Transformer : A necessary Electro-magnetic Device
A transformer is a static electromagnetic device containing two to several windings located on a common magnetic circuit and inductively coupled, thus, to each other. A transformer is used to convert the electrical energy of an alternating current by means of electromagnetic induction without changing the frequency of the current. Transformers are used both for the conversion of alternating voltage and for galvanic isolation in various fields of electrical engineering and electronics.
For the sake of convenience, we note that in some cases the transformer can contain only one winding (autotransformer), and the core may be completely absent (HF transformer), but most of the transformers have a core (magnetic core) made of soft magnetic ferromagnetic material, and two or more isolated taps or wire windings, covered by a common magnetic flux, but arranged and overlapped in order. Now, what kinds of transformers are, how they are arranged and what they are used for.
Here we illustrate the types of transformers
Power Transformers
This type of low-frequency (50-60 Hz) transformers serves in electrical networks, as well as in installations for receiving and converting electrical energy. Why is it called power? Because it is this type of transformer that is used to supply and receive electricity for power lines and with power lines, where the voltage can reach 1150 kV.
In urban electric networks, the voltage reaches 10 kV. By means of power low-frequency transformers, the voltage also drops to 0.4 kV, 380/220 volts, which are necessary for consumers.
Structurally, a typical power transformer may comprise two, three or more windings located on an armored core of electrical steel, with some of the lower voltage windings being able to be supplied in parallel (a transformer with split windings).
This is convenient for increasing the voltage received simultaneously from several generators. Typically, a power transformer is placed in a tank with transformer oil, and in the case of particularly powerful specimens, an active cooling system is added.
Three-phase power transformers up to 4000 kVA are installed in substations and power stations. Three-phase are more common, since losses are obtained up to 15% less than with three single-phase losses.
Transformer network
Often, network transformers are made with several secondary windings, so that several voltage sources can be used to power the various parts of the circuit. In particular, transformers TH (transformer incandescent) could always be (and now can) be found in the schemes where radio tubes were present.
Modern network transformers are structurally implemented on Ш-shaped, rod or toroidal cores from a set of electrotechnical steel plates, onto which coils are wound. The toroidal shape of the magnetic circuit makes it possible to obtain a more compact transformer.
If we compare transformers of equal overall power on the toroidal and on the Ш-shaped cores, the toroidal will take up less space, besides, the surface area of the toroidal magnetic core is completely enclosed by windings, there is no empty yoke, as in the case of armor Ш-shaped or core cores. Welding transformers with a power of up to 6 kW can be classified as mains. Network transformers, of course, belong to low-frequency transformers.
Autotransformer
One type of low-frequency transformer is an autotransformer, in which the secondary winding is part of the primary or primary is part of the secondary. That is, in the autotransformer, the windings are connected not only magnetically, but also electrically. Several leads are made from a single winding, and allow only one winding to receive a different voltage.
The main advantage of the autotransformer is a lower cost, since less wire is used for windings, less steel for the core, and the result is less weight than a conventional transformer. The disadvantage is that there is no galvanic isolation of the windings.
Autotransformers find application in automatic control devices, and are also widely used in high-voltage power networks. Three-phase autotransformers with the connection of windings in a triangle or in a star in electrical networks are in high demand today.
Power autotransformers are produced at power up to hundreds of megawatts. Autotransformers are also used to start powerful AC motors. Autotransformers are particularly suitable at low conversion ratios.
Laboratory Autotransformer
A special case of an autotransformer is a laboratory autotransformer (LATR). It allows you to smoothly adjust the voltage supplied to the consumer. The design of the LATR is a toroidal transformer with a single winding, which has an uninsulated "path" from the turn to the turn, that is, it is possible to connect to each of the turns of the winding. Contact with the track is provided by a sliding coal brush, which is controlled by a rotary knob.
So you can get on the load the operating voltage of different sizes. Typical single-phase LATRs allow to receive voltage from 0 to 250 volts, and three-phase ones - from 0 to 450 volts. LATRs with a power from 0.5 to 10 kW are very popular in laboratories for the purpose of setting up electrical equipment.
Current transformer
A current transformer is a transformer whose primary winding is connected to a current source and a secondary winding to a protective or measuring device having small internal resistances. The most common type of current transformer is a current measuring transformer.
The primary winding of the current transformer (usually only one turn, one wire) is connected in series to the circuit in which it is required to measure the alternating current. It turns out that the secondary winding current is proportional to the primary current, and the secondary winding must necessarily be loaded, because otherwise the voltage of the secondary winding can turn out to be high enough to break through the insulation. In addition, if the secondary winding of the TT is opened, the magnetic circuit will simply burn out from the induced uncompensated currents.
The design of the current transformer is a core of blended siliceous cold-rolled electrical steel, onto which one or several isolated windings are reeled, which are secondary. Primary winding is often just a bus, or a wire with a measured current passed through the magnetic wire window (this is the principle, by the way, that current clamps work). The main characteristic of a current transformer is the transformation coefficient, for example 100/5 A.
For current measurement and in relay protection circuits, current transformers are used quite widely. They are safe, because the measured and secondary circuits are galvanically isolated from each other. Typically, industrial current transformers are available with two or more groups of secondary windings, one of which is connected to protective devices, the other to a measuring device, for example to counters.
Pulse transformer
Almost all modern power supply units, various inverters, welding machines, and other power and low-power electrical converters use pulse transformers. Today, pulsed circuits almost completely replaced the heavy low-frequency transformers with cores made of bent steel.
A typical pulse transformer is a transformer made on a ferrite core. The shape of the core (magnetic core) can be quite different: a ring, a rod, a cup, an S-shaped, a U-shaped one. The advantage of ferrites over transformer steel is obvious - transformers on ferrite can operate at frequencies up to 500 kHz or more.
Since the pulse transformer is a high-frequency transformer, its dimensions also decrease with increasing frequency. Less winding is required on the windings, and a field, IGBT or bipolar transistor is sufficient to obtain a high-frequency current in the primary circuit, sometimes several, depending on the topology of the impulse power circuit (direct-travel - 1, push-pull - 2, half bridge - 2, bridge - 4) .
For the sake of fairness, we note that if a backward power scheme is used, then the transformer is essentially a dual throttle, since the processes of accumulation and return of electricity to the secondary circuit are separated in time, that is, they do not flow simultaneously, so with a flyback control it is still a choke, and not a transformer.
Pulse circuits with transformers and chokes on ferrite are found today everywhere, from ballasts of energy-saving lamps and chargers of various gadgets to welding machines and powerful inverters.
Pulse current transformer
To measure the magnitude and (or) direction of the current in pulse circuits, pulse current transformers are often used, which are a ferrite core, often - a ring (toroidal), with a single winding. A wire is passed through the core ring, the current in which it is necessary to investigate, and the winding itself is loaded onto a resistor.
For example, the ring contains 1000 turns of wire, then the ratio of the currents of the primary (lead wire) and the secondary winding will be 1000 to 1. If the winding of the ring is loaded with a resistor of known denomination, the measured voltage on it will be proportional to the winding current, more current through this resistor.
The industry produces pulsed current transformers with different transformation ratios. The developer only needs to connect a resistor and a measurement circuit to this transformer. If you want to know the direction of the current, and not its magnitude, then the winding of the current transformer is loaded simply by two counter-diodes.
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