Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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- 1 - Case 2595
CAPACITIVE BRUSH CONSTRUCTION FOR A DC MACHINE
This invention relates to a brush construction
for brushes used in DC dynamoelectric machines.
In DC dynamoelectric machines having a
commutator and electrical brushes contacting segments
of the commutator, there is a tendency for sparking to
occur at the edge of a brush as a commutator segment is
leaving -the brush face. In such a dynamoelectric machine
there is a coil or winding connecting each commutator
segment to another to form a curr~nt path from a first
brush, through the commutator segment under the brush
face, through the winding connected between that
commutator segment to another segment underneath a second
brush, and out from that second brush. The winding has
inductance and, as a commutator segment connected to the
winding leaves a brush face and interrupts the current
through the inductive winding, there is a tendency for
sparking. The sparking is undesirable.
It is a feature of the present invention to
provide a brush construction which reduces or eliminates
sparking when a commutator segment Ieaves a brush
face.
It is another feature of the invention to
provide an auxiliary brush-adjacent the trailing edge
of the main brush and having a capacitive connection
with the main brush to reduce spar~ing.
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Case 2595
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The present invention thus proposes an
auxiliary brush, which for convenience and strength
may be secured to the main brush by an insulating
means and interconnected with the main brush by a
capacitor. For example, the auxiliary brush may be
cemented to the trailing side of the main brush using
an insulating cement. The auxiliary brush must, of
course, be insulated from the brush holder.
While it is not usual, it is possible that
sparking may occur at the leading edge of a brush as
a commutator segment moves under the brush. This might
occur in a machine with a high flux density and there-
fore a high voltage gradient at the brush. If there is
a sparking problem at the leading edge of a brush,
then an auxiliary brush could be provided closely
adjacent the leading edge and connected to the main
brush by a capacitor.
Therefore, in accordance with one form of
the present invention there is provided a brush
construction for a DC dynamoelectric machine, comprising
a main brush portion having opposite main surfaces, and
an auxiliary brush portion mounted adjacent one of
said surfaces and insulated therefrom.
In accordance with another form of the
invention there is provided in combination with a
brush holder for a DC dynamoelectric machine having
a commutator, a brush construction for reducing
sparking comprising, a main brush portion having a
width for extending across said commutator and a
thickness for extending in the direction of rotation
of said commutator, the said thickness being defined
by a leading side and a trailing side with respect
to said direction of rotation, a layer of insulating
material on said trailing side, an auxiliary brush
portion mounted adjacent said layer of insulating
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Case 2595
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material and having substantially the same width as
said width of said main brush portion and having a
thickness smaller than the thickness of said main
brush portion, means for insulating said auxiliary
brush portion from said brush holder, and a capacitor
connecting said main brush portion and said au~iliary
brush portion.
The present invention will be described
with reference to the accompanying drawings, in
which:
Figure 1 is a schematic cross-sectional
view of a brush and a portion of commutator,
Figure 2 is a schematic cross-sectional
view, similar to Figure 1, showing in addition an
auxiliary brush portion, and
Figure 3 is an isometric view of the brush
according to an embodiment of the invention.
Referring now to Figure 1, there is shown
schematically a brush 10 resting on a surface 11 of
a commutator 12. The brush is mounted in a brush
holder which is not shown and does not form part of
the invention. The commutator ]2 has a plurality
of commutator segments 14 separated from one another
by insulation 13. The commutator segments 14 are
joined by coils or windings 15 which, as is well
known, carry the current from one brush to another.
Only one brush is shown in Figure 1. The commutator
12 is, of course, mounted on the rotor of a DC dynamo-
electric machine (not shown~ in a manner well known
in the art. The direction of rotation of the
commutator is indicated by arrow 16. The brush 10
has opposite main surfaces 18a and 18b facing the
direction of rotation and the opposite direction.
Sparking occurs at area 17, i.e., on the side of the
brush facing the direction of rotation and referred
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Case 2595
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to as the trailing side, when a commutator segment,
shown as segment 14a in Figure 1, moves away from
the brush. This is due to the inductance of the
coils.
Figure 2 shows a similar arrangement,
however the brush lOa now has an auxiliary brush
20 which is conveniently mounted to the trailing
side of brush lOa using an insulating adhesive 21.
The brush 20 must be insulated from the brush holder
in which it is mounted. The auxiliary brush 20 may,
of course, be mounted by other means such as an
insulating tape encircling brush lOa and auxiliary
brush 20 with an insulating material 21 sandwiched
between them. Again the auxiliary brush should be
insulated from the brush holder when a common brush
holder is used. The auxiliary brush 20 could be
mounted in a separate brush holder, if desired,
provided the separate brush holder could be mounted
closely adjacent so the brush 20 engaged the
2Q commutator closely adjacent brush 10. The auxiliary
brush 20 may be relatively thin as it carries only
small amounts of current as compared to main brush lOa.
The brush lOa and brush 20 are connected by
a capacitor 22 which is conveniently connected at the
brush pigtails 23. The capacitance value of~capacitor
22 is not critical. In fact, there is frequently a
reduction of sparking when no external capacitor is
used and the auxiliary brush 20 is not connected to
anything, i.e., the brush 20 engages the commutator
but is insulated from brush lOa and from the brush
holder and no external capacitor used. Presu~ably
the sma]l amount of inherent capacitance bet~een the
brushes lOa and 20 is sufficient to cause some
reduction in sparking. However, the use of an
external capacitor is preferred.
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Case 2595
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Capacitors having values between 50 micro-
farads and 0.1 microfarads were tested on a 100 HP
motor under various loads. All the values tested,
including 0.1 microfarad capacitors, eliminated the
sparking for all practical purposes. The size of the
capacitor probably has a relationship to the amount
of inductance in the windings and the level of current.
However, the value is not critical and a suitable
value can readily be found by trial.
It was previously mentioned that sparking
could also occur at the leading edge of a brush in
a machine with a very high flux density. If there is
sparking at the leading edge, a similar auxiliary
brush and connecting capacitor can be used to
eliminate it.
Figure 3 is an isometric view of one form
of brush according to the invention. The brush lOa
in Figure 3 i5 of a normal carbon composition and has
auxiliary brush 20 of the same material mounted to
the trailing side. A capacitor 22 extends to connect
the brush 20 to pigtails 23 of brush lOa.
It is believed that the brush has been
adequately described and variations in form and
structure suitable for different installations will
be apparent.
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