Electric current is the main form of energy that does useful work in all areas of human life. It sets in motion various mechanisms, gives light, heats homes and revitalizes a whole host of devices that ensure our comfortable existence on the planet. Truly, this type of energy is universal. From it you can get anything you want, and even great destruction with inept use.
But there was a time when electrical effects were still present in nature, but did not help a person. What has changed since then? People began to study physical phenomena and came up with interesting machines - converters, which, in general, made a revolutionary leap in our civilization, allowing a person to receive one energy from another.
So people learned how to generate electricity from ordinary metal, magnets and mechanical motion - that's all. Generators were built that were capable of delivering enormous power flows of energy, calculated in megawatts. But it is interesting that the principle of operation of these machines is not so complicated and may well be understood even by a teenager. What is an electric current generator? Let's try to figure this out.
Electromagnetic induction effect
The basis for the appearance of an electric current in a conductor is an electromotive force - EMF. It is able to make charged particles, which are many in any metal, move. This force appears only if the conductor experiences a change in the intensity of the magnetic field. The effect itself is called electromagnetic induction. EMF is greater, the greater the rate of change of the magnetic flux. That is, it is possible to move the conductor near a permanent magnet, or to influence a stationary wire by the field of an electromagnet, changing its strength, the effect will be the same - an electric current will appear in the conductor.
Scientists Oersted and Faraday worked on this issue in the first half of the 19th century. They also discovered this physical phenomenon. Subsequently, based on electromagnetic induction, current generators and electric motors were created. Interestingly, these machines can easily be converted into each other.
How AC and DC generators work
It is clear that an electric current generator is an electromechanical machine that generates current. But in fact, it is a converter of energy: wind, water, heat, anything in the EMF, which already causes a current in the conductor. The device of any generator is fundamentally no different from a closed conductive circuit that rotates between the poles of a magnet, as in the first experiments of scientists. Only much greater is the magnitude of the magnetic flux created by powerful permanent or more often electric magnets. The closed loop has the form of a multi-turn winding, of which in a modern generator there are not one, but at least three. All this is done in order to obtain the largest possible emf.
A standard electric alternating current (or direct) generator consists of:
- Housings . It performs the function of a frame inside which a stator with poles of an electromagnet is mounted. The roller bearings of the rotor shaft are installed in it. It is made of metal, it also protects the entire internal filling of the machine.
- Stator with magnetic poles. A magnetic flux field winding is fixed to it. It is made of ferromagnetic steel.
- Rotor or anchors. This is the moving part of the generator, the shaft of which causes a foreign force to rotate. A self-excitation winding is located on the core of the armature, where an electric current is generated.
- Node switching. This structural element serves to divert electricity from the movable rotor shaft. It includes conductive rings that are movably connected to graphite collector contacts.
DC generation
In a generator producing direct current, the conductive circuit rotates in space of magnetic saturation. Moreover, for a certain moment of rotation, each half of the circuit is close to one or another pole. The charge in the conductor during this half-revolution moves in one direction.
To obtain the removal of particles, an energy removal mechanism has been made. Its peculiarity is that each half of the winding (frame) is connected to a conductive half ring. The half rings are not closed to each other, but are fixed on a dielectric material. For the period when one part of the winding begins to pass a certain pole, the half-ring is closed in the electrical circuit by brush contact groups. It turns out that only one type of potential comes to each terminal.
It is more correct to call the energy not constant, but pulsating, with an unchanged polarity. The ripple is caused by the fact that the magnetic flux on the conductor during rotation has both maximum and minimum effects. To equalize this ripple, several windings on the rotor and powerful capacitors at the input of the circuit are used. To reduce magnetic flux losses, the gap between the armature and the stator is minimized.
Alternator circuit
When the moving part of the current-generating device rotates, an EMF is also induced in the frame conductors, as in a DC generator. But a small feature - the alternator, the collector unit device has another. In it, each terminal is connected to its conductive ring.
The principle of operation of the alternator is as follows: when half the winding passes near one pole (the other, respectively, near the opposite pole), the current moves in the circuit in one direction from its minimum to its highest value and again to zero. As soon as the windings change their position relative to the poles, the current begins to move in the opposite direction with the same regularity.
At the same time, at the input of the circuit, a waveform is obtained in the form of a sinusoid with a half-wave frequency corresponding to the period of rotation of the rotor shaft. In order to obtain a stable signal at the output, where the frequency of the alternator is constant, the rotation period of the mechanical part must be unchanged.
Gas type magnetic generators
Designs of current generators, where instead of a metal frame a conductive plasma, liquid or gas is used as a charge carrier, are called MHD generators. Substances under pressure are driven away in a magnetic field. Under the influence of the same EMF induction, charged particles acquire directional motion, creating an electric current. The magnitude of the current is directly proportional to the speed of passage through the magnetic flux, as well as its power.
MHD generators have a simpler design solution - they do not have a rotor rotation mechanism. Such power sources are capable of delivering large power capacities in short periods of time. They are used as backup devices and in emergency situations. The coefficient determining the beneficial effect (Efficiency) of these machines is higher than that of an electric alternator.
Synchronous alternator
There are such types of alternators:
- Synchronous machines.
- Asynchronous machines.
A synchronous alternator has a strict physical relationship between the rotational movement of the rotor and the generated frequency of electricity. In such systems, the rotor is an electromagnet assembled from cores, poles and exciting windings. The latter are powered from a constant current source by means of brushes and ring contacts. The stator is a coil of wire, interconnected by the principle of a star with a common point - zero. EMF is already being induced in them and current is being generated.
The rotor shaft is driven by an external force, usually turbines, the frequency of which is synchronized and constant. The electric circuit connected to such a generator is a three-phase circuit, the current frequency in a separate line of which is shifted by a phase of 120 degrees relative to other lines. To obtain the correct sinusoid, the direction of the magnetic flux in the lumen between the stator and rotor parts is controlled by the design of the latter.
Excitation of an alternator is implemented by two methods:
- Contact.
- Non-contact.
In the contact excitation circuit, the electromagnet windings are supplied with electric power from another generator through a brush pair. This generator can be combined with the main shaft. It, as a rule, has less power, but sufficient to create a strong magnetic field.
The non-contact principle provides that the synchronous alternating current generator on the shaft has additional three-phase windings, in which EMF is induced during rotation and electricity is generated. It through the rectifying circuit enters the excitation coils of the rotor. Structurally, in such a system there are no movable contacts, which simplifies the system, making it more reliable.
Asynchronous generator
There is an asynchronous alternator. Its device is different from synchronous. There is no exact dependence of the emf on the frequency with which the rotor shaft rotates. There is such a thing as “glide S”, which characterizes this difference in influence. The amount of slip is determined by calculation, so it is wrong to think that there is no regularity of the electromechanical process in an induction motor.
If the generator idling is loaded, then the current flowing in the windings will create a magnetic flux that impedes the rotation of the rotor with a given frequency. This forms a slip, which, of course, affects the production of EMF.
The modern asynchronous alternator the device of the moving part has three different designs:
- Hollow rotor.
- Squirrel cage rotor.
- Phase rotor.
Such machines can have self- and independent excitation. The first circuit is implemented by including capacitors and semiconductor converters in the winding. An independent type of excitation is created by an additional AC source.
Generator switching circuits
All powerful power supplies of power lines generate a three-phase electric current. They contain three windings in which alternating currents are formed with a phase shifted from each other by 1/3 of the period. If we consider each individual winding of such a power source, then we get a single-phase alternating current going into the line. A voltage of tens of thousands of volts can be generated by a generator. The consumer receives 220 V from a distribution transformer.
Any alternator winding device has a standard, but there are two types of connection to the load:
The principle of operation of an alternating current generator switched on by a star involves the union of all wires (zero) into one that goes from the load back to the generator. This is due to the fact that the signal (electric current) is transmitted mainly through the output wire of the winding (linear), which is called the phase. In practice, this is very convenient, because you do not need to pull three additional wires to connect the consumer. The voltage between the line wires and the line and neutral wires will be different.
Connecting the generator windings with a triangle, they are closed with each other sequentially in one circuit. From the points of their connection, lines are drawn to the consumer. Then you do not need a neutral wire at all, and the voltage on each line will be the same regardless of the load.
The advantage of a three-phase current over a single-phase current is its less ripple during rectification. This has a positive effect on powered devices, especially DC motors. Also, a three-phase current creates a rotating magnetic field flux, which is capable of driving powerful asynchronous motors.
Where AC and DC generators are applicable
DC generators are significantly smaller in size and mass than AC machines. Having a more complex design than the latter, they still found application in many industries.
They received the main distribution as high-speed drives in machines where speed control is required, for example, in metalworking mechanisms, mine hoists, rolling mills. In transport, such generators are installed on diesel locomotives and various ships. Many models of wind generators are assembled on the basis of constant voltage sources.
Special-purpose direct current generators are used in welding, for exciting the windings of synchronous-type generators, as direct current amplifiers, for supplying galvanic and electrolysis plants.
The purpose of an alternator is to generate electricity on an industrial scale. This type of energy was presented to humanity by Nikola Tesla. Why is it that a polarity-changing current, rather than a constant one, is widely used? This is due to the fact that during the transmission of direct voltage there are large losses in the wires. And the longer the wire, the higher the loss. Alternating voltage can be transported over vast distances at much lower cost. Moreover, it is easy to convert an alternating voltage (lowering and increasing it), which has developed a 220 V generator.
Conclusion
Man did not fully understand the nature of magnetism, which pervades everything around. And electric energy is only a small part of the open secrets of the universe. The machines that we call energy generators are essentially very simple, but what they can give us is simply amazing. Nevertheless, the real miracle here is not in technology, but in the thought of a person who was able to penetrate into an inexhaustible reservoir of ideas spilled in space!