Three-phase power is a method of electrical power transmission that makes use of three wires to deliver three independent alternating electrical currents. The current in each wire is set off from the others by one-third of a complete cycle, with each current representing one phase. This means that a device operating off this type of power source receives a more stable flow of electricity than it would from single-phase distribution system. Some three-phase power systems actually have four wires; the fourth is a neutral wire that allows the system to use a higher voltage.
Purpose
The three currents, together, deliver a balanced load, something not possible with single-phase alternating current. In alternating current (AC), the current alternates direction, flowing back and forth in the circuit; this means that the voltage alternates as well, constantly changing from maximum to minimum. Three-phase power combines the three wires to off-set the maximum and minimum oscillations, so that a device receiving this type of power does not experience such a wide variation in voltage. This makes three-phase power a very efficient form of electrical power distribution. Consequently, a three-phase electric motor uses less electricity and normally lasts longer than a single-phase motor of the same voltage and rating.
Origins
Three-phase power flow begins in a power station, where an electrical power generator converts mechanical power into alternating electrical currents. After numerous conversions in the distribution and transmission network, the power is transformed into the standard voltage supplied to homes and businesses, 230 volts in Europe or 120 volts in North America. The output of the transformer usually connects to the power system using three live wires tied to a single grounded return. This is called a star connection.
Applications
This type of system does not usually provide power to domestic houses, but when it does, a main distribution board splits the load. Most domestic loads use single-phase power because of the lower cost of distribution. Three-phase power is most common in industrial settings, or where more power is needed to operate heavy machinery, though there are exceptions.
Running electric motors are the most frequent use for three-phase power. A three-phase induction motor combines high efficiency, a simple design, and a high starting torque. Industrial fans, blowers, pumps, compressors, and many other kinds of equipment commonly use this type of electric motor. Other systems that may use three-phase power include air conditioning equipment, electric boilers, and large rectifier systems used for converting alternating current to direct current.
While most motors that run on three-phase power are quite big, there are examples of very small motors, such as those that power computer fans, which work on this type of power. An inverter circuit inside the fan converts direct current (DC) to a three-phase AC current. This serves to decrease noise, as the torque from a three-phase motor is very smooth.
Standards
The wires, called conductors, used in a three-phase power system are normally color-coded, although the colors vary greatly by location, and most countries have their own codes. North America traditionally uses black, red, and blue to represent the three phases, for example, while white represents the neutral wire. In Europe, by contrast, brown, black, and grey represent the phases, and the neutral wire is blue. Even with these national standards, there tend to be a lot of irregularities in day-to-day applications. It is not a good idea for anyone working with three-phase power to make assumptions without consulting the diagram for the individual installation or system in question.