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The why and how of transposed conductors

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.

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