This invention relates to assembled commutators, particularly of the
type shown in GB 223 889A.
Commutators for fractional horsepower motors are generally of two
types, moulded or assembled. Moulded commutators are superior in fixing of the
segments to the commutator base as the base is moulded directly to the segments
but this process is difficult and thus, slow and expensive. Assembled commutators
are cheaper but have a problem of how to reliably secure the segments to the preformed
Assembled commutators such as those shown in GB 223889A have addressed
this problem by using commutator segments with fixing means or limbs in the form
of fingers and loops for mating with recesses and projections formed on the base
to secure the segments. The loops are placed over the projections and the fingers
are plastically deformed into the recesses to secure the segments to the base.
While this works very well when the various dies and assembly machines
are brand new, with time the segments are not secured as well as they might be
and after use, the segments may become loose allowing vibrations or minute movement
between the segments and the base which in turn can lead to metal fatigue and failure
of the fixing limbs resulting in premature failure of the commutator.
However, it has been found that by coating the fixing means at one
or both ends of the commutator (depending on construction) with a bonding material
such as the insulating resin commonly used to relieve stresses in the armature
lead wires, the minute movement or vibrations which lead to the early failure of
the commutator can be prevented. An advantage of using this resin is that simplified
winding techniques can be used without fear of broken lead wires due to vibration
According, the present invention provides an assembled commutator
having a preformed base; a plurality of segments fitted to the base; and bonding
material wherein, each segment has a brush contact portion for making sliding contact
with a brush, a terminal for making electrical contact with an armature winding,
and fixing means extending from the brush contact portion and formlocked with the
base to fix the segment to the base, and the bonding material at least partially
covers the fixing means to bond the segments to the base.
Preferably, the base is cylindrical with first and second axial ends
and fixing means include fingers plastically deformed into at least one recess
formed in the first axial end of the base.
Preferably, the fixing means further includes a limb extending from
the brush contact portion and engaging detent means formed on the second axial
end of the base for fixing the segment to the base.
Preferably, the detent means comprises a plurality of axially extending
projections and the limb forms a loop through which the projections extend.
Preferably, the base has an annular groove formed in the second axial
end adjacent the root of the projections and the limbs extend into the groove,
the groove forming a reservoir for further bonding material bonding the limbs to
According to a second aspect, the present invention provides a wound
rotor for an electric motor comprising a shaft; an armature core mounted on the
shaft for rotation therewith; armature windings wound around the armature core;
a commutator mounted on the shaft adjacent the armature core, the commutator having
a preformed base and a plurality of segments fitted to the base, each segment having
a brush contact portion for making sliding contact with a brush connected to a
source of electrical power, a terminal electrically connecting the segment to the
armature windings and a finger extending from the brush contact portion and plastically
deformed into a recess in the base thereby fixing the segment to the base; and
bonding material disposed on the fingers and the base in the region of the recess
to prevent relative movement between the base and the segments.
In addition, the present invention provides an electric motor incorporating
a wound rotor including a commutator as herein described.
A preferred embodiment of the invention will now be described by way
of example only with reference to the accompanying drawings in which:
- Figure 1 is a partially cut-away view of a fractional horsepower universal motor
incorporating a commutator according to the present invention;
- Figure 2 is an end view of a commutator with a part cut away;
- Figure 3 is an opposite end view of the commutator with a part away; and
- Figure 4 is a sectional view of the commutator taken along line A-A of Figure
The fractional horsepower universal motor 10 of Figure 1 incorporates
a commutator 30 according to the present invention. The motor has a stator assembly
including a stator core 11 and a stator winding 12 (only part of the coil is visible
and the connecting leads have been omitted for clarity but are well known). Mounted
directly on the stator core 11 at either end are bearing brackets 13, 14. Bearing
bracket 14 supports brush gear comprising two carbon brushes 15 positioned for
contact with the commutator and slidably disposed in insulating brush holders 16
fitted in apertures in the bearing bracket 14. Springs located inside the brush
holders 16 urge the brushes 15 into contact with the commutator surface.
The bearing brackets 13, 14 support bearings 17 in which motor shaft
18 is journalled. The shaft supports the commutator 30 and armature core 19 on
which armature windings 20 are wound. The armature windings 20 are connected to
the commutator 30 by lead wires 21 which are terminated on terminals or tangs of
the commutator. The shaft also supports a fan 25 for cooling the motor 10.
A spacer hidden in Figure 1 by the lead wires 21 and resin 22 separates
the commutator and the armature core to allow room for the armature windings. The
lead wires 21 wrap around the spacer to provide support for the lead wires which
are subjected to vibration and centrifugal forces. A trickling resin 22 coats the
lead wires as they extend from the spacer to the commutator terminals to provide
The commutator 30 is more clearly shown in Figures 2, 3 and 4. The
commutator has a base 31 of moulded insulating material on which a plurality of
commutator segments 32 are fitted.
The base 31 is cylindrical in shape with a central bore 33 for fitting
to the motor shaft 18. The base 31 has first and second axial ends 34, 35 and an
outer cylindrical surface 36 covered by the segments. The bore 33 has a step 37
in the first axial end 34 for receiving the spacer. The first axial end which is
located adjacent the commutator core is formed with a plurality of axially extending
The second axial end 35 which is located remote from the armature
core, has a plurality of axially extending projections 39 and a recess or an annular
groove 40 radially inward of the projections. A lip 41 defines the inner edge of
the groove. The lip 41, groove 40 and projections 39 form a reservoir 42 for bonding
material as will be described later.
Each segment 32 has an elongate brush contact portion 45 which is
curved to sit on the surface 36 of the base 31 and extends axially along the base.
At one end of the brush contact portion is a tang or terminal 46 for making electrical
connection with an armature lead wire 21. The terminal shown is a weldable tang
in the form of a U-shaped hook around which a lead wire 21 is looped and the tang
is then pressed to collapse the hook around the wire while sufficient current is
passed through the tang to burn off insulating varnish on the lead wire to ensure
good electrical contact between the lead wire and the segment.
Each segment 32 is secured to the base 31 by fixing limbs comprising
two fingers 47 extending axially from the brush contact portion 45 either side
of the terminal 46 and a loop 48 extending radially from the opposite end of the
brush contact portion 45. To hold the segment in place, the loop 48 is placed over
a projection 39 on the second end 35 of the base 31. The fingers 47 are plastically
deformed into the recesses 38 in the first end 34 of the base 31 and into engagement
with the wall of recess to secure the segment in place. The segment is thus formlocked
to the base. At the same time, the loop may be squeezed or pressed further into
the annular groove 40 to secure the other end of the segment.
Bonding material 50, preferably the same resin used to coat the lead
wires 21, is coated over the connection between the fixing limbs 47, 48 and the
base 31. Depending on the construction, the resin may be applied to both ends or
only one end of the commutator. Ideally, the resin may be applied to the first
end of the commutator at the same time the resin is being applied to the lead wires.
Alternatively, it could be applied to one or both ends after the commutator 30
has been assembled but before it is fitted to the shaft 18.
The first end of the commutator 30 as shown in Figure 2 has a plurality
of recesses equal in number to the number of segments and adjacent fingers 47 of
adjacent segments 32 share a common recess 38. The resin 50 as shown in the upper
portion of Figure 2, covers the recesses 38 and part of the pairs of fingers 47.
The resin bonds the fingers to the base to prevent any minute movement or vibration
which may otherwise occur due to relaxation of the grip of insufficiency of the
grip by the fingers on the base. The number of recesses 38 of course could be varied
between a separate recess for each finger to multiple fingers per recess including
a single recess accommodating all the fingers. The resin is omitted from the lower
part of Figure 2 to show detail of the finger/recess arrangement.
The second end of the commutator as shown in Figure 3 has the loops
48 extending into the annular groove 40 and the resin 50 coats at least that part
of the loops 48 within the groove to bond the loops to the base to prevent minute
movement therebetween. Resin 50 fills the reservoir 42 as shown in the upper portion
of Figure 3. The resin has been omitted or cut away from the lower portion to show
detail of the loops, groove and projections.
The preferred bonding material is a thixotropic polyester resin such
as DOLPHON CC-1133/513-D Polyester Resin by John C. Dolph Company of U.S.A.
While the resin may provide only a weak bond between the segments
and the base, it is sufficient to prevent the vibrations or minute movements which
can lead to premature commutator failure. At the same time, this arrangement is
superior to gluing the segments directly to the commutator base as that method
is difficult to do properly due to possible contamination of the commutator surface
by the glue and the difficulty in achieving consistently reliable bonding due to
the heat developed by the commutator in use and variations in the thickness of
the glue which also affects the positioning of the segments. With the present invention,
these problems are avoided.
Various modification to the described embodiments will be obvious
to the reader and it is desired to include all such modifications as fall within
the scope of the claims. For example, loops or fingers may be used as the fixing
limbs at either or both ends of the commutator.