Note: Descriptions are shown in the official language in which they were submitted.
95~
RD-12559
MAGNETIC SLOT WEDGE WITH LOW AVERAGE PERMEABILITY
-
AND HIGH MECHANICAL STRENGTH
Background of the Invention
-
This invention relates to AC machines and
more particularly to slot wedges therefor, or top
sticks as they are sometimes called.
In an AC machine having a toothed stator,
slot wedges are used to hold the stator windings in
the slots formed between the stator teeth. Slot
wedges made of magnetic material are a significant
means to improve the efficiency of an ac motor.
Magnetic slot wedges reduce slot ripple in the
air gap flux caused by the changing reluctance due
to the slots, and also reduce the associated eddy
current losses due to the interaction of the
harmonics in the air gap flux with the conducting
surface of the rotor. The magnetizing current
required in the stator windings to generate the
desired air gap flux is less with magnetic slot
wedges, since more of the air gap flux is
available for useful power production. However,
closing the slot completely with magnetic material
increases the leakage reactance of the motor, which
in the case of an induction motor results in a
reduction of power factor and of peak torque, and
in a synchronous motor a reduction of peak torque
and slower dynamic re~ponse.
The present methods of making magnetic
slot wedges with wire or iron powder embedded in
a carrier, generally do not allow easy shaping of
the magnetic material in a way which reduces
slotting harmonics or, as they are sometimes called,
RD-12559
space harmonics, wlth a minirnum increase in slot leakage
reactance. Another problem is that attempts to
manufacture magnetic slot wedges that are structurally
sound and do not fail during operation have only been
partially successful. Nonmaynetic slot wedges do no-t
have the failure problems that magnetic slot wedges
have. Thus, it is expected that the different
maynetic forces, the different loss characteristics
and the different thermal characteristics of the
magnetic slot wedges are responsible for their
limited life in actual operation.
To achieve reduced slo-t harmonic losses,
stators with semiclosed slots have been used.
Semiclosed slots, as the name implies, provide a
narrow opening at the top of -the slot and require
random widnings. Random windings are windings in
which the relative position of one wire to another
is not known until the wires are pushed through the
narrow opening and pressed into the slot. Formed
coil windings cannot be inserted into semiclosed
slots. Formed coils are used :Eor high voltage
(above 600 volts) applica-tions because of their
superior turn and ground insulation properties
and for larger machines (above 600 HP) because
of their superior reliability, heat transfer
capability and easy manufacturability in the laryer
coi] sizes. When open slots are used in con~unction
with-magnetic slot wedyes, formed coils can be used
withou-t sacrificing the advantages of semiclosed
slots. In a Eormed coil the windinys are performed
RD-12559
and the position of each wire relative to each of
the other wires is known prior to insertion in the
slot.
It is an object of the present invention
to provide a magnetic slot wedge that results in
reduced slot harmonic losses and sufficient strength
to carry all the forces the wedge is exposed to.
It is a further object of the present
invention to provide a magnetic slot wedge that
results in reduced space harmonic losses and permits
formed windings to be used.
It is a still further object of the present
invention to provide a magnetic slot wedge which
reduces the slotting harmonics with a minimum
increase in slot leakage reactance.
Summary of the Invention
In one aspect of the present invention a
magnetic wed~e is provided for use in a toothed
stator. The magnetic wedges hold the stator
windings in the slots formed between the teeth
of the stator. The magnetic slot wedge comprises
a nonmagnetic body with a width corresponding to
the width of the slot. The nonmagnetic body has
parallel laminatlons of magnetic material extendlng
part way through the width of the magnetic body
from both sides of the magnetic body and perpendicular
to the top surface. A central nonmagnetic region is
thereby created with a region on either side of the
central portion having an average permeability along
its lenyth substantially in the range of about 5-10.
~3--
5~
RD-12559
Brief Description of the Drawing
While the speci:Eication concludes with
claims particularly pointing out and distinctly
claiming the present invention, the objects and
advantages of the in~ention can be more readily
ascertained from the following description of a
preferred embodiment when used in conjunction with
the accompanying drawing in which:
Fi~ure l is a perspective view of the
magnetic slot wedge according to the present
invention;
Fi~ure 2 is a section of a toothed stator
core with the slot wedge of Figure 1 positioned
between the teeth;
Figure 3 is a graph showing the relation-
ship among different values of permeability in the
regions on either side of the central nonmagnetic
region and harmonic losses, effective air yap and
slot leakage reactance;
Figure 4 is a graph showing the flux
density in the air gap between the rotor and stator
of a machi.ne adjacent to a stator slot having a
semiclosed slot wedge;
Figure 5 is a graph showing the flux density
in the air gap between the rotor and stator of a
machine adjacent to a stator slot having a magnetic
slot wedge which has an average permeability oE 5 in
the two regions on either slde of a nonmagnetic central
portion as in the present invention; and
Fiyure 6 is a graph showing t~e flux density
in the air gap between the rotor and stator of a
machine, adjacen-t to a stator slot having a nonmagnetic
s ],o t wed ge .
--4--
5~
RD-12559
Detailed Description of the Invention
Referring now to the drawing wherein like
numerals indicate like elements, ~here is shown in
Figure 1 a magnetic slot wedge 1. The magnetic slot
wedge comprises a nonmagnetic body 3 of epoxy in a
preferred embodiment but can alternatively be
,` ` j fabricated reinforced plastic or fibergl~ss. The
width of the nonmagnetic body corresponds to the
width of the slot in the stator into which the wedge
is to be inserted. The nonmagnetic body 3 includes
a plurality of parallel slits extending part way
through its width from both sides, with the slits
perpendicular to the top surface of the nonmagnetic
body. The top surface of the nonmagnetic body is
defined as that portion facing the air gap when the
wedge is positioned in a slot of a toothed rotor.
The slits 7 each have a lamina-tion of magnetic
material 9 such as silicon steel inserted in them.
~lternatively, the laminations can be molded in
place in the nonmagnetic body. The relative
permeability of -the cen-tral region of the width
of the magnetic slot wedge is unity since it
contains no magnetic material. The average perme-
ability on either side of the central region is
dependent on the stacking factor, which is determined
by the distance between successive parallel
laminations.
Referring now to Figure 2, a portion of a
laminated toothed stator core 10 is shown. Positioned
between teeth 11 are slots 13. Formed windings 15 are
in each slot. Near the open end of each slot on
either side are notches 17 into which the magnetic slot
5~i
RD-12559
wedges 1 are slid. The magnetic slot wedges extend
the axial distance of the slot. The top of the
magnetic slot wedges are flush with tops of the
teeth 11. The outline of a rotor 19 is shown with
the distance bewteen the inside diameter of the
stator and the outer surface of the rotor shown as
21, representing the air gap.
Investigations of the air gap field between
the rotor and the stator in ac machines have shown
that the harmonic content of the semiclosed slot is
not as low as it could be. Figure 3 shows that in
a slot wedge geometry of the type shown in Figure 1
the harmonic content of the air gap flu~ is a
function of the average permeability of the regions
on either side of the nonmagnetic central portion.
It can also be seen in Figure 3 that the value
of permeability can be chosen to adjust the leakage
reactance of the motor, the leakage reactance being
slightly greater than the semiclosed slot configuration.
A permeability of only approximately 5 is required for
the particular geometry analyzed ~the variable that
would result in different permeabilities in different
geometries is slot opening to air gap length which
effects slot harmonic losses and leakage reactance)
to obtain minimum slot harmonics. A permeability of
5 can be obtained by using magnetic laminations
fabricated of 19 mil thick silicon steel separated
by a nonmagnetic spacer three times the lamination
thickness. In general, an average permeability
substantially in the range of about 5 to 10 should
be suitable for most confiyurations. The ra-tio of
radial thickness of the slot wedge to the slot
56
RD-12559
opening influences leakage reactance, but not
harmonic losses. The shape of the lamination
also affects leakage reactance but not harmonic
losses.
In a semiclosed slot the average
permeability is approximately 10,000 while a
nonmagnetic slot wedge has a permeability of
1. Figures ~, 5 and 6 show the magnetic field
in the air gap adjacent to a toothed stator having
a semiclosed slo~ (ur = 108), a magnetic slot
wedge having an average permeability in each of
the regions on either side of the nonmagnetic
central portion of u = 5 and for a nonmagnetic
slot wedge (,ur = 1). The ordinate in each of the
graphs in the Figures 4, 5 and 6 indicates the
relative flux density and the abscissa indicates
the distance, with zero representing the center
of the slot and one representing the center of the
tooth. X represents the actual circumferential
distance from the center of the slot and ts the
slot pitch. X is divided by t , and multiplied
by .5 to achieve a normalized value of 1 at the
center of the tooth. In Figure 4 a permeability
of lO~ was used as a approximation of infini-te
permeability for the semiclosed slot, though
actual permeability is approximately 10,000 in
the semiclosed slot. By examining the flux
curves of Figures 4, 5 and 6, it can be seen
that the u = 5 case in Fiyure 5 has the most
yradual transition, and smallest rate of flux
change with distance from the center of the slot,
resultiny in the lowest harmonic content. A
~3~
RD-12559
Fourier analysis of the three curves would confirm
this result.
Since the magnetic slot wedge fits into
notches in the stator slot 17 of Figure 2, formed
windings can be used where required without the
penalty of relatively high harmonic losses and can
further provide the advantages of allowing greater
slot fill and predetermined positioning between coil
wires which is not possible if a semiclosed slot
design is used.
The nonmaynetic body, which includes the
spacers between the laminations, carries the
mechanical load across the slot and insures that
there are no interlaminator currents and losses
in the slot wedge. The magnet~c wedge of Figure 1
appears from the air gap to be a wedge having a
region of low permeability magnetic material on
either side of a nonmagnetic central portion. The
leakage reactance of the ac machine increases with
decreasing width of the nonmagnetic central portion
of the slot wedge. Slotting harmonic losses of the
ac machine increase with increasing width of the
nonmagnetic central portion of the slot wedge past
the op-timum permeability, which is subs-tantially in
the range of 5 to 10.
The foregoing describes a magnetic slot
wedge that results in reduced slotting harmonic losses
and sufficient strength to carry all the forces the
7~edye is exposed to. The magnetic slot wedge also
permits the use of formed windings.
While the invention has been particularly
shown and described with reference to a preferred
--8--
RD-12559
embodiment thereof, it will be understood by those
skilled in the art that various changes in form and
details may be made therein without departing from
the spirit and scope of the invention.
