August 2012 Electrical Apparatus

August 2012 Electrical Apparatus

This is a summary of the Electrical Apparatus August 2012 featured technical article,  by Richard L. Nailen, P.E.     

Medium-voltage a-c motor stator windings are normally form-wound, requiring open stator slots to permit coil insertion. Once in place, coils are secured against damaging vibration by wedges fitted into the slot openings.

Because these openings create a magnetic discontinuity around the inner circumference of the stator, magnetic flux pulsation and harmonics are present to generate unwanted energy losses in and around the air gap. To minimize this effect, slot wedges containing magnetic material have been designed to simulate the effect of a closed or semi-closed slot, limiting all or most of the discontinuity.

The results are increased efficiency, with reduced core and stray load losses, magnetizing current, accelerating torque, and winding temperature. The magnitude of these changes varies widely with individual motor design as well as with wedge composition.

Many different wedge designs have been proposed during the past hundred years of a-c machine development. Most have involved encasing or molding iron or steel wire, fiber, strips, flakes, or powder into a plastic strip physically resembling the reinforced plastic laminates used for non-magnetic wedges.

Magnetic wedges are costly. In a large motor, the efficiency improvement justifies the cost premium. However, magnetic wedge materials, particularly when iron content is high, tend to be mechanically weaker. Loosening and disintegration of magnetic wedges continue to be reported worldwide.

Besides the usual forces exerted during wedge installation, and the thermal cycling of normal service, magnetic wedges are subject to strong electromagnetic vibratory forces. Fatigue failure results in premature motor breakdown and expensive repairs, which can far outweigh the energy saving resulting from higher efficiency.

Low-speed motors with small air gaps, and motors subject to severe starting duty or fluctuating loads, are especially susceptible to wedge failure. Complete vacuum impregnation of the wound stator, or at least the fixing in place of each wedge using epoxy resin, is important for long wedge life.

Most magnetic wedge materials in common use today are powder-filled, with components by weight 70% to 75% iron, 5% to 10% woven or mat reinforcement, and the remainder a cured epoxy.

To order a back issue with the full article, “Magnetic slot wedges: a mature technology?” call 312-321-9440 or visit our online webstore.

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