Magnetic slot wedge with low average permeability and high mechanical strength

A magnetic wedge for use in toothed stators holds the stator windings in the slots formed between the teeth of the stator. The magnetic slot wedge comprises a stack of magnetic material with a width corresponding to the width of the slot. The laminations each have central bridge portions connecting a first and second magnetic region, the bridge portion saturating during machine operation. The bridge and the first and second magnetic regions define an opening which, when the laminations are stacked, form a channel having a narrowed entrance. A fiberglass rod fits inside the channel and provides axial stiffness to the stacked laminations which clamp on the fiberglass rod.

This invention relates to a copending application Ser. No. 353,278 filed 
Mar. 1,1982 and assigned to the same assignee as the present invention. 
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 
results in a reduction of peak torque and slower dynamic response. 
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, space harmonics, with a minimum 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. Nonmagnetic slot wedges do not have the 
failure problems that magnetic slot wedges have. Thus, it is expected that 
the different magnetic 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 slot 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 windings. 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 for high voltage (above 600 volts) 
applications 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 larger coil sizes. When open slots are used in conjunction with 
magnetic slot wedges, formed coils can be used without sacrificing the 
advantages of semiclosed slots. In a formed coil the windings are 
preformed 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 mechanical 
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. 
It is another object of the present invention to provide a magnetic slot 
wedge that is easily manufacturable and the resulting magnetic slot wedge 
is easily assembled in the stator slots. 
SUMMARY OF THE INVENTION 
In one aspect of the present invention magnetic wedges are provided for use 
in a toothed stator. The magnetic wedges hold the stator windings in the 
slots formed between the teeth of the stator. Each magnetic slot wedge 
comprises a stack of laminations of magnetic material, each lamination 
having a width corresponding to the width of the slot. The laminations 
each have a central bridge portion that is sufficiently narrow in the 
radial direction to saturate during machine operation. A support means of 
nonmagnetic and nonconductive material is located adjacent to the narrow 
central bridge portion of the stack to provide stiffness in the direction 
perpendicular to the plane of the laminations.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
Referring now to the drawing wherein like numerals indicate like elements, 
there is shown in FIG. 1 a magnetic slot wedge 2. The magnetic slot wedge 
has a stack of laminations 3, each lamination having a central bridge 
portion 5 connecting a first and second magnetic region 7 and 9, 
respectively. One side of the bridge 5 of each lamination forms a straight 
edge with the top of the first and second region across the width of the 
wedge providing a flat to to the wedge. The first and second regions and 
the bridge define an opening. The opening of each lamination when stacked 
aligns with the opening of each other lamination to form a channel with a 
narrowed entrance. A nonmagnetic nonconductive rod which in the present 
embodiment is a fiberglass reinforced plastic rod 10, is shaped to tightly 
fit in the channel. Since the entrance to the channel is narrower than the 
maximum width of the channel, the laminations clamp about the rod. The 
channel is not round, preventing the laminations from turning about the 
rod. The fiberglass reinforced plastic such as polyester fiberglass, can 
be pultruded to have the desired cross section. Pultruding is a continuous 
process of drawing numerous glass monofilaments through a resin bath and 
into a curing and shaping die. 
Referring now to FIG. 2, a portion of a laminated toothed stator core 11 is 
shown. Positioned between teeth 12 are slots 13. Formed windings 14 are in 
each slot. Near the open end of each slot on either side are wedge grooves 
15 into which protrusions 17 on the magnetic slot wedges of FIG. 1 are 
slid. The magnetic slot wedges extend the axial distance of the slot. The 
tops of the magnetic slot wedges are flush with the tops of the teeth or 
can be recessed slightly to obtain some mechanical protection for the slot 
wedges. The recess would be on the order of 5 mils for a core with an 
inside diameter of 10". The outline of a rotor 18 is shown with the 
distance between the inside diameter of the stator and the outside surface 
of the rotor shown as 19, representing the air gap. 
The wedges can be degreased in a suitable degreaser such as Freon.RTM. and 
bonded using vacuum pressure impregnation before assemblying in the core. 
Alternatively, the magnetic slot wedges could be placed in the core and 
the core bonded using a hot shot vacuum pressure impregnation as described 
in U.S. Pat. No. 3,904,785, issued Sept. 9, 1975 and assigned to the 
instant assignee hereby incorporated by reference. 
Since the magnetic slot wedge fits into notches in the stator slot, 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 would not 
be possible if a semiclosed slot design was used. 
In operation, the narrow portion of the bridge, which is narrow in the 
radial direction, saturates in the presence of the magnetic field produced 
by the stator windings during machine operation, causing a gradual 
transition of flux density across the slot and thereby reducing the higher 
order slotting harmonics. The saturation of the central bridge portion 5 
also results in a smaller leakage reactance than that which would result 
in a design without saturation. A smaller leakage reactance is desirable. 
The laminations of the magnetic slot wedge reduce the current flowing in 
the magnetic slot wedges and therefore reduce the losses due to the AC 
fields. 
An alternative embodiment of the laminated magnetic slot wedge is shown in 
FIG. 3. A central bridge portion 5 which saturates during operation is 
wider in a circumferential direction than the central bridge portion in 
FIG. 2 that saturates, resulting in a larger effective slot opening. The 
larger effective slot opening results in a lower leakage reactance with a 
slight increase in harmonic losses as compared to the embodiment shown in 
FIG. 1. The top of the wedge in FIG. 1 has a narrower saturated region and 
is more resistant to fatigue failure from magnetic and mechanical sources. 
The foregoing describes a magnetic slot wedge that results in reduced 
slotting harmonic losses and sufficient mechanical strength to carry all 
the force the wedge is exposed to. The magnetic slot wedge permits the use 
of form windings and is easy to manufacture and insert into a stator core. 
While the invention has been particularly shown and described with 
reference to several embodiments thereof, it will be understood by those 
skilled in the art that various changes in the form and details may be 
made therein without departing from the spirit and scope of the invention.