A split-phase or single-phase three-wire system is a form of single-phase electric power distribution. It is the alternating current (AC) equivalent of the original three-wire DC system developed by the Edison Machine Works. The main advantage of split-phase distribution is that, for a given power capacity, it requires less conductor material than a two-wire single-phase system. Split-phase distribution is widely used in North America for residential and light commercial service. A typical installation supplies two 120 V AC lines that are 180 degrees out of phase with each other (relative to the neutral), along with a shared neutral conductor. The neutral is connected to ground at the transformer's center tap. In North America, standard household circuits for lighting and small appliances are connected between one line and the neutral, providing 120 V. Higher-demand appliances such as ovens, dryers, or water heaters are powered by 240 V circuits, connected between the two 120 V lines. These 240 V loads are either hard-wired or use outlets designed to be non-interchangeable with 120 V outlets. Split-phase systems are also used in some specialized applications to reduce the risk of electric shock or to minimize electromagnetic noise.
Connections
A transformer supplying a three-wire distribution system has a single-phase input (primary) winding. The output (secondary) winding has a center tap connected to a grounded neutral. As shown in Fig. 1, either end to center has half the voltage of end-to-end. Fig. 2 illustrates the phasor diagram of the output voltages for a split-phase transformer. Since the two phasors do not define a unique direction of rotation for a revolving magnetic field, a split single-phase is not a two-phase system. In the United States and Canada, the practice originated with the DC distribution system developed by Thomas Edison. By connecting pairs of lamps or groups of lamps on the same circuit in series, and doubling the supply voltage, the size of conductors was reduced substantially. When there are two circuits, each with unique loads, the common wire for each circuit can be replaced by a single conductor returned to the center tap of the supply voltage, stabilizing the branch circuit voltages from changes when loads are switched on and off. The current carried in the neutral is only the imbalance of current flowing from one group of loads to the other. This arrangement reduces the number of conductors from four to three along with a significant labor and material cost saving. The line-to-neutral voltage is half the line-to-line voltage. Lighting and small appliances may be connected between a line wire and the neutral. Higher-power appliances, such as cooking equipment, space heating, water heaters, clothes dryers, air conditioners and electric vehicle charging equipment, are connected to the two line conductors. This means that, for the supply of the same amount of power, the current is halved. Smaller conductors may be used than would be needed if the appliances were designed to be supplied by the lower voltage.
If the load were guaranteed to be balanced (the same current drawn from each line), then the neutral conductor would not carry any current and the system would be equivalent to a single-ended system of twice the voltage with the line wires taking half the current. This would not need a neutral conductor at all, but would be impractical for varying loads; just connecting the groups in series would result in excessive voltage and brightness variation as lamps are switched on and off. By connecting the two lamp groups to a neutral, intermediate in potential between the two live legs, any imbalance of the load will be supplied by a current in the neutral, giving substantially constant voltage across both groups. The total current carried in all three wires (including the neutral) will always be twice the supply current of the most heavily loaded half. For short wiring runs limited by conductor current carrying capacity, this allows three half-sized conductors to be substituted for two full-sized ones, using 75% of the copper of an equivalent single-phase system. Long wiring runs are limited by the permitted voltage drop limit in the conductors. Because the supply voltage is doubled, a balanced load can tolerate double the voltage drop, allowing quarter-sized conductors to be used; this uses 3/8 the copper of an equivalent single-phase system. In practice, some intermediate value is chosen. For example, if the imbalance is limited to 25% of the total load (half of one half) rather than the absolute worst-case 50%, then conductors 3/8 of the single-phase size will guarantee the same maximum voltage drop, totalling 9/8 of one single-phase conductor, 56% of the copper of the two single-phase conductors.
Balanced power In a so-called balanced power system, sometimes called "technical power", an isolation transformer with a center tap is used to create a separate supply with conductors at balanced voltages with respect to ground. The purpose of a balanced power system is to minimize the noise coupled into sensitive equipment from the power supply. Unlike a three-wire distribution system, the grounded neutral is not distributed to the loads; only line-to-line connections at 120 V are used. A balanced power system is used only for specialized distribution in audio and video production studios, sound and television broadcasting, and installations of sensitive scientific instruments. The U.S. National Electrical Code provides rules for technical power installations. The systems are not to be used for general-purpose lighting or other equipment and may use special sockets to ensure that only approved equipment is connected to the system. Additionally, technical power systems pay special attention to the way the distribution system is grounded. A risk of using a balanced power system in an installation that also uses "conventional" power in the same rooms is that a user may inadvertently interconnect the power systems together via an intermediate system of audio or video equipment, elements of which might be connected to different power systems. The chance of this happening may be reduced by appropriate labelling of the balanced power outlets and by the use of a type of power outlet socket for the balanced system that is physically different from that of the "conventional" power system to further differentiate them.
Applications
… excerpt ends here. Continue reading the full article.
