Carbon Brushes Notes

1.  INTRODUCTION

Brushes provide connection between rotating armatures and external circuitry, and play a  major  role  in  satisfactory commutation of DC machines.

During commutation,  in the armature coil under short circuit by the brush,  the current reverses from +I to -I.  Since the change of current takes place in a very short period, an emf is induced in the armature coil undergoing commutation.  Commutating poles are provided to nullify this emf by creating an equal and opposite voltage in the same coil.  However due to  design limitations/manufacturing  tolerances,  it  is  not  possible  to totally balance out the induced emf (known as reactance voltage), and   therefore  the  residual  voltage  in  the  coil  causes  a circulation  of current,  which appears in the form  of  sparking under  the  brushes.  As the process of commutation  became  more apparent,  it  was realised that a brush of comparatively  higher resistance could materially assist the commutation.

In  the early experimentally period, before 1880,  when DC motors were under development,  copper brushes, in the form of brush and not  as a solid block,  were used.  It was from the early  period that  the term brush emanated,  and is still  continuing.  Copper brushes used to cause high commutator wear,  heavy sparking,  and even welding into the commutator surface.  these problems and the fact that higher resistance of the brush assists commutation, led to the use of the carbon as a brush material.

The  other  reason  for using carbon for  brushes  on  electrical machines  is  that  the wear of the carbon brush  and  electrical erosion,  considerably  exceeds that of commutator  resulting  in higher commutator life.

Charles  Van  Depoele,  one  of the early  traction  pioneers  in America,  was the first to try brushes made of carbon on traction motors after successful trials in 1884.

2.  BRUSH MATERIALS

Carbon is used for the brush in the following forms:

     1. Natural Graphite

     2. Hard Carbon

     3. Electro-graphite Carbon

     4. Metalized Carbons and Graphite

These grades of carbon are obtained by varying the combination of raw materials,  and the processes followed for mixing  them.  The following  chart  shows  the cycle used for  production  of  non-metallic carbon brush material. flow sheet for the production of non-metallic brush material

A  few examples of processes and material variants and  their usual effect on the performance are given below:

Raw materials

Graphite        -   decrease friction

Copper, Silver   -   decrease contact drop

Pressing

Pressing  at higher pressures reduce the porosity,  give  greater strength, increased brush life and narrower blackened results.

Graphitisation

Reduces the hardness, friction and specific resistance.

Impregnation

·       Oils and waxes generally improve friction, stability and increase contact drop slightly.

·       Resins strengthen brush material so that it becomes more resistant to breaking and chipping.

·       PTFE reduces friction under humid conditions.

·       Barium floride reduces friction and wear at very low humidity.

In   view  of  the  above  an  exceedingly  complex  multivariant relationship  exists between the various aspects  of  performance requirements,  specifications  of  raw materials and  processing. Therefore,  stability  of a particular make and grade of  brushes can only be established after extensive tests and trials.

For  traction  machines,  the Electro-graphite  grades  are  most suitably  used.  Technical data on some of the most commonly used brush grades for traction machines is given in annexure.

3.  BRUSH ANGLES

Brushes  are often defined by the methods of applying them to the commutator. They are three main classes:

     1. Reaction

     2. Trailing

     3. Radial

(No. 1 & 2 are used only on non-reversing machines.)

Reaction Brushes

The  brushes  are  said to be `reaction' or  `leading'  when  the commutator is rotated against the angle of tilt i.e.  the brushes are inclined in a leading direction. The angle between the centre line of the brush and the normal lines between 30 to 40 degrees.

Trailing Brushes

The brushes are said to be `trailing' when the commutator is  run in  the same direction as the brushes are tilted.  The tilt angle usually lies between 7 to 15 degrees.

Radial Brushes

Traction  motors are invariably fitted with radial  brushes  i.e. their  centre  line is radial to the  commutator,  which  permits operation   under  similar  conditions  for  both  direction   of rotation.

4.  BRUSH TYPES

Split Brushes

Commutator, howsoever well designed and manufactured,  losses its truness  in the long run of service and high/low spots are  often formed  on its surface.  The unavoidable commutator  eccentricity gives  rise to radial forces,  which tend to break commutator  to brush contact.

The split brush arrangements gives some freedom to each piece  of carbon  to  move independently so that the commutator surface  is closely  followed  and  electrical  contact  is  maintained.  The biggest  advantage is the resistance between leading and trailing edge of the split brush tends to reduce circulating currents.

Rubber-Top Brushes

Apart  from damping the radial forces,  the  rubber-tops  prevent passage of current through the brush holder springs.  The springs thus do not get over heated and loose their tensions.

5.  ELECTRICAL CHARACTERISTICS

(BRUSH TO COMMUTATOR CONTACT)

It  is  perhaps surprising that very little is known  even  today regarding brush to commutator phenomenon.

Microscopic study has revealed that area of the contact initially is only of the order of 1/4000th of brush area. As the machine is started,  due  to  very  high current density  at  these  contact points,  the  carbon gets heated up and a gaseous layer is formed between  the  brush  and  commutator,   which  helps  in  current conduction.  The commutator losses its fresh copper  colour,  and initial  high  brush wear (due to initial high friction and  high current density), gradually comes down. The colour of the film on commutator  becomes  stable after some hours,  or in  some  cases after  several  days  of  running,  depending  on  the  operating

conditions.  If no mechanical/electrical or thermal  disturbances occur,  brush tracks present an uniform polished colour,  varying from dark chocolet to mild black.

During  the  course  of service,  the first  indications  of  any commutation problem due to internal or external factors are often revealed  from  the  condition of  the  commutator  film.  It  is therefore  extremely necessary to have adequate familiarity of the different  types  of the commutator films.  This  information  is usually given in the brush literature. IEC specification No. 276 gives illustrations of some typical films.  Part 4.  of  IS-3003, also includes some of such specifications.

6.  SELECTION OF BRUSH GRADES

Brush  grade  selection involves considerable tests both  on  the test  bed  and under actual service conditions.  It is  sometimes found that brushes which are considered satisfactory on the  test bed  do not operate satisfactorily in service.  In view of  this, the  proper  grade  can only be selected after  suitable  service trials and evaluation.

Indian   Standard   Specification  IS-3003   covers   dimensions, requirements and test procedures for carbon brushes.

Divergence  in the physical properties and dimensions  of  carbon brushes  can cause considerable trouble in service.  Verification of  the  properties involves exhaustive testing,  and  since  the carbon brushes are required to be procured rather frequently,  it is  not practicable to carry out such a large amount of tests  on each  lot purchased.  It is extremely  important,  therefore,  to restrict  the brush procurement from established and well  proven sources only,  even if the prices may be higher. Also, whenever a new  supply source or a new brush grade is  considered,  detailed tests/service  trials should be carriied out before approving  the same for bulk use.

Some of the defects usually noticed on the carbon brushes are:

     1. Dimensions not confirming to the drawing

     2. Bowed/Curved and chipped carbons

     3. Poor quality of pig tails, which results in their getting frayed/broken in service

4. Bad joints between pigtail and carbons, resulting in high unequal voltage drops across     the same

     5. Hair line, invisible cracks at pigtail to carbon joints

     6. Physical properties not conforming to the grade

7.  SERVICE PERFORMANCE

Howsoever good may be the design/manufacture of the machine,  and the  quality  of  the brushes,  satisfactory  performance  cannot continue  to be obtained without resorting to regular and  proper maintenance of the brush-gear and commutator.

The  importance of early detection of commutation troubles cannot be over emphasized.  As such it is imperative that from the  time the  machines are commissioned,  suitable statistical information should  be  collected  on  the  basis  of  regular  observations. Analysis  of  the  data thus collected will  help  to  avoid  the possibility   of  any  particular  commutation  problem  assuming epidemic proportions.

Section  II  of BHEL's Workshop Manual covers the  aspects  which govern  the  satisfactory  commutating  performance  of  traction machines.  In this section guidelines for operation,  maintenance and trouble shooting are also covered.

8.  ANALYSIS OF COMMUTATION PROBLEMS

The  commutation problems are caused by several factors,  some of which are enumerated here:

8.1 Carbon brushes

Poor quality of brushes,  bad carbon to pigtail joints, wrong brush grades,  mixing of grades on same machine,  brushes too loose or tight in brush holders,  improper  bedding,  brushes too thin or thick, brush angles not correct, etc.

8.2 Brush Gear

Brushes in incorrect positions, low or high spring tensions, unequal current sharing by brushes of the same arm, incorrect brush stagger,  spring carrying current, excessive vibrations due  to poor/defective mounting of brush holders,  high brush box  to commutator clearance,  unequal pressure on  parts  of split brush,  brush holders prone to flashover damages,  poor accessibility for maintenance, etc.

8.3 Commutator

Eccentricity, ovality,  high   and  low  bars, flats  on commutator,  pround mica,  oily or dirty surface, bridging of mica grooves,  rough surface,  high commutator  temperatures, inadequate stability due to poor seasoning, etc.

8.4 Machine Faults

Compole strength and gaps not correct,  clogged  ventilation ducts, poor ventilation, defective armature bearings, dynamic unbalance,   wrong  connections  of  compole  or  main  field windings,   armature  or  field  winding  faults,  inadequate equalization,  commutating  zone too narrow or unsymmetrical, poor commutation performance in general,  saturated compoles, low field/armature ampere turns ratio, etc.

8.5 External Causes

Excessive  vibrations  due to defects in machine mounting  or defective  bogie  designs or poor  rail track, leakage  from ventilation  ducts,  collapsed bellows,  prolonged light load running,  rapidly fluctuating or excessive loads,  faults  in control circuitry,  mal-operation of line contactors,  unequal  load  sharing  by machines,  excessive wheel slips  or  wheel locking,   humid   or   corrosive  atmosphere,   towing   of motors without lifting the brushes, oil/water/brake-shoe-dust  coming with cooling air, high voltage transients, high ripple contents,   poor   maintenance,   inadequate  facilities   in maintenance depots, etc.

9.  SUMMARY

In DC-DC and AC-DC diesel locomotives, a large number of DC machines have been used. Carbon brushes play an important role in these machines. Understanding the characteristics and its working helps the maintainers/users to run the machines trouble free. The brush material, brush rigging, types of brushes and electrical characteristics help the users in selecting proper grade for a particular application. The service performance is recorded and monitored in order to decide the proper selection of brush grade too.

Commutation in DC machines is a critical phenomenon. Proper analysis of commutation problem helps in minimising the troubles. This unit also contains technical data of different carbon brushes, which are in use. A chart showing grades of brushes for specific application is given to help the reader .

1.       SELF ASSESSMENT EXERCISE

1.     Describe, How do you select a brush grade for an application.

2.     Why is it necessary to monitor service performance of brush.

3.     How do you analyse the commutation problem of a DC machine.

4.     Why are the brushes placed at an angle in unidirectional machines.

5.     Describe the process to obtain an Electro-graphite brush material.