Polyphase system
Polyphase electrical systems supply alternating current electrical power in overlapping phases. The most common, and almost universal, examples are three-phase power systems. Three phase power is commonly found in industrial applications and LCR circuits.
Phases
These systems exhibit three-phase induction or more. Three phase power is commonly found in industrial applications. Normal household supply consists of a sine wave of alternating polarity, with the voltage rising and falling over time. This is called single phase electric power.
A few older installations in the U.S. used two-phase four-wire systems for motors. The chief advantage of these was that the winding configuration was the same as for a single-phase capacitor-start motor. Most of these have been upgraded to three-phase systems. A two-phase supply with 90 degrees between phases can be derived from a three-phase system using transformers in a Scott connection. In North America most households are equipped with single-phase circuits supplied from a transformer in which the secondary winding is 120/240 volts (nominal) three-wire with neutral. The phase alignment is 180°. These are used in high-energy appliances such as electric Oven/ranges and clothes dryers.
Motors
Polyphase power is particularly useful in AC motors, where it can be used to generate a rotating magnetic field. Alternate current sine wave voltage points, when the field is graphed and analyzed, show a result of a magnetic field that is rotating (thus producing the alternating polarity). When a three-phase voltage completes one full cycle, the magnetic field has rotated through 360°. In a polyphase system several such supplies are overlapped out of phase to smooth out the total voltage.
Invention
Nikola Tesla invented this type of system. Polyphase electrical systems operate on the principle of the lead-lag effect in the rotating magnetic field produced by an alternating-current circuit. He found these systems during his research into harmonic oscillators.
See also
