Polyphase electrical systems supply alternating current electrical power in overlapping phases. The most common example are three-phase power systems used in industrial applications.
Polyphase systems have two or more phases. The voltage on each phase is a sine wave, with a fixed time offset, or phase shift, between the phases. Modern utility power generation and distribution is almost universally three phase, with the phases separated by 120° or one third of an AC cycle. It is commonly used in industry, as it is ideally suited to powering the 3-phase induction motor. Residences and small businesses are usually supplied with a single phase taken from one of the three utility phases. Individual customers are distributed among the three phases to balance the loads.
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 houses, small apartments and small businesses are supplied from a single-phase utility transformer with a three-wire 120/240 volt (nominal) secondary. One of the wires -- the neutral -- is grounded; the other two are hot. Each hot wire is at 120V with respect to the neutral, but the phase offset between the two hot wires is 180° so that the AC voltage between the hot wires is 240V. This supply is used by high-power appliances such as electric Ovens/ranges, clothes dryers and central air conditioning systems. Such systems are sometimes referred to as split phase and are sometimes incorrectly referred to as two phase.
Many larger apartment buildings are fed with 3-phase transformers, with two of the three phases fed to each unit. The phase offset between the two hot wires is therefore only 120 degrees, so that while the voltage from each "hot" wire to neutral is 120V, the voltage between hot wires is only 208V. Most appliances rated for 240V will run satisfactorily on 208V, but special autotransformers can boost 208V to 240V if necessary.
Polyphase power is particularly useful in AC motors, particularly the induction motor, where it generates a rotating magnetic field. When a three-phase supply completes one full cycle, the magnetic field has rotated through 360°.
Last updated: 08-10-2005 09:10:57