IS 9919: Guide for Selection and Use of Carbon Brushes in Electrical Rotating Machines

24 Pages·1999·2.7 MB·English

by  Bureau of Indian Standards

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IS 9919 (1999): Guide for Selection and Use of Carbon
Brushes in Electrical Rotating Machines [ETD 15: Rotating
Machinery]
“!ान $ एक न’ भारत का +नम-ण”
Satyanarayan Gangaram Pitroda
““IInnvveenntt aa NNeeww IInnddiiaa UUssiinngg KKnnoowwlleeddggee””
“!ान एक ऐसा खजाना > जो कभी च0राया नहB जा सकता हहहहै””ै”
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“Knowledge is such a treasure which cannot be stolen”

SI
9919 : 1999
vRl=hm
.

Indian Standard
GUIDE FOR SELECTION AND USE OF CARBON
BRUSHES IN ELECTRICAL ROTATING MACHINES
(First Revision )
ICS 29.160

0 BIS 1999
BUREAU OF INDIAN STANDARDS
MANAK BHAVAN, 9 BAHADUR SHAHZAFARMARG
NEW DELHI 110002
April 1999 Price Group 8
Rotating Machinery Sectional Committee, ET 15
FOREWORD
This Indian Standard (First Revision) was adopted by the Bureau of Indian Standards, after the draft finalized
by the Rotating Machinery Sectional Committee had been approved by the Electrotechnical Division Council.
This guide has been prepared with a view to provide guidance for selection and use of carbon brushes for
different electrical rotating machines.
This revision has been undertaken to delete the reference to brush grades and the information on the carbon
brush manufacturers included in the earlier version.
For the purpose of deciding whether a particular requirement of this standard is complied with, the final value,
observed or calculated, expressing the result of a test or analysis shall-be rounded off in accordance with
IS 2 : 1960 ‘Rules for rounding off numerical values (revised)‘. The number of significant places retained in the
rounded off value should be the same as that of the specified value in this standard.
Is 9919:1999
Indian Standard
GUIDEFORSELECTIONANDUSEOFCARBON
BRUSHESINELECTRICALROTATINGMACHINES
(F irst Revision )
1 SCOPE machines, ac commutator machine, amplier dyres and
starters used in amplidynes and starters used in
This standard provides guidance for selection and
automobiles.
use of carbon brushes for application in electrical
rotating machines. 6 BRUSH SELECTION
2 REFERENCES 6.1 For brush selection, IS 13466, IS 13584 and
IS 13586 should be referred to.
The following Indian Standards are necessary
adjuncts to this standard: 6.2 Characteristics of Carbon Brushes
IS No. Title 6.2.1 Contact Voltage Drop
4722 : 1992 Rotating electrical machines tsrif( The figures of contact voltage drop be&veen rotating
commutator/slip-ring and the brushes are generally
revision)
stated by the brush manufacturer at normal current
13466: 1992 Specificationf or brushes for electrical density recommended for the brush. The high
machines gnidesrepus[ IS 3003 contact resistance assists in commutationby decreasing
(Parts3 and 4) :1977] the circulating current in the coils that are shorted by
the brushes. On machines that have difficulty in
13584 : 1993 Specification for brush materials for tcatnoc
commutating, higher egatlov drop brush grades
electrical machinery
are usually necessary to minimize or eliminating
13586: 1993 Definitions and nomenclature for sparking. Higher than necessary contact drop reduces
carbon brushes, brush holders, machine efficiency.
commutators and slip-rings for
6.2.2 Specific Resistance
rotating electrical machinery
The specific resistance of a brush material has a much
14376 : 1996 Brush holders for electrical machines
smaller influence in determining the curremcarrying
3TERMINOLOGY capacity of a brush than the contact drop, coefficient
of friction and thermal conductivity of the material.
For the purpose of this standard, definitions given in Measurement of specific resistance is more useful from
IS 13586 shall apply. the point of view for uniformity of quality of carbon
4 GENERAL grade.
6.2.3 Co-efficient of Friction
4.1 The selection of a suitable grade of brush for a
particular application is very vital from the point of Lesser co&cient of friction is better since it will give
view of reliability and trouble-free service since the low wear of brush and also of commutator or slip-ring.
damage caused~bya wrong choice of brush ultimately
6.2.4 Current Density
may prove very costly.
Different grades of brushes have different values of
4.2 Recommendation of suitable brush grades for a
rated current densities for satisfactory operation.
particular electrical machine is the responsibility of
the machine manufacturer. 6.2.5 Mechanical Shocks and Mbrations
NOTE - Users should not normally introduce new brush Strength of brush grade~is an important factor to be
~grades without consulting&e machine manufacturer. considered when selecting brush grades for such
applications as tractionmotors, reversible rolling mill
5 USE OF BRUSHES
motors, train lighting generators, etc, which are
The carbon brushes are extensively used in dc motors, subjected to mechanical shocks and vibrations in
dc generators, ac synchronous and asynchronous services. In such cases, it is often necessary to use
1
IS 9919 : 1999
specially resin impregnated brushes or brushes with 6.3.5 Sandwich Brush
highertransverse strength.
A sandwich brush is made up from two or more
6.2.6 Hardness parallel layers of brush material bonded together as
shown in Fig. 1. The bond line is at right angles to
tI is the elastic property of the material. It determines the brush ‘t’ dimension. Each part has separate
the ‘riding’ ability of the brush over the collector flexible connection. The parts are of the same grade
surface. The higher the shore hardness, the lesser of material, but are not necessarily of equal thickness.
will be its ability to maintain intimacy with the The bond between the parts can be with a suitable
collector surface. The brushes with higher shore adhesive or with an insulation sheet having
hardness, because of their compratively lower polishing capability. Due to machine characteristics
elasticity, tend to bounce more on commutators and in abnormal service conditions, excessive
having surface irregularities, eccentricity and oval&y. collector skin build-up or commutator segment
blackening can take place. Under this situation
6.3 Constructional Features
cleaning action of the bonding layer can control the
The choice of radial, trailing and reaction brushes collector surface. These brushes are particularly reco-
should be made according to the guidelines given mmended for use on difficult commutating machines
in 6.3.1 to 6.3.5 (see also Annex A). such as amplidyne and ac commutator motors of high
bar 10 bar voltage creating circulating currents
6.3.1 Radial Brush
between the commutator bars under the-brushes.
Most commonly used on reversible machines.
6.3.2 The brushes with contact level angle are used
for better mechanical stability and increased arc of
contact. These may be trailing or reaction brushes or OND
a combination of both.
6.3.2.1 The trailing brushes, preferably in tandem, are
most suitable for large unidirectional machines. The
trailing angle may be 7.5 to 15 degrees.
6.3.2.2 The reaction brushes are normally fitted on
large heavily loaded machines running continuously
for long periods. Ifused on reversible machines angle
FIG. 1 SANDWICHB RUSHW ITHS AMEM ATERIAL
should be large say 22.5 to 30 degrees.
6.3.6 Composite Brush
6.3.2.3 In the case of special rolling mill drives, a com-
The composite brush is of similar construction as the
bination of trailing and reaction bru shes in a ‘V’ shape

sandwich brush, but with a difference~that it is made
at a suitable angle are used.
of layers of different grades of materials, as shown
6.3.3 Insulated Resilient Top Brushes in Fig. 2. The combination of different grades gives
hybrid contact drop characteristics not obtainable from
Wear due to passage of current at the point where the a single grade brush. This can be used to improve
pressure finger bears against the brush top and wear commutation. The problem of unstable friction with
of the pressure figner pivot may be prevented by the electrographite grades, particularly under low load
use of insulated resilient top brush. Fibrous conditions can be overcome with a combination of
materials bonded with resilient materials like rubber EG with natural graphite or special resin impregnated
are used for insulating the taps of brushes. Resilient grades. As in the case of sandwich brush, the
insulated top also enables shock absorption circulating currents between the commutator bars
ensuring better riding ability. under the brushes can be reduced with the use of
composite brush, and with polishing ability of the
6.3.4 Split Brushes bonding layer collector surface skin can be controlled.
In the case of split brushes, two or more brushes are 6.4 Operating Conditions
fitted into one brush box and acted on by a common
6.4.1 Current Density
pressure finger. This arrangement allowsa limited degree
of independent movement between two or more brush Size and grade of the brush is selected based on the
halves which enables them to, ride over commutator machine rating since different brush grades have
irregularities and increases the effective contact. different current densities.
2
IS 9919 : 1999
4 Heavy shock load Motors and generators
followed by no-load of rolling mills, motors
very heavy starting of E. 0. T. cranes, auto-
OND current for short mobile starter motors,
duration followed traction machines and
by sustained rated train lighting genera-
currents in one dir- tors.
ection, nocload or
reverse currents in
the other direction
d) Frequent and sudd- Mill duty reversible
FIG. 2 COMPOSITEB RUSHW ITHT wo DIFFERENT
en reversion loading motors.
GRADESO F MATERIALS
e) Widely varying Traction duty machines
6.4.2 Speed
speed and load and rolling mill motors
No matter how high the standard of design, construction which have to operate
and maintenance may be, no commutator or slip-ring under a very wide oper-
surface can be free from geometric irregularities, ating zone, with current
namely, ovality, eccentricity, high/low surfaces and and speed varying 200
flats. As the machine speed increases, effect of these to 800 percent of the
amplify and affect riding of the brushes. The brush- rated values. Therefore,
holder spring pressure mechanism may not be able to brush grades for such
respond, particularly to thL high frequency vibrations, duty should have a
therefore, resilience and damping are required to be wide range of commuta-
provided by means of brush construction and ting ability.
selecting a suitable brush material with elastic
6.4.4 Environmental %onditions
property. Generally for high surface speed
applications lower specific density and softer brushes The dust, high or lowhumidity and~times which may
perform better. corrode the commutator are factors that should be
considered for selection of optimum brush grade. If
6.43 Duty Cycle
conditions are such that excessive film~thickness is
Light loading of the machine may lead to threading developed, a brush with a more cleaning action is
on the commutator and slip-ring surface, and-copper normally required. On the other hand, a brush with
drag on the commutator surface. It also restricts film more film forming ability may be required for low
formation and increase brush wear. Overloading of humidity or other conditions when insufficient
the machine for long duration ma y create problems commutator film is maintained.
such as overheating, higher sparking and excessive
6.4.4.1 The rotating machines-may have to operate
wear ofbrushes, commutator and slip-ring. With rapid
under different environmental conditions as giveribelow:
current and speed changes satisfactory operation of
brush may become difficult. ‘Intermittent duty cycle a) Dry, hot and cold Cement mill drives,
may permit use of brushes grades at higher than climate and dusty rolling mill drives,
normal current densities. Long off-periods may atmosphere traction motors and
require an abrasive grade of efficient starting excavator machines.
capability. Sudden overloads and heavy current-surges
b) High humidity and Paper mill drives,
will require a very porous/open structure grade and
corrosive chemical chemical plant drives
ability of rapid recovery of collector surfaces.
fumes and textile mill drives.
6.4.3.1 Typical load applications (duties) are as
c) High humidity Power stations.
follows:
d) Rarefied conditions Machines used in
a) Continuous susta- Induction motors, syn-
aircraft and sub-zero
ined full load chronous motors, paper
temperature application.
mill ac drives and
chemical plant drives. 6.4.5 Machine Ventilation Conditions and Cooling
Factors
b) Intermittent load- Auxiliary drives of mills.
ing, rapid rise in Slip-ring induction The machine ventilation conditions and cooling
speeds motors. factors should aid be considered while selecting the
3!I 9919 : 1999
brushes particularly in regard to current density, 7.4 Brush Gear
coefficient of friction and contact voltage drop.
Brush holder and brush design including flexible,
7 FACTORSAFFECTING BRUSH fitting and brush to brush-holder connection
PERFORMANCE arrangement are important aspects for the brush
performance. Brush grade, sizes of brush and brush
7.1 The factors which affect the performance of the
box on the brush-holder, and the value of spring
brush are given below:
pressure should be as per the design and
)a Design parameters of a machine; whether specification. If the brushes are not properly bedded,
electrical, magnetic or mechanical unbalances current collection and commutation cannot be
exist or not; and whether the design constants satisfactory.
approach the limit values or not.
7.5 Clearance Between Brush Holder Box and Brush
b) Interval between major maintenance
Due to sudden change of speed of the machine the
operations.
brush is thrown against the side of the brush holder
4 Maintenance which the machine requires box. If the clearance between brush-holder box and
and gets. brush is high, this may result in chipping of the brush
4 Commutating ability ofbrushes. contact face. Recommended values of the clearances
are given in Table 1.
)e Duty cycle to which the machine is subjected.
Table 1 Clearance Between BrushHolder Box
0 Environmental conditions, temperature,
and Brush
humidity, atmospheric contamination by
gases or dust under which a machine is
Nominal Brush Holder to Brush Clearance (mm)
working, oil land dirt on the commutator
Values
or slip-ring surface. Max Min
f9 Current collecting ability of brushes and 1.6 144 44
2
distribution of current between brushes.
2.5
)h Electrical and frictional losses during opera- 3.2 158 50
tion responsible for heating of brushes and 4 178 50
commuter or/slip-ring. The frictional losses 5
are dependent of the speed and brush pres- 6.3 193 55
sure of the machine. 8
10
J) Brush holder design.
12.5 232 72
k) Brush design including flex i bles and fittings. 16
n3 Brush vibration due to the ov a lity of comm- 20 254 - 80
25
utator/slip-ring, stick slip phenomenon and
32 300 100
external shocks.
40
n) Tolerances on inherent characteristics of 50
brushes.
7.6 Effect of Aerodynamic Pressure
P) ‘TLpe~opf ower supply for motors.
At high speeds, the brush tends to get lifted and
s) Brush bedding.
reduce brush pressure due to air pressure acting
r) Commutator/slip-ring material. upwards. For such machines brush grade should be
porous. Also split brushes with slight clearance
7.2 Design Parameters
between adjoining surfaces in the form of groove along
Satisfactory performance of the brushes depends upon the length of the brush would be helpful.
soundness of the machine design, and design
7.7 Load Sharing Between Brushes
margins available on electrical, magnetic and
mechanical parameters. The load sharing between the brushes, connected in
parallel on a brush arm, is affected by:
7.3 Maintenance of the Machine
a) brush pressure,
It is very importantt o identify the maintenances chedule
which the machine requires, and its strict compliance. b) contact voltage drop,
4
SI 9919: 1999
c) brush resistance, and in Annex A. Brush manufacturer based on this and
the tests given in IS 13584 gives recommendations to
d) pig-tail connection voltage drop.
the machine manufacturer regarding the brush grades
A variation in these characteristics among different to be adopted. This questionnaire can also be used
brushes would lead to different load sharing and by the users and the machine manufacturers to
result in sparking and unequal wear. obtain assistance in diagnosing any cause of difficulty
on the existing machine.
8 ASSESSMENT OF BRUSH PERFORMANCE
10.1.2 Based on the suggestions of the brush
When judging the operation of brushes the various
manufacturer, commutation tests on the machine for
points should be taken into consideration as
which grade selection is under consideration or a similar
mentioned below:
electrical machine, or the brush performance data
)a Wear, chipping, break and chattering of available for a similar machine, the machine
brushes; manufacturer selects a suitable brush grade.
However, in case of doubt, it is desirable to prove
+I Wear of commutator/slip-ring; suitability of this grade by actual service trial before
its adoption in bulk, for selection and field evaluation
)c Noiseless operation of brushes;
of a brush grade cooperation between machine
4 Burning and fraying of flexibles; manufacturer, brush manufacturer and the machine
user is essential.
6 Intensity of sparking at the brush contact
surface (see IS 4722); 10.2 Black Band Test
Temperature-rise of brushes, commutator/ 10.2.1 During the tests for selection of carbon grade,
slip-ring~and brush holder; the black band curves should be taken with new grades
and established grades on the same machine at the
fz) Type of film formed by brushes on commut-
premises of machine manufacturer or machine user.
ator/slip-ring (see Annex B);
10.2.2 This test is basically meant to find the interpole
)h Copper dragging of commutator segments due
strength, however, it also gives an idea for the width
to brushes (see Annex B);
of band for a particular brush grade since the width
3 Blackening of commutator (see Annex B); and of band will vary for different grades.
k) Other factors shown on commutator, such as 10.2.3 Circuit diagrams for the black band test are
serrations, grooving, burning of commutator given in Fig. 3 and Fig. 4.
segments and bar marking.
10.2.4 Motors which are tested at a specified speed
9 FAULTANALYSISANDREMED IALMEASURES should have their fields set for full load and the field

kept constant, the speed being maintained constant

9.1 The commutator check chart whi ch illustrates the by variation of the armature voltage since varying the
film formation and performance of electrical machine interpole field affects the speed also.
is given in Annex B. By comparing the surface
10.2.5 Load the machine to about 25 percent fullload,
condition with check chart, it is possible to adopt
remedial measures given therein! and connect the booster across the interpoles. A
suitable range voltmeter also to be connected across
9.2 The common terms used for the commutator sur- the interpoles. Now increase the booster excitation
face and commutator film are given in Fig. 15 ( see and note whether the interpole voltage increases or
AnnexB ). decreases. If the voltage increases the interpoles are
being boosted and if the voltage decreases, the
10 RECOMMENDED PROCEDURE OF
interpoles are being bucked or diverted. Make a careful
ESTABLISfWG THE BRUSH GRADE
note of these conditions and the manner in which the
FORTHEMACXUNE
booster controls are operated to produce them.
10.1 General
10.2.6 Assuming that the machine is not sparking,
10.1.1 Machine manufacturer usually consults brush adjust the interpole strength by means of booster until
manufacturer for his recommendation for a suitable the sparking is just visible. Also check whether this
grade for existing machine, by furnishing the minimum condition is boosting or diverting and record the~load
essential details as per technical questionnaire included current and booster current. Then reverse the booster