The principle of operation of the transformer diagram. How does a transformer work? Out of service mode

The principle of operation of the transformer is related to the principle of electromagnetic induction. The current supplied to the primary winding creates a magnetic flux in the magnetic circuit.

The operation of the transformer is based on the phenomenon of electromagnetic induction. One of the windings, called the primary winding, is energized from an external source. The alternating current flowing through the primary winding creates an alternating magnetic flux in the magnetic circuit, phase-shifted, with a sinusoidal current, by 90 ° with respect to the current in the primary winding. As a result of electromagnetic induction, an alternating magnetic flux in the magnetic circuit creates in all windings, including in the primary, the EMF of induction proportional to the first derivative of the magnetic flux, with a sinusoidal current shifted by 90 ° with respect to the magnetic flux. When the secondary windings are not connected to anything (no-load mode), the induction EMF in the primary winding almost completely compensates for the power supply voltage, therefore the current through the primary winding is small, and is mainly determined by its inductive resistance. The induction voltage on the secondary windings in no-load mode is determined by the ratio of the number of turns of the corresponding winding w2 to the number of turns of the primary winding w1: U2 = U1w2 / w1.

When the secondary winding is connected to the load, current begins to flow through it. This current also creates a magnetic flux in the magnetic circuit, and it is directed opposite to the magnetic flux created by the primary winding. As a result, in the primary winding, the compensation of the EMF of induction and the EMF of the power source is disturbed, which leads to an increase in the current in the primary winding, until the magnetic flux reaches almost the same value. In this mode, the ratio of the currents of the primary and secondary winding is equal to the inverse ratio of the number of winding turns (I1 = I2w2 / w1,), the voltage ratio in the first approximation also remains the same.

Schematically, the above can be depicted as follows:

U1> I1> I1w1> Ф> ε2> I2.

The magnetic flux in the magnetic core of the transformer is phase-shifted relative to the current in the primary winding by 90 °. The EMF in the secondary winding is proportional to the first derivative of the magnetic flux. For sinusoidal signals, the first derivative of sine is cosine, the phase shift between sine and cosine is 90 °. As a result, when the windings are connected, the transformer shifts the phase by approximately 180 °. When the windings are connected oppositely, an additional 180 ° phase shift is added and the total phase shift of the transformer is approximately 360 °.

Idling experience

An open-circuit test and a short-circuit test are used to test the transformer.

When the transformer is idle, its secondary winding is open and there is no current in this winding (/ 2-0).

If the primary winding of the transformer is included in the network of an alternating current electric power source, then an open-circuit current I0 will flow in this winding, which is a small value compared to the rated current of the transformer. In high-power transformers, the no-load current can reach values of the order of 5-10% of the rated current. In low-power transformers, this current reaches 25-30% of the rated current. The no-load current I0 creates a magnetic flux in the magnetic circuit of the transformer. To excite the magnetic flux, the transformer consumes reactive power from the network. As for the active power consumed by the transformer during no-load operation, it is spent to cover the power losses in the magnetic circuit caused by hysteresis and eddy currents.

Since the reactive power at no-load of the transformer is much higher than the active power, its power factor cos φ is very small and is usually equal to 0.2-0.3.

A transformer, a device that transfers electrical energy from one part of a circuit to another through magnetic induction and, as a rule, with a change in voltage. Transformers only work with alternating electric current (AC).

Transformers are essential in power distribution. They increase the voltage generated in power plants to high values in order to efficiently transfer electricity. Other transformers reduce this voltage at the point of consumption.

Many household appliances are equipped with transformers in order to increase or decrease the voltage supplied from the household power grid as needed. For example, the TV and the audio amplifier need a voltage boost, and the doorbell or thermostat needs a low voltage to operate.

How does a transformer work

Typically, a simple transformer consists of two coils wound with insulated wire. In most transformers, wires are wound around a rod of iron called a core.

One of the windings, also called the primary winding, is connected to an alternating current source, which in turn results in a permanently alternating magnetic field around the winding. This alternating magnetic field, in turn, creates an alternating current in the other winding (secondary winding).

The value, defined as the ratio of the number of turns in the primary winding to the number of turns in the secondary winding, determines the scale of the decrease or increase in voltage in the secondary winding. This value is also called the transformation ratio.

For example, if the transformer has 3 turns in the primary winding and 6 turns in the secondary winding, then the voltage in the secondary winding will be 2 times higher than in the primary. Such a transformer is called a step-up transformer.

And on the contrary, if there are 6 turns in the primary winding and 3 turns in the secondary, then the voltage removed from the secondary winding will be 2 times lower than in the primary winding. This type of transformer is called a step-down transformer.

It should also be borne in mind that the ratio of the current in both coils is inversely related to the ratio of their voltages. Thus, the electrical power (voltage times amperage) is the same in both coils.

The impedance (resistance to AC current) of the primary coil depends on the secondary impedance and the transformation ratio. With the correct ratio of the turns of the transformer, almost the same resistance of both circuits can be achieved.

Matching impedances are essential in stereo systems and other electronic systems to allow maximum energy transfer from one circuit block to another.

Content:

The transformer belongs to the category of static electromagnetic devices capable of converting alternating current with one voltage value into alternating current with another voltage, while maintaining the same frequency. These devices are successfully used in electrical networks for the transmission and distribution of energy, and are also an integral part of many electrical installations. In this regard, the question of how the transformer works, depending on the number of windings, phases, cooling methods and other design features, on which the use of these devices directly depends, becomes especially relevant.

Step-down transformer action

There are various types of step-down transformers. They can be one-, two- or, which allows them to be used in various fields of energy. The design of these devices includes two windings and a laminated core, for the manufacture of which electrical steel is used. A distinctive feature of the step-down transformer is the different number of turns in the primary and secondary windings. In order to use the device correctly, you need to have a good understanding of how a step-down transformer works.

The voltage applied to the input of the transformer causes the appearance of an electromotive force in the winding, which, in turn, leads to the appearance of a magnetic field. As a result of the intersection of the turns of the second coil by this field, its own electromotive force of self-induction appears in it. Under its influence, a voltage appears in the second coil that differs from the primary one by the difference in the number of turns in both windings.

To determine the exact parameters, it is necessary to perform the calculations of the step-down transformer. It should be borne in mind that the emergence of the electromotive force of self-induction is possible only under the action of an alternating voltage. Therefore, all household electrical networks work only on.

In modern conditions, it is increasingly necessary to convert high voltage to low voltage. This is due to the fact that power plants generate high voltage current that meets the needs of a site. Therefore, at each such section, the initial voltage is converted to a value that is acceptable for use in domestic conditions. In addition, step-down transformers are quite often used in domestic conditions to adapt low-voltage devices to 220V mains current. They are structural elements of various power supplies, adapters, stabilizers and other similar devices.

When purchasing a step-down transformer, you should pay attention to parameters such as power and the number of turns in both windings. It is necessary to take into account an important indicator - the voltage transformation ratio. This parameter depends on the ratio of the number of turns in the primary and secondary windings of the transformer. Thus, the ratio of the voltages on both windings is determined.

In a step-down transformer, the number of turns in the primary winding exceeds the number of turns in the secondary winding, which produces a lower output voltage. Some devices have multiple pins, meaning there are several groups of connections at once. The formation of the required circuit in them is carried out depending on the magnitude of the input and output current. Such transformers are universal and multifunctional, which are very popular with consumers.

Voltage transformer working principle

The main function of voltage transformers is to convert the energy from the source to the desired voltage value. These devices can only operate with an alternating voltage with a constant frequency.

According to the transformation ratio, there are three types of voltage transformers:

Downward. In these devices, the output voltage is less than the input voltage. Used in power supplies, stabilizers, etc.
Boosting. Here, the output current is greater than that of the input. It is mainly used in amplifying devices.
Coordinator. The operation of these devices takes place without changing the voltage parameters, all actions are limited only by galvanic isolation. Used in audio amplifier circuits.

In order to use a particular design correctly, you need to know exactly how a current transformer works. It is known that the basis of the operation of these devices is. Magnetic cores are used in transformers to reduce losses during transformation and maximize energy transfer. There is one primary coil in the design, while there are several secondary coils, depending on the purpose of each device.

After the appearance of an alternating current in the primary winding, a magnetic flux appears in the magnetic circuit, exciting the voltage in the secondary winding. The main parameter is the transformation ratio, which is equal to the ratio of the voltage in the primary winding to the voltage in the secondary winding. In the same way, the number of turns available in the first and second coils is related.

This factor is used to calculate the parameters for a specific transformer. For example, if there are 2000 turns in the primary winding, and 100 in the secondary, the transformation ratio will be 20. Therefore, with an input mains voltage of 240 V, the output voltage will be 12 V. voltage.

One of the types of such devices, which are widely used in practice, are voltage measuring transformers. They are used in equipment consuming high currents and high operating voltages for the purpose of performing control measurements. With the help of these devices, the measured values are reduced to a level that allows the necessary measurements to be made.

Instructions

The transformer is based on a phenomenon called electromagnetic induction. When a conductor is exposed to a changing magnetic field, a voltage arises at the ends of this conductor, which corresponds to the first derivative of the change in this field. Thus, when the field is constant, no voltage arises at the ends of the conductor. This voltage is very small, but it can be increased. To do this, instead of a straight conductor, it is sufficient to use a coil consisting of the desired number of turns. Since the turns are connected in series, the voltages across them are summed up. Therefore, all other things being equal, the voltage will be greater than a single turn or straight conductor in the number of times corresponding to the number of turns.

You can create an alternating magnetic field in different ways. For example, rotating a magnet next to the coil will create a generator. In the transformer, for this, another winding is used, called the primary winding, and a voltage of one form or another is applied to it. A voltage arises in the secondary winding, the shape of which corresponds to the first derivative of the voltage waveform in the primary winding. If the voltage on the primary winding changes in a sinusoidal manner, on the secondary it will change in a cosine manner. The transformation ratio (not to be confused with the efficiency) corresponds to the ratio of the number of turns of the windings. It can be either less or more than one. In the first case, the transformer will be step-down, in the second - step-up. The number of turns per volt (the so-called "number of turns per volt") is the same for all transformer windings. For power frequency transformers, it is at least 10, otherwise the efficiency drops and heating increases.

The magnetic permeability of air is very low, therefore, coreless transformers are used only when operating at very high frequencies. In industrial frequency transformers, cores made of steel plates covered with a dielectric layer have been used. Thanks to this, the plates are electrically isolated from each other, and eddy currents do not occur, which can reduce efficiency and increase heating. In transformers of switching power supplies operating at high frequencies, such cores are not applicable, since significant eddy currents can occur in each individual plate, and the magnetic permeability is excessive. Ferrite cores are used here - dielectrics with magnetic properties.

Losses in the transformer, which reduce its efficiency, arise due to the emission of an alternating electromagnetic field by it, small eddy currents that still arise in the core despite the measures taken to suppress them, as well as the presence of active resistance in the windings. All these factors, except for the first, lead to heating of the transformer. The active resistance of the winding should be negligible compared to the internal resistance of the power supply or load. Therefore, the greater the current through the winding and the lower the voltage across it, the thicker the wire is used for it.

The principle of operation and the device of a single-phase transformer

In fig. 1a shows the simplest transformer, consisting of two coils I and II, located coaxially one above the other. Coil I is supplied with alternating current from an alternator G. This coil is called the primary coil or primary winding. With a coil II, called a secondary coil or secondary winding, a circuit is connected by receivers of electrical energy.

The principle of operation of the transformer

The action of the transformer is as follows. When a current passes in the primary coil, it creates a magnetic field, the lines of force of which penetrate not only the coil that created them, but partially also the secondary coil. An approximate picture of the distribution of the lines of force created by the primary coil is shown in the figure.

As can be seen from the figure, all the lines of force are closed around the conductors of the coil, but some of them in Fig. 1b power lines 1, 2, 3, 4 are also closed around the coil conductors. Thus, coil I is magnetically coupled to coil II by means of magnetic lines of force.

The degree of magnetic coupling of coils I and II, with their coaxial arrangement, depends on the distance between them: the farther the coils are from each other, the less the magnetic coupling between them, because the less field lines of coil I are coupled with coil II.

Since, as we assume, an alternating current passes through the coil I, i.e., a current that changes in time according to some law, for example, according to the sine law, then the magnetic field created by it will also change in time according to the same the law.

For example, when the current in the coil I passes through the largest value, then the magnetic flux generated by it also passes through the largest value; when the current in the coil I passes through zero, changing its direction, then the magnetic flux also passes through zero, also changing its direction.

As a result of a change in the current in coil I, both coils I and II are penetrated by a magnetic flux that continuously changes its value and direction. According to the basic law of electromagnetic induction, with any change in the magnetic flux penetrating the coil, a variable electromotive force is induced in the coil. In our case, the self-induction electromotive force is induced in the coil I, and the mutual induction electromotive force is induced in the coil II.

If the ends of coil II are connected to a circuit of receivers of electrical energy (see Fig. 1a), then a current will appear in this circuit; hence the receivers will receive electrical energy. At the same time, energy will be directed to coil I from the generator, almost equal to the energy given to the circuit by coil II. Thus, electrical energy from one coil will be transmitted to the circuit of the second coil, which is completely unconnected to the first coil galvanically (metallic). In this case, the means of energy transmission is only an alternating magnetic flux.

Shown in fig. 1a, the transformer is very imperfect, because there is little magnetic coupling between the primary coil I and the secondary coil II.

The magnetic coupling of two windings, generally speaking, is estimated by the ratio of the magnetic flux coupled to both windings to the flux created by one coil.

From fig. 1b, it can be seen that only part of the field lines of coil I are closed around coil II. Another part of the lines of force (in Fig. 1b - lines 6, 7, 8) is closed only around coil I. These lines of force do not participate at all in the transfer of electrical energy from the first coil to the second, they form the so-called stray field.

In order to increase the magnetic coupling between the primary and secondary windings and at the same time reduce the magnetic resistance for the passage of the magnetic flux, the windings of technical transformers are placed on completely closed iron cores.

The first example of the implementation of transformers is shown schematically in Fig. 2 single-phase transformer of the so-called rod type. His primary and secondary coils c1 and c2 are located on iron rods a - a, connected at the ends with iron plates b - b, called yokes. Thus, two rods a, a and two yokes b, b form a closed iron ring, in which the magnetic flux passes, interlocking with the primary and secondary windings. This iron ring is called the transformer core.

The second example of the implementation of transformers is shown schematically in Fig. 3 single-phase transformer of the so-called armored type. In this transformer, the primary and secondary windings c, each consisting of a row of flat coils, are located on a core formed by two rods of two iron rings a and b. Rings a and b, surrounding the windings, cover them almost entirely, as it were, with armor, therefore the described transformer is called armored. The magnetic flux passing inside the windings c is split into two equal parts, each closing in its own iron ring.

The use of iron closed magnetic circuits in transformers achieves a significant reduction in the leakage flux. In such transformers, the fluxes coupled with the primary and secondary windings are almost equal to each other. Assuming that the primary and secondary windings are penetrated by the same magnetic flux, we can write the expressions based on the general induction gap for the instantaneous values of the electromotive forces of the windings:

The expressions w1 and w2 are the number of turns of the primary and secondary windings, and dФt is the magnitude of the change in the penetrating coil of the magnetic flux per time element dt, therefore there is the rate of change in the magnetic flux. From the last expressions, the following ratio can be obtained: e1 / e2 = w1 / w2

that is, the instantaneous electromotive forces indicated in the primary and secondary coils I and II relate to each other in the same way as the number of turns of the coils. The last conclusion is valid not only in relation to the instantaneous values of electromotive forces, but also to their largest and effective values.

The electromotive force induced in the primary coil, being the electromotive force of self-induction, almost completely balances the voltage applied to the same coil. If through E1 and U1 we denote the effective values of the electromotive force of the primary coil and the voltage applied to it, then we can write: E1 = U1

The electromotive force induced in the secondary coil is, in the case under consideration, equal to the voltage at the ends of this coil.

If, similarly to the previous one, through E2 and U2 we denote the effective values of the electromotive force of the secondary coil and the voltage at its ends, then we can write: E2 = U2

Therefore, by applying a certain voltage to one coil of the transformer, any voltage can be obtained at the ends of the other coil, it is only necessary to take a suitable ratio between the numbers of turns of these coils. This is the main property of the transformer.

The ratio of the number of turns of the primary winding to the number of turns of the secondary winding is called the transformation ratio of the transformer. The transformation ratio will be denoted by kt.

Therefore, we can write: Е1 / Е2 = U1 / U2 = w1 / w2 = kт

A transformer whose transformation ratio is less than one is called a step-up transformer, because its secondary voltage, or the so-called secondary voltage, is higher than the primary voltage, or the so-called primary voltage. A transformer with a transformation ratio greater than one is called a step-down transformer, because its secondary voltage is less than the primary one.

Single-phase transformer operation under load

During idle operation of the transformer, the magnetic flux is created by the current in the primary winding, or rather, by the magnetomotive force of the primary winding. Since the magnetic circuit of the transformer is made of iron and therefore has a low magnetic resistance, and the number of turns of the primary winding is usually taken to be large, the no-load current of the transformer is small, it is 5-10% of the normal one.

If you close the secondary winding to any resistance, then with the appearance of a current in the secondary winding, the magnetomotive force of this winding will also appear.

According to Lenz's law, the magnetomotive force of the secondary winding acts against the magnetomotive force of the primary winding

It would seem that the magnetic flux in this case should decrease, but if a constant voltage is applied to the primary winding, then there will be almost no decrease in the magnetic flux.

Indeed, the electromotive force induced in the primary winding when the transformer is loaded is almost equal to the applied voltage. This electromotive force is proportional to the magnetic flux. Consequently, if the primary voltage is constant in magnitude, then the electromotive force under load should remain almost the same as it was during the idle operation of the transformer. This circumstance results in almost complete constancy of the magnetic flux at any load.

The operation of a single-phase transformer under load So, at a constant primary voltage, the magnetic flux of the transformer almost does not change with a change in load and can be taken equal to the magnetic flux during idle operation.

The magnetic flux of a transformer can maintain its value under load only because with the appearance of a current in the secondary winding, the current in the primary winding also increases, and so much so that the difference between the magnetomotive forces or ampere turns of the primary and secondary windings remains almost equal to the magnetomotive force or ampere turns during idle operation ... Thus, the appearance of a demagnetizing magnetomotive force or ampere-turns in the secondary winding is accompanied by an automatic increase in the magnetomotive force of the primary winding.

Since, as mentioned above, a small magnetomotive force is required to create a magnetic flux of a transformer, it can be said that an increase in the secondary magnetomotive force is accompanied by an increase in the primary magnetomotive force that is almost the same in magnitude.

Therefore, we can write: I2w2 = I1w1

From this equality, the second main characteristic of the transformer is obtained, namely, the ratio: I1 / I2 = w2 / w1 = 1 / kt, where kt is the transformation ratio.

Thus, the ratio of the currents of the primary and secondary windings of the transformer is equal to one divided by its transformation ratio.

So, the main characteristics of the transformer are in the relationship E1 / E2 = w1 / w2 = kt and I1 / I2 = w2 / w1 = 1 / kt

If we multiply the left sides of the relations with each other and the right sides with each other, then we get I1E1 / I2E2 = 1 and I1E1 = I2E2

The last equality gives the third characteristic of the transformer, which can be expressed in words as follows: the power delivered by the secondary winding of the transformer in volt-amperes is almost equal to the power supplied to the primary winding also in volt-amperes.

If we neglect the energy losses in the copper of the windings and in the iron of the transformer core, then we can say that all the power supplied to the primary winding of the transformer from the energy source is transferred to its secondary winding, and the transmitter is the magnetic flux.

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