Two-phase electrical power was an early 20th-century
polyphase alternating current
Alternating current (AC) is an electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction. Alternating current is the form in w ...
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. Active two-phase distribution systems remain in
Center City Philadelphia
Philadelphia ( ), colloquially referred to as Philly, is the List of municipalities in Pennsylvania, most populous city in the U.S. state of Pennsylvania and the List of United States cities by population, sixth-most populous city in the Unit ...
, where many commercial buildings are permanently wired for two-phase, and in
Hartford, Connecticut
Hartford is the List of capitals in the United States, capital city of the U.S. state of Connecticut. The city, located in Hartford County, Connecticut, Hartford County, had a population of 121,054 as of the 2020 United States census, 2020 ce ...
.
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
Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems that use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...
, it was easier to analyze and design two-phase systems where the phases were completely separated.
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
A split-phase or single-phase three-wire system is a type of single-phase electric power distribution. It is the alternating current (AC) equivalent of the original Edison Machine Works three-wire direct-current system. Its primary advantage i ...
*
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
{{Electric machines
Electric power
AC power