How the generator works: physical principles, device, features of AC and DC electric generators

Electricity is not the primary energy freely present in nature in significant quantities, and it must be produced to be used in industry and everyday life. Most of it is created by devices that convert the driving force into electric current - this is how generators work, sources of mechanical energy for which can be steam and water turbines, internal combustion engines and even muscle strength person.

History and evolution

Discovered by Michael Faraday in 1831 laws of electromagnetic induction became the basis for the construction of electrical machines. But before the advent of electric lighting, there was no need to commercialize the technology. In the early consumers of electricity, for example, in the telegraph, galvanic batteries were used as a power source. This was a very expensive way to generate electricity.

At the end of the 19th century, many inventors were looking for an application of the Faraday principle of induction to generate electricity mechanically. Some of the important achievements were the development of the dynamo by Werner von Siemens and the production by Hippolyte Fontaine of working models of Theophilus Gramm generators. The first devices were used in conjunction with outdoor arc lighting devices known as Yablochkov candles.

They were replaced by Thomas Edison's highly successful incandescent lamp system. Its commercial power plants were based on powerful generators, but the circuit built in production direct current, was poorly suited for power distribution over long distances due to the impressive heat loss.

Nikola Tesla developed an improved alternator as well as a practical induction motor. These electrical machines, along with step-up and step-down transformers, provided the basis for for the creation of larger distribution networks by power companies using powerful power plants. In large AC power systems, generation and transportation costs were several times lower, than in Edison's scheme, which stimulated the demand for electricity and, as a result, the further evolution of electrical machines. The main dates in the history of generators can be considered:

• 1820 g. - André-Marie Ampere discovered that an electric current affects a magnetic field;
• 1832 g. - Faraday's creation of the simplest unipolar generator;
• 1849 g. - the first use for powering arc lamps of beacons;
• 1866 g. - the simultaneous discovery of the dynamo-electric principle by several inventors;
• 1891 g. - demonstration of a commercial machine for the production of multiphase voltage;
• 1895 g. - launched a hydroelectric power station in Niagara.

Principle of operation

Electromagnetic induction generators do not generate electricity. With the help of mechanical energy, they only set in motion electrical charges that are always present in conductors. The principle of operation of an electric generator can be compared to a water pump that causes water to flow, but does not create water in the pipes. Overwhelming most induction generators are rotary electric machinesconsisting of two main components:

• stator (stationary part);
• rotor (rotating part).

To illustrate how an electric generator works, a simple electrical machine, consisting of a coil of wire and a U-shaped magnet, can serve. The main fundamental elements of this model:

• a magnetic field;
• movement of a conductor in a magnetic field.

A magnetic field is the area around a magnet where its strength is felt. To better understand the operation of the model, you can imagine the lines of force going out from the north pole of the magnet and returning to the south pole. The stronger the magnet, the more lines of force it creates. If the coil begins to rotate between the poles, then both sides of it will begin to intersect the imaginary magnetic lines. This causes the movement of electrons in the conductor (generation of electricity).

In accordance with the rule of the right hand, when the coil rotates, a current will be induced in it, changing its direction every half turn, since the lines of force by the sides of the loop will intersect in one or the other direction. Twice in each revolution, the coil passes through positions (parallel to the poles) at which electromagnetic induction does not occur. Thus, the simplest generator works like an electrical machine that produces alternating current. The stress it creates can be changed by:

• magnetic field strength;
• coil rotation speed;
• the number of turns of wire crossing the lines of force of the magnetic field.

The loop of a conductor turning between the poles of a magnet has another important effect. When a current flows in a loop, it creates an electromagnetic field opposite to the field of a permanent magnet. And the more electricity is induced in the coil, the stronger the magnetic field and the resistance to turning the conductor. The same magnetic force in the turns causes the rotor of the electric motor to rotate, that is, under certain conditions, generators can work as motors and vice versa.

Features of AC generators

Alternating current (AC) is produced by the simplest generator described. In order for the generated electricity to be usable, it must somehow be delivered to the load. This is done with a contact assembly on the shaft, which consists of rotating rings and fixed carbon parts, called brushes, sliding over them. Each end of the rotating conductor is connected to a corresponding ring, and the current thus created in the coil is passed through the rings and brushes to the load.

Construction of industrial machines

Practical generators differ from the simplest ones. They are usually equipped with an exciter - an auxiliary generator that supplies direct current to electromagnets used to create a magnetic field in the generator.

Instead of a coil in the simplest model, practical devices are equipped with copper wire windings, and the role of a magnet is played by coils on iron cores. In most alternators, electromagnets that create an alternating field are placed on the rotor and electrical energy is induced in the stator coils.

In such devices, the collector is used to transfer direct current from the exciter to the magnets. This greatly simplifies the design, since it is more convenient to transmit weak currents through the brushes and to receive high voltage from the stationary stator windings.

Application in networks

In some machines, the number of winding sections is the same as the number of electromagnets. But most AC generators are equipped with three sets of coils for each pole. Such machines produce three streams of electricity and are called three-phase. Their power density is significantly higher than that of single-phase ones.

In power plants, AC generators are used as converters of mechanical energy into electrical energy. This is because the AC voltage can be easily increased or decreased using a transformer. In large generators, a voltage of about 20 thousand meters is produced. volt. Then it rises by more than an order of magnitude for the possibility of transporting electricity over long distances. A series of step-down transformers create a voltage suitable for use at the point where the electricity is used.

Dynamo device

A coil of wire rotating between the poles of a magnet changes the poles at the ends of the conductor twice for each revolution. To turn the simplest model into a DC generator, you need to do two things:

• take the current from the loop to the load;
• organize the flow of the diverted current in only one direction.

The role of the collector

A device called a manifold can do both. Its difference from the contact brush assembly is that its base is not a conductor ring, but a set of segments isolated from each other. Each end of the rotating circuit is connected to the corresponding sector of the collector, and two stationary carbon brushes remove the electric current from the switch.

The collector is designed in such a way that, regardless of the polarity at the ends of the loop and the phase of rotation of the rotor, the contact group provides the current with the desired direction when transferring it to the load. The windings in practical dynamos consist of many segments, therefore, for DC generators, due to the need for their commutation, the circuit in which the armature with inducible coils rotates in a magnetic field turned out to be preferable.

Power supply of electromagnets

Classic dynamos use a permanent magnet to induce a field. The rest of the DC generators need power for the electromagnets. In so-called separately excited generators, external direct current sources are used for this. Self-excited devices use some of their self-generated electricity to drive electromagnets. Starting such generators after stopping depends on their ability to accumulate residual magnetism. Depending on the method of connecting the excitation coils with the armature windings, they are divided:

• shunt (with parallel excitation);
• Serial (with sequential excitation);
• mixed excitation (with a combination of shunt and sequential).

Excitation types are applied depending on the required voltage control. For example, generators used to charge batteries need simple voltage control. In this case, the shunt type would be a suitable type. A separately excited generator is used as a machine that generates energy for a passenger elevator, since such systems require complex control.

The use of collector generators

Many DC generators are powered by AC motors in combinations called motor generator sets. This is one way to change AC to DC. Plating plants that produce aluminum, chlorine and some other materials electrochemically require a lot of direct current.

Diesel generators also supply DC power to locomotives and ships. Since collectors are complex and unreliable devices, DC generators are often replaced by machines that produce AC in combination with electronic ones. Switching generators have found application in low-power networks, allowing the use of a permanent magnet dynamo without excitation circuits.

There are other types of devices that are capable of producing electricity. These include electrochemical batteries, thermoelectric and photovoltaic cells, fuel converters. But compared to AC / DC induction generators, their share in global energy production is negligible.