The common reference is usually connected single phase motor wiring diagram pdf ground and often to a current-carrying conductor called the neutral. Due to the phase difference, the voltage on any conductor reaches its peak at one third of a cycle after one of the other conductors and one third of a cycle before the remaining conductor. This phase delay gives constant power transfer to a balanced linear load.
In a three-phase system feeding a balanced and linear load, the sum of the instantaneous currents of the three conductors is zero. In other words, the current in each conductor is equal in magnitude to the sum of the currents in the other two, but with the opposite sign. The return path for the current in any phase conductor is the other two phase conductors. Thus, the ratio of capacity to conductor material is doubled. The ratio of capacity to conductor material increases to 3:1 with an ungrounded three-phase and center-grounded single-phase system, and 2. 25:1 when both employ grounds of the same gauge as the conductors. Three-phase systems may also have a fourth wire, particularly in low-voltage distribution.
The connections are arranged so that, as far as possible in each group, equal power is drawn from each phase. Transformers may be wired in a way that they have a four-wire secondary but a three-wire primary while allowing unbalanced loads and the associated secondary-side neutral currents. The phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load. This makes it possible to reduce the size of the neutral conductor because it carries little or no current. With a balanced load, all the phase conductors carry the same current and so can be the same size. Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations. Most household loads are single-phase.
In North American residences, three-phase power might feed a multiple-unit apartment block, but the household loads are connected only as single phase. In lower-density areas, only a single phase might be used for distribution. Some high-power domestic appliances such as electric stoves and clothes dryers are powered by two or three phases instead of one in order to use lower currents, so households with such appliances must be provided with more than one phase. Wiring for the three phases is typically identified by color codes which vary by country.
Connection of the phases in the right order is required to ensure the intended direction of rotation of three-phase motors. For example, pumps and fans may not work in reverse. Left image: elementary six-wire three-phase alternator with each phase using a separate pair of transmission wires. Right image: elementary three-wire three-phase alternator showing how the phases can share only three wires.
After further voltage conversions in the transmission network, the voltage is finally transformed to the standard utilization before power is supplied to customers. A “delta” connected transformer winding is connected between phases of a three-phase system. A “wye” transformer connects each winding from a phase wire to a common neutral point. A single three-phase transformer can be used, or three single-phase transformers. In an “open delta” or “V” system, only two transformers are used.
A closed delta made of three single-phase transformers can operate as an open delta if one of the transformers has failed or needs to be removed. Another variation is a “corner grounded” delta system, which is a closed delta that is grounded at one of the junctions of transformers. The fourth wire, if present, is provided as a neutral and is normally grounded. A four-wire system with symmetrical voltages between phase and neutral is obtained when the neutral is connected to the “common star point” of all supply windings.
In such a system, all three phases will have the same magnitude of voltage relative to the neutral. Other non-symmetrical systems have been used. The four-wire wye system is used when a mixture of single-phase and three-phase loads are to be served, such as mixed lighting and motor loads. When a group of customers sharing the neutral draw unequal phase currents, the common neutral wire carries the currents resulting from these imbalances.
Electrical engineers try to design the system so the loads are balanced as much as possible within premises where three-phase power is used. These same principles apply to the wide scale distribution of power to individual premises. Hence, every effort is made by supply authorities to distribute all three phases over a large number of premises so that, on average, as nearly as possible a balanced load is seen at the point of supply. 3 phases and neutral to each customer, but at a lower fuse rating, typically 40 or 63 A per phase, and “rotated” to avoid the effect that more load tends to be put on the first phase. Three-phase loads such as motors connect to L1, L2 and L3. Single-phase loads would be connected between L1 or L2 and neutral, or between L1 and L2.