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Slot Wedges -- Essential, yet Often Misunderstood

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.

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