Design of Switches Use to Disconnect Generator Motor Field
Three-phase electric power is a common method of electric power transmission. It is a type of polyphase system mainly used to power motors and many other devices. A three-phase system uses less conductor material to transmit electric power than equivalent single-phase, two-phase, or direct-current systems at the same voltage.
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).
Three phase has properties that make it very desirable in electric power systems. First, 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 eliminate the neutral conductor on some lines; all the phase conductors carry the same current and so can be the same size, for a balanced load. Second, power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations. Finally, three-phase systems can produce a magnetic field that rotates in a specified direction, which simplifies the design of electric motors. Three is the lowest phase order to exhibit all of these properties.
Most domestic loads are single phase. Generally three phase power either does not enter domestic houses at all, or where it does, it is split out at the main distribution board.
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 offset in time to give different phases. In a three-phase system the phases are spaced equally, giving a phase separation of one-third cycle. The power frequency is typically 50 Hz in Asia, Europe, South America and Australia, and 60 Hz in the US and Canada (but see Mains power systems for more detail).
Generators output at a voltage that ranges from hundreds of volts to 30,000 volts. At the power station, transformers "step-up" this voltage to one more suitable for transmission.
After numerous further conversions in the transmission and distribution network the power is finally transformed to the standard mains voltage (i.e. the "household" voltage). The power may already have been split into single phase at this point or it may still be three phase. Where the stepdown is 3 phase, the output of this transformer is usually star connected with the standard mains voltage (120 V in North America and 230 V in Europe and Australia) being the phase-neutral voltage. Another system commonly seen in North America is to have a delta connected secondary with a center tap on one of the windings supplying the ground and neutral. This allows for 240 V three phase as well as three different single phase voltages (120 V between two of the phases and the neutral, 208 V between the third phase (known as a high leg) and neutral and 240 V between any two phases) to be made available from the same supply.
Large air conditioning, etc. equipment use three-phase motors for reasons of efficiency, economy and longevity.
Resistance heating loads such as electric boilers or space heating 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. Fluorescent lighting systems also benefit from reduced flicker if adjacent fixtures are powered from different phases.
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.
An interesting example of a three-phase load is the electric arc furnace used in steelmaking and in refining of ores.
In much of Europe stoves are designed for a three phase feed. Usually the individual heating units are connected between phase and neutral to allow for connection to a single phase supply. In many areas of Europe, single phase power is the only source available.
Occasionally the advantages of three-phase motors make it worthwhile to convert single-phase power to three phase. Small customers, such as residential or farm properties may not have access to a three-phase supply, or may not want to pay for the extra cost of a three-phase service, but may still wish to use three-phase equipment. Such converters may also allow the frequency to be varied allowing speed control. Some locomotives are moving to multi-phase motors driven by such systems even though the incoming supply to a locomotive is nearly always either DC or single phase AC.
Because single-phase power goes to zero at each moment that the voltage crosses zero but three-phase delivers power continuously, any such converter must have a way to store energy for the necessary fraction of a second.
One method for using three-phase equipment on a single-phase supply is with a rotary phase converter, essentially a three-phase motor with special starting arrangements and power factor correction that produces balanced three-phase voltages. When properly designed these rotary converters can allow satisfactory operation of three-phase equipment such as machine tools on a single phase supply. In such a device, the energy storage is performed by the mechanical inertia (flywheel effect) of the rotating components. An external flywheel is sometimes found on one or both ends of the shaft.
A second method that was popular in the 1940s and 50s was a method that was called the "transformer method". In that time period capacitors were more expensive relative to transformers. So an autotransformer was used to apply more power through fewer capacitors. This method performs well and does have supporters, even today. The usage of the name transformer method separated it from another common method, the static converter, as both methods have no moving parts, which separates them from the rotary converters.
Another method often attempted is with a device referred to as a static phase converter. This method of running three phase equipment is commonly attempted with motor loads though it only supplies 2/3 power and can cause the motor loads to run hot and in some cases overheat. This method will not work when sensitive circuitry is involved such as CNC devices, or in induction and rectifier type loads.
Some devices are made which create an imitation three-phase from three-wire single phase supplies. This is done by creating a third "subphase" between the two live conductors, resulting in a phase separation of 180° - 90° = 90°. Many three-phase devices will run on this configuration, but at lower efficiency.
Variable-frequency drives (also known as solid-state inverters) are used to provide precise speed and torque control of three phase motors. Some models can be powered by a single phase supply. VFDs work by converting the supply voltage to DC and then converting the DC to a suitable three phase source for the motor.
Digital phase converters are a recent development in phase converter technology that utilizes software in a powerful microprocessor to control solid state power switching components. This microprocessor, called a digital signal processor (DSP), monitors the phase conversion process, continually adjusting the input and output modules of the converter to maintain balanced three-phase power under all load conditions.
- Three-wire single-phase distribution is useful when three phase power is not available, and allows double the normal utilization voltage to be supplied for high-power loads.
- Two phase power, like three phase, gives constant power transfer to a linear load. For loads which connect each phase to neutral, assuming the load is the same power draw, the two wire system has a neutral current which is greater than neutral current in a three phase system. Also motors aren't entirely linear, which means that despite the theory, motors running on three phase tend to run smoother than those on two phase. The generators at Niagara Falls installed in 1895 were the largest generators in the world at the time and were two-phase machines. True two-phase power distribution is essentially obsolete. Special purpose systems may use a two-phase system for control. Two-phase power may be obtained from a three-phase system using an arrangement of transformers called a Scott-T transformer.
- Monocyclic power was a name for an asymmetrical modified two-phase power system used by General Electric around 1897 (championed by Charles Proteus Steinmetz and Elihu Thomson; this usage was reportedly undertaken to avoid patent infringement). In this system, a generator was wound with a full-voltage single phase winding intended for lighting loads, and with a small (usually ¼ of the line voltage) winding which produced a voltage in quadrature with the main windings. The intention was to use this "power wire" additional winding to provide starting torque for induction motors, with the main winding providing power for lighting loads. After the expiration of the Westinghouse patents on symmetrical two-phase and three-phase power distribution systems, the monocyclic system fell out of use; it was difficult to analyze and did not last long enough for satisfactory energy metering to be developed.
- High phase order systems for power transmission have been built and tested. Such transmission lines use 6 or 12 phases and design practices characteristic of extra-high voltage transmission lines. High-phase order transmission lines may allow transfer of more power through a given transmission line right-of-way without the expense of a HVDC converter at each end of the line.
A polyphase system is means of distributing alternating current electrical power. Polyphase systems have three or more energized electrical conductors carrying alternating currents with a definite time offset between the voltage waves in each conductor. Polyphase systems are particularly useful for transmitting power to electric motors. The most common example is the three-phase power system used for most industrial applications.
One voltage cycle of a three-phase system
In the very early days of commercial electric power, some installations used two phase four-wire systems for motors. The chief advantage of these was that the winding configuration was the same as for a single-phase capacitor-start motor, and, by using a four-wire system, conceptually the phases were independent and easy to analyze with mathematical tools available at the time. Two-phase systems have been replaced with three-phase systems. A two-phase supply with 90 degrees between phases can be derived from a three-phase system using a Scott-connected transformer.
Polyphase power is particularly useful in AC motors, such as the induction motor, where it generates a rotating magnetic field. When a three-phase supply completes one full cycle, the magnetic field of a two-pole motor has rotated through 360° in physical space; motors with more pairs of poles require more power supply cycles to complete one physical revolution of the magnetic field, and so these motors run more slowly. Nikola Tesla and Michail Dolivo-Dobrovolsky invented the first practical induction motors using a rotating magnetic field - previously all commercial motors were DC, with expensive commutators, high-maintenance brushes, and characteristics unsuitable for operation on an alternating current network. Polyphase motors are simple to construct, are self-starting and have little vibrations.
Higher phase numbers than three have been used. A common practice for rectifier installations and in HVDC converters is to provide six phases, with 60 degree phase spacing, to reduce harmonic generation in the AC supply system and to provide smoother direct current. Experimental high-phase-order transmission lines have been built with up to 12 phases. These allow application of Extra High Voltage (EHV) design rules at lower voltages, and would permit increased power transfer in the same transmission line corridor width.
Residences and small businesses are usually supplied with a single phase taken from one of the three utility phases. Individual customers are distributed among the three phases to balance the loads. Single-phase loads such as lighting may be connected from an energized phase to the circuit neutral, allowing the load in a large building to be balanced over the three supply phases. The phase offset of the line-to-neutral voltages is 120 degrees; the voltage between any two live wires is always v3 times between a live and neutral wire. See the article Mains power systems for a list of single-phase distribution voltages around the world; the three-phase line-to-line voltage will be v3 times these values.
In North America, residential apartment blocks may have 120 volt (line to neutral), 208 Volt (line to line) distribution. Major single-phase appliances such as ovens or cook tops intended for the 240 Volt split phase system usually used in single-family dwellings may not operate well when connected to 208 Volts; heating appliances will develop only 3/4 of their rated power, and electric motors will not operate correctly with 13% lower voltage applied.
Design of Switches Use to Disconnect Generator Motor Field
Source: https://www.cableorganizer.com/learning-center/articles/three-phase-electric-power.php
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