Two-phase electric power

Two-phase electrical power was an early 20th-century polyphase alternating current electric power distribution system. Two circuits were used, with voltage phases differing by one-quarter of a cycle, 90°. Usually circuits used four wires, two for each phase. Less frequently, three wires were used, with a common wire with a larger-diameter conductor. Some early two-phase generators had two complete rotor and field assemblies, with windings physically offset to provide two-phase power. The generators at Niagara Falls installed in 1895 were the largest generators in the world at that time and were two-phase machines. Three-phase systems eventually replaced the original two-phase power systems for power transmission and utilization. There remain few two-phase distribution systems, with examples in Philadelphia, Pennsylvania; many buildings in Center City are permanently wired for two-phase and Hartford, Connecticut.

Comparison with single-phase power

The advantage of two-phase electrical power over single-phase was that it allowed for simple, self-starting electric motors. In the early days of electrical engineering, it was easier to analyze and design two-phase systems where the phases were completely separated.Thomas J. Blalock "The first polyphase system: a look back at two-phase power for AC distribution", in ''IEEE Power and Energy Magazine'', March–April 2004, p. 63 It was not until the invention of the method of symmetrical components in 1918 that polyphase power systems had a convenient mathematical tool for describing unbalanced load cases. The revolving magnetic field produced with a two-phase system allowed electric motors to provide torque from zero motor speed, which was not possible with a single-phase induction motor (without an additional starting means). Induction motors designed for two-phase operation use a similar winding configuration as capacitor start single-phase motors. However, in a two-phase induction motor, the impedances of the two windings are identical.

Two-phase circuits also have the advantage of constant combined power into an ideal load, whereas power in a single-phase circuit pulsates at twice the line frequency due to the zero crossings of voltage and current.

Comparison with three-phase power

Three-phase electric power requires less conductor mass for the same voltage and overall power, compared with a two-phase four-wire circuit of the same carrying capacity.Terrell Croft and Wilford Summers (ed), ''American Electricans' Handbook'', Eleventh Edition, McGraw Hill, New York (1987) page 3–10, figure 3–23 It has replaced two-phase power for commercial distribution of electrical energy, but two-phase circuits are still found in certain control systems.

Two-phase circuits typically use two separate pairs of current-carrying conductors. Alternatively, three wires may be used, but the common conductor carries the vector sum of the phase currents, which requires a larger conductor. The vector sum of balanced three-phase currents, however, is zero, allowing for the neutral wires to be eliminated. In electrical power distribution, a requirement of only three conductors, rather than four, represented a considerable distribution-wire cost savings due to the expense of conductors and installation.

While both two-phase and three-phase circuits have a constant combined power for an ideal load, practical devices such as motors can suffer from power pulsations in two-phase systems. These power pulsations tend to cause increased mechanical noise in transformer and motor laminations due to magnetostriction and torsional vibration in generator and motor drive shafts.

Two-phase power can be derived from a three-phase source using two transformers in a Scott connection: One transformer primary is connected across two phases of the supply. The second transformer is connected to a center-tap of the first transformer, and is wound for 86.6% of the phase-to-phase voltage on the three-phase system. The secondaries of the transformers will have two phases 90 degrees apart in time, and a balanced two-phase load will be evenly balanced over the three supply phases.

See also

*Polyphase system
*Rotary converter
* Single-phase electric power
*Split-phase electric power
* Three-phase electric power

References

; Notes:

; Specific references:

; General references:

*Donald G. Fink and H. Wayne Beaty, ''Standard Handbook for Electrical Engineers'', Eleventh Edition, McGraw-Hill, New York, 1978,
* Edwin J. Houston and Arthur Kennelly, ''Recent Types of Dynamo-Electric Machinery'', copyright American Technical Book Company 1897, published by P. F. Collier and Sons New York, 1902

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