Brushholders and the Performance fo Carbon Brushes
As presented at the EASA International Convention
June 28, 1994
Jeff D. Koenitzer, P.E., Vice President of Engineering
Helwig Carbon Products Inc.
Milwaukee, Wisconsin
1. Introduction
2. Holder Size Dimensions
3. Holder Position
4. Holder Angle
5. Holder Mounting Height
6. Spring Force
7. Spring Pressure Recommendation
8. Electrical Connections
9. Summary
Introduction
A carbon brush is an electrical contact which makes a connection with
a moving surface. Optimal performance on motors, generators and other
types of moving contact applications will be attained only when the
carbon brush, the brushholder and the contact surface are properly
designed and maintained. All three components are critical factors
in a complex electro-mechanical system.
The brushholder, as the name suggests, holds the brush so that
the brush can perform properly. Holders provide stable support in
the proper position in relation to the contact surface and often
provide the means for application of the contact force on the brush.
For many decades brushholders had received little attention. New
rotating equipment was supplied with copies of the same old brushholder
designs. Typically, when performance problems occurred the focus
had been on the brush as this was the part exhibiting rapid wear.
In the early 1980’s Helwig Carbon led the industry towards
the consideration of brushholders and particularly spring pressure
as a common cause of many brush problems. Further, recent holder
developments and the coordination of the designs of constant pressure
holders with Red Top brushes have resulted in significant advancements
in performance and life.
The purpose of this paper is to review the critical areas of consideration
for brushholders in relation to the proper functioning of brushes.
The most important factors are 1) maximum stability of the carbon
in the holder, 2) proper positioning of the brush on the contact
surface, and 3) minimum resistance through the brush and holder
portion of the electrical circuit.
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Holder Size Dimensions
The fit of the carbon portion of the brush in the holder is critical
for stable electrical contact. If there is inadequate space between
the holder walls and the thickness and width of the brush, there
is potential for binding of the brush in the holder particularly
with increased temperature and contamination. On the other hand,
an excess amount of space between the holder and the carbon will
result in an unstable electrical contact as the brush face can move
tangentially or axially within the holder.
The holder and brush tolerances on the thickness and width therefore
must be well coordinated. Brushes are machined undersize per NEMA
tolerances or per drawing specifications while brushholders are
made oversize. As a general guideline for brushholders, industrial
sizes typically should be held oversize to a tolerance of +.002/+.008".
Smaller frame units with a brush thickness less than .500"
and greater than .125" should have holders with a tolerance
of +.001/+.005". Micro size units with brushes of thickness
.125" or less should have holders held to a tolerance of +.001/+.003".
Over a long period of usage the thickness dimension on a holder
can become worn from brush movement or distorted from heat. Therefore,
it is important to periodically measure the thickness and width
dimensions on the top and bottom of the holders to ensure they are
within tolerance and that the brush will have adequate support for
a stable electrical contact.
When motor and generator brushholders are subjected to high temperatures,
it may be necessary to provide extra compensation for thermal expansion
depending on the temperature rise and the degree of heat dissipation.
In these cases it is easier to reduce the brush thickness and width
dimensions slightly to avoid sticking in the holder rather than
adjusting holder dimensions. Metal graphite brushes with over 50%
metal content by weight are manufactured with an increased undersize
tolerance per NEMA standards as they usually carry higher current,
generate more heat, and have a higher coefficient of thermal expansion
than non-metal grades.
Brush and holder length can also have a significant effect on the
stability and performance of the brush. Most often the length is
limited due to the space available within the frame. There are,
however, also practical length limitations due to the excess resistance
of a long piece of carbon. As the carbon length is increased the
resistance of the current path from the shunt to the contact surface
is increased. At the same time the amount of contact area between
the carbon and the longer holder is increased and the corresponding
contact resistance is decreased. This then creates the potential
for distorted current flow directly between the holder and the carbon
rather than through the shunting.
On the other hand, short brush and holder designs are more susceptible
to instability at the contact surface. There is potential for a
higher degree of brush tilt in the holder since the length of support
is less in relation to the brush thickness.
In addition to dimensional concerns the insides of the holder must
be smooth and free of all obstructions including burrs. If a used
brush has any straight scratches down the sides of the carbon then
there are protrusions inside the brush box, which will restrict
the brush from making proper electrical contact. Rough handling
of brushholders can cause distortion of the metal and effect the
critical inside dimensions of the brush cavity. Holders made from
metal stampings are particularly susceptible to irregularities on
the inside dimensions and on squareness. Broaching is generally
accepted as the best manufacturing method for assurance of consistent
inside dimensions and a smooth finish.
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Holder Position
The holder position will determine the location of the brush on
the moving contact surface. For slip ring applications the holders
are usually located around the top portion of the ring for ease
of access. In this position the weight of the brush contributes
to the contact force. If holders are mounted on the underside of
a contact surface then additional spring force may be necessary
to compensate for the weight of the brush.
On DC machines with commutators proper positioning of the holders
in relation to the field poles is critical. The brushes should be
equally spaced around the commutator. This spacing can be checked
by wrapping a paper tape around the commutator, marking the location
of the same edge of each brush, and then measuring the distance
between marks on the paper.
The brushes must also contact the commutator within the neutral
zone where voltage levels are near zero. When the holder position
allows the brush to make contact outside the neutral zone there
will be higher bar to bar voltages under the brush, circulating
currents, bar edge burning, and damage from arcing.
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Holder Angle
The most common angle for holder mounting is 0 degrees, i.e. perpendicular
to the contact surface. Most slip rings and reversing commutator
applications make use of this so-called radial mount. The advantages
are ease of holder installation, maximum spring force transferred
to the contact surface, and fair stability of brush contact upon
reversal of direction.
Any brush face movement within the holder will result in a change
in the contact surface. The most stable surface contact will occur
when the top and bottom of the brush are always held to the same
side of the holder regardless of the direction of rotation. Angle
holder mountings were developed to increase this stability and the
effective area of the brush contact. However stability will occur
only when the correct angles are used in relation to the direction
of rotation.
When the entering edge is the short side of the brush or a trailing
position the face angle should be 20 degrees or less. At greater
angles the action of the rotation and the spring force wedges the
brush into the bottom corner of the holder and causes high friction
and an unstable contact. Normally trailing brushes also have a shallow
top bevel.
When the entering edge is the long side of the brush or a leading
position the face angle should be 25 degrees or more. At angles
of 20 degrees and less the action of the rotation pulls the bottom
of the brush to the opposite side of the holder from the top of
the brush. Leading brushes should have a top bevel of 20 to 30 degrees.
A stable contact can be maintained in either or both directions
of rotation with brush face angles between 20 and 25 degrees. The
potential disadvantage of holder angles is the loss of effective
downward force of the spring. A portion of the spring force is dissipated
in holding the brush stable to one side for the holder. The loss
in downward contact force for various angles are as follows:
Angle Degrees Loss in Downward Force
5 0.4%
0.4%
10 1.5%
15 3.4%
20 6.0%
25 9.4%
30 13.4%
35 18.1%
40 23.4%
45 29.3%
The spring force should be increased to compensate for the loss
of effective downward force from the action of the brush angle in
holding the brush to the side of the holder. If a brush has bevels
of 20 degrees on the top and 30 degrees on the bottom then the spring
force should be increased 6.0% + 13.4% or about 20% to maintain
the proper level of effective downward contact force at the brush
face.
In the special case of post mounted double holders commonly used
on slip rings, the best design would allow both brushes to make
contact at zero degrees or perpendicular to the ring. Any angle
will result in one brush in the pair operating with less contact
stability.
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Holder Mounting Height
The vertical position of the holders above the contact surface is
very important in assuring proper brush support throughout the wearable
length of the rush and for proper positioning on the contact surface.
When a brushholder is mounted too high above the contact surface
or when the surface has been turned down to a significantly smaller
diameter, there will not be adequate support for the carbon as the
brush wears to a short length. This will contribute to increased
electrical wear due to the instability of the contact.
The holder mounting height should be proportional to the size of
the unit. On the large frame sizes the holders should be mounted
a maximum of .125" above the contact surface. In a few cases
units operated with intentional runout of the contact surface which
must be taken into consideration. The small micro frame sizes should
have a holder mounting height of approximately .032". During
holder mounting a flexible mounting pad of the appropriate thickness
can be placed on the contact surface to ensure consistent height
and spacing. This pad also helps protect the commutator from damage
during mounting.
There are several common problems related to excess height of the
holder. When a commutator has been turned down several times angled
brushes will make contact in a different position. With steep bottom
bevels and significant decreases in diameter the location of the
brush contact could even move outside the neutral zone. There will
be a significant increase in wear unless the holder is moved closer
to the commutator or the neutral is adjusted.
Although single post mounted holders can be rotated to move the
holder closer to the commutator, the position of brush contact will
change. As above it is very likely that adjustment of the neutral
position will be required to avoid edge arcing.
On V-shaped toe-to-toe holders which are mounted too high above
the commutator the brushes can interfere at the toes. This will
result in one or both brushes not making contact with the commutator.
It is especially important that these old style holders are mounted
sufficiently close to the commutator to avoid this problem.
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Spring Force
Many inventive methods have been used for the application of the contact
force on brushes. These included clock type springs, torsion bars,
lever springs, helical coil springs, and constant force negator springs.
As noted in the graph shown below the brush wear rate will change
as the spring pressure changes. This is one of the most important
concepts in understanding brush performance.
There has always been a problem with an accelerating rate of wear
as the brush gets shorter due to the declining spring force and
the dramatic increase in electrical wear. The most consistent brush
performance will be attained when the spring force is virtually
constant at the correct level throughout the wear length of the
brush.
The use of the proper constant force springs can be a significant
advantage with consistent minimal wear rate of the brushes, reduced
wear of the contact surface, less carbon dust, and much lower overall
maintenance costs on the unit.
Testing and application experience have resulted in the following
recommended ranges of spring pressure:
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Spring Pressure Recommendation
Application Spring Pressure
General Industrial
4.0-6.0 PSI
Fractional HP Motors 4.0-7.0 PSI
Traction 5.0-8.0 PSI
Induction & Sync Motors 3.5-4.5 PSI
High Speed Slip Rings 2.25-2.75 PSI
Elevator Generators 3.5-4.0 PSI
35 18.1%
40 23.4%
45 29.3%
When operating conditions vary from the standard then some adjustment
in spring force can improve performance. If the current density
is very low, the humidity is very low, or the speed is extremely
high then a slightly lower spring force than above can be an advantage.
However if the current loads are high, the speed low, there is contamination
causing over filming, or where external vibration and roughness
of the contact surface are affecting the brush, then a spring force
near the high end of each range is recommended.
The unique set of conditions on each application will result in
its own specific graph and numbers for the ideal spring force to
obtain minimum wear of the brushes and the contact surface. Often
times a change in spring force will have a far more dramatic effect
than a change in brush grade. Several original equipment manufacturers
test for the ideal spring force prior to testing different brush
materials.
The springs on all holders should be checked every 2 or 3 brush
changes to ensure the pressure is still within the recommended tolerance
and the that the force is consistent on all holders. The force of
the spring must first be measured with an accurate scale. This value
is then used to calculate spring pressure as shown below.
If the spring pressure value is below the recommended range then
the springs should be replaced to avoid accelerated wear of the
brush and the contact surface.
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Electrical Connections
The primary function of the brush involves conducting current. In
many cases the brush holder is also a part of this electrical circuit.
Therefore it is necessary that all electrical connections are of minimal
resistance to provide the best path for current flow from the main
lead connection to the contact surface. Corrosion, contamination,
or electrolytic action over a period of time can cause dramatic increases
in resistance which then requires cleaning. Careless installation
of the brushes or the holders can lead to loose connections. Any high
resistance in the brush circuit will result in excess heat or an undesirable
path of current flow and unequal loading of the brushes.
On fractional horsepower cartridge style brushholders with captive
coil spring type brushes the current should flow from the clip connector
at the bottom of the holder up the brass insert to the cap on the
end of the brush and then down through the shunt to the carbon.
The brushes fail very quickly if the round or eared cap on the end
of the brush does not make proper contact with the brass holder
insert. When this condition exists current will flow directly from
the brass insert to the spring or to the carbon. In either case
there will be extreme heat, loss of brush contact, commutator wear,
and eventually motor failure.
Another problem with larger frame sizes can occur when the holder
mounting is part of the electric circuit. If the holder mounting
surface becomes dirty, corroded, or even painted over then current
will again need to follow another path and thereby cause problems.
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Summary
The general knowledge and experience in the field on rotating equipment
has been slowly declining for many years. In addition brushholders
have seldom ever received proper attention during trouble shooting
or as part of a maintenance program. Therefore it is hoped that the
above information will be helpful in creating awareness of the potential
problems with brushholders as a very critical component in the satisfactory
performance of carbon brushes on motors, generators, and other types
of sliding contacts. The important factors to check for proper functioning
of the holder and brush are:
1. Inside holder dimensions
2. Holder spacing
3. Holder angle
4. Holder height
5. Spring force
6. Electrical connections
When there is an opportunity to implement new holders, the use
of the principles mentioned above along with the coordination of
the latest constant pressure holder and Red Top