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Schunk Kohlenstofftechnik
Frequently asked questions - FAQ
SKT / GB4
Frequently asked questions
Page: 1
Content
Schunk Kohlenstofftechnik
1. General
2. Brush manufacturing, design and
application
3. Theory
4. Brush holders
5. Maintenance
6. Common problems
SKT / GB4
Frequently asked questions
Page: 2
1. General
Schunk Kohlenstofftechnik
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?
SKT / GB4
Frequently asked questions
Page: 3
2. Brush manufacturing, design and
application
Schunk Kohlenstofftechnik
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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Frequently asked questions
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3. Theory and characteristics
Schunk Kohlenstofftechnik
1. What is commutation ?
13. Why do some slip rings have spiral grooves ?
2. What role does resistivity play in a brush ?
14. What is field weakening ?
3. How important is Hardness ?
15. What is the neutral zone ?
4. What is contact drop or voltage drop
16. Which limiting values of the leak resistance should be
kept ?
5. How does the voltage drop influence commutation ?
17. How does the controller influence brush performance ?
6. What is current density ?
18. How does the brush design influence brush
performance ?
7. How is current density calculated ?
19. What are the main factors influencing commutation ?
8. What is the reason for low load problems ?
9. What causes brush wear ?
10. What is reasonable brush life ?
11. What causes the commutator or slip—ring film ?
12. What is a black band ?
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4. Brush holders
Schunk Kohlenstofftechnik
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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Page: 6
5. Maintenance
Schunk Kohlenstofftechnik
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
Schunk Kohlenstofftechnik
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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Schunk Kohlenstofftechnik
Chapter 1
General
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Page: 9
1. Why are they called brushes ?
Schunk Kohlenstofftechnik
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 ?
Schunk Kohlenstofftechnik
Carbon has some unique properties which makes it the preferable
material for electrical sliding contacts
•
•
•
•
•
•
•
Good electrical and thermal conductivity
Low shear strength of graphite crystal
Low friction coefficient
Low modulus of elasticity
Retains moderate strength at high temperature
No melting point, passing from solid to vapour at 3500°C
No welding between carbon and
counter material
• Wide band of phys. Characteristics by
means of
- Raw materials
- Process
- Design
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1.3. How is carbon made ?
Schunk Kohlenstofftechnik
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-graphitisation. 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.
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1.4 What additives are used
Schunk Kohlenstofftechnik
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.
.
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1.5. How many brush grades are
there?
Schunk Kohlenstofftechnik
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.
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Page: 14
1.6. What general groups of brush
grades do exist ?
Schunk Kohlenstofftechnik
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 ?
Schunk Kohlenstofftechnik
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 ?
Schunk Kohlenstofftechnik
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 ?
Schunk Kohlenstofftechnik
4. Metal-Graphite
These materials are composed of graphite and metal powders, preferably copper, and
thus have a relatively high electrical conductivity. Depending on the proportion of meta1
and the structure, the specific e1ectrica1 resistance of meta1-graphite is in the region of
O.1 to 10 µΩm. This results in low contact resistance and voltage drop. Its hardness is
relatively low. The graphite incorporated in the material gives the good friction properties
which are necessary for satisfactory operation. With a high proportion of metal, metalgraphite 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 ?
Schunk Kohlenstofftechnik
5.Metal-impregnated Graphite
The grades in this class have its porous structure impregnated with a metal.
The essential character of the base material is relatively unaffected by the comparatively small proportion of
metal. However, the increase in mechanical strength given by this metal reinforcement, plus an increased
thermal and electrical conductivity, has substantially extended the uses for electro-graphite based carbon. The
grades are used for brushes on high current duty machines such as low voltage motors for battery driven
vehicles and current collectors (for example pantographs for trains and trams).
Carbon bearings with white metal, antimony and other impregnations are used in mechanical applications.
6. Resin Bonded Graphite
As a result of the resin bonding, this materia1 has a relatively high intrinsic resistivity (of the order of100 to 350
µΩm) and also a high ratio of transverse to longitudinal resistance. The latter is due to the laminar structure of
the graphite used. In conjunction with a high contact voltage, the material is therefore capable of greatly
attenuating short circuit currents between segments bridged by the carbon brushes. It is therefore particularly
suitable for three-phase commutator machines. But due to the improved load capacity at the present day, carbon
brushes of this kind have also proved themselves increasingly on small and some medium sized D.C. machines
and are used in relatively large quantities.
Through the resin bonding, however, the lead capacity and especially the overload capacity is still low in
comparison with electrographite brushes. Specific continuous current densities of 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 ?
Schunk Kohlenstofftechnik
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 ?
Schunk Kohlenstofftechnik
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 ?
Schunk Kohlenstofftechnik
Electro-graphite grades are manufactured at a temperature of 3000°C and are more or
less “dead material”. Some additives may lose some of their effectiveness. However,
this should not greatly affect the general operation, though the benefit of the additive
may be reduced or lost.
Metal graphite brushes or the cooper flexible connection may be subject to corrosion
if stored in an unsuitable atmosphere. If corrosion is present, a possibility may be to
order brushes with tinned shunts which will assist in protecting the leads against
corrosion if stored for long periods
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Page: 22
1.9. Are any safety measures
necessary while cleaning motors ?
Schunk Kohlenstofftechnik
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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Schunk Kohlenstofftechnik
Chapter 2
Brush manufacturing
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2.1. How are brushes made ?
Schunk Kohlenstofftechnik
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 ?
Schunk Kohlenstofftechnik
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 ?
Schunk Kohlenstofftechnik
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
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In case of problems additional information is
necessary.
•Description of the problem
•Present brush grade
•Actual number of brushes
Frequently asked questions
Page: 27
2.4. How may I select the right grade?
Schunk Kohlenstofftechnik
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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Page: 28
2.5. Why do brushes have top or
bottom angles ?
Schunk Kohlenstofftechnik
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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Page: 29
2.6. What is the purpose of a rubber
pad on the top?
Schunk Kohlenstofftechnik
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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Page: 30
2.7. Why do some brushes have saw
cuts in the face ?
Schunk Kohlenstofftechnik
These saw cut, which generally is only applied to slip ring brushes, serves two functions.
1.
2.
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 ?
Schunk Kohlenstofftechnik
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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Page: 32
2.9. How is the cable size and
number determined ?
Schunk Kohlenstofftechnik
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 ?
Schunk Kohlenstofftechnik
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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Page: 34
2.10. Are there any alarm devices to
indicate short brushes ?
Schunk Kohlenstofftechnik
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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Schunk Kohlenstofftechnik
Chapter 3
Theory
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Page: 36
3.1. What is commutation ?
Welsch & Partner, scientific media
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Frequently asked questions
Schunk Kohlenstofftechnik
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
Page: 37
3.1. What is commutation ?
Schunk Kohlenstofftechnik
Switching-off
+ IA ½
Switching-on
+ IA ½
is
0
0
is
- IA ½
- IA ½
2 IA ½
2 IA ½
VK
VK
+ IA ½
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is
- IA ½
Frequently asked questions
+ IA ½
is
Page: 38
- IA ½
3.1. What is commutation ?
Schunk Kohlenstofftechnik
► 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
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Page: 39
3.1. What is commutation ?
Schunk Kohlenstofftechnik
► When the armature becomes horizontally
aligned the commutator reveres the direction
of current through the coil, reversing the
magnetic field. The process then repeats.
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Page: 40
3.2. What role does resistivity play
in a brush ?
Schunk Kohlenstofftechnik
Obviously the resistance of the material differs from one type to another depending
on its ingredients and its production process, especially the final temperature
treatment.
Although carbon is unique among the non-metals in being a fairly good conductor of
electricity, it is a poor conductor when compared with metals. The low resistivity
electrographite materials are of the order of 10 µΩm. The corresponding figure for
copper is 0.0178 µΩm. Thus the resisitivity of electrographite material is over 450
times that of copper while hard carbon grades can be ten times higher than this
factor. Even so, the energy loss arising from this resistance of the brush material is
only 10% of that which is dissipated at the brush contact surface due to contactresistance 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.
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Page: 41
3.3. How important is brush
hardness?
Schunk Kohlenstofftechnik
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.
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Page: 42
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
U1
U
U2
U3
SKT / GB4
Schunk Kohlenstofftechnik
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
Frequently asked questions
Page: 43
3.5. How does the voltage drop
influence commutation ?
Schunk Kohlenstofftechnik
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.
SKT / GB4
Frequently asked questions
Page: 44
3.5. What is current density
Schunk Kohlenstofftechnik
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 wellventilated 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.
SKT / GB4
Frequently asked questions
Page: 45
3.7. How is current density
calculated ?
Schunk Kohlenstofftechnik
DC Motors
S 
I
N
t
a
I
( N / 2)  t  a
= Current [A]
= Number of Carbon Brushes
= Tangential Dimension [cm]
= Axial Dimension [cm}
1000 A – 6 pole 5 cb’s each, i.e. 30 brushes,
i.e. calculation with 15 brushes
20 x 32 x 50 mm³
1000 A
 10.4 A / cm²
15  2cm  3.2cm
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
1000 A
 12.5 A / cm²
10  2,5cm  3,2cm
SKT / GB4
Frequently asked questions
Page: 46
3.7. How is current density
calculated?
Schunk Kohlenstofftechnik
Slip ring drives
I
S 
N t a
Asynchronous-Slip-Ring Drive
500A - 3 rings 5 cb’s, i.e. calculation with
5 brushes – 40 x 20 x 40 mm³
500 A
 12.5 A / cm²
5  4cm  2cm
I
N
t
a
=
=
=
=
Current [A]
Number of Carbon Brushes
Tangential Dimension [cm]
Axial Dimension [cm}
Turbogenerator
1000A - 2 rings with 10 cb’s each , i.e.
calculation with 10 brushes – 32 x32
x64mm³
1000 A
 9.7 A / cm²
10  3.2cm  3.2cm
SKT / GB4
Frequently asked questions
Page: 47
3.8. What is the reason for low load
problems and what are remedies ?
Schunk Kohlenstofftechnik
Standard conditions
Carbon
brush
Low load
Carbon
brush
Contact point
Contact point
Patina
Collector
Patina
Collector
Carbon
brush
Plasma
⇒ The film consists of Graphite and
copper oxides
⇒ copper oxide is a semiconductor
⇒ High el. resistance at low temperatures
⇒ current goes via some frit bridges only
Collector
SKT / GB4
Frequently asked questions
Page: 48
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
Schunk Kohlenstofftechnik
Carbon
brush
Collector
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
SKT / GB4
Frequently asked questions
Page: 49
3.9. What causes brush wear ?
Schunk Kohlenstofftechnik
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.
10
9
Brush wear mm/1000h
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
8
7
External effects
6
5
4
3
Probably error in measurement
Possibly collector wear
2
1
0
0
10
20
30
40
Peripheral speed m/s
SKT / GB4
Frequently asked questions
Page: 50
50
60
3.9. What causes brush wear ?
Schunk Kohlenstofftechnik
Electrical load
A film consisting mainly of metal oxides and graphite from the carbon brushes builds up
on the surface of the collector. The motor current is transferred through this skin via
metallic bridges formed by the process of “fritting”.
Under low-load conditions and at low rotor surface temperature only a small number of
“fritting” bridges develop with corresponding high, localized current densities. This causes
copper particles to melt and deposit themselves in the brush contact surface. This
phenomenon causes grooves and ridges on the collector surface.
Sharp edges of these frit points increase the mechanical brush wear.
Possible remedial actions are;
•Reduce the number of brushes track wise.
•Reduce the rate of cooling-air flow
The commutator temperature should not drop below 60°C or exceed 90°C
•Use a low load resistant brush grade
SKT / GB4
Frequently asked questions
Page: 51
3.9. What causes brushes to wear ?
Schunk Kohlenstofftechnik
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.
SKT / GB4
Frequently asked questions
Page: 52
3.9. What causes brush wear ?
Schunk Kohlenstofftechnik
Humidity
If the humidity is too low (< 3g water per m³ air), friction increases and causes chattering,
which might destroy the brushes. As a remedial action specially impregnated brushes
should be used.
If the humidity is too high (>25g water per m³ air) the patina on the collector surface
becomes too thick, resulting in too few fritting bridges and high localized current
densities. Tracks then begin to form on the collector surface. As a remedy a different
brush grade can be used.
unacceptable
unacceptable
wear
acceptable
3
SKT / GB4
8
Frequently asked questions
ideal
acceptable
15
Absolute Humidity
25 g/m³ air
Page: 53
3.9. What causes brush wear ?
Schunk Kohlenstofftechnik
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.
SKT / GB4
Frequently asked questions
Page: 54
3.9. What causes brush wear ?
Schunk Kohlenstofftechnik
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…
SKT / GB4
Frequently asked questions
Page: 55
3.9. What causes brushes to wear ?
Schunk Kohlenstofftechnik
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
SKT / GB4
Frequently asked questions
Page: 56
3.10. What is reasonable brush life ?
Schunk Kohlenstofftechnik
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.
.
SKT / GB4
Frequently asked questions
Page: 57
3.10. What is reasonable brush life ?
Schunk Kohlenstofftechnik
Brush wear depends mainly on the distance the brush travels on the collector.
An empirical formula for industrial application is:
Brush wear (mm/1000h) =
0,2 π  d  n
60 *1000
d = diameter (mm)
n = speed rpm
SKT / GB4
Frequently asked questions
Page: 58
3.11. What causes the commutator
or slip-ring film ?
Schunk Kohlenstofftechnik
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.
Water
Brush
Graphite
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.
Metal oxides
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.
SKT / GB4
Frequently asked questions
Page: 59
3.12. What is a black band ?
Schunk Kohlenstofftechnik
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.
SKT / GB4
Frequently asked questions
Page: 60
3.13. Why do some slip-rings have
spiral grooves ?
Schunk Kohlenstofftechnik
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.
SKT / GB4
Frequently asked questions
Page: 61
3.14. What is field weakening ?
Schunk Kohlenstofftechnik
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.
SKT / GB4
Frequently asked questions
Page: 62
3.14. What is field weakening ?
Schunk Kohlenstofftechnik
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
SKT / GB4
Frequently asked questions
Page: 63
3.15. What is the neutral position ?
Schunk Kohlenstofftechnik
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.
SKT / GB4
Frequently asked questions
Page: 64
3.16. Which limiting values of the
leak resistance have to be kept ?
Schunk Kohlenstofftechnik
The insulation resistance of windings can deteriorate while the machine is running as a
result of ambient and operating conditions. The critical insulation value at a winding
temperature of 25 °C must be calculated by multiplying the rated voltage Urated (kV) with
the critical resistivity (MΩ/kV), e.g. critical resistance for Urated = 690 V:
0,69 kV x 0,5 MΩ/kV = 0,345 MΩ
If the measured value is close to the critical value, the insulation resistance should be
regularly checked thereafter or the winding should be cleaned.
After cleaned windings have been dried, it is important to remember that the insulation
resistance is lower when the winding is warm. Insulation resistance can be accurately
measured only when the winding is allowed to cool down to room temperature
(approximately 20 to 30 °C).
Measuring voltage: 500 V – (at least 100 V ) at a winding temperature of 25 °C The
minimum insulation resistance of new, cleaned or repaired windings must be > 10 MΩ.
SKT / GB4
Frequently asked questions
Page: 65
3.17. How does the controller
influence brush performance ?
Schunk Kohlenstofftechnik
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.
SKT / GB4
Frequently asked questions
Page: 66
3.17. How does the controller
ínfluence brush performance ?
Schunk Kohlenstofftechnik
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.
SKT / GB4
Frequently asked questions
Page: 67
3.18. How does the brush design
influence brush performance?
Schunk Kohlenstofftechnik
Block brush
That is the most simple brush design, suitable for many applications.
Block Brush
Twin Brush
Tripple Brush
SKT / GB4
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.
Frequently asked questions
Page: 68
3.18. How does the brush design
influence brush performance?
Split Brush
Sandwich Brush
SKT / GB4
Schunk Kohlenstofftechnik
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.
Insulating layer
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.
Frequently asked questions
Page: 69
3.19. What are the main factors
influencing commutation ?
Schunk Kohlenstofftechnik
“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:
1.
The inductance of the commutation coil
2.
Additional induced voltages in this coil
3.
The Ohmic resistance of this coil
4.
Concentricity and surface quality of the commutator and any vibrations of the brush holders
From the viewpoint of the carbon brushes
1.
The electrical contact resistance between the carbon brush and the segment
2.
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
3.
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.
SKT / GB4
Frequently asked questions
Page: 70
Schunk Kohlenstofftechnik
Chapter 4
Brush holders
Design an adjustment
SKT / GB4
Frequently asked questions
Page: 71
4.1. What types of brush holders are
there ?
Flanschbürstenhalter
Flange
brush holder
Double clamp brush
Doppel-Klemm-Bürstenhalter
holder
SKT / GB4
Schenkel-Bürstenhalter
Leg type holder
Steck-Bürstenhalter /
Plug in holder
-elemente
Frequently asked questions
Schunk Kohlenstofftechnik
Tubular
holder
Köcher-Büstenhalter
Taschen-Bürstenhalter
Single brush holder
Tandem-Bürstenhalter
Tandem holder
Page: 72
Bürstenbrücken
Brush
rocker
4.2 What is the pressure curve of a
brush holder ?
Schunk Kohlenstofftechnik
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.
SKT / GB4
Frequently asked questions
Page: 73
4.2 What is the pressure curve of a
brush holder ?
Schunk Kohlenstofftechnik
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%)
SKT / GB4
Frequently asked questions
Page: 74
4.2. What is the pressure curve of a
brush holder ?
Schunk Kohlenstofftechnik
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%)
SKT / GB4
Frequently asked questions
Page: 75
4.3.Do brush holders have a
corrosion protection ?
Schunk Kohlenstofftechnik
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)
SKT / GB4
Frequently asked questions
Page: 76
4.4. What is the correct spring
force?
Schunk Kohlenstofftechnik
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 troublefree 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.
SKT / GB4
Frequently asked questions
Page: 77
4.4. What is the correct spring
force?
Schunk Kohlenstofftechnik
Stationery commutator machines – Data in cN/cm² (PSI)
Commutator speed
Up to 30
30 . 50
> 50
m/s
m/s
m/s
Single / multi holder
200
200
-
Normal demand
(2.8)
(2.8)
Tandem holder
200
250
300
t = 10 – 12,5mm
(2.8)
(3.6)
(4.3)
Tandem holder
200
200
250
t > 12,5mm
(2.8)
(2.8)
(3.6)
Holder with large
250
250
-
mechanical stress
(3.6)
(3.6)
All data in cN/cm²
SKT / GB4
Frequently asked questions
Page: 78
4.4. What is the correct spring
force?
Schunk Kohlenstofftechnik
Slip ring drives – Data in cN/cm² (PSI)
Counter material
Standard holder
Leg type holder
200 – 250
250 – 400
(2.8 - 3.6)
(3.6 – 5.7)
Ungrooved steel
200 – 250
250 – 400
rings
(2.8 – 3.6)
((3.6 – 5.7)
Bronze and copper
max 30 m/s
SKT / GB4
Grooved steel rings
130 – 160
F brush grades
(1.8 – 2.3)
Grooved steel rings
200 – 220
E brush grades
(2.8 – 3.1)
Frequently asked questions
-
Page: 79
4.5. How is the spring force
measured and adjusted ?
Schunk Kohlenstofftechnik
pressure sensor
brush
brush holder
During QC inspection a computer controlled pressure sensor passes through the
entire wear length of the brush and creates a diagram of the spring force.
SKT / GB4
Frequently asked questions
Page: 80
4.5. How is the spring force
measured and adjusted ?
Schunk Kohlenstofftechnik
New
measurement
with sensors
Old measurement
SKT / GB4
Frequently asked questions
Page: 81
4.6. Are there any standards for
brush holders ?
Schunk Kohlenstofftechnik
The dimensions of the carbon brushes are standardized according to DIN 43.000.
Also single standards for brushes in different holder designs exist
All usual holder types with connection dimensions and accessories are
defined in DIN 43 037 to DIN 43 080 in single standards
The dimensions and tolerances of brushes and brush holders are
defined in IEC 136.
The measurement of the brush pressure takes place according to DIN 43 031
SKT / GB4
Frequently asked questions
Page: 82
4.7. How important is brush holder
spacing ?
Schunk Kohlenstofftechnik
A large gap between brush holder and
commutator is disadvantageous as there would
be a high turning moment caused by the
friction force pivoted on the bottom edge of the
brush box and the brush is not guided properly
by the brush box.
Max 2 mm
SKT / GB4
Frequently asked questions
Page: 83
4.8. What is axial staggering ?
Schunk Kohlenstofftechnik
In order that the copper wear resulting from the brush
friction may be uniform on the whole surface of a
commutator, it would be necessary that the covering rate
of the brushes be the same on all parts during the full
rotation.
This is a theoretical condition and, in fact, it is not
workable. However, there is a quite satisfying solution
which is to stagger the brushes laterally and by pairs, with
a distance of a/2, according to the figure:
By this method, the intervals between the brushes, for
each pair of lines, are covered systematically by the brush
pairs of the following and preceding lines.
On the other hand, in placing in the same track two
brushes successively positive and negative, in order that
each track may be covered by an equal number of brushes
for each polarity, a frequent cause of apparition of stripes
on the commutators is suppressed.
This ordering of brushes on the commutator surface is
called staggering or axial or lateral staggering. But this
operation, easy to make, can be undertaken only on new
commutators or renovated by the usual ways of grinding
(lathe, grinding wheels, abrasive stone, etc)
SKT / GB4
Frequently asked questions
Page: 84
4.9. What is circumferential
staggering?
Schunk Kohlenstofftechnik
Circumferential brush staggering on a commutator is a method for
improving the commutating ability of a machine, mainly when on
overload. Circumferential brush stagger increases the commutator arc
covered by the brushes by staggering tangentially one or more brushes
on each arm. There are two types of staggering:
•Asymetrical according to the neutral line (b) for unidirectional
machines with staggering in the direction of rotation; hence the name
"advanced brushes" sometimes given to the staggered brushes.
•Symetrical according to the neutral line (d) for reversing machines.
These two arrangements can be obtained by "all or nothing" as shown
on b and c, or by progressive stagger as shown on d.
By increasing in this way the number of bars covered by the brushes, the
commutating time is increased and the speed of current reversal
decreased in the commutated coil. The com-mutating difficulties of the
machine are accordingly reduced.
The effect of circumferential stagger is especially sensitive when:
• the number of commutator bars is great,
• the speed of the machine is high,
• the thickness of the brush is small ('t' dimension), i.e. the number of
bars covered by a brush is smaller.
SKT / GB4
Frequently asked questions
Page: 85
4.10. How much clearance should a
brush have in a holder ?
Schunk Kohlenstofftechnik
Nominal
values
Unless otherwise specified,
brushes are machined with
the tolerances given in the
table in conformity with the
recommendation IEC 60136.
2
Brush tolerance on
t and a dimensions
-0,03 / -0,09
Min/Max
clearance on
t and a
dimensions
0,044 / 0,144
2,5
3,2
-0,03 / - 0,09
0,050/0,158
4
-0,03 / - 0,11
0,050/0,178
-0,03 / - 0,11
0,055 / 0,193
-0,04 / -0,13
0,072 / 0,232
-0,04 / - 0,13
0,080 / 0,254
-0,05 / - 0,15
0,100 / 0,300
-0,05 / - 0,15
0,110 / 0,330
5
6,3
8
10
12,5
16
20
25
32
40
Metal free brush grades
50
64
80
SKT / GB4
Frequently asked questions
Page: 86
4.10. How much clearance should a
brush have in a holder ?
Nominal values
2
Schunk Kohlenstofftechnik
Brush tolerance on
t and a dimensions
Min/Max
clearance on
t and a
dimensions
0,074/ 0,174
-0,06 / -0,12
2,5
3,2
-0,07 / - 0,15
0,090/0,218
4
-0,07 / - 0,15
0,090/0,218
-0,08 / - 0,17
0,105 / 0,253
-0,15 / -0,26
0,182 / 0,362
-0,16 / - 0,29
0,200 / 0,414
-0,17 / - 0,33
0,220 / 0,480
5
6,3
8
10
12,5
16
20
25
32
Metal brush grades
40
0,220 / 0,480
50
0,230 / 0,490
64
-0,19 / - 0,29
0,250 / 0,560
80
SKT / GB4
Frequently asked questions
0,260 / 0,570
Page: 87
Schunk Kohlenstofftechnik
Chapter 5
Brush and
machine maintenance
SKT / GB4
Frequently asked questions
Page: 88
5.1. Which regular machine checks
should be done ?
Schunk Kohlenstofftechnik
Preventive maintenance is an important tool to prevent brush problems. The rules for preventive
maintenance vary necessarily according to the type and size of machines, the particular conditions
of service, the length of the running and rest periods
The following points should be part of routine preventive maintenance:
1.
Watch for vibrations and noises which may appear during slow speed running and which may affect
either the frame, the bearings, or even the brushes.
2.
By means of a location spot traced on the shaft of the machine check that the rotor does not stop in
a preferential position
3.
As soon as the machine stops, measure by any appropriate means the temperature of the
commutator and/or each of the rings.
4.
Thoroughly blow out the rotor and stator with dry compressed air from both ends of the machine
5.
in order to be effective, the blowing should remove all the dust to the outside of the machine
6.
Blow out and clean the filters with a solvent and dry.
7.
Wipe all insulators with a dry rag.
8.
Brush between the commutator bars with a fibre glass or nylon brush.
9.
Check for concentricity of the commutator (or ring) by applying a comparator to the top of a brush.
10. Insulation resistance of windings to be measured four times. When warm, when cold, before and
after blowing.
11. Measure with a spring balance the pressure applied by the brush holder pressure fingers on the top
of the brushes.
SKT / GB4
Frequently asked questions
Page: 89
5.1. Which regular machine checks
should be done ?
Schunk Kohlenstofftechnik
12. If necessary, measure the wear of the bearings by checking clearances at 90° apart (3 - 6 - 9 and 12
o’clock) with a set of appropriate feelers.
13. Measure, if necessary, the length of all brushes on one arm to see if there is abnormal wear
15. Examine the bar chamfers and look for incipient pitting or shading of the bars or burning of the
trailing edges and oil leakage. (If the skin appears a little too thick, it is advantageous to pass a
flexible abrasive cleaner over the commutator or the rings before putting the machine back into
operation).
16. Examine the groove edges of helically grooved rings (a cutting edge on the rim of a helical groove
always brings about rapid wear of the brush) and for incipient marks or burns).
17. Carefully check that surface of the commutator or slip-rings is not polluted with oil
18. Note first that there is no copper dust deposited on the brush-holders
19. Check that the contact edges of the brushes are not chipped or burnt and that the surfaces do not
have any vibration or burn marks.
20. Examine if necessary the interior of the brush boxes for smoothness and cleanliness.
21. Check that the brush flexibles are not oxidised, burnt or frayed.
22. After putting the brushes back into their brush holders verify that they are sliding normally in their
boxes, that the pressure fingers are in their correct position at the middle of the top of the brushes,
that the flexibles are not pinched by the pressure fingers.
23. Remove all brushes especially if the machine has to spend some time in a humid, salty or chemical
atmosphere or put a piece of paper between the brush and the commutator or rings.
SKT / GB4
Frequently asked questions
Page: 90
5.2. What are suitable parameters to
indicate motor performance ?
Schunk Kohlenstofftechnik
Measurement
Category
Very suitable
Suitable
Less suitable
Vibration
Shifting
Expansion
Process factors
Temperature
Noise
Optical changes
Engine speed
Capacity
Change of
Run-up period
Losses
confirmation
Run-down period
Fill level
Pressure
Electr. factors
Power
Torque
SKT / GB4
Frequently asked questions
Page: 91
5.3. At what length should a brush
be replaced ?
Schunk Kohlenstofftechnik
Normally a brush should have any wear mark, either a special wear mark at one side
surface or stamped lines on the front surface indicating the remaining length.
Sometimes also the lower edge of the brush marking indicates the wear mark.
Usually this wear mark is set 2 – 3mm below the lowest point of the tamped contacts.
Electronic brush wear indicators also may give an alarm, short before the tamped contact
touches the collector.
SKT / GB4
Frequently asked questions
Page: 92
5.4. Do I need to bed a brush in ?
Schunk Kohlenstofftechnik
From the outset, the carbon brushes must be as fully as possible in contact with the
commutator or ring surface both over their whole tangential width and their axial length. In
most cases it will therefore be necessary to bed in new carbon brushes, and in any case
when a complete set is changed.
However, if the diameter of the commutator or ring is very large in comparison with the
size of the brushes, bedding-in may be omitted in certain circumstances. This also applies
when only individual items of larger set have to be changed.
SKT / GB4
Frequently asked questions
Page: 93
5.5. How are brushes bedded in ?
Schunk Kohlenstofftechnik
Various methods of bedding-in are used in practice. One very attractive
method for large machines is to fix an emery cloth with adhesive film. By
slowly turning the rotor in the normal direction the carbon brushes are
then bedded-in. After removing the emery cloth, the commutator or ring
surface must be carefully cleaned of any remaining adhesive.
Wrong
O.k.
SKT / GB4
Another possibility is to draw a strip of emery cloth to and from beneath
the carbon brushes. In the case of non-reversing machines it is essential
however that the last grinding operation should be carried out so that the
emery cloth is only drawn beneath the brushes in the direction of rotation
of the machine. When the cloth is drawn back, the brushes should be
lifted.
If, on switching on the machine, the carbon brushes are to be in effective
contact over their whole surface, this can be achieved by using a soft
pumice stone which is held against the ring or commutator in front of the
brushes while the rotor rotates. The pumice powder thus produced finds
its way under the brushes and beds them in.
The disadvantage of this method is the relatively large accumulation of
dust which makes a subsequent thorough cleaning essential. It has the
advantage that any patina which exists becomes roughened making for an
improvement of the running conditions.
Frequently asked questions
Page: 94
5.6. What should be done if
changing grades ?
Schunk Kohlenstofftechnik
When the brushes being fitted are of different grade from those previously used, then it is
advisable to remove the collector patina left by the previous grade brushes. Assuming work
on the machine has not already involved turning or cleaning the collector, this skin removal
should be done by scouring with an abrasive stone while the collector is rotating. Do not
finish with a lapped slightly polished surface; a moderately abraded collector surface will
help the new brushes to run in and the collector will acquire readily the patina appropriate
to the new brush grade.
When the machine is running well and it is simply a matter of replacing worn brushes by
new ones of the same grade, it is best to leave the developed film of the collector
completely untouched.
SKT / GB4
Frequently asked questions
Page: 95
5.7. What happens if grades are
mixed ?
Schunk Kohlenstofftechnik
It is highly inadvisable to use different brush grades on a machine, mainly because it is then
rarely possible to get a uniform distribution of current between the brushes. Main reason is
the different voltage drop of different brush grades. Burnt shunts and destroyed brushes can
give rise to major malfunction of the motor.
When there has been such intermixing of grades it is often difficult to determine which grade
is the correct one; the grade giving the worst performance may well be the one which would
behave the best if used alone !
SKT / GB4
Frequently asked questions
Page: 96
5.8. How can I set the neutral
position?
Machine at rest, armature disconnected and
locked.
The poles are fed with low voltage alternating
current rendering unnecessary the switching
required when using D.C.
The brushes are left in place on the commutator.
An auto ranging voltmeter is connected to the
terminals of the armature. Rotate the brush
holder ring until the voltmeter, set to its most
sensitive scale, shows a minimum reading in mV
In each instance it is advisable to repeat the
operation for two or three different positions of
the armature. It is useful in practice to mark the
yoke indicating the best brush rocker position
for each measurement.
This will result in a series of marks surrounding
the true neutral position or they may coincide.
Repeating the process by moving the rocker in
the opposite direction will produce a point
coincident with the first series. This will be the
neutral.
SKT / GB4
Frequently asked questions
Schunk Kohlenstofftechnik
AC current
~
V
Page: 97
5.9. Can commutators and slip-rings
be ground in the machine ?
Schunk Kohlenstofftechnik
A small degree of out-of-roundness can generally be eliminated by grinding the commutator or
ring surface with a silicon-carbide grinding stone which is applied manually to the collector surface.
With the machine running at rated speed, if this is possible, the grinding stone should be guided with
uniform light pressure and an oscillatory motion over the whole contact surface to ensure uniform
grinding.
In order to avoid increasing the out-of-roundness when grinding, the tangential dimension of the stone
should be at least 2 x as large as the defective place to be removed on the rotor. It is expedient to
move the stone along a guiding aid which for example can take the form of a rigid straight sheet metal
strip fixed to a bar or straight edge. Tangential movements or oscillations of the manually guided
stone can thus be avoided.
If the defective places are very wide or if the commutator is severely out-of-round, grinding must
be carried out with a stone having a fixed means of guidance. For this purpose it is best to use
suitable equipment which can also be fitted with rotating grinding wheels. A rotating grinding
wheel must turn in the opposite direction to the rotation of the rotor.
For the machining of steel rings coarser stones should be used than for copper or non-ferrous metals.
In our experience when machining copper and non-ferrous metal, an SC 80 K4 BA stone is very suitable
for the preliminary grinding and an SC 220 K3 BA (DIN 69 100 designation) for finish grinding. If steel
rings have to be machined an SC 46 K5 BA stone should be used for preliminary grinding and
an SC 80 K4 BA stone for the final machining. When the latter grindstones in each case are correctly
applied to the ground surface, they will produce a surface structure with peak-to-valley heights
of the order of RZ = 5 – 8 µm (Ra = 0,8 – 1,2 µm) which is favourable for the running-in of carbon
brushes
SKT / GB4
Frequently asked questions
Page: 98
5.10. When and how should
commutators be turned ?
Schunk Kohlenstofftechnik
If the out-of-roundness or collector run-out is very great, it is recommended on account of the higher
metal removal rate, that the commutators or rings should be skimmed. Furthermore, skimming
produces less contamination (no grinding dust). Skimming can either be carried out in situ (mainly in
the case of medium size or large machines) or by turning the dismantled rotor in a lathe. If the rotor is
not skimmed while on its own bearings, but is held for example between centres, a check must
first be made after setting up in the lathe to ensure true running in the bearings. If this shows up
deviations greater than 0.01mm, re-centring is required. If skimming is carried out in situ, any axial play
in the bearings must be removed.
If there is a risk that the insulation bars between the segments are no longer sufficiently undercut
after turning or are even flush, they must be re-cut.
Both diamond and carbide tipped cutting tools can be used for turning the machines in question
when they have undercut segment insulation bars. When the insulation bars are flush only carbide
should be used. Diamond is used in metal cutting predominantly to give a super-finish, i.e. it has
advantages when surface with minimum peak-to-valley heights have to be produced at high speeds.
Since we are not aiming here for any mirror smooth surface finish, carbide may be preferred unless the
greater service life of diamond is the decisive factor.
Our experience shows that cutting speeds of the order of 160 – 200 m/min are advisable with carbide
tools and 300 – 350 m/min with diamond tools. The feed should be about 30 µm at a cutting depth of
0.05 – 0.1 mm.
SKT / GB4
Frequently asked questions
Page: 99
5.11. Is undercutting necessary ?
Schunk Kohlenstofftechnik
On machines used today in industry and on the railways, practically the only
carbons used are of a quality which is not capable of wearing down the
insulation bars between the commutator segments.
The insulation bars must therefore be neatly undercut. Imperfectly undercut
insulation or even mica which remains standing proud leads to contact
problems creating collector flaws and high brush wear.
There are not fast rules to be observed for the form of the segment grooves,
the sawing, scraping or milling out of the mica, or for the most favourable
depth of the insulation groove, or for the edge chamfer. It has generally been
found that a milled-out depth equal to the width of the insulation is sufficient.
It is important to ensure that no mica or insulation residue remains at the
edges. The milling out should therefore be wider by about 1/10 mm than the
insulation bar which also takes account of possible dividing errors; see the
illustration at the left side. The most usual angles for the edge chamfer lie
between 60° and 90°. Undercutting the segment insulation bars of small
machines can be done manually. It is however advisable where a higher
quantity of pieces is involved, to use special machines which are offered by
the industry for this purpose.
If the edges of the segments have to be chamfered, this should be done
before the last turning or grinding operation so that any flash which may
stand proud can be removed. The grooves must be thoroughly cleaned after
machining.
SKT / GB4
Frequently asked questions
Page: 100
5.12. How does oil influence
commutators and brushes ?
Schunk Kohlenstofftechnik
Oil, grease and other non-conductive substances which get on to collectors form an
insulating film so that the number of electrical contact points are decreased. The very
high temperatures occurring locally at the remaining contact points lead to melting of the
collector metal so that we get a similar effect to that obtained with fritting. In addition the
oil and grease is “cracked” by the thermal conditions and very hard oil coke formed which
increases the grooving and the brush wear.
SKT / GB4
Frequently asked questions
Page: 101
5.13. Can brushes be washed in
solvent ?
Schunk Kohlenstofftechnik
Carbon materials are like a sponge. The fine capillaries connecting the pores would be filled
immediately with any solvent and would never let it come out. .
Since the solvent might have a negative influence on film performance it is not advisable to
wash brushes with any cleaning agent.
If the collector surface has to be cleaned from oil etc only solvents recommended by brush
suppliers should be used.
During the cleaning operation the brushes should be removed from the brush boxes and the
motor.
SKT / GB4
Frequently asked questions
Page: 102
5.14. What atmospheric
contaminants affect brushes?
Schunk Kohlenstofftechnik
Chemically aggressive gases like H2S, SO2, HCl e.g. in the paper industry quickly build
up a badly conducting layer on the collector surface, which are destroyed by intensive
fritting. This of course leads to grooving and high brush wear.
The counter measures depend on the concentration of the aggressive substance, In case
of slight contamination only a so called “self cleaning” brush variant or a sandwich
brush design can be used.
It is also appropriate in such cases to use carbon brush materials that lay down a thick
graphitic patina as a protecting film.
SKT / GB4
Frequently asked questions
Page: 103
5.15. What happens if the machine is
subject to vibration ?
Schunk Kohlenstofftechnik
Brush oscillation can, dependent on intensity, lead relatively quick to
damaging the brush flex as well as to step formation at the base of
the brush. Similarly rattling can lead to breakage and splitting off of
the brush particularly in the running surface area.
Brush oscillation and especially rattling also lead to contact
interruption between carbon brush and collector.
If a commutator or slip ring is perfectly round and the brushes rattle
then the general cause is excessive friction (out-of-roundness
protruding mica, etc. naturally give rise to vibrations).
Very smooth, polished patina whose formation has been encouraged
by light load or even no load running leads to the sliding of the
carbon brush bordering on the limit of static friction. With other
systems this creates the so-called “stick-slip-effect”. Carbon brushes
with a correspondingly higher friction are excided into a rocking
oscillation which we refer to as “rattling”. The brush rattling can be
reduced if the pressure finger of the brush holder presses on the
brush top such that the direction of the force on the brush top bears
on the leading edge area. In this case the reaction moment at the
brush becomes very small – this is of particular importance when
there is variable friction. When the friction force oscillates the
reaction moment has the effect of stabilizing the brush and
compensates for the friction.
SKT / GB4
Frequently asked questions
Page: 104
5.16. How to measure the collector
temperature ?
Schunk Kohlenstofftechnik
The commutator temperature is very critical for brush performance.
The optimal temperature range is 60 – 90°C .
There are three common methods how to measure temperature:
1. Infrared instruments
Since the measuring principle of those instruments is based on the extinction
coefficient, they are very suitable if the material to be checked has a uniform
coloration. Unfortunately slip-rings or commutators are quite un-uniform in colour.
So the IR-instruments do not give an accurate measurement !
2. Thermo couples
The measurement with thermocouples is the standard test method of carbon brush
field engineers. Disadvantage is that the measurement can be done only during a
machine stop. Due to some heat dissipation until the stop and during the
measurement the value is also not very accurate, but at least reproducable.
3. Temperature test points
There are test points for different temperature ranges. These points change colour
with temperature They are put onto the collector service and can be checked after
some time of running. They give a relatively exact hint of temperature conditions
while machine operating.
SKT / GB4
Frequently asked questions
Page: 105
5.17. What has to be done if the motor
has to be stored for a longer period ?
Schunk Kohlenstofftechnik
The following procedures are recommend in case the motor is stored for a longer period in the
warehouse :
•Motors must be stored in a dry, well ventilated area which is free of vibration and dust.
•Do not remove the rotor shipping brace (if fitted)
•Open the pressure fingers of the brush holders and take the brushes out of the holder boxes.
•Remove fitted keys.
•Protective unit against condensation caused by temperature fluctuations.
•Protect unit against ingress of aggressive gases such as hydrogen sulfide, chlorine or ammonia.
•Once a month, remove the rotor shipping brace, turn the rotor by 360° and replace the brace again.
•Replace the anti-corrosion protection on the shaft extension if necessary.
Before commissioning, remember to:
•Measure the insulation resistance (see Operating Guide).
•Check the commutator surface and clean if necessary.
•Mount the brushes and close the pressure fingers.
•Lubricate the bearings, clean and grease the bearings if necessary.
•Insert fitted keys.
SKT / GB4
Frequently asked questions
Page: 106
5.18 Why is surface
roughness so important ?
Schunk Kohlenstofftechnik
Graphite
Surface roughness is one of the main factors
determining brush performance.
Rough surface
Smooth Surface
SKT / GB4
Frequently asked questions
If the surface is too smooth, the coefficient of friction
may periodically become excessive at fairly low speeds
since the carbon brushes are almost subject to static
friction, while at higher sliding speeds there is a risk of
aerodynamic effects. . Both lead to poor contact over
the circumference of the collector. In addition, the
carbon or graphite particles which are abraded from
the running surfaces of the carbon brushes do not
adhere sufficiently to smooth surfaces, so that a patina
only forms very slowly and incompletely.
This then quickly causes damage to the surface of the
segments in the form of flaws especially in the case of
highly stressed commutator machines, resulting in
sparking at the brushes
Page: 107
5.18 Why is surface
roughness so important ?
Schunk Kohlenstofftechnik
If the surface of the collector is rough, high initial wear may
occur. But since the surface is getting smooth quite fast, this
problem is only minor. But on a rough surface the graphite of the
carbon brush can be deposited much faster.
Generally speaking a somewhat rougher surface is considerably
less critical for performance than a smooth one.
Before skimming
After skimming and grinding
SKT / GB4
For optimum operation of the brushes, the surface roughness of
the collectors must therefore lie within the limits of being not too
smooth and not too rough. Trials and experience have shown
that a peak-to-valley height RZ of the order of 5 – 8 µm
should be aimed at.
After machining, the collector surface must not have a very
mirror like appearance. Such a surface is a certain sign of too low
surface roughness It should rather have a tendency towards a
mat finish.
If slip rings or commutators are found to be in satisfactory
condition before making a replacement and if a good patina has
formed on the surface it may be possible to leave them as they
are. If, however, they are very smooth and have a mirror finish, it
is advisable as already mentioned above to roughen this patina
slightly with a grindstone so as to avoid problems in operation
Frequently asked questions
Page: 108
Schunk Kohlenstofftechnik
Chapter 5
Common
operating problems
SKT / GB4
Frequently asked questions
Page: 109
6.1. Why do shunts fall out of
brushes ?
Schunk Kohlenstofftechnik
Modern brushes have a so called tamped connection of
fkexible and brush body. Its mechanically strong and of
low electrical resistance. There are supplementary
strengthening processes.
Although brush manufacturers use machines for effective
and safe tamping procedure, the contact can fail due to
heavy vibrations in service. If a certain limit is exceeded,
the mechanical strength of the connection is not strong
enough. In some cases the embedded connection shakes
loose and in others the flexible frayes and brakes from
metal fatigue.
An other possible reason for a failed tamped connection
is overheating due to commutation problems. In this case
mostly the cable at the trailing side is overloaded and the
current limit for the cable exceeded. Then the tamped
connection acts like a small oven and the connection will
fail. The same happens in case of uneven current
distribution.
SKT / GB4
Frequently asked questions
Tamping
Tube
Flexible
Brush
Page: 110
6.2. What causes commutator
grooving and threading ?
Schunk Kohlenstofftechnik
Grooving - is the uniform circumferential wear, the width of the brush that is
exhibited on the commutator. Excessive abrasive dust in the atmosphere or on
abrasive brush can cause this condition. Extreme light spring pressure can also
cause this condition. Proper brush applications and filtering the air on force
ventilated motors can reduce the commutator wear, Some people call this
"Ridging" because of the resulting ridges on each side of the groove.
Threading - is damaging of the commutator by copper particles in the brush
face. The excessive copper transfer occurs due to low spring pressure, light
loading or contamination. These particles are trapped in the porous carbon
brush and work harden, creating a tool that machines or Gaul's the commutator
surface. The machine can operate for long periods of time with this condition,
but reduced commutator and brush life will be experienced.
SKT / GB4
Frequently asked questions
Page: 111
6.3. Why do commutators get flat
spots ?
Schunk Kohlenstofftechnik
The development of flats on a collector is a very good example of the chain reaction started by a small cause
and building up to a major fault. A flat gives rise to destructive sparking which increases the size of the flat by
burning. The brush bounce caused by the larger flat promotes increased sparking and burning and then
damage to the brush holders caused by vibration and the trouble is aggravated still further.
Flats may be caused by vibration originating from worn bearings, defective gearings or couplings, machine outof-balance or disturbance fed back from connected plant.
Flats may also be caused by the burning which arises from a disturbance of the brush contact by high intersegment insulation particularly at the edges of segments where the recessing of the insulation has been
imperfectly carried out. But more frequently flats are the cumulative result of electrical faults which in the early
stages can be recognized by segment burning, by blackening on commutators or by blackened patches on slip
rings.
Loose or high resistant joints between the armature winding and the commutator cause burning which starts at
the segment in advance of the bad joint and then spreads as the burning destroys the contact between
commutator and brush. Flats resulting from this cause often have “sympathetic” flats spread at one pole pitch
apart around the commutator and are due to the unbalance in the armature circuit arising from the bad current
collection at the original flat.
Sometimes bar marking occurs in a regular sequence in which every third, fourth or fifth segment either is
burnt or is darker or cleaner than the intervening segments. The segments thus marked occur at slot pitch
frequency, according to the number of coils per slot, and this phenomenon is a symptom of unsatisfactory
commutation conditions.
Slip rings may develop flats from the same mechanical causes as for commutators and an additional electrical
reason may be unequal current sharing between brushes.
An occasional cause of flatting on slip rings is the tarnishing or rusting of the exposed portion of the rings
whilst the machine is hut down, particularly if it is out of service for a considerable time.
SKT / GB4
Frequently asked questions
Page: 112
6.4. What causes brushes to get
stuck in holders ?
Schunk Kohlenstofftechnik
The mechanism causing the problems is different for the main
materials for industrial and traction brush grades.
Metal graphite brush grades consist of different portions metal
powder and graphitic components. Additionally additives can be used
to improve the performance. Depending on the metal content metal
bridges are formed during the sinter treatment (see micro
photography) or the metal particles are more singularly distributed.
These metal particles have a large, active surface. At elevated
temperature, possibly in combination with high humidity, this surface
is covered by a layer of metal oxides, even in the inner spheres of the
grade.
The oxidation goes with an enlargement of the volume. The brushes
get larger, are swelling and get stuck in the brush holder box. Finally
they will be destroyed by arcing. Collectors and slip rings are heavily
damaged by the arcing as well.
Typical problems arise, if brushes are electrically overloaded, i.e. are
permanently used above the current density, permitted by the brush
manufacturer. The swelling is triggered by high humidity.
Electrographite grades don’t show any dimensional change while
heated. So Brush made of electrographite grades normally get stuck
only if brush dust, mostly in combination with oil, adheres at the brush
side surface and causes additional friction in the brush box.
SKT / GB4
Frequently asked questions
Page: 113
6.5. Why do brushes sometimes wear
differently in the same machine ?
Schunk Kohlenstofftechnik
Many customers ask : „Why do brushes on a specific machine show a non-uniform brush wear ?“
Fluctuations in the brush grade are often held responsible. The list below shows some possible causes:
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Non-uniform out-of-roundness from one brush track to the next
Non-uniform brush pressure
Non-uniform total down-force
Non-uniform characteristics of the spring pressure with the brush wear
Variable friction in the brush box
Differences in temperature
Non-uniform current distribution
Non-uniform commutation
Stability of the brush holders and brush stud assembly
Differential polarity
Non-uniform brush spacing ( pole pitch)
Non-uniform brush orientation
Non-uniform brush seating
Non-uniform electrical connection
SKT / GB4
Frequently asked questions
Page: 114
6.6. Why do some brushes sometimes
have overheated flexibles ?
Schunk Kohlenstofftechnik
Overheating of individual brushes and burning of flexibles arise from non-uniform current distribution
between brushes. The reason for this may be:
Brush mounting and maintenance: Brushes sticking in holders
Uneven brush pressure
Loose terminal screws, dirty terminals
Flexibles to short or stiff
Indifferent brush bedding
Machine Adjustment:
Unequal brush spacing around the commutator
Incorrect commutation conditions
Discoloration of the flexibles in one brush wafer of a twin brush is a typical indication for commutation
problems, since the trailing halve of the brush is normally overloaded in case o commutation problems.
SKT / GB4
Frequently asked questions
Page: 115
6.7. What causes brushes to wear
and dust excessively ?
Schunk Kohlenstofftechnik
The important word here is “excessive”, A certain amount of wear is inevitable when there is current collection
through sliding contacts. It occurs as a result of mechanical abrasion and of electrical erosion, and by burning
from sparking , whether visible or not. A rate of wear which can be considered reasonable for one type of machine
could equally well be regarded as excessive for other conditions. It largely depends on the load and duty of the
machine concerned. For examples machines subject to high current overload peaks of short duration interspersed
with periods of lighter load, e.g. traction motors or generators supplying rolling mill motors, generally have a
better brush life than those machines which run mostly at full load on industrial applications.
The rate of wear is determined more by the current loading in the most worked portion of the brush contact face
than by the apparent load as indicated by the machine current and brush cross-sectional area.
Excessive brush wear is more often electrical rather than mechanical in origin, but it must be remembered that
mechanical imperfections and bad maintenance often give rise to wear which is electrical in character.
Among the electrical causes the most frequent are:
• Insufficient brush pressure
• Current overload
•due to machine overload,
•Incorrect current distribution in the brush surface because of commutation conditions,
• Unequal load distribution between brushes caused by
• non uniform brush pressure,
•Unequal circumferential spacing of brushes
• non uniform air gaps
•Indiscriminate mixing of brush grades
•Some brushes sticking in their boxes, giving abnormal heavy wear on individual brushes
Apart from these machine conditions, high brush wear may be caused by the use of an unsuitable brush grade.
SKT / GB4
Frequently asked questions
Page: 116
6.8. Why do some commutators show
regular light and dark patterns on the
segments ?
Schunk Kohlenstofftechnik
Sometimes bar marking occurs in a regular sequence in which every third,
fourth or fifth segment either is burnt or is darker or cleaner than the
intervening segments. The segments thus marked occur at slot pitch
frequency, according to the number of coils per slot, and this
phenomenon is a symptom of unsatisfactory commutation conditions, i.e.
incorrect commutating poles strength or brush position.
Because the armature slot is moving with respect to the commutating pole
whilst the coils in the slot are passing through their cycle in succession,
the commutating field conditions are not necessarily the same for all the
coils in the same slot. A small brush shift may assist.
When cooling conditions permit, any tendency to produce slot-pitch “bar
marking” can be reduced by the use of a brush grade of higher contact
drop. Alternatively a brush grade may be used which has a controlled
degree of mild abrasive and polishing action. Should either of these
methods fail, steps must be taken to widen the interpole flux zone so that
a brush position may the be found at which commutation conditions for
all the coils in the slot are more nearly alike.
This involves changing the commutating pole shoes or commutating pole
air gap or both and subsequent re-adjustment of the commutating pole
ampere-turns.
SKT / GB4
Frequently asked questions
Page: 117
6.9. What causes differential wear of
slip rings ?
Schunk Kohlenstofftechnik
The amount of collector wear which takes place under the cathodic brush (i.e. current flow
from collector to brush) is several times as much as that under the opposite polarity. This is
due to the transfer of metal by ionic conduction in a manner very similar to that of
electroplating. This effect is most readily to be seen on synchronous motors, where the two
polarities are completely separated. Fortunately there is only a very small proportion of the
total current conducted this way, otherwise the rate of polarity wear could be very much
greater than is actually experienced.
On these synchronous machines the rotor DC excitation current is fed in by means of slip
rings. Generally the wear on negative rings, i.e. current from ring to brush, is more than that
on positive rings. To prevent this disparity becoming too pronounced it is often advisable to
change the polarity of the rings at regular intervals if at all practicable. Where brush current
densities and ring cooling conditions permit, brush grades without metal content should be
used because ring wear problems are then much less pronounced than with grades of the
Metal-graphite class.
Brush instability from poor mechanical conditions and insufficient brush pressure can
aggravate this ring polarity wear effect.
SKT / GB4
Frequently asked questions
Page: 118
6.10. What causes selective action ?
3
Voltage drop V
2,5
2
1,5
1
0,5
0
0
3
6
9
12
15
18
Current density A/cm²
SKT / GB4
Frequently asked questions
Schunk Kohlenstofftechnik
If simultaneous readings are taken of the currents
carried by the individual brushes of a set at any one
instant, it will generally be found, that the values differ
greatly. At another instant this pattern of non-uniform
current distribution will be quite different and a brush
which formerly carried little current may now carry far
more than its share.
The graph shows, that a slight change in the voltage
drop of one brush e.g. by slightly different brush
pressure or a temperature difference can cause quite a
big difference in current density.
If there is any factor which causes a permanent
difference in the current carried by each of the brushes
of a set, then the brush carrying the most current will
run hotter and wear faster than the remainder. A brush
which is running hot tends to have a lower contact drop
than its neighbor.
Sometimes one brush arm will carry more current than
other arms of the same polarity. The reason should be
sought in some fault in the machine adjustment, for
example the non-uniform spacing of brushes round the
periphery of a commutator.
Page: 119
6.11. What is out-of-roundness ?
Schunk Kohlenstofftechnik
Flat spots, segments which protrude or are too low, flaws etc. – in short, all deviations
from circular form, whether they occur during skimming or grinding, or during
operation – are designated out-of-roundness. They cause violent brush movement
and sparking especially at high circumferential speeds. This results in increased out-ofroundness, greater wear of the carbon brushes, and, in some circumstances,
severe damage to commutators, rings and carbon brushes.
SKT / GB4
Frequently asked questions
Page: 120
6.11. What is out-of-roundness ?
As a guide line in the case of mean circumferential
speeds, a total out-of- roundness of 0.1 mm
should not be exceeded and a difference in height
of 0.01 – 0.03 mm between adjacent segments
should be regarded as the limit. The value for the
maximal permitted bar-to-bar distance depends
on the frame size and the peripheral speed.
Schunk Kohlenstofftechnik
12
10
8
6
200
180
160
140
120
100
80
60
40
20
0
4
2
0
<200
Permissible bar to bar distance
<200
200 - 450
>450
Permissible total out-of-roundness
SKT / GB4
200 - 450
Frequently asked questions
Page: 121
>450
6.12. What are reasons for
copper drag ?
Schunk Kohlenstofftechnik
The term copper dragging describes a situation in which
a fin of copper grows from the leaving edge of a
commutator bar toward the next segment, thus reducing
the gap between them and making the machine prone to
flashover.
Copper drag is often associated with high frequency
vibrations to give a hammering effect on the
commutator. This is sometimes met on machines with
periods of low load running such as rolling mill motors.
In case of low load conditions the film disappears and the
friction coefficient increases, causing the friction
problems.
So all action working against the low load problem, do
also help against copper drag.
Correct bevelling of the segment edges play an important
part in delaying the onset of copper drag.
SKT / GB4
Frequently asked questions
Page: 122