March 2008 Electrical Apparatus

March 2008 Electrical Apparatus

This is a summary of the Electrical Apparatus March 2008 featured technical article,  by Richard L. Nailen, P.E.    

A three-phase motor lap winding normally includes identical groups of interconnected coils, each group containing the same number of coils and each coil having the same number of turns.

This pattern can be varied in several ways. The most familiar one involves differing numbers of coils per group, when the number of slots per pole per phase is fractional rather than integral. The grouping is selected to provide a symmetrical or balanced magnetic field throughout each phase.

In a second alternative connection, the number of coils per group is constant, but the coils throughout each group do not all lie in consecutive adjacent slots. Instead, one or more coils at the ends of each group are shifted into slots within adjacent groups. Such an “interspersed” winding connection changes the overall distribution factor of each group to reduce objectionable harmonics in the magnetic field.

In a third alternative, one or more coils within each group contain one fewer turn than in the remaining coils. This can be a simple pattern such as alternate 3- and 4-turn coils, providing the equivalent of 3-1/2 turn coils throughout, or a more complex pattern to achieve some other fraction or to reduce harmonics.

Still another connection option is the “overlap” winding, in which one or more coils at the end of each group are each replaced by two coils of fewer turns, which are intermingled in slots with corresponding coils added to adjacent groups. Although each slot throughout the winding contains the same number of turns, some slots contain four coil sides instead of two. This can greatly reduce stray load loss, increasing motor efficiency and lowering temperature rise.

Because such changes will generally add some material and labor cost, they must be justified by enhanced performance. Care must be taken to retain electromagnetic balance throughout the winding so as to avoid undue vibration, noise, and circulating currents. Some of the options are suited only to random-wound low-voltage machines and are most often used in two-pole air conditioning system drives.

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