressure fingers on the top of the brushes. Frequently asked questions

90 5.1. Which regular machine checks should be done ?
12. If necessary, measure the wear of the bearings by checking clearances at 90° apart ( and 12 o’clock) with a set of appropriate feelers. Measure, if necessary, the length of all brushes on one arm to see if there is abnormal wear Examine the bar chamfers and look for incipient pitting or shading of the bars or burning of the trailing edges and oil leakage. (If the skin appears a little too thick, it is advantageous to pass a flexible abrasive cleaner over the commutator or the rings before putting the machine back into operation). Examine the groove edges of helically grooved rings (a cutting edge on the rim of a helical groove always brings about rapid wear of the brush) and for incipient marks or burns). Carefully check that surface of the commutator or slip-rings is not polluted with oil Note first that there is no copper dust deposited on the brush-holders Check that the contact edges of the brushes are not chipped or burnt and that the surfaces do not have any vibration or burn marks. Examine if necessary the interior of the brush boxes for smoothness and cleanliness. Check that the brush flexibles are not oxidised, burnt or frayed. After putting the brushes back into their brush holders verify that they are sliding normally in their boxes, that the pressure fingers are in their correct position at the middle of the top of the brushes, that the flexibles are not pinched by the pressure fingers. Remove all brushes especially if the machine has to spend some time in a humid, salty or chemical atmosphere or put a piece of paper between the brush and the commutator or rings. Frequently asked questions

91 5.2. What are suitable parameters to indicate motor performance ?
Frequently asked questions

92 5.3. At what length should a brush be replaced ?
Normally a brush should have any wear mark, either a special wear mark at one side surface or stamped lines on the front surface indicating the remaining length. Sometimes also the lower edge of the brush marking indicates the wear mark. Usually this wear mark is set 2 – 3mm below the lowest point of the tamped contacts. Electronic brush wear indicators also may give an alarm, short before the tamped contact touches the collector. Frequently asked questions

93 5.4. Do I need to bed a brush in ?
From the outset, the carbon brushes must be as fully as possible in contact with the commutator or ring surface both over their whole tangential width and their axial length. In most cases it will therefore be necessary to bed in new carbon brushes, and in any case when a complete set is changed. However, if the diameter of the commutator or ring is very large in comparison with the size of the brushes, bedding-in may be omitted in certain circumstances. This also applies when only individual items of larger set have to be changed. Frequently asked questions

94 5.5. How are brushes bedded in ?
Various methods of bedding-in are used in practice. One very attractive method for large machines is to fix an emery cloth with adhesive film. By slowly turning the rotor in the normal direction the carbon brushes are then bedded-in. After removing the emery cloth, the commutator or ring surface must be carefully cleaned of any remaining adhesive. Another possibility is to draw a strip of emery cloth to and from beneath the carbon brushes. In the case of non-reversing machines it is essential however that the last grinding operation should be carried out so that the emery cloth is only drawn beneath the brushes in the direction of rotation of the machine. When the cloth is drawn back, the brushes should be lifted. If, on switching on the machine, the carbon brushes are to be in effective contact over their whole surface, this can be achieved by using a soft pumice stone which is held against the ring or commutator in front of the brushes while the rotor rotates. The pumice powder thus produced finds its way under the brushes and beds them in. The disadvantage of this method is the relatively large accumulation of dust which makes a subsequent thorough cleaning essential. It has the advantage that any patina which exists becomes roughened making for an improvement of the running conditions. Wrong O.k. Frequently asked questions

95 5.6. What should be done if changing grades ?
When the brushes being fitted are of different grade from those previously used, then it is advisable to remove the collector patina left by the previous grade brushes. Assuming work on the machine has not already involved turning or cleaning the collector, this skin removal should be done by scouring with an abrasive stone while the collector is rotating. Do not finish with a lapped slightly polished surface; a moderately abraded collector surface will help the new brushes to run in and the collector will acquire readily the patina appropriate to the new brush grade. When the machine is running well and it is simply a matter of replacing worn brushes by new ones of the same grade, it is best to leave the developed film of the collector completely untouched. Frequently asked questions

96 5.7. What happens if grades are mixed ?
It is highly inadvisable to use different brush grades on a machine, mainly because it is then rarely possible to get a uniform distribution of current between the brushes. Main reason is the different voltage drop of different brush grades. Burnt shunts and destroyed brushes can give rise to major malfunction of the motor. When there has been such intermixing of grades it is often difficult to determine which grade is the correct one; the grade giving the worst performance may well be the one which would behave the best if used alone ! Frequently asked questions

97 5.8. How can I set the neutral position?
Machine at rest, armature disconnected and locked. The poles are fed with low voltage alternating current rendering unnecessary the switching required when using D.C. The brushes are left in place on the commutator. An auto ranging voltmeter is connected to the terminals of the armature. Rotate the brush holder ring until the voltmeter, set to its most sensitive scale, shows a minimum reading in mV In each instance it is advisable to repeat the operation for two or three different positions of the armature. It is useful in practice to mark the yoke indicating the best brush rocker position for each measurement. This will result in a series of marks surrounding the true neutral position or they may coincide. Repeating the process by moving the rocker in the opposite direction will produce a point coincident with the first series. This will be the neutral. V ~ AC current Frequently asked questions

98 5.9. Can commutators and slip-rings be ground in the machine ?
A small degree of out-of-roundness can generally be eliminated by grinding the commutator or ring surface with a silicon-carbide grinding stone which is applied manually to the collector surface. With the machine running at rated speed, if this is possible, the grinding stone should be guided with uniform light pressure and an oscillatory motion over the whole contact surface to ensure uniform grinding. In order to avoid increasing the out-of-roundness when grinding, the tangential dimension of the stone should be at least 2 x as large as the defective place to be removed on the rotor. It is expedient to move the stone along a guiding aid which for example can take the form of a rigid straight sheet metal strip fixed to a bar or straight edge. Tangential movements or oscillations of the manually guided stone can thus be avoided. If the defective places are very wide or if the commutator is severely out-of-round, grinding must be carried out with a stone having a fixed means of guidance. For this purpose it is best to use suitable equipment which can also be fitted with rotating grinding wheels. A rotating grinding wheel must turn in the opposite direction to the rotation of the rotor. For the machining of steel rings coarser stones should be used than for copper or non-ferrous metals. In our experience when machining copper and non-ferrous metal, an SC 80 K4 BA stone is very suitable for the preliminary grinding and an SC 220 K3 BA (DIN designation) for finish grinding. If steel rings have to be machined an SC 46 K5 BA stone should be used for preliminary grinding and an SC 80 K4 BA stone for the final machining. When the latter grindstones in each case are correctly applied to the ground surface, they will produce a surface structure with peak-to-valley heights of the order of RZ = 5 – 8 µm (Ra = 0,8 – 1,2 µm) which is favourable for the running-in of carbon brushes Frequently asked questions

99 5.10. When and how should commutators be turned ?
If the out-of-roundness or collector run-out is very great, it is recommended on account of the higher metal removal rate, that the commutators or rings should be skimmed. Furthermore, skimming produces less contamination (no grinding dust). Skimming can either be carried out in situ (mainly in the case of medium size or large machines) or by turning the dismantled rotor in a lathe. If the rotor is not skimmed while on its own bearings, but is held for example between centres, a check must first be made after setting up in the lathe to ensure true running in the bearings. If this shows up deviations greater than 0.01mm, re-centring is required. If skimming is carried out in situ, any axial play in the bearings must be removed. If there is a risk that the insulation bars between the segments are no longer sufficiently undercut after turning or are even flush, they must be re-cut. Both diamond and carbide tipped cutting tools can be used for turning the machines in question when they have undercut segment insulation bars. When the insulation bars are flush only carbide should be used. Diamond is used in metal cutting predominantly to give a super-finish, i.e. it has advantages when surface with minimum peak-to-valley heights have to be produced at high speeds. Since we are not aiming here for any mirror smooth surface finish, carbide may be preferred unless the greater service life of diamond is the decisive factor. Our experience shows that cutting speeds of the order of 160 – 200 m/min are advisable with carbide tools and 300 – 350 m/min with diamond tools. The feed should be about 30 µm at a cutting depth of 0.05 – 0.1 mm. Frequently asked questions

100 5.11. Is undercutting necessary ?
On machines used today in industry and on the railways, practically the only carbons used are of a quality which is not capable of wearing down the insulation bars between the commutator segments. The insulation bars must therefore be neatly undercut. Imperfectly undercut insulation or even mica which remains standing proud leads to contact problems creating collector flaws and high brush wear. There are not fast rules to be observed for the form of the segment grooves, the sawing, scraping or milling out of the mica, or for the most favourable depth of the insulation groove, or for the edge chamfer. It has generally been found that a milled-out depth equal to the width of the insulation is sufficient. It is important to ensure that no mica or insulation residue remains at the edges. The milling out should therefore be wider by about 1/10 mm than the insulation bar which also takes account of possible dividing errors; see the illustration at the left side. The most usual angles for the edge chamfer lie between 60° and 90°. Undercutting the segment insulation bars of small machines can be done manually. It is however advisable where a higher quantity of pieces is involved, to use special machines which are offered by the industry for this purpose. If the edges of the segments have to be chamfered, this should be done before the last turning or grinding operation so that any flash which may stand proud can be removed. The grooves must be thoroughly cleaned after machining. Frequently asked questions

101 5.12. How does oil influence commutators and brushes ?
Oil, grease and other non-conductive substances which get on to collectors form an insulating film so that the number of electrical contact points are decreased. The very high temperatures occurring locally at the remaining contact points lead to melting of the collector metal so that we get a similar effect to that obtained with fritting. In addition the oil and grease is “cracked” by the thermal conditions and very hard oil coke formed which increases the grooving and the brush wear. Frequently asked questions

102 5.13. Can brushes be washed in solvent ?
Carbon materials are like a sponge. The fine capillaries connecting the pores would be filled immediately with any solvent and would never let it come out. . Since the solvent might have a negative influence on film performance it is not advisable to wash brushes with any cleaning agent. If the collector surface has to be cleaned from oil etc only solvents recommended by brush suppliers should be used. During the cleaning operation the brushes should be removed from the brush boxes and the motor. Frequently asked questions

103 5.14. What atmospheric contaminants affect brushes?
Chemically aggressive gases like H2S, SO2, HCl e.g. in the paper industry quickly build up a badly conducting layer on the collector surface, which are destroyed by intensive fritting. This of course leads to grooving and high brush wear. The counter measures depend on the concentration of the aggressive substance, In case of slight contamination only a so called “self cleaning” brush variant or a sandwich brush design can be used. It is also appropriate in such cases to use carbon brush materials that lay down a thick graphitic patina as a protecting film. Frequently asked questions

104 5.15. What happens if the machine is subject to vibration ?
Brush oscillation can, dependent on intensity, lead relatively quick to damaging the brush flex as well as to step formation at the base of the brush. Similarly rattling can lead to breakage and splitting off of the brush particularly in the running surface area. Brush oscillation and especially rattling also lead to contact interruption between carbon brush and collector. If a commutator or slip ring is perfectly round and the brushes rattle then the general cause is excessive friction (out-of-roundness protruding mica, etc. naturally give rise to vibrations). Very smooth, polished patina whose formation has been encouraged by light load or even no load running leads to the sliding of the carbon brush bordering on the limit of static friction. With other systems this creates the so-called “stick-slip-effect”. Carbon brushes with a correspondingly higher friction are excided into a rocking oscillation which we refer to as “rattling”. The brush rattling can be reduced if the pressure finger of the brush holder presses on the brush top such that the direction of the force on the brush top bears on the leading edge area. In this case the reaction moment at the brush becomes very small – this is of particular importance when there is variable friction. When the friction force oscillates the reaction moment has the effect of stabilizing the brush and compensates for the friction. Frequently asked questions

105 5.16. How to measure the collector temperature ?
The commutator temperature is very critical for brush performance. The optimal temperature range is 60 – 90°C . There are three common methods how to measure temperature: Infrared instruments Since the measuring principle of those instruments is based on the extinction coefficient, they are very suitable if the material to be checked has a uniform coloration. Unfortunately slip-rings or commutators are quite un-uniform in colour. So the IR-instruments do not give an accurate measurement ! Thermo couples The measurement with thermocouples is the standard test method of carbon brush field engineers. Disadvantage is that the measurement can be done only during a machine stop. Due to some heat dissipation until the stop and during the measurement the value is also not very accurate, but at least reproducable. Temperature test points There are test points for different temperature ranges. These points change colour with temperature They are put onto the collector service and can be checked after some time of running. They give a relatively exact hint of temperature conditions while machine operating. Frequently asked questions

106 5.17. What has to be done if the motor has to be stored for a longer period ?
The following procedures are recommend in case the motor is stored for a longer period in the warehouse : Motors must be stored in a dry, well ventilated area which is free of vibration and dust. Do not remove the rotor shipping brace (if fitted) Open the pressure fingers of the brush holders and take the brushes out of the holder boxes. Remove fitted keys. Protective unit against condensation caused by temperature fluctuations. Protect unit against ingress of aggressive gases such as hydrogen sulfide, chlorine or ammonia. Once a month, remove the rotor shipping brace, turn the rotor by 360° and replace the brace again. Replace the anti-corrosion protection on the shaft extension if necessary.    Before commissioning, remember to:      Measure the insulation resistance (see Operating Guide). Check the commutator surface and clean if necessary. Mount the brushes and close the pressure fingers. Lubricate the bearings, clean and grease the bearings if necessary. Insert fitted keys. Frequently asked questions

107 5.18 Why is surface roughness so important ?
Graphite Surface roughness is one of the main factors determining brush performance. If the surface is too smooth, the coefficient of friction may periodically become excessive at fairly low speeds since the carbon brushes are almost subject to static friction, while at higher sliding speeds there is a risk of aerodynamic effects. . Both lead to poor contact over the circumference of the collector. In addition, the carbon or graphite particles which are abraded from the running surfaces of the carbon brushes do not adhere sufficiently to smooth surfaces, so that a patina only forms very slowly and incompletely. This then quickly causes damage to the surface of the segments in the form of flaws especially in the case of highly stressed commutator machines, resulting in sparking at the brushes Rough surface Smooth Surface Frequently asked questions

108 5.18 Why is surface roughness so important ?
If the surface of the collector is rough, high initial wear may occur. But since the surface is getting smooth quite fast, this problem is only minor. But on a rough surface the graphite of the carbon brush can be deposited much faster. Generally speaking a somewhat rougher surface is considerably less critical for performance than a smooth one. For optimum operation of the brushes, the surface roughness of the collectors must therefore lie within the limits of being not too smooth and not too rough. Trials and experience have shown that a peak-to-valley height RZ of the order of 5 – 8 µm should be aimed at. After machining, the collector surface must not have a very mirror like appearance. Such a surface is a certain sign of too low surface roughness It should rather have a tendency towards a mat finish. If slip rings or commutators are found to be in satisfactory condition before making a replacement and if a good patina has formed on the surface it may be possible to leave them as they are. If, however, they are very smooth and have a mirror finish, it is advisable as already mentioned above to roughen this patina slightly with a grindstone so as to avoid problems in operation Before skimming After skimming and grinding Frequently asked questions

109 Common operating problems
Chapter 5 Common operating problems Frequently asked questions

110 6.1. Why do shunts fall out of brushes ?
Tamping Tube Flexible Brush Modern brushes have a so called tamped connection of fkexible and brush body. Its mechanically strong and of low electrical resistance. There are supplementary strengthening processes. Although brush manufacturers use machines for effective and safe tamping procedure, the contact can fail due to heavy vibrations in service. If a certain limit is exceeded, the mechanical strength of the connection is not strong enough. In some cases the embedded connection shakes loose and in others the flexible frayes and brakes from metal fatigue. An other possible reason for a failed tamped connection is overheating due to commutation problems. In this case mostly the cable at the trailing side is overloaded and the current limit for the cable exceeded. Then the tamped connection acts like a small oven and the connection will fail. The same happens in case of uneven current distribution. Frequently asked questions

111 6.2. What causes commutator grooving and threading ?
Grooving - is the uniform circumferential wear, the width of the brush that is exhibited on the commutator. Excessive abrasive dust in the atmosphere or on abrasive brush can cause this condition. Extreme light spring pressure can also cause this condition. Proper brush applications and filtering the air on force ventilated motors can reduce the commutator wear, Some people call this "Ridging" because of the resulting ridges on each side of the groove. Threading - is damaging of the commutator by copper particles in the brush face. The excessive copper transfer occurs due to low spring pressure, light loading or contamination. These particles are trapped in the porous carbon brush and work harden, creating a tool that machines or Gaul's the commutator surface. The machine can operate for long periods of time with this condition, but reduced commutator and brush life will be experienced. Frequently asked questions

112 6.3. Why do commutators get flat spots ?
The development of flats on a collector is a very good example of the chain reaction started by a small cause and building up to a major fault. A flat gives rise to destructive sparking which increases the size of the flat by burning. The brush bounce caused by the larger flat promotes increased sparking and burning and then damage to the brush holders caused by vibration and the trouble is aggravated still further. Flats may be caused by vibration originating from worn bearings, defective gearings or couplings, machine out-of-balance or disturbance fed back from connected plant. Flats may also be caused by the burning which arises from a disturbance of the brush contact by high inter-segment insulation particularly at the edges of segments where the recessing of the insulation has been imperfectly carried out. But more frequently flats are the cumulative result of electrical faults which in the early stages can be recognized by segment burning, by blackening on commutators or by blackened patches on slip rings. Loose or high resistant joints between the armature winding and the commutator cause burning which starts at the segment in advance of the bad joint and then spreads as the burning destroys the contact between commutator and brush. Flats resulting from this cause often have “sympathetic” flats spread at one pole pitch apart around the commutator and are due to the unbalance in the armature circuit arising from the bad current collection at the original flat. Sometimes bar marking occurs in a regular sequence in which every third, fourth or fifth segment either is burnt or is darker or cleaner than the intervening segments. The segments thus marked occur at slot pitch frequency, according to the number of coils per slot, and this phenomenon is a symptom of unsatisfactory commutation conditions. Slip rings may develop flats from the same mechanical causes as for commutators and an additional electrical reason may be unequal current sharing between brushes. An occasional cause of flatting on slip rings is the tarnishing or rusting of the exposed portion of the rings whilst the machine is hut down, particularly if it is out of service for a considerable time. Frequently asked questions

113 6.4. What causes brushes to get stuck in holders ?
The mechanism causing the problems is different for the main materials for industrial and traction brush grades. Metal graphite brush grades consist of different portions metal powder and graphitic components. Additionally additives can be used to improve the performance. Depending on the metal content metal bridges are formed during the sinter treatment (see micro photography) or the metal particles are more singularly distributed. These metal particles have a large, active surface. At elevated temperature, possibly in combination with high humidity, this surface is covered by a layer of metal oxides, even in the inner spheres of the grade. The oxidation goes with an enlargement of the volume. The brushes get larger, are swelling and get stuck in the brush holder box. Finally they will be destroyed by arcing. Collectors and slip rings are heavily damaged by the arcing as well. Typical problems arise, if brushes are electrically overloaded, i.e. are permanently used above the current density, permitted by the brush manufacturer. The swelling is triggered by high humidity. Electrographite grades don’t show any dimensional change while heated. So Brush made of electrographite grades normally get stuck only if brush dust, mostly in combination with oil, adheres at the brush side surface and causes additional friction in the brush box. Frequently asked questions

114 6.5. Why do brushes sometimes wear differently in the same machine ?
Many customers ask : „Why do brushes on a specific machine show a non-uniform brush wear ?“ Fluctuations in the brush grade are often held responsible. The list below shows some possible causes: Non-uniform out-of-roundness from one brush track to the next Non-uniform brush pressure Non-uniform total down-force Non-uniform characteristics of the spring pressure with the brush wear Variable friction in the brush box Differences in temperature Non-uniform current distribution Non-uniform commutation Stability of the brush holders and brush stud assembly Differential polarity Non-uniform brush spacing ( pole pitch) Non-uniform brush orientation Non-uniform brush seating Non-uniform electrical connection Frequently asked questions

115 6.6. Why do some brushes sometimes have overheated flexibles ?
Overheating of individual brushes and burning of flexibles arise from non-uniform current distribution between brushes. The reason for this may be: Brush mounting and maintenance: Brushes sticking in holders Uneven brush pressure Loose terminal screws, dirty terminals Flexibles to short or stiff Indifferent brush bedding Machine Adjustment: Unequal brush spacing around the commutator Incorrect commutation conditions Discoloration of the flexibles in one brush wafer of a twin brush is a typical indication for commutation problems, since the trailing halve of the brush is normally overloaded in case o commutation problems. Frequently asked questions

116 6.7. What causes brushes to wear and dust excessively ?
The important word here is “excessive”, A certain amount of wear is inevitable when there is current collection through sliding contacts. It occurs as a result of mechanical abrasion and of electrical erosion, and by burning from sparking , whether visible or not. A rate of wear which can be considered reasonable for one type of machine could equally well be regarded as excessive for other conditions. It largely depends on the load and duty of the machine concerned. For examples machines subject to high current overload peaks of short duration interspersed with periods of lighter load, e.g. traction motors or generators supplying rolling mill motors, generally have a better brush life than those machines which run mostly at full load on industrial applications. The rate of wear is determined more by the current loading in the most worked portion of the brush contact face than by the apparent load as indicated by the machine current and brush cross-sectional area. Excessive brush wear is more often electrical rather than mechanical in origin, but it must be remembered that mechanical imperfections and bad maintenance often give rise to wear which is electrical in character. Among the electrical causes the most frequent are: Insufficient brush pressure Current overload due to machine overload, Incorrect current distribution in the brush surface because of commutation conditions, Unequal load distribution between brushes caused by non uniform brush pressure, Unequal circumferential spacing of brushes non uniform air gaps Indiscriminate mixing of brush grades Some brushes sticking in their boxes, giving abnormal heavy wear on individual brushes Apart from these machine conditions, high brush wear may be caused by the use of an unsuitable brush grade. Frequently asked questions

117 6.8. Why do some commutators show regular light and dark patterns on the segments ?
Sometimes bar marking occurs in a regular sequence in which every third, fourth or fifth segment either is burnt or is darker or cleaner than the intervening segments. The segments thus marked occur at slot pitch frequency, according to the number of coils per slot, and this phenomenon is a symptom of unsatisfactory commutation conditions, i.e. incorrect commutating poles strength or brush position. Because the armature slot is moving with respect to the commutating pole whilst the coils in the slot are passing through their cycle in succession, the commutating field conditions are not necessarily the same for all the coils in the same slot. A small brush shift may assist. When cooling conditions permit, any tendency to produce slot-pitch “bar marking” can be reduced by the use of a brush grade of higher contact drop. Alternatively a brush grade may be used which has a controlled degree of mild abrasive and polishing action. Should either of these methods fail, steps must be taken to widen the interpole flux zone so that a brush position may the be found at which commutation conditions for all the coils in the slot are more nearly alike. This involves changing the commutating pole shoes or commutating pole air gap or both and subsequent re-adjustment of the commutating pole ampere-turns. Frequently asked questions

118 6.9. What causes differential wear of slip rings ?
The amount of collector wear which takes place under the cathodic brush (i.e. current flow from collector to brush) is several times as much as that under the opposite polarity. This is due to the transfer of metal by ionic conduction in a manner very similar to that of electroplating. This effect is most readily to be seen on synchronous motors, where the two polarities are completely separated. Fortunately there is only a very small proportion of the total current conducted this way, otherwise the rate of polarity wear could be very much greater than is actually experienced. On these synchronous machines the rotor DC excitation current is fed in by means of slip rings. Generally the wear on negative rings, i.e. current from ring to brush, is more than that on positive rings. To prevent this disparity becoming too pronounced it is often advisable to change the polarity of the rings at regular intervals if at all practicable. Where brush current densities and ring cooling conditions permit, brush grades without metal content should be used because ring wear problems are then much less pronounced than with grades of the Metal-graphite class. Brush instability from poor mechanical conditions and insufficient brush pressure can aggravate this ring polarity wear effect. Frequently asked questions

119 6.10. What causes selective action ?
If simultaneous readings are taken of the currents carried by the individual brushes of a set at any one instant, it will generally be found, that the values differ greatly. At another instant this pattern of non-uniform current distribution will be quite different and a brush which formerly carried little current may now carry far more than its share. The graph shows, that a slight change in the voltage drop of one brush e.g. by slightly different brush pressure or a temperature difference can cause quite a big difference in current density. If there is any factor which causes a permanent difference in the current carried by each of the brushes of a set, then the brush carrying the most current will run hotter and wear faster than the remainder. A brush which is running hot tends to have a lower contact drop than its neighbor. Sometimes one brush arm will carry more current than other arms of the same polarity. The reason should be sought in some fault in the machine adjustment, for example the non-uniform spacing of brushes round the periphery of a commutator. Frequently asked questions

120 6.11. What is out-of-roundness ?
Flat spots, segments which protrude or are too low, flaws etc. – in short, all deviations from circular form, whether they occur during skimming or grinding, or during operation – are designated out-of-roundness. They cause violent brush movement and sparking especially at high circumferential speeds. This results in increased out-of-roundness, greater wear of the carbon brushes, and, in some circumstances, severe damage to commutators, rings and carbon brushes. Frequently asked questions

121 6.11. What is out-of-roundness ?
As a guide line in the case of mean circumferential speeds, a total out-of- roundness of 0.1 mm should not be exceeded and a difference in height of 0.01 – 0.03 mm between adjacent segments should be regarded as the limit. The value for the maximal permitted bar-to-bar distance depends on the frame size and the peripheral speed. Permissible bar to bar distance Permissible total out-of-roundness Frequently asked questions

122 6.12. What are reasons for copper drag ?
The term copper dragging describes a situation in which a fin of copper grows from the leaving edge of a commutator bar toward the next segment, thus reducing the gap between them and making the machine prone to flashover. Copper drag is often associated with high frequency vibrations to give a hammering effect on the commutator. This is sometimes met on machines with periods of low load running such as rolling mill motors. In case of low load conditions the film disappears and the friction coefficient increases, causing the friction problems. So all action working against the low load problem, do also help against copper drag. Correct bevelling of the segment edges play an important part in delaying the onset of copper drag. Frequently asked questions

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