Alternating current generator: operating principle. Operating principle of the generator

If a permanent magnet rotates above a core with a coil attached to it, the magnetic field around the coil will continuously change and, due to the phenomenon of electromagnetic induction, an alternating induced current will appear in it. An induction alternating current generator operates on this principle, in which mechanical energy is converted into electrical energy.

Alternating current has a number of properties similar to those of direct current, but some of its properties are different from those of direct current.

So, flowing through the conductors, alternating current heats them (just like direct current). This property is used in electric heating devices and incandescent electric lamps.

Around the conductors through which alternating current passes, there is necessarily a magnetic field, but it, like the current, is variable. In an electromagnet powered by alternating current from the mains, the polarity of the ends of the magnetic circuit (core) changes 50 times per second.

It is easy to verify that a series-wound commutator motor can operate when powered by alternating current. Such motors are used in many household appliances (vacuum cleaner, juicer, fan, etc.). Indeed, when the polarity of the poles of the inductor changes, the direction of the current in the armature simultaneously changes, so the armature will continue to rotate in the same direction.

TEST QUESTIONS

1. What is the operating principle of an induction generator?
2. What properties of alternating current do you know?
3. What are the devices of an induction turbo and hydrogen generator? Explain with pictures.

4. Why does the turbogenerator rotor have one pair of poles, while the hydrogenerator has many?

Exercises

1. Prove that the hydraulic generator of the Bratsk hydroelectric power station produces alternating current with a frequency of 50 Hz. Its rotor, rotating at a frequency of 125 rpm, has 24 pairs of poles.
2. How many pairs of poles should a hydraulic generator have if its rotor rotates at a frequency of 5 rps? The frequency of the induced current is 50 Hz.
3. Prove that magnetoelectric instruments are unsuitable for measurements in alternating current circuits, but electromagnetic and electrodynamic instruments are suitable.
4. The figure shows a graph taken from the oscilloscope screen. Each cell corresponds horizontally to 0.01 s, and vertically to 20 V. Determine the voltage and frequency of the electric current.

A generator converts mechanical energy into electrical energy by rotating a wire coil in a magnetic field. An electric current is also generated when the field lines of a moving magnet intersect the turns of a wire coil (picture on the right). Electrons (blue balls) move towards the positive pole of the magnet, and electric current flows from the positive pole to the negative pole. As long as the magnetic field lines cross the coil (conductor), an electric current is induced in the conductor.

A similar principle also works when moving a wire frame relative to a magnet (far figure on the right), i.e., when the frame intersects the magnetic field lines. The induced electric current flows in such a way that its field repels the magnet when the frame approaches it and attracts it when the frame moves away. Each time the frame changes orientation relative to the poles of the magnet, the electric current also changes its direction to the opposite direction. As long as the source of mechanical energy rotates the conductor (or magnetic field), the generator will generate alternating electric current.

Operating principle of an alternator

The simplest alternating current generator consists of a wire frame rotating between the poles of a stationary magnet. Each end of the frame is connected to its own slip ring, which slides along an electrically conductive carbon brush (picture above the text). The induced electric current flows to the inner slip ring when the half of the frame connected to it passes the north pole of the magnet, and vice versa to the outer slip ring when the other half of the frame passes the north pole.

Three Phase Alternator

One of the most cost-effective ways to generate high alternating current is to use a single magnet rotating across multiple windings. In a typical three-phase generator, the three coils are located equidistant from the axis of the magnet. Each coil produces alternating current when a magnet pole passes by it (right picture).

Changing the direction of electric current

When a magnet is pushed into a wire coil, it induces an electric current in it. This current causes the galvanometer needle to deviate away from the zero position. When the magnet is removed from the coil, the electric current reverses its direction and the galvanometer needle moves away from the zero position.

AC

The magnet will not induce electric current until its lines of force begin to cross the wire loop. When a magnet pole is pushed into a wire loop, an electric current is induced in it. If the magnet stops moving, the electric current (blue arrows) also stops (middle diagram). When a magnet is removed from a wire loop, an electric current is induced in it, flowing in the opposite direction.

For those who are unfamiliar with generators, we explain that this is a unit in which another type of energy is obtained from one type of energy. Or, more precisely, from mechanical to electrical. Moreover, these devices can generate both direct current and alternating current. Until the mid-twentieth century, mainly direct current generators were used. These were large devices that did not work very well. The appearance of semiconductor-type diodes on the market made it possible to invent a three-phase alternating current generator. It is diodes that allow alternating current to be rectified.

Operating principle

The operation of a three-phase generator is based on Faraday's law - the law of electromagnetic induction, which states that an electromotive force will necessarily be induced in a rotating rectangular frame, which is installed between two magnets. The caveat is that the magnets will create a rotating magnetic field. The direction of rotation of both the frame and the magnetic field must coincide. But an electromotive force will also arise if the frame remains stationary and the magnet rotates inside it.

To understand how the generator works, pay attention to the figure below. This is the simplest way it works.

Here you can clearly see magnets with different poles, a frame, a shaft and slip rings, with the help of which current is drained.

Of course, this is just a diagram, although laboratory generators were created this way. In practice, ordinary magnets are replaced by electromagnets. The latter are copper windings or inductors. When an electric current passes through them, the necessary magnetic field is formed. Such generators are installed in all cars (this is just an example); to start them, a battery is installed under the hood, that is, a direct current source. Some generator models are started using the principle of self-excitation or using low-power generators.

Varieties

The classification is based on the principle of operation, therefore these AC units are divided into two classes:

• Asynchronous. These are the most reliable generators, small in size and weight, simple in design. They cope well with overloads and short circuits. However, it is necessary to take into account that this type will immediately fail if it is subject to a large overload. For example, the starting current of electrical equipment. Therefore, it is worth taking this fact into account, for which you will have to purchase a generator with a power three or four times greater than the power consumed by the equipment at startup.
• Synchronous. But this type easily copes with short-term loads. Such a generator can withstand an overload of five or six times. True, it does not differ in high reliability compared to asynchronous options; moreover, it is large in size and weight.

Of course, this division is based on the operating principle of the unit. But there are other criteria.

• Single phase.
• Two-phase.
• Three-phase.
• Multiphase (usually six phases).
• Welding.
• Linear.
• Induction.
• Stationary.
• Portable.

Three-phase generator device

In principle, the design of a three-phase alternating current generator is quite simple. This is a housing with two covers on opposite sides. Each of them has holes for ventilation. The covers contain niches for bearings in which the shaft rotates. A transmission element is installed on the front end of the shaft. For example, a car generator has a pulley that transmits rotation from the internal combustion engine to the generator. At the opposite end of the shaft, electric current is transmitted, because the shaft in this case acts as an electromagnet with one winding.

The transmission is carried out through graphite brushes and slip rings (they are made of copper). The brushes are connected to an electrical regulator (essentially a regular relay), which regulates the supply of 12 volts with the required deviations. The most important thing is that the relay does not increase or decrease the voltage depending on the speed of rotation of the shaft itself.

So, if we talk about three-phase alternating current generators, then these are three such single-phase ones. Only a three-phase unit has a winding not on the rotor (shaft), but in the stator. And there are three such windings, which are shifted relative to each other in phase. The shaft, as in the first design, performs the functions of an electromagnet, which is powered by direct current through sliding contacts.

The rotation of the shaft creates a magnetic field in the windings. Electromotive force begins to be induced when the magnetic field of the windings intersects with the rotor. And since the windings are located symmetrically on the stator, that is, every 120º, then, accordingly, the electromotive force will have the same amplitude value.

Humanity has been using electricity in all areas of activity for more than a century. Without him it is simply impossible to imagine a normal life. With the help of special machines, mechanical energy is converted into alternating or direct current. To better understand how this happens, it is necessary to understand what the generator consists of and how it works.

Conversion of mechanical energy into electrical energy

The basis of the operation of any generator lies the principle of magnetic induction. The first electric cars appeared in the second half of the 19th century. Their inventors were Michael Faraday and Hippolyte Pixie. In 1886, there was a public demonstration of an alternator, a device capable of generating current from mechanical movement.

The first three-phase alternating current generator was developed by the Russian Dolivo-Dobrovolsky. In 1903, he built the very first industrial power plant on Earth, which became the power source for the elevator.

The simplest circuit of an alternating current generator is a wire coil rotating in a magnetic field. An alternative option is when the coil remains motionless, and a magnetic field crosses it. In both cases, electrical energy will be generated. While the movement continues, an alternating current is generated in the conductor. Generators are used to generate electricity all over the world. They are part of the global power supply system of the globe.

How the generator is designed depends on its purpose, and various modifications are possible. However there are two main components:

1. The rotor is a moving element made of solid iron.
2. The stator is stationary and is assembled from insulated iron sheets. Inside there are grooves in which the wire winding passes.

To obtain the greatest magnetic induction, the distance between these parts of the unit should be as small as possible. The field winding located on the rotor is powered through a brush system.

There are two types of construction:

• with a rotating armature and a stationary magnetic field;
• the magnetic field rotates, but the armature remains in place.

The most widely used machines are those with movable magnetic poles. It is much more convenient to remove electricity from the stator than from the rotor. In general, the generator is built in the same way as an electric motor.

Classification and types of units

Units for converting mechanical energy into electrical energy have a similar design. They may differ in the principle of operation of the generator and field winding:

By design:

• salient poles;
• not expressed.

According to the method of connecting the windings:

Depending on the number of phases:

• single-phase;
• two-phase;
• three-phase.

DC units are designed in such a way that the mechanism for removing energy consists of two isolated half-rings, each of which receives a charge of a certain potential. The output produces a pulsating current of one direction.

Synchronous generators have an armature with a winding to which direct current is supplied. By adjusting its value, you can change the strength of the magnetic field and control the output voltage. Asynchronous ones do not have a winding; instead, the magnetization effect is used.

Main Applications

It is worth remembering that ordinary electricity in sockets appears due to the operation of huge alternating current generators at thermal power plants. Scope of use of these electric machines includes all types of human activity:

• used as a backup source of energy at facilities where power supply interruptions cannot be tolerated;
• indispensable in places where there are no power lines;
• Most vehicles are equipped with a generator; it generates electricity for the on-board network;
• power supply for hydrolysis plants;
• industry;
• at nuclear and hydroelectric power plants.

Recently, household units for generating electricity have become increasingly popular. They are distinguished by their compact size and low fuel consumption. They can run on gasoline and diesel. They are used in camping conditions, at the dacha or as an emergency power source.

The invention of a method for generating electricity from mechanical movement was of epochal importance for the development of modern civilization. The world around us is full of mysteries, the answers to which are unknown, but perhaps other important discoveries await people that can change their lives.

Story

Systems producing alternating current have been known in simple forms since the discovery of magnetic induction of electric current. Early machines were developed by pioneers such as Michael Faraday and Hippolyte Pixie.

Faraday developed a "rotating triangle" whose action was multipolar- each active conductor was passed sequentially through an area where the magnetic field was in opposite directions. The first public demonstration of the most powerful "alternator system" took place in 1886. A large two-phase alternating current generator was built by British electrician James Edward Henry Gordon in 1882. Lord Kelvin and Sebastian Ferranti also developed an early alternator that produced frequencies between 100 and 300 hertz. In 1891, Nikola Tesla patented a practical "high-frequency" alternator (which operated at a frequency of about 15,000 hertz). After 1891, multiphase alternators were introduced.

The principle of operation of the generator is based on the action of electromagnetic induction - the occurrence of electrical voltage in the stator winding located in an alternating magnetic field. It is created with the help of a rotating electromagnet - rotor when direct current passes through its winding. The alternating voltage is converted to direct voltage by a semiconductor rectifier.

Car generator

Car alternator. The drive belt has been removed.

The alternator is used in modern cars to charge the battery and to supply power to the car's electrical system. Alternating current generators do not use a commutator, this gives a great advantage over direct current generators: they are simpler, lighter and cheaper. Automotive alternators use a set of rectifiers (diode bridge) to convert alternating current to direct current. To produce direct current with low ripple, automotive alternators have a three-phase winding and a three-phase rectifier.

Modern automobile alternators have a voltage regulator built into them. Previously, only analog voltage regulators were installed. At the moment, relay regulators have switched to a digital channel, the so-called CAN bus.

Marine alternators

Marine alternating current generators in yachts with appropriate adaptation to salt-water environments.

Brushless Alternators

A brushless generator consists of two generators on one shaft. Small brushless generators may look like one unit, but the two parts are easily identified on larger generators. The larger part of the two is the main generator and the smaller part is the exciter. The exciter has stationary field coils and a rotating armature (power coils). The main generator uses opposing rotating field configurations and stationary coils. The bridge rectifier (rotating rectifier) ​​is mounted on a plate attached to the rotor. No brushes or slip rings are used, reducing the number of wearing parts.

Induction generator

Unlike other generators, the operation of an induction generator is based not on a rotating magnetic field, but on a pulsating one, in other words, the field changes not as a function of displacement, but as a function of time, which ultimately (induction of EMF) gives the same result.

The design of induction generators involves the placement of both a constant field and coils for inducing EMF on the stator, while the rotor remains free from windings, but must have a tooth shape, since the entire operation of the generator is based on the tooth harmonics of the rotor.

Generators for small energy

For powers up to 100 kW, single- and three-phase generators with excitation from permanent magnets are widely used. The use of high-energy permanent magnets of the neodymium-iron-boron composition made it possible to simplify the design and significantly reduce the size and weight of generators, which is critical for small-scale wind energy.

Alternator Design

In the most general case, the most commonly used three-phase alternating current generator consists of a salient-pole rotor with one pair of poles (low-power high-speed generators) or 2 pairs of them, arranged crosswise (the most common generators with powers up to several hundred kilowatts. This design not only allows for more efficient use material, but also for an industrial AC frequency of 50 Hz gives an operating rotor speed of 1500 rpm, which is in good agreement with the traction speed of diesel engines of this power), as well as a stator with 3 (in the first case) or 6 (in the second) power windings and poles. The voltage from the power windings is that which is supplied to the consumer.

The rotor can be made with permanent magnets only for very low-power generators; in all other cases it has a so-called winding. excitation winding, that is, it is a direct current electromagnet, powered in a rotating rotor through a brush-commutator assembly with simple ring contacts that are more resistant to wear than the split lamella commutator of DC machines.

In any powerful alternating current generator with an excitation winding on the rotor, the question inevitably arises - what magnitude of excitation current should be supplied to the coil? After all, the output voltage of such a generator depends on this. And this voltage must be maintained within certain limits, for example, 380 Volts, regardless of the current in the consumer circuit, a significant value of which can also significantly reduce the output voltage of the generator. In addition, the load across phases can generally be very uneven.

This issue is solved in modern generators, as a rule, by introducing electromagnetic current transformers into the output circuits of the generator phases, connected by secondary windings in a triangle or star, and producing at the output an alternating three-phase voltage with an amplitude of one - tens of volts, strictly proportional and phase-matched with the value of the load current of the phases generator - the greater the current currently consumed in a given phase, the greater the voltage at the output of the corresponding phase of the corresponding current transformer. This achieves a stabilizing and auto-regulating effect. All three regulating phases from the secondary windings of the current transformers are then connected to a conventional 3-phase rectifier made of 6 semiconductor diodes, and its output produces a direct current of the required value, which is supplied to the rotor excitation winding through a brush-collector assembly. The circuit can be supplemented with a rheostat unit for some freedom in regulating the excitation current.

In outdated or low-power generators, instead of current transformers, a system of powerful rheostats was used, with the isolation of the operating excitation current by changing the voltage drop across the resistor when the current through it changes. These schemes were less accurate and much less economical.

In both cases, there is the problem of the appearance of an initial voltage on the power windings of the generator at the moment it begins to operate - indeed, if there is no excitation yet, then the current in the secondary windings of the current transformers has nowhere to come from. The problem, however, is solved by the fact that the iron of the rotor yoke has some capacity for residual magnetization; this residual magnetization is sufficient to excite a voltage of several volts in the power windings, sufficient to self-excite the generator and reach its operating characteristics.

In self-excited generators, a serious danger is posed by the accidental supply of external voltage from an industrial electrical network to the stator power windings. Although this does not lead to any negative consequences for the generator windings themselves, the powerful alternating magnetic field from the external network effectively demagnetizes the stator, as a result of which the generator loses its ability to self-excite. In this case, an initial supply of excitation voltage from some external source, for example, a car battery, is required; sometimes this procedure completely cures the stator, but in some cases the need to supply external excitation remains forever.

Main alternator

The main generator consists of a rotating magnetic field, as stated earlier, and a stationary armature (generator windings)

Hybrid cars

See also

Links

• Alternators. Integrated Publishing (TPub.com).
• Wooden Low-RPM Alternator. ForceField, Fort Collins, Colorado, USA.