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Electricity Meters
| Electricity Single-Phase Meters | |
Single-phase electricity is often used in homes or business where electricity is mostly used for general purposes such as lighting and heating.
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| Electricity Three-Phase Meters | |
| Power stations supply three-phase electricity which is three times as much electricity
along three wires as can be supplied through two wires, without having
to increase the thickness of the wires. Large buildings, shopping centers, factories, office blocks, and multiple-unit apartments generally have three-phase electricity. The most important class of a three-phase load is the electric motor. Such motors are applied in industry for pumps, fans, blowers, compressors, conveyor drives etc. Large air conditioning equipment (units above 8.8 kW cooling capacity) use three-phase motors for reasons of efficiency and economy. Resistance type heating such as electric boilers may be connected to three-phase systems. Electric lighting may also be similarly connected. These types of loads do not require the revolving magnetic field characteristic of three-phase motors but take advantage of the higher voltage and power level usually associated with three-phase distribution. Large rectifier systems may have three-phase inputs. The resulting DC current is easier to filter (smooth) than the output of a single-phase rectifier. Such rectifiers may be used for battery charging, electrolysis processes such as aluminum production or for operation of DC motors. Three phase electricity is a smoother form of electricity than single-phase or two-phase power. It is this more consistent electrical power that allows machines to run more efficiently and last many years longer. Some applications are able to work with three-phase power in ways that would not work on single phase at all. At the power station, an electrical generator converts mechanical power into a set of alternating electric currents, one from each electromagnetic coil or winding of the generator. The currents are sinusoidal functions of time, all at the same frequency but with different phases. In a three-phase system the phases are spaced equally, giving a phase separation of 120°. The frequency is typically 50 Hz in Europe and 60 Hz in the US. The currents in each conductor reach their peak instantaneous values sequentially, not simultaneously; in each cycle of the power frequency, first one, then the second, then the third current reaches its maximum value. The waveforms of the three supply conductors are offset from one another in time (delayed in phase) by one-third of their period. Three phase has properties that make it very desirable in electric power systems. Such as: The phase currents tend to cancel one another (summing to zero in the case of a linear balanced load). This makes it possible to eliminate the neutral conductor on some lines; Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations; Three-phase systems can produce a magnetic field that rotates in a specified direction, which simplifies the design of electric motors. Three phase systems may or may not have a neutral wire. A neutral wire allows the three phase system to use a higher voltage while still supporting lower voltage single phase appliances. In high voltage distribution situations it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection). A single-phase load may be powered from a three-phase distribution system either by a connection between a phase and neutral or by connecting the load between two phases. The load device must be designed for the voltage in each case. In places using a 415 volt 3 phase system, the phase-to-neutral voltage is 240 volts, allowing single-phase lighting to be connected phase-to-neutral while three-phase motors to be connected to all three phases. In North America, a typical three-phase system will have 208 volts between the phases and 120 volts between the phase and neutral. If heating equipment designed for the 240-volt three-wire single phase system is connected to two phases of a 208 volt supply, it will only produce 75% of its rated heating effect. Single-phase motors may have taps to allow their use on either 208 V or 240 V supplies. |
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