Everything about Electrical Generator totally explained
In
electricity generation, an
electrical generator is a device that converts
mechanical energy to
electrical energy, generally using
electromagnetic induction. The reverse conversion of electrical energy into mechanical energy is done by a
motor, and motors and generators have many similarities. The
source of mechanical energy may be a reciprocating or turbine
steam engine, water falling through a
turbine or waterwheel, an
internal combustion engine, a
wind turbine, a hand
crank, the
sun or
solar energy,
compressed air or any other source of mechanical energy.
Historic developments
Before the connection between
magnetism and
electricity was discovered,
electrostatic generators were invented that used
electrostatic principles. These generated very high
voltages and low
currents. They operated by using moving
electrically charged belts, plates and disks to carry charge to a high potential electrode. The charge was generated using either of two mechanisms:
Because of their inefficiency and the difficulty of
insulating machines producing very high voltages, electrostatic generators had low power ratings and were never used for generation of commercially-significant quantities of electric power. The
Wimshurst machine and
Van de Graaff generator are examples of these machines that have survived.
Faraday's disk
In
1831-
1832 Michael Faraday discovered the operating principle of electromagnetic generators. The principle, later called
Faraday's law, is 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 DC voltage, and large amounts of current.
This design was inefficient due to self-cancelling counterflows of current in regions not under the influence of the magnetic field. While current flow was induced directly underneath the magnet, the current would circulate backwards in regions outside the influence of the magnetic field. This counterflow limits the power output to the pickup wires, and induces waste heating of the copper disc.
Later homopolar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field effect in one current-flow direction.
Dynamo
Main article Dynamo
The
Dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses
electromagnetic principles to convert mechanical rotation into a pulsing direct electric
current through the use of a
commutator.
Through a series of accidental discoveries, the dynamo became the source of many later inventions, including the DC
electric motor, the AC
alternator, the AC
synchronous motor, and the
rotary converter.
A dynamo machine consists of a stationary structure, which provides a constant magnetic field, and a set of rotating windings which turn within that field. On small machines the constant magnetic field may be provided by one or more permanent magnets; larger machines have the constant magnetic field provided by one or more electromagnets, which are usually called field coils.
Large power generation dynamos are now rarely seen due to the now nearly universal use of
alternating current for power distribution and
solid state electronic power conversion. But before the principles of AC were discovered, very large direct-current dynamos were the only means of power generation and distribution. Now power generation dynamos are mostly a novelty.
Other Rotating Electromagnetic Generators
Without a
commutator, the dynamo is an example of an
alternator, which is a
synchronous singly-fed generator. With an electromechanical commutator, the dynamo is a classical direct current (DC) generator. The alternator must always operate at a constant speed that's precisely synchronized to the electrical frequency of the power grid for non-destructive operation. The DC generator can operate at any speed within mechanical limits but always outputs a direct current waveform.
Other types of generators, such as the
asynchronous or induction singly-fed generator, the
doubly-fed generator, or the
brushless wound-rotor doubly-fed generator, don't incorporate permanent magnets or field windings (i.e, electromagnets) that establish a constant magnetic field, and as a result, are seeing success in variable speed constant frequency applications, such as
wind turbines or other
renewable energy technologies.
The full output performance of any generator can be optimized with electronic control but only the
doubly-fed generators or the
brushless wound-rotor doubly-fed generator incorporate electronic control with power ratings that are substantially less than the power output of the generator under control, which by itself offer cost, reliability and efficiency benefits.
MHD generator
A
magnetohydrodynamic generator directly extracts electric power from moving hot gases through a magnetic field, without the use of rotating electromagnetic machinery. MHD generators were originally developed because the output of a plasma MHD generator is a flame, well able to heat the boilers of a
steam power plant. The first practical design was the AVCO Mk. 25, developed in 1965. The Indian government funded substantial development, culminating in a 25Mw demonstration plant in 1987. MHD generators operated as a
topping cycle are currently (2007) less efficient than combined-cycle
gas turbines.
Concepts
The generator moves an electric current, but doesn't
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 doesn't create the water inside.
Other types of electrical generators 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.
Terminology
The two main parts of a generator or motor can be described in either mechanical or electrical terms:
Mechanical:
Rotor: The rotating part of an alternator, generator, dynamo or motor.
Stator: The stationary part of an alternator, generator, dynamo or motor.
Electrical:
Armature: The power-producing component of an alternator, generator, dynamo or motor. In a generator, alternator, or dynamo the armature windings generate the electrical current. The armature can be on either the rotor or the stator.
Field: The magnetic field component of an alternator, generator, dynamo or motor. The magnetic field of the dynamo or alternator can be provided by either electromagnets or permanent magnets mounted on either the rotor or the stator. (For a more technical discussion, refer to the Field coil article.)
Since power transferred into the field circuit is much less than in the armature circuit, AC generators nearly always have the field winding on the rotor and the stator as the armature winding. Only a small amount of field current must be transferred to the moving rotor, using slip rings. Direct current machines necessarily have the commutator on the rotating shaft, so the armature winding is on the rotor of the machine.
Excitation
Main article Excitation (magnetic)
An electric generator or electric motor that uses field coils rather than permanent magnets will require a current flow to be present in the field coils for the device to be able to work. If the field coils are not powered, the rotor in a generator can spin without producing any usable electrical energy, while the rotor of a motor may not spin at all. Very large power station generators often utilize a separate smaller generator to excite the field coils of the larger.
In the event of a severe widespread power outage where islanding of power stations has occurred, the stations may need to perform a black start to excite the fields of their largest generators, in order to restore customer power service.
Equivalent circuit
The equivalent circuit of a generator and load is shown in the diagram to the right. To determine the generator's and parameters, follow this procedure: -
Before starting the generator, measure the resistance across its terminals using an ohmmeter. This is its DC internal resistance are equal.
Note 2: If the generator is an AC type, use an AC voltmeter for the voltage measurements.
The maximum power 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. This is inefficient since half the power is wasted in the generator's internal resistance; practical electric power generators operate with load resistance much higher than internal resistance, so the efficiency is greater.
Vehicle-mounted generators
Early motor vehicles until about the 1960's tended to use DC generators with electromechanical regulators. These were not particularly reliable or efficient and have now been replaced by alternators with built-in 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 designed electrical load within the vehicle - some cars now have electrically-powered steering assistance and air conditioning, which places a high load on the electrical system. Commercial vehicles are more likely to use 24 V to give sufficient power at the starter motor to turn over a large diesel engine without the requirement for unreasonably thick cabling. Vehicle alternators don't use permanent magnets and are typically only 50-60% efficient over a wide speed range. Motorcycle alternators often use permanent magnet stators made with rare earth magnets, since they can be made smaller and lighter than other types. See also hybrid vehicle.
Some of the smallest generators commonly found power bicycle lights. These tend to be 0.5 ampere, 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 are designed and manufactured with great precision. Nevertheless, the maximum efficiency is only around 60% for the best of these 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.
Sailing yachts may use a water or wind powered generator to trickle-charge the batteries. A small propeller, wind turbine or impeller is connected to a low-power alternator and rectifier to supply currents of up to 12 A at typical cruising speeds.
Engine-generator
An engine-generator is the combination of an electrical generator and an engine (prime mover) mounted together to form a single piece of self-contained equipment. The engines used are usually piston engines, but gas turbines can also be used. Many different versions are available - ranging from very small portable petrol powered sets to large turbine installations.
Human powered electrical generators
» Main article: Self-powered equipment
A generator can also be driven by human muscle power (for instance, in field radio station equipment).
Human powered direct current generators are commercially available, and have been the project of some DIY enthusiasts. Typically operated by means of pedal power, a converted bicycle trainer, or a foot pump, such generators can be practically used to charge batteries as large as 12 volts, and in some cases are designed with an integral inverter. Portable radio receivers with a crank are made to reduce battery purchase requirements, see clockwork radio.
Patents
-- Magneto-Electric Machines : Thomas Edison's main continuous current dynamo. The device's nickname was the "long-legged Mary-Ann". This device has large bipolar magnets. It is inefficient.
-- Dynamo-Electric Machine : Edison's improved dynamo which includes an extra coil and utilizes a field of force.
-- Dynamo Electric Machine - Nikola Tesla's construction of the alternating current induction motor / generator.
-- Dynamo Electric Machine - Tesla's "Unipolar" machine (for example, a disk or cylindrical conductor is mounted in between magnetic poles adapted to produce a uniform magnetic field).
-- Armature for Electric Machines -Tesla's construction principles of the armature for electrical generators and motors. (Related to patents numbers US327797, US292077, and GB9013.)
-- Method of Operating Arc-Lamps - Tesla's alternating current generator of high frequency alternations (or pulsations) above the auditory level.
-- Alternating Electric Current Generator - Tesla's generator that produces alternations of 15000 per second or more.Further Information
Get more info on 'Electrical Generator'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://electrical_generator.totallyexplained.com">Electrical generator Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
