User:Modal Jig/Bernadette O'Farrell
Bernadette O'Farrell was an Irish actress chiefly remembered for playing Maid Marian in The Adventures of Robin Hood television series. She may very well have been my first object of lust. I
Generation and distribution[edit]
At the power station, an electrical generator converts mechanical power into a set of three alternating electric currents, one from each coil (a.k.a. "winding") of the generator. The windings are arranged such that the currents vary sinusoidally at the same frequency, but with the peaks and troughs of their wave forms offset to provide three complementary currents with a phase separation of one-third cycle (120° or 2π/3 radians). The generator frequency is typically 50 Hz or 60 Hz, varying by country. (See Mains power systems for more detail.)
Large power generators provide an electrical current at a potential of a few hundred to about 30,000 volts. At the power station, transformers step this voltage up to one suitable for transmission.
Single-phase loads[edit]
Single-phase loads may be connected to a three-phase system in two ways. A load may be connected across two of the three phase conductors, or a load can be connected from a live phase conductor to the system neutral. Single-phase loads should be distributed evenly between the phases of the three-phase system for efficient use of the supply transformer and supply conductors. Where the line-to-neutral voltage is a standard utilization voltage (for example in a 240 V/415 V system), individual single-phase utility customers or loads may each be connected to a different phase of the supply. Where the line-to-neutral voltage is not a common utilization voltage, for example in a 347/600 V system, single-phase loads must be supplied by individual step-down transformers.
In a symmetrical three-phase system, the system neutral has the same magnitude of voltage to each of the three phase conductors. The voltage between phase conductors is √3 times the line to neutral voltage.
In multiple-unit residential buildings in North America, three-phase power is supplied to the building but individual units have only single-phase power formed from two of the three supply phases. Lighting and convenience receptacles are connected from either phase conductor to neutral, giving the usual 120 V. High-power loads such as cooking equipment, space heating, water heaters, or air conditioning can be connected across both live conductors to give 208 V. This practice is common enough that 208 V single-phase equipment is readily available in North America. Attempts to use the more common 120/240 V equipment intended for three-wire single-phase distribution may result in poor performance since 240 V heating equipment will only produce 75% of its rating when operated at 208 V.
Where three phase at low voltage is otherwise in use, it may still be split out into single phase service cables through joints in the supply network or it may be delivered to a master distribution board (breaker panel) at the customer's premises. Connecting an electrical circuit from one phase to the neutral generally supplies the country's standard single phase voltage (120 V AC or 230 V AC) to the circuit.
The currents returning from the customers' premises to the supply transformer all share the neutral wire. If the loads are evenly distributed on all three phases, the sum of the returning currents in the neutral wire is approximately zero. Any unbalanced phase loading on the secondary side of the transformer will use the transformer capacity inefficiently.
If the supply neutral of a three-phase system with line-to-neutral connected loads is broken, the voltage balance on the loads will no longer be maintained. The neutral point will tend to drift toward the most heavily loaded phase, causing undervoltage conditions on that phase only. Correspondingly, the lightly-loaded phases may approach the line-to-line voltage, which exceeds the line-to-neutral voltage by a factor of √3, causing overheating and failure of many types of loads.
For example, if several houses are connected through a 240 V transformer, which is connected to one phase of the three phase system, each house might be affected by the imbalance on the three phase system. If the neutral connection is broken somewhere in the system, all equipment in a house might be damaged due to over-voltage. A similar phenomenon can exist if the house neutral (connected to the center tap of the 240 V pole transformer) is disconnected. This type of failure event can be difficult to troubleshoot if the drifting neutral effect is not understood. With inductive and/or capacitive loads, all phases can suffer damage as the reactive current moves across abnormal paths in the
References[edit]
- ^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed., 1917, vol. 4, Ch. 46: Alternating Currents, p. 1026, fig. 1260.
- ^ Hawkins Electrical Guide, Theo. Audel and Co., 2nd ed., 1917, vol. 4, Ch. 46: Alternating Currents, p. 1026, fig. 1261.