Trystan Casey
The term 2-phase in electricity refers to an early 20th-century polyphase alternating current electric power distribution system. It is a legacy term associated with the original polyphase system developed by Nikola Tesla and is rarely used today, except in a few places like Philadelphia and Niagara Falls. 2-phase power systems used two circuits with voltage phases differing by 90 degrees, typically requiring four wires (two for each phase) or, less frequently, three wires with a common conductor. It offered advantages over single-phase power, enabling the use of simple, self-starting electric motors. However, 3-phase power systems have largely replaced 2-phase systems due to their higher efficiency and capacity to handle higher loads.
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What You'll Learn
- Two-phase power is a legacy system, rarely used today
- It was an early 20th-century polyphase alternating current system
- Two-phase power is derived from three-phase power using two transformers
- It was easier to analyse and design two-phase systems in early electrical engineering
- Two-phase circuits are still found in certain control systems
Two-phase power is a legacy system, rarely used today
Two-phase electrical power systems were an early 20th-century power distribution system. Two circuits were used, with voltage phases differing by one-quarter of a cycle, 90°. Usually, these circuits used four wires, two for each phase, but sometimes three wires were used, with a common wire with a larger-diameter conductor.
The two-phase system had advantages over single-phase systems, as it allowed for simple, self-starting electric motors. 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.
However, two-phase power systems have largely been replaced by three-phase power systems, which are more efficient and can transmit three times as much power as a single-phase power supply with only one additional wire. Three-phase systems also have better consistency in the delivery of power, as a single-phase system has peaks and dips in voltage.
Today, true two-phase power is rarely used, with the exception of some legacy systems in certain parts of the world, such as Center City Philadelphia, where many commercial buildings are permanently wired for two-phase, and in Hartford, Connecticut. In other parts of the world, the term "2 phase" is sometimes used to describe "2 out of 3 phases", meaning there is a 3-phase distribution system, and the load or machine is only using two of those three phases.
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It was an early 20th-century polyphase alternating current system
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 degrees. Typically, two-phase circuits use two separate pairs of current-carrying conductors. Circuits usually used four wires, two for each phase, but sometimes three wires were used, with a common wire with a larger-diameter conductor.
The two-phase system was advantageous over single-phase systems as it allowed for simple, self-starting electric motors. In the early 20th century, it was easier to analyse and design two-phase systems where the phases were completely separated. 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.
The two-phase system was replaced by three-phase systems, which are more efficient and require fewer conductors. Three-phase systems can deliver more power with fewer wires and lower transmission losses. However, two-phase systems are still found in certain control systems and active two-phase distribution systems remain in Center City Philadelphia and Hartford, Connecticut.
The term "2 phase" can be misleading as, outside of the legacy systems in Philadelphia and Niagara Falls, it is rarely used in North America. In other parts of the world, "2 phase" is used to describe "2 out of 3 phases", meaning a 3-phase distribution system where the load or machine is only using two of the three phases.
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Two-phase power is derived from three-phase power using two transformers
Two-phase power is a term that can be misleading. True two-phase power exists in very limited places, such as in Center City Philadelphia, where many commercial buildings are permanently wired for two-phase power, and in Hartford, Connecticut. In other parts of the world outside of North America, the term "two phase" is used to describe "two out of three phases", meaning you have a three-phase distribution system, and your load/machine is only using two of those three phases.
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.
A three-phase electrical system is used to generate and transmit electric power over long distances for use by offices and industry. Three-phase voltages (and currents) are raised or lowered by means of three-phase transformers, since a three-phase transformer can have its windings connected in various ways. The three-limb core-type three-phase transformer is the most common method of three-phase transformer construction, allowing the phases to be magnetically interlinked. The magnetic flux flowing around each limb uses the other two limbs for its return path. The three magnetic fluxes within the core generated by the line voltages differ in time-phase by 120 degrees (120o).
In a single-phase supply, the energy flows in via the active, through your meter and main switch, and onward via RCD/circuit breakers, into the wiring and to your appliances. That same energy returns via the neutral wires to complete the circuit. If any of that energy "leaks" out of the system, then the RCD/safety switch will trip off because it senses an imbalance between the active and neutral.
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It was easier to analyse and design two-phase systems in early electrical engineering
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, two-phase circuits used four wires, two for each phase. Less frequently, three wires were used, with a common wire with a larger-diameter conductor.
In the early days of electrical engineering, it was easier to analyse and design two-phase systems where the phases were completely separated. This was because two-phase electrical power had several advantages over single-phase power. Firstly, it allowed for simple, self-starting electric motors. 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 mechanism. Secondly, two-phase circuits had 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.
However, 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. This invention made it easier to analyse and design three-phase systems, which eventually replaced two-phase power systems for power transmission and utilisation. Three-phase electric power requires less conductor mass for the same voltage and overall power compared to a two-phase four-wire circuit of the same carrying capacity. In electrical power distribution, the requirement of only three conductors for three-phase power, rather than four for two-phase power, represented considerable distribution-wire cost savings due to the expense of conductors and installation.
Today, true two-phase power is rare and only exists in very limited places, such as Center City Philadelphia and Niagara Falls, as a legacy of the original polyphase system developed by Nikola Tesla. In other parts of the world outside of North America, the term "2 phase" is sometimes used to describe "2 out of 3 phases", meaning there is a three-phase distribution system, and the load or machine is only using two of those three phases.
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Two-phase circuits are still found in certain control systems
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, i.e., 90°. Typically, two-phase circuits use two separate pairs of current-carrying conductors. Less frequently, three wires were used, with a common wire with a larger-diameter conductor. The two-phase system allowed for simple, self-starting electric motors. 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. Induction motors designed for two-phase operation use a similar winding configuration as capacitor start single-phase motors.
However, two-phase power systems have been replaced by three-phase systems for power transmission and utilization. Three-phase electric power requires less conductor mass for the same voltage and overall power compared to a two-phase four-wire circuit with the same carrying capacity. Three-phase systems can deliver more power with fewer wires and lower losses in power transmission.
Despite the advantages of three-phase systems, two-phase circuits are still found in certain control systems and specific locations. Active two-phase distribution systems remain in Center City Philadelphia, where many commercial buildings are permanently wired for two-phase power, and in Hartford, Connecticut. Additionally, a visitor to the Peco Energy facility in Philadelphia's Old City section in 2020 reported that 4 kV two-phase three-wire service distribution transformers were still in use.
The term "two-phase" can be misleading, as it is sometimes used to describe "two out of three phases" in a three-phase distribution system. In North America, this is referred to as "single-phase" power to differentiate it from the true two-phase legacy systems.
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Frequently asked questions
The term "2-phase" in electricity refers to a type of polyphase alternating current electric power distribution system that uses two circuits with voltage phases differing by one-quarter of a cycle, or 90 degrees.
An example of a 2-phase power system is the generators at Niagara Falls installed in 1895, which were the largest generators in the world at that time.
2-phase power offers advantages over single-phase power, such as enabling simple, self-starting electric motors and easier analysis and design of electrical systems. However, single-phase power is more commonly used for residential homes, while commercial and industrial facilities often use 3-phase power.
The term "2-phase" is not commonly used in North America, where true 2-phase power systems have largely been replaced by 3-phase systems. However, it may be used to describe "2 out of 3 phases" in a 3-phase distribution system, where a load or machine is using only two of the three available phases.
