Electric generator

In electricity generation, an electrical generator is a device that converts kinetic 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, or any other source of mechanical energy.

Historic developments

Electrostatic generators are used for scientific experiments requiring high voltages. Because of the difficulty of insulating machines producing very high voltages, electrostatic generators are made only with low power ratings and are never used for generation of commercially-significant quantities of electric power. Before the connection between magnetism and electricity was discovered, generators used electrostatic principles. The Wimshurst machine used electrostatic induction or "influence". Some electrostatic machines (such as the more modern 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)

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 DC voltage, and large amounts of current.

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. 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.

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.

Unlike the Faraday disc, many turns of wire connected in series can be used in the moving windings of a dynamo. This allows the terminal voltage of the machine to be higher than a disc can produce, so that electrical energy can be delivered at a convenient voltage.

Reversible power conversion

The relationship between mechanical rotation and electric current in a dynamo is reversible; the principles of the electric motor were discovered when it was found that one dynamo could cause a second interconnected dynamo to rotate if current was fed through it.

The transformative ability of a dynamo to change energy from electrical power to mechanical power and back again could be exploited as a current-compensation and balancing device to even out power distribution on interconnected, unbalanced circuits.

Jedlik's dynamo

In 1827, Hungarian Anyos Jedlik started experimenting with electromagnetic rotating devices which he called electromagnetic self-rotors. In the prototype of the single-pole electric starter (finished between 1852 and 1854) both the stationary and the revolving parts were electromagnetic. He formulated the concept of the dynamo at least 6 years before Siemens and Wheatstone. In essence the concept is that instead of permanent magnets, two electromagnets opposite to each other induce the magnetic field around the rotor.

File:Jedlikdynamo.GIF

Ányos Jedlik's single pole electric starter (dynamo) (1861)

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.

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 is 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, do not 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 techologies.

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 U.S. government performed 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 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 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.

Excitation

A generator that uses field coils instead of permanent magnets requires a current flow to be present in the field coils for the generator to be able to produce any power at all. If the field coils are not powered, the rotor can spin without the generator producing any usable electrical energy.

For older and very large power generating equipment, it has been traditionally necessary for a small separate exciter generator to be operated in conjunction with the main power generator. This is a small permanent-magnet generator which produces the constant current flow necessary for the larger generator to function.

Most modern generators with field coils feature a capability known as self-excitation where some of the power output from the rotor is diverted to power the field coils. Additionally the rotor or stator contains a small amount of magnetizable metal, which retains a very weak residual magnetism when the generator is turned off. The generator is turned on with no load connected, and the initial weak field creates a weak flow in the field coils, which in turn begins to slightly affect the rotor to begin to produce current that then further strengthens the field. This feedback loop continues to increase field voltage and output power until the generator reaches its full operating output level.

This initial self-excitation feedback process does not work if the generator is started connected to a load, as the load will quickly dissipate the slight power production of the initial field buildup process.

It is additionally possible for a self-exciting generator either turned off or started with a load connected to result in dissipation of the residual magnetic field, resulting in complete non-function of the generator. In the case of a 220v portable generator commonly used by consumers and construction contractors, this loss of the residual field can usually be remedied by shutting down the generator, disconnecting all loads, and connecting what are normally the high-voltage/amperage generator outputs to the terminals of a common 9-volt battery. This very small current flow from the battery (in comparison with normal generator output) is enough to restore the residual self-exciting magnetic field. Usually only a moment of current flow, just briefly touching across the battery terminals, is enough to restore the field.

Terminology

The parts of a dynamo or related equipment can be expressed in either mechanical terms or electrical terms. Although distinctly separate, these two sets of terminology are frequently used interchangeably or in combinations that include one mechanical term and one electrical term. This causes great confusion when working with compound machines such as a brushless alternator or when conversing with people who work on a machine that is configured differently than the machines that the speaker is used to.

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. 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.

Equivalent circuit

The equivalent circuit of a generator and load is shown in the diagram to the right. To determine the generator's $V_{G}$ and $R_{G}$ parameters, follow this procedure: -

Before starting the generator, measure the resistance across its terminals using an ohmmeter. This is its DC internal resistance $R_{GDC}$ .
Start the generator. Before connecting the load $R_{L}$ , measure the voltage across the generator's terminals. This is the open-circuit voltage $V_{G}$ .
Connect the load as shown in the diagram, and measure the voltage across it with the generator running. This is the on-load voltage $V_{L}$ .
Measure the load resistance $R_{L}$ , if you don't already know it.
Calculate the generator's AC internal resistance $R_{GAC}$ from the following formula:

$R_{GAC}={R_{L}}\left({{{V_{G}} \over {V_{L}}}-1}\right)$

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 $R_{GAC}$ and assume that $R_{GAC}$ and $R_{GDC}$ 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 and Lorentz force or back emf 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.

Vehicle-mounted generators

Early motor vehicles 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 do not 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 mounted together to form a single piece of equipment. This combination is also called an engine-generator set or a gen-set. 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 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 temporary installations of lighting, sound amplification systems, amusement rides etc.

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, sewage 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.

The generator can also be driven by the human muscle power (for instance, in the field radio station equipment).

The generator voltage (volts), frequency (Hz) and power (watts) ratings are selected to suit the load that will be connected.

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).

There are only a few portable three-phase generator models available in the US. Most of the portable units available are single phase power only and most of the three-phase generators manufactured are large industrial type generators.

Portable engine-generators may require an external power conditioner to safely operate some types of electronic equipment.

Human powered electrical generators

Human powered direct current generators are commercially available, and have been the project of some homebrew 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.

A preferred hand-held generator is one that has an inverter. They are the smallest, quietest, and most fuel-efficient generators. Small portable generators have a standard A.C. alternator and run at a faster R.P.M. to generate power. Inverter models can run at slower RPMs to generate the power that is necessary, thus reducing the noise of the engine and making it more fuel-efficient. Inverter generators are best to power sensitive electronic devices such as computers and lights that use a ballast. The power output of a true sine wave inverter is more stable and is equal to or better than household electrical power. Regular generators that do not have AVR (Automatic Voltage Regulation) or which use low cost non-sine or modified sine wave inverters can damage electronics such as computers and sound systems due to their varying voltage.

Mid-size stationary engine-generator

The mid-size stationary engine-generator pictured here is a 100 kVA set which produces 415 V at around 110 A per phase. It is 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. Diesel engines in the UK run on red diesel and rotate at 1500 rpm. This produces power at a frequency of 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 divisor of 3600. Diesel engine-generator sets operated at their peak efficiency point can produce between 3 and 4 kilowatthours of electrical energy for each litre of diesel fuel consumed, with lower efficiency at part load.

Patents

U.S. patent 222,881 -- 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.
U.S. patent 373,584 -- Dynamo-Electric Machine : Edison's improved dynamo which includes an extra coil and utilizes a field of force.
U.S. patent 359,748 -- Dynamo Electric Machine - Nikola Tesla's construction of the alternating current induction motor / generator.
U.S. patent 406,968 -- 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 417,794 -- 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 447,920 -- Method of Operating Arc-Lamps - Tesla's alternating current generator of high frequency alternations (or pulsations) above the auditory level.
U.S. patent 447,921 -- Alternating Electric Current Generator - Tesla's generator that produces alternations of 15000 per second or more.

External links