A polyphase system is a means of distributing alternating-current (AC) electric power using multiple conductors carrying AC voltages that are shifted in phase relative to each other. This contrasts with a single-phase system, which uses only one alternating voltage. Polyphase systems are overwhelmingly the most common method for power generation, transmission, and distribution worldwide, primarily due to their efficiency and the ability to produce a rotating magnetic field in electric motors.
Principles
In a polyphase system, multiple AC voltage waveforms are generated, each with a specific phase angle displacement from the others. These voltages are typically produced by a single alternator (generator) that has multiple windings, each displaced mechanically.
- Phase: Each individual AC voltage and its associated circuit is referred to as a "phase."
- Phase Shift: The phase shift between the voltages is usually equal, determined by dividing 360 degrees by the number of phases. For example, in a three-phase system, the three voltages are shifted by 120 degrees from each other (360°/3 = 120°).
Advantages
Polyphase systems offer several significant advantages over single-phase systems, particularly for high-power applications:
- Constant Power Delivery: For a balanced load, the total instantaneous power delivered by a polyphase system is constant, unlike single-phase power, which pulsates. This reduces vibrations in generators and motors.
- Efficient Power Transmission: For the same amount of power, a polyphase system (especially three-phase) requires less conductor material than a single-phase system, leading to reduced line losses and lower infrastructure costs.
- Self-Starting Motors: Polyphase motors (like induction motors) can generate a rotating magnetic field without additional starting mechanisms, making them simpler, more robust, and efficient compared to single-phase AC motors which often require auxiliary windings or capacitors for starting.
- Higher Power Density: Generators and motors designed for polyphase power are generally smaller and more efficient for a given power output.
- Reduced Ripple: When rectified to DC, polyphase AC produces a smoother DC output with less ripple, requiring smaller and less costly filtering components for high-power DC applications.
Types of Polyphase Systems
While systems with various numbers of phases have been explored, the most prevalent types are:
- Two-Phase System: Historically, some early power systems, like those developed by Westinghouse for the Niagara Falls power plant, utilized two-phase power. In a two-phase system, two AC voltages are displaced by 90 degrees. It is largely obsolete for general power distribution but can still be found in specialized motor designs or control systems.
- Three-Phase System: This is by far the most common polyphase system used globally for electricity generation, transmission, and distribution. It uses three AC voltages, each 120 degrees out of phase with the others.
- Wye (Star) Configuration: In a Wye connection, the ends of the three phase windings are connected to a common point called the neutral point. This configuration allows for both phase-to-phase voltages (line voltages) and phase-to-neutral voltages (phase voltages), providing two voltage levels from a single source. It typically uses four wires (three phase conductors and one neutral).
- Delta Configuration: In a Delta connection, the windings are connected end-to-end to form a closed loop. This configuration does not have a neutral point. It typically uses three wires (three phase conductors), and only line voltages are available.
- Higher-Phase Systems (e.g., Six-Phase, Twelve-Phase): These systems are typically used in specialized applications, such as for high-power rectifiers where a very smooth DC output is required, or in large industrial drives. They are generally derived from a three-phase supply using specialized transformers.
Applications
- Power Generation: Large alternators in power plants are invariably three-phase.
- Power Transmission: High-voltage transmission lines are almost exclusively three-phase, carrying power over long distances.
- Power Distribution: Industrial and commercial facilities, and increasingly residential areas, are supplied with three-phase power.
- Electric Motors: The vast majority of industrial motors are three-phase induction or synchronous motors due to their efficiency, reliability, and self-starting capability.
- Rectifiers: High-power DC applications, such as industrial electrolysis, variable-speed drives, and HVDC transmission, utilize three-phase AC rectified to DC.
History
The concept of polyphase power was independently developed by several individuals in the late 19th century. Notable pioneers include Nikola Tesla, who conceived of the rotating magnetic field and developed the polyphase induction motor (patented in 1888), and Mikhail Dolivo-Dobrovolsky, who developed the three-phase generator and transformer and demonstrated the first long-distance three-phase AC power transmission in 1891. George Westinghouse was instrumental in commercializing Tesla's polyphase system in North America. The superior advantages of three-phase power quickly led to its adoption as the standard for modern electrical grids.
See Also
- Alternating current
- Single-phase electric power
- Three-phase electric power
- Electric power transmission
- Induction motor