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Frequently asked questions - FAQ
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Content

• General
• Brush manufacturing, design and application
• Theory
• Brush holders
• Maintenance
• Common problems

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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?


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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 ?
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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 slipring film ?
12. What is a black band ?
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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 ?

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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 ?
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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 ?
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Chapter 1 General
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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

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1.2. Why carbon ?

• Carbon has some unique properties which makes it
  the preferablematerial 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 3500C
• No welding between carbon and counter
  material
• Wide band of phys. Characteristics by means
  of - Raw materials - Process - Design

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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.
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. .
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. 
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.
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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
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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 0.25. 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 (77- 103
A/in²). Depending on the duration and type of
material, peak loadings up to 60 A/cm² (387
A/in²) are possible.
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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 µOm. 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
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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 µOm) 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 8 -10 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.
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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.
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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.
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1.8. Is there any shelf live for carbon brushes ?
Electro-graphite grades are manufactured at a
temperature of 3000C 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
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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.
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Chapter 2 Brush manufacturing
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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.
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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.
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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

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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.
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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.
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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.
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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.

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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.
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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.
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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.
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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.
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Chapter 3 Theory
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
38
3.1. What is commutation ?
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
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.

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 µOm. The corresponding figure
for copper is 0.0178 µ?m. 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.
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.
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 80
-85 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
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.
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.
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 cbs each, i.e. 30
brushes,i.e. calculation with 15 brushes 20
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
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 Drive500A - 3 rings 5
cbs, i.e. calculation with 5 brushes 40 x
20 x 40 mm³

Turbogenerator1000A - 2 rings with 10 cbs each
, i.e. calculation with 10 brushes 32 x32
x64mm³
48
3.8. What is the reason for low load problems and
what are remedies ?
Standard conditions
Low load

• 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

49
3.8. What is the reason for low load problems and
what are remedies ?

• 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

50
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 measurementPossibly collector
wear
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 flowThe
  commutator temperature should not drop below 60C
  or exceed 90C
• Use a low load resistant brush grade

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.
53
3.9. What causes brush wear ?
Humidity If the humidity is too low (lt 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
(gt25g 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.
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.
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
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

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.
• .

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

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.
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.
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.
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.
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
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.
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 (MO/kV), e.g. critical
resistance for Urated 690 V 0,69 kV x 0,5
MO/kV 0,345 MO 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 gt 10  MO.  
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.
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.
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
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
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.

71
Chapter 4 Brush holders Design an adjustment
72
4.1. What types of brush holders are there ?
Tubular holder
Köcher-Büstenhalter
Flange brush holder
Leg type holder
Single brush holder
Double clamp brush holder
Tandem holder
Brush rocker
Plug in holder
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.
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)

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 5 - 10)

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)

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.
78
4.4. What is the correct spring force?
Stationery commutator machines Data in cN/cm²
(PSI)
All data in cN/cm²
79
4.4. What is the correct spring force?
Slip ring drives Data in cN/cm² (PSI)
80
4.5. How is the spring force measured and
adjusted ?
pressure sensor brush brush holder
During