Electrical Apparatus magazine, October 2005 Catching the wave in a-c machines

In three-phase motors above the smallest sizes, the usual stator winding is of the “lap” type, consisting of multi-turn diamond-shaped coils. In the alternative “wave” winding, each coil turn does not double back upon itself, but is connected to a turn in the next coil. The current path thus proceeds around the stator like a periodic wave until returning to a slot adjacent to that in which the path began. The process repeats, filling all the slots, to create a series of single-turn coils connected in each phase. Interconnection of two separate conductors in each slot can similarly create a group of two-turn coils.

Such turns filling a reasonably-proportioned slot are necessarily much deeper than turns of a lap-wound stator coil. That would be unacceptable in a stator, because of high eddy current losses at line frequency. Wave windings are therefore confined to rotors, where the frequency is so low that eddy currents are insignificant.

The major advantage of wave winding in a rotor is that a balanced three-phase connection includes only three interconnecting jumpers plus three leads to collector rings. These will be spaced 120 degrees apart, minimizing mechanical unbalance in the assembly. Also, a small number of rotor connections is desirable because each is subjected to centrifugal stress.

Pole-phase groups exist in the wave-wound rotor just as in a lap-wound stator. Alternate groups must exhibit opposite polarity. Hence, each wave-wound phase is wound in two halves, each constituting a circuit path following the opposite director from the other. That limits the number of possible parallel circuits – a further obstacle to use of such windings in stators.

Phases may be joined in wye or in delta. For either one, three options exist within the phases. The span or slot pitch of certain coils may return each circuit path to one slot behind where it began (retrogressive connection); to one slot ahead (progressive); or to both options, one in each half of the winding. The first version is most common because it minimizes the number of different coil shapes required.

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Featured Technical Article
October 2005

On this page is a summary of the Electrical Apparatus October 2005 featured technical article, by Richard L. Nailen, P.E. , "Catching the wave in a-c machines" Über die 'Welle' in Wechselstrommaschinen ... Ondulation des machines á courant alternatif ... 'A Coger la Onda' en las Máquinas de CA... ..

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