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Summary: “Catching the
wave in a-c machines" With
trilingual summary
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.
From
“Catching the wave in a-c
machines"- published in Electrical Apparatus
October 2005
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