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
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.
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