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Summary:
"The why and how of transposed conductors" With
trilingual summary
By
Richard L. Nailen, EA Engineering Editor
Alternating
current flow through any conductor will be surrounded by an
alternating magnetic field. As that varying field moves relative to
the conductor or any others nearby, it will induce voltages in them.
A closed circuit path within any such conductors will then experience
current flow.
Because
of that complex set of interrelationships, different areas of a
conductor will tend to carry differing amounts of current. That
so-called "skin effect" influences design of high voltage
transmission cables as well as squirrel-cage motor rotors. When large
cables are connected in parallel to transmit high current, their
relative orientation can result in unequal sharing of the current,
with some cables underloaded while others overheat. Differing voltage
drops lead to phase unbalance.
The
same phenomenon is responsible for the circulation of parasitic or
eddy currents within conductors in large motors or generators. These
reduce efficiency and lead to overheating. The deeper the coil turn
conductor, the more severe the penalty. Manufacturing and cost
concerns dictate making large conductors of many strands of wire that
are insulated from one another. But in extremely deep conductors even
that does not solve the eddy current problem, because all the strands
must join together at the coil ends.
The
solution to all those problems is transposition--periodic shifting of
the relative positions of individual strands through the coil length.
That equalizes induced voltages and impedances. In power circuits,
individual cables are bundled and rotated. On transmission towers,
phase conductor locations are periodically interchanged. In rotating
machine windings using deep multi-turn coils, an end-turn
transposition shifts strand position at each interconnection between
coils.
The
half-turn or bar coils in much larger machines, which may be 20 or
more strands in depth, typically use a Roebel transposition in which
strands periodically cross over from one tier to another, so that
throughout the bar length each strand may occupy every possible
position at least once.
In
any winding transposition, care must be taken to maintain the
integrity of all insulation separating components subject to eddy
currents, even though they are all theoretically at the same
potential.
Copyright 2003, Barks Publications, Inc., Chicago.
Reproduction by any means prohibited.
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