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Slot Wedges -- Essential, yet Often Misunderstood With
trilingual summary
In
each slot of an a-c motor or generator stator, a strip of insulating
material is fitted into lamination grooves to wedge the coils tightly
in place. Although such a "slot wedge" need not exhibit high
dielectric strength, it must withstand the electromagnetic force
tending to push coils out of the slot. (In a wound rotor or armature,
centrifugal force acts in the same direction.)
Although precise calculation of the cyclic forces
acting on a stator slot wedge is not feasible, even conservative
estimates are far below material stress limits. However, the fit
between lamination grooves and the edges of a wedge is neither
uniform nor exactly predictable, and is subject to wear during
service. Wedge looseness is therefore a major concern in large
machines not subject to global impregnation. Periodic testing of
wedge tightness is essential in large generator maintenance.
Because the extension of the wedge into its supporting
grooves necessarily blocks off part of the air space on either side
of the coils at each vent space in the core, wedges are often notched
out at those spaces to remove that restriction. An alternative
(unacceptable to some users) is to place only a few short wedges in
each slot, counting on VPI treatment to bond the coils in place.
Other special machining of wedge contours has been used to better
direct ventilating air flow from the rotor into the passages between
stator coils.
Some manufacturers (particularly in Europe) have used
wedges made of magnetic material, such as iron powder in an epoxy
compound, to improve machine performance by creating a highly
permeable path across the otherwise open slot adjacent to the air
gap. Although significant efficiency improvements have been claimed,
rigorous calculation of the performance benefits does not seem
possible.
Many different materials have been tried, from rigid
molded strips to paste mixtures placed over the coil side and then
cured. Besides the difficulty of achieving the right balance between
magnetic permeability and electrical conductivity (which tends to
short-circuit the laminations), problems have included brittleness of
material; difficulty in machining rigid wedges; and looseness caused
by magnetic forces and thermal cycling.
From “Slot
Wedges -- Essential, yet Often Misunderstood," to
be published in Electrical Apparatus
April 2006
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copy. © 2006
Barks Publications, Inc. All Rights Reserved.
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