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“Tricks and traps in three-phase winding connections"
With
trilingual summary
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.
From “Tricks
and traps in three-phase winding connections"
from the
Electrical Apparatus March 2008 issue
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copies. © 2008
Barks Publications, Inc. All Rights Reserved.
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