The Background and History of Electrical Generators
An electrical generator is a device that moves electrical energy from a mechanical energy source using electromagnetic induction. The process is known as electricity generation and is analogous to a water pump. 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, or any other source of mechanical energy.
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
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. It is the most common way to generate electrical energy for bicycle lighting.
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.
Jedlik's dynamo
In 1827, 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.
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
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 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.
Misconception
Electric currents in copper wires are a flow of electrons, but these electrons are not created, they already pre-exist. Generators do not 'generate' them. Instead the electrons come from the wire. In copper wire, copper atoms supply the flowing electrons. The electrons in a circuit were already there before the generator was connected. They were even there before the copper was mined and made into wires! Generators do not create these electrons, they merely pump them, and the electrons act like a pre-existing fluid which is always found within all wires. In order to understand electric circuits, we must imagine that all the wires are pre-filled with a sort of "liquid electricity."
Using a hand-cranked generator as your power supply. Ask yourself exactly where the flowing "electricity" comes from when a generator powers a light bulb. A hand-cranked generator contains a coil and some magnets. When cranked, it takes electrons in from one terminal and simultaneously spits them out the other terminal. At the same time, the generator pushes electrons through the rotating coil of wire inside itself. It also pushes them through the rest of the circuit. So where did these electrons come from? Unlike the situation with a battery-powered circuit, all we have here is wires. Inside the generator is just more wires. Where is the source of this flowing "electricity?"
When we include the generator in the circuit, we find that the circuit is a continuous closed loop, and we can find no single place where the "electricity" originates. A generator is like a closed-loop pump, but it does not supply the substance being pumped. Batteries are like this as well. The liquid between the battery plates is an electrolyte, and electrolytes are conductors. Some batteries contain acid, others are alkaline batteries, and still others use conductive salt water. Flowing charges go through the battery, and no charges build up inside.
But weren't we all taught during grade-school that "generators create Current Electricity"? This phrase forms a serious conceptual stumbling block (at least it did for me!) To fix it, get rid of the bogus idea called "Current Electricity". Instead change the statement to read like this:
"Generators cause electric charge to flow."
To complete the picture, add this: all conductors are full of movable charge. That's what a conductor is, it's a material which contains movable charge.
A generator is like your heart: it moves blood, but it does not create blood. When a generator stops, or when the metal circuit is opened, all the electrons stop where they are, and the wires remain filled with electric charges. But this isn't unexpected, because the wires were full of vast quantities of charge in the first place.
Equivalent circuit
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 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 forecast 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 usually do not use permanent magnets; they can achieve efficiencies of up to 90% over a wide speed range by control of the field voltage. Motorcycle alternators often use permanent magnet stators made with rare earth magnets, since they can be made smaller and lighter than other types.
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 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
Engine - generator of the radio station (Dubendorf museum of the military aviation). The generator worked only when sending the radio signal (the receiver could operate on the battery power)
Hand-driven electric generator of the radio station (Dubendorf museum of the military aviation)
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 gen-set. In many contexts, the engine is taken for granted and the combined unit is simply called a generator. or a
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. 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.
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).
When using engine-generators, you must be aware of the quality of the electrical wave it outputs. This is particularly important when running sensitive electronic equipment. A power conditioner can take the square waves generated by many engine-generators and smooth it out by running it through a battery in the middle of the circuit. Using an inverter rather than a generator may also produce clean sinusoidal waves. There are several quiet running inverters available that produce clean sinusoidal wave power suitable for use with computers and other sensitive electronics, however some low cost inverters do not produce clean sinusoidal waves and may damage certain electronic charging equipment.
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, 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).
Mid-size stationary engine-generator
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. Diesel engine-generator sets operated at their best 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.