Electric generators are devices for producing electrical energy from another form of energy. In contrast, a device that consumes electrical energy is called an electrical receiver.
It enables an almost constant voltage to be supplied without loss of load. It is a theoretical model which makes it possible to overcome the internal resistance of the generator. (Analogue for current generators)
Symbol of a voltage generator in a circuit
Electrochemical accumulators are direct current generators, rechargeable, used in electrotechnical and portable electronic applications.
DC rotary machine
A direct current generator popularly called a dynamo is, like many electric generators, a rotating machine. It was invented by Zénobe Gramme.
This machine works as a generator but is reversible as a motor. The constituent details of the machine are detailed in the article direct current machine.
This type of generator being reversible, it easily becomes an electric motor, which implies that when stopping the dynamo must be disconnected from its load if the latter can provide it with a current in return: accumulator battery, other dynamo.
- This last feature was used in small automobiles of the 1970s, a relay system connects the battery to supply current to the dinastar which starts the internal combustion engine and automatically switches to a dynamo when it reaches a certain speed.
DC machine: The dynamoabbas
Alternating Current Rotary Machine
The discovery in 1832 by Faraday of the phenomena of electromagnetic induction allowed him to consider producing alternating electric voltages and currents using magnets. Pixii, on the instructions of Ampère built the same year a first machine which was then perfected (1833 – 1834) by Sexton and Clarke. An alternator is a rotating machine that converts the mechanical energy supplied by an engine (turbine, diesel, wind turbine, etc.) into alternating current electrical energy.
More than 95% of electrical energy is produced by synchronous alternators: electromechanical machines providing voltages of frequencies proportional to their speed of rotation. These machines are less expensive and have a better efficiency than direct current machines (dynamos) which deliver continuous voltages (95% instead of 85%).
Asynchronous machines in hypersynchronous operation (rotation frequency greater than the synchronous frequency) also supply energy to the AC network to which they are connected. They are used more and more as generators thanks to recent progress in power electronics.
Principle of synchronous alternator
Exploded view generator
As the machines are all reversible, reference is made to the synchronous machine article.
This machine consists of a rotor (rotating part) and a stator (fixed part).
- The rotor is the inductor.
- It can be a permanent magnet, the regulation of the output voltage will be done by regulating the speed of rotation of the alternator, (the frequency of the current will also vary). This is the principle of the bicycle dynamo, which is in fact a small alternator.
- More commonly an electromagnet provides induction. This winding is supplied with direct current, either by means of a rotary ring collector (a double ring with brushes) bringing an external source, or by a rotating diode and brushless exciter. A regulation system allows adjustment of the voltage and phase of the current produced.
- The stator is the armature. It consists of windings which will be the seat of alternating electric current induced by the magnetic field created by the inductor in motion..
Different types of alternators
- In industrial alternators, the armature consists of three windings arranged at 120 ° which provide a system of three-phase alternating currents.
- In thermal power plants (nuclear or conventional), a steam turbine or a gas turbine rotating at high speed is coupled to a turbo-generator. This type of generator generally rotates at 1500 revolutions / minute (4-pole rotor) or 3000 revolutions / minute (two-pole rotor), for distribution networks at 50 Hz. The electrical power supplied by one of the turbogenerators of a nuclear power plant can reach 1600 megawatts.
- Hydraulic power stations, whose turbines rotate more slowly, have rotors with a large number of poles (14, 16 poles). The axis of rotation of the shaft can be vertical or horizontal, and the diameter of this shaft is large.
- Large generator sets generally use a slow diesel engine. In this case, the rotor of the alternator looks a lot like that of a ‘hydraulic’ alternator, with a high number of poles, a large diameter and a large movement of inertia absorbing variations in the speed of rotation of the shaft. diesel engine.
- In domestic alternators (single-phase generator), the armature consists of a single winding.
- The on-board alternators, among others on motor vehicles, are three-phase alternators equipped with a rectifier system (diodes), which delivers a direct current at a voltage of about 14 V or 28 V for trucks, providing the electric power from the car and recharging the battery which supplies it with power when the vehicle’s engine is stopped.