Polyphase coils are electrical coil

Coil

A coil is a series of loops. A coiled coil is a structure in which the coil itself is in turn also looping.-Electromagnetic coils:An electromagnetic coil is formed when a conductor is wound around a core or form to create an inductor or electromagnet...

s (phases) connected together in a polyphase system

Polyphase system

A polyphase system is a means of distributing alternating current electrical power. Polyphase systems have three or more energized electrical conductors carrying alternating currents with a definite time offset between the voltage waves in each conductor. Polyphase systems are particularly useful...

 such as a generator

Electrical generator

In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

 or motor

Electric motor

An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

. In modern systems, the number of phases is usually three or a multiple of three. Each phase carries a sinusoidal alternating current

Alternating current

In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 whose phase

Phase (waves)

Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...

 is delayed relative to one of its neighbours and advanced relative to its other neighbour. The phase currents are separated in time evenly within each period of the alternating current. For example, in a three-phase system, the phases are separated from each other by one-third of the period.

Coil construction

Like all coils used in electrical machinery, polyphase coils (made from insulated conducting wire

Wire

A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...

) are wound around ferromagnetic

Ferromagnetism

Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...

 armatures with radial projections and maximum core-surface exposure to the magnetic field.

The windings are physically separated around the circumference of an electrical machine. The result of such an arrangement is a rotating magnetic field

Rotating magnetic field

A rotating magnetic field is a magnetic field which changes direction at a constant angular rate. This is a key principle in the operation of the alternating-current motor. Nikola Tesla claimed in his autobiography that he identified the concept of the rotating magnetic field in 1882. In 1885,...

 that is used to convert electrical power

Electric power

Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...

 to rotary mechanical work

Mechanical work

In physics, work is a scalar quantity that can be described as the product of a force times the distance through which it acts, and it is called the work of the force. Only the component of a force in the direction of the movement of its point of application does work...

, or vice versa.

Polyphase motors and generators

Compared to single-phase motors and generators, polyphase motors are simpler, because they do not require external circuitry (using capacitor

Capacitor

A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

s and inductor

Inductor

An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...

s) to produce a starting torque

Torque

Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....

. Polyphase machines can deliver constant power over each period of the alternating current, eliminating the pulsations found in a single-phase machine as the current passes through zero amplitude.

History

The use of polyphase coils in electrical power systems was pioneered by the engineers Nikola Tesla

Nikola Tesla

Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...

, Galileo Ferraris

Galileo Ferraris

Galileo Ferraris was an Italian physicist and electrical engineer, noted mostly for the studies and independent discovery of the rotating magnetic field, a basic working principle of the induction motor...

, and Michail Dolivo-Dobrovolsky

Michail Dolivo-Dobrovolsky

Mikhail Osipovich Dolivo-Dobrovolsky , was a Russian engineer, electrician, and inventor. Born in a mixed family between a Polish noble family originating from Mazowsze, and a Russian noble family. Emigrated to Germany due to political persecution after the assassination of Alexander II of Russia...

.