Generators
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Wikipedia-Article "Generators"
- "Generator" redirects here. For other uses, see generator (disambiguation)
An electrical generator is a device that produces electrical energy from a mechanical energy source. The process is known as electricity generation.
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Historic Developments
Before the connection between magnetism and electricity was discovered, generators used electrostatic principles. The Wimshurst machine used electrostatic induction or "influence". The Van de Graaff generator uses either of two mechanisms:
- Charge transferred from a high-voltage electrode
- Charge created by the triboelectric effect using the separation of two insulators (the belt leaving the lower pulley)
Electrostatic generators are inefficient and are useful only for scientific experiments requiring high voltages.
Faraday
In 1831-1832 Michael Faraday discovered that a potential difference is generated between the ends of an electrical conductor that moves perpendicular to a magnetic field. He also built the first electromagnetic generator called the Faraday disc, a type of homopolar generator, using a copper disc rotating between the poles of a horseshoe magnet. It produced a small direct current.
Dynamo
The dynamo was the first electrical generator capable of delivering power for industry, and is still the most important generator in use in the 21st century. The dynamo uses electromagnetic principles to convert mechanical rotation into an alternating electric current.
The first dynamo based on Faraday's principles was built in 1832 by Hippolyte Pixii, a French instrument maker. It used a permanent magnet which was rotated by a crank. The spinning magnet was positioned so that its north and south poles passed by a piece of iron wrapped with wire. Pixii found that the spinning magnet produced a pulse of current in the wire each time a pole passed the coil. Furthermore, the north and south poles of the magnet induced currents in opposite directions. By adding a commutator, Pixii was able to convert the alternating current to direct current.
Gramme dynamo
Both of these designs suffered from a similar problem: they induced "spikes" of current followed by none at all. Antonio Pacinotti, an Italian scientist, fixed this by replacing the spinning coil with a toroidal one, which he created by wrapping an iron ring. This meant that some part of the coil was continually passing by the magnets, smoothing out the current. Zénobe Gramme reinvented this design a few years later when designing the first commercial power plants, which operated in Paris in the 1870s. His design is now known as the Gramme dynamo. Various versions and improvements have been made since then, but the basic concept of a spinning endless loop of wire remains at the heart of all modern dynamos.
Concepts
It is important to understand that the generator creates an electric current, but does not create electric charge, which is already present in the conductive wire of its windings. It is somewhat analogous to a water pump, which creates a flow of water but does not create the water itself.
Other types of electrical generator exist, based on other electrical phenomena such as piezoelectricity, and magnetohydrodynamics. The construction of a dynamo is similar to that of an electric motor, and all common types of dynamos could work as motors. Also, all common types of electric motors could work as generators.
The generator rotor is turned by a device termed a prime mover, often a Diesel engine, steam turbine, water turbine or gas turbine coupled to the rotor shaft.
Equivalent circuit
G = generator
VG=generator open-circuit voltage
RG=generator internal resistance
VL=generator on-load voltage
RL=load resistance
The equivalent circuit of a generator and load is shown in the diagram to the right. To determine the generator's VG and RG parameters, follow this procedure: -
- Before starting the generator, measure the resistance across its terminals using an ohmmeter. This is its DC internal resistance RGDC.
- Start the generator. Before connecting the load RL, measure the voltage across the generator's terminals. This is the open-circuit voltage VG.
- Connect the load as shown in the diagram, and measure the voltage across it with the generator running. This is the on-load voltage VL.
- Measure the load resistance RL, if you don't already know it.
- Calculate the generator's AC internal resistance RGAC from the following formula:
Note 1: The AC internal resistance of the generator when running is generally slightly higher than its DC resistance when idle. The above procedure allows you to measure both values. For rough calculations, you can omit the measurement of RGAC and assume that RGAC and RGDC are equal.
Note 2: If the generator is an AC type (distinctly not a dynamo), use an AC voltmeter for the voltage measurements.
Maximum power
The maximum power theorem applies to generators as it does to any source of electrical energy. This theorem states that the maximum power can be obtained from the generator by making the resistance of the load equal to that of the generator. However, under this condition the power transfer efficiency is only 50%, which means that half the power generated is wasted as heat inside the generator. For this reason, practical generators are not usually designed to operate at maximum power output, but at a lower power output where efficiency is greater.
Low-power
Early motor vehicles tended to use DC generators with regulators. These were not particularly reliable or efficient and have now been replaced by alternators with inbuilt rectifier circuits. These power the electrical systems on the vehicle and recharge the battery after starting. Rated output will typically be in the range 50-100 A at 12 V, depending on the forecast electrical load within the vehicle - some cars now have electrically powered superchargers and airconditioning, which places a high load on the electrical system. Commercial vehicles are more likely to use 24 V to give sufficient torque at the starter motor to turn over a large diesel engine. Vehicle alternators do not use permanent magnets; they can achieve efficiencies of up to 90% over a wide speed range by control of the field voltage.
Some of the smallest generators commonly found are used to power bicycle lights. These tend to be 0.5 A permanent-magnet alternators, supplying 3-6 W at 6 V or 12 V. Being powered by the rider, efficiency is at a premium, so these may incorporate rare-earth magnets and be designed and manufactured with great precision. Nevertheless, the maximum efficiency is only around 60% for the best generators - 40% is more typical - due to the use of permanent magnets. A battery would be required in order to use a controllable electromagnetic field instead, and this is unacceptable due to its weight and bulk.
Aircraft have also switched from DC generators to alternators; these are typically powered by a takeoff from an engine.
Sailing yachts may use a water or wind powered generator to trickle-charge the batteries. A small propellor, wind turbine or impeller is connected to a low-power alternator and rectifier to supply currents of up to 10 A at typical cruising speeds.
Engine-generator
An engine-generator is the combination of an electrical generator and an engine mounted together to form a single piece of equipment. This combination is also called an engine-generator set or a genset. In many contexts, the engine is taken for granted and the combined unit is simply called a generator.
In addition to the engine and generator, engine-generators generally include a fuel tank, an engine speed regulator and a generator voltage regulator. Many units are equipped with a battery and electric starter. Standby power generating units often include an automatic starting system and a transfer switch to disconnect the load from the utility power source and connect it to the generator.
Engine-generators produce alternating current power that is used as a substitute for the power that might otherwise be purchased from a utility power station. The generator voltage (volts), frequency (Hz) and power (watts) ratings are selected to suit the load that will be connected. Both single-phase and three-phase models are available.
Engine-generators are available in a wide range of power ratings. These include small, hand-portable units that can supply several hundred watts of power, hand-cart mounted units, as pictured above, that can supply several thousand watts and stationary or trailer-mounted units that can supply over a million watts. The smaller units tend to use gasoline (petrol) as a fuel, and the larger ones have various fuel types, including diesel, natural gas and propane (liquid or gas).
Engine-generators are often used to supply electrical power in places where utility power is not available and in situations where power is needed only temporarily. Small generators are sometimes used to supply power tools at construction sites. Trailer-mounted generators supply power for lighting, amusement rides etc. for traveling carnivals.
Standby power generators are permanently installed and kept ready to supply power to critical loads during temporary interruptions of the utility power supply. Hospitals, communications service installations, sewerage pumping stations and many other important facilities are equipped with standby power generators.
Small and medium generators are especially popular in third world countries to supplement grid power, which is often unreliable. Trailer-mounted generators can be towed to disaster areas where grid power has been temporarily disrupted.
Mid-size stationary engine-generator
The mid-size stationary engine-generator pictured here is a 100 kVA set which produces 415 V at around 100 A per phase. It's powered by a 6.7 litre turbocharged Perkins Phaser 1000 Series engine, and consumes approximately 27 litres of fuel an hour, on a 400 litre tank. Stationary generators used in the US are used in size up to 2800 kW. These diesel engines are run in the UK on red diesel and rotate at 1500 rpm. This produces power at 50 Hz, which is the frequency used in the UK. In areas where the power frequency is 60 Hz (United States), generators rotate at 1800 rpm or another even multiple of 60.
Patents
- U.S. Patent 222,881 -- Magneto-Electric Machines : Thomas Edison's main continous current dynamo. The device's nickname was the "long-legged Mary-Ann". This device has large bipolar magnets. It is inefficient.
- U.S. Patent 373,584 -- Dynamo-Electric Machine : Edison's improved dynamo which includes an extra coil and ultilizes a field of force.
- U.S. Patent 359748 -- Dynamo Electric Machine - Nikola Tesla's construction of the alternating current induction motor / generator.
- U.S. Patent 406968 -- Dynamo Electric Machine - Tesla's "Unipolar" machine (i.e., a disk or cylindrical conductor is mounted in between magnetic poles adapted to produce a uniform magnetic field).
- U.S. Patent 417794 -- Armature for Electric Machines -Tesla's construction principles of the armature for electrical generators and motors. (Related to patents numbers US327797, US292077, and GB9013.)
- U.S. Patent 447920 -- Method of Operating Arc-Lamps - Tesla's alternating current generator of high frequency alternations (or pulsations) above the auditory level.
- U.S. Patent 447921 -- Alternating Electric Current Generator - Tesla's generator that produces alterations of 15000 per second or more.