18 autotransformers design features operating principle. Autotransformer - device, economical principles of operation and regulation. What happens when voltage is applied to an autotransformer

Autotransformer- a transformer variant in which the primary and secondary windings are connected directly, they are wound on one rod, power is transferred between the windings in a combined way - by electromagnetic induction and electrical connection. The autotransformer winding has several terminals (at least 3), by connecting to which, you can receive different voltages.

In some cases it may be necessary to change the voltage within small limits. The easiest way to do this is not with two-winding transformers, but with single-winding ones, called autotransformers. If the transformation ratio differs little from unity, then the difference between the magnitude of the currents in the primary and secondary windings will be small. What happens if you combine both windings? The result is an autotransformer circuit (Fig. 1).

Autotransformers are classified as special purpose transformers. Autotransformers differ from transformers in that their low-voltage winding is part of a higher-voltage winding, that is, the circuits of these windings have not only a magnetic, but also a galvanic connection.

Depending on the inclusion of the windings of the autotransformer, you can get an increase or decrease in voltage.

Rice. 1 Schemes of single-phase autotransformers: a - step-down, b - step-up.

If you connect an alternating voltage source to points A and X, then an alternating magnetic flux will appear in the core. In each of the turns of the winding an EMF of the same magnitude will be induced. Obviously, between points a and X an emf will arise equal to the emf of one turn multiplied by the number of turns enclosed between points a and X.

If you attach some load to the winding at points a and X, then the secondary current I2 will pass through part of the winding and precisely between points a and X. But since the primary current I1 also passes through the same turns, both currents will add up geometrically, and a very small current will flow through the section aX, determined by the difference between these currents. This allows part of the winding to be made from thin wire to save copper. If we take into account that this section makes up the majority of all turns, then the copper savings are quite noticeable.

Thus, it is advisable to use autotransformers for a slight decrease or increase in voltage, when a reduced current is installed in the part of the winding that is common to both circuits of the autotransformer, which allows it to be made with a thinner wire and save non-ferrous metal. At the same time, the steel consumption for the manufacture of the magnetic core, the cross-section of which is smaller than that of the transformer, is reduced.

In electromagnetic energy converters - transformers - the transfer of energy from one winding to another is carried out by a magnetic field, the energy of which is concentrated in the magnetic circuit. In autotransformers, energy is transferred both by a magnetic field and by electrical connection between the primary and secondary windings.

Electrical consumers need to transform the current to the required voltage value. If such changes are not detected within a small limit, a special unit can be used. A conventional transformer has two coils. A special device may have only one combined winding. This is an autotransformer. It is used if the conversion index is not more than 1.

In this case, the difference between the current level in the primary and secondary windings will be small. What an autotransformer is, as well as the basic principles of its operation, will be discussed further.

Device principle

Autotransformers are characterized by a specific device and principle of operation. Their first winding is part of the second circuit or vice versa. Such circuits are characterized by electromagnetic and galvanic coupling. Increasing and decreasing units are used in many areas of human activity. Moreover, its characteristics are determined by the characteristics of the windings.

When connected to an AC coil, a magnetic flux is detected in the core. In each of the existing turns, an electromotive force will be induced at this moment. Moreover, its value will be identical.

The autotransformer diagram explains the operating principle of the unit. When a load is connected, the secondary electrical flow will move through the winding. At this moment, the primary current also moves along the same conductor. Both flows add up geometrically. Therefore, a very small electric current will be supplied to the winding.

Peculiarities

The equivalent circuit of the autotransformer allows you to save on the amount of copper conductor. For such equipment, wire of a smaller cross-section is required. This ensures significant savings in materials and a relatively low cost of the device. It is possible to reduce the cost of manufacturing the presented equipment by reducing the amount of steel for the manufacture of the magnetic drive. Power transformers and autotransformers differ significantly in the size of the core cross-section.

The design of a modern autotransformer makes the equipment in demand if the transformation coefficient approaches 1 or is in the range from 1.5 to 2. If the coefficient is greater than 3, the use of such a device becomes unjustified.

In many respects, the operating principle of an autotransformer, its design and parts differ little from conventional two-winding transformers.

Various operating modes of autotransformers can eliminate the shortcomings of the household electrical network. This is necessary, for example, when the voltage does not reach or, conversely, slightly exceeds the standard norm of 220 V. The design features of the autotransformer allow adjustments to be made in a certain step. An electronic autotransformer, which includes a switching and control system, performs this process automatically.

Varieties

The choice of type of autotransformer is influenced by its purpose and operating conditions. The eight types of units presented are most often used:

VU-25-B. Designed to equalize secondary winding currents when using a differential protection circuit for power transformers.
ATD. The power is at 25W. Has an outdated design type. It takes a long time to saturate and is used quite rarely.
LATR-1. The operating principle of this autotransformer allows it to be used with a 127V load.
LATR-2. Manufactured for household network (220V). In LATR, it is possible to regulate the voltage using a contact sliding along the turns of the coil.
DATR-1. Used for light loads in special equipment.
RNO. Used under high load conditions.
RNT. Operated under the heaviest loads in special-purpose networks.
ATNC. Used for telemetering instruments.

There is also a division into low power units (up to 1 kV), medium power units (more than 1 kV) and power types.

Single-phase varieties

Today, single-phase and three-phase autotransformers are used. In the first case, the equipment is represented by such a variety as LATR. It is used for low-voltage networks. At higher voltages, a step-down design is required, for example, an autotransformer of the 220/110 or 220/100 type. In this case, the secondary winding is part of the primary circuit. The step-up type of autotransformers, on the contrary, includes the primary winding in the secondary circuit.

In both types of devices, regulation is carried out by sliding the movable contact along the winding turns. LATRs consist of a ring-shaped magnetic drive. Its winding includes one layer. It consists of an insulated copper wire.

Single-phase autotransformers have several taps that extend from the winding. It is these design elements that determine whether the unit will operate to increase or decrease the network voltage. To achieve smooth adjustment of the secondary voltage, a small track is created on the surface of the winding. It is cleared of the insulation layer. A roller or brush contact moves along this track. Adjustment is carried out within the range from 0 to 250 V.

Three-phase varieties

Along with single-phase devices, three-phase devices are also used. They differ in the type of winding. There is a three-phase type autotransformer with two and three circuits.

Most often, the windings in such devices are connected in the form of a star. They have a separate neutral point. Using the direction of voltage supply, a decrease or increase is performed. This principle is the basis for starting the operation of a powerful engine and regulating the electric current using a step system. The three-phase type of autotransformers is used for heating elements of furnaces.

Devices with three windings are used in high-voltage networks. In this case, on the higher voltage side, the unit is connected to the neutral wire in a star. This type of contact can reduce voltage, taking into account the insulation characteristics of the equipment. The use of such devices can increase the level of system efficiency, as well as save costs for electricity transmission. However, in this case the number of short circuit currents increases.

The presence of a galvanic connection between combined circuits does not allow the use of the presented equipment in power networks (6-10 kV) if the voltage drops to 0.38 kV. In this case, three-phase voltage 380V is supplied directly to electrical consumers. People can work on such equipment. To avoid accidents, other types of units are used in such conditions.

Flaws

Before putting the presented equipment into operation, it is necessary to study its main disadvantages:

A low voltage type circuit will be significantly affected by high voltage levels. To avoid network failure, you will need to create a well-designed low voltage supply system. Only in this case will the device be able to withstand increased loads.
The flux dissipated between the windings is negligible. If certain faults occur, a short circuit may occur. Its probability in this case increases significantly.
The connections that are made between the secondary and primary windings must be identical. Otherwise, some problems may occur during the operation of the unit.
It is impossible to create a system with grounding on one side. Both blocks must have a neutral.
The presented system makes it difficult to maintain electromagnetic balance. To improve this indicator, you will need to increase the body of the device. If the transformation range is significant, resource savings will be negligible.

It should also be noted that when repairing an autotransformer, eliminating problems and emergencies that have arisen, the safety of the operating personnel may be reduced. Higher voltage can also be observed on the lower winding. In this case, all elements of the system will be connected to the high-voltage part. According to safety rules, this state of affairs is unacceptable. In this case, there is a possibility of breakdown of the insulation of conductors that are connected to electrical equipment.

Having examined the main features of the operation and design of autotransformers, we can draw conclusions about the advisability of using them for our own purposes.

Purpose, design and principle of operation of autotransformers

In some cases it may be necessary to change the voltage within small limits. The easiest way to do this is not with single-winding transformers, called autotransformers. If the transformation ratio differs little from unity, then the difference between the magnitude of the currents in the primary and secondary windings will be small. What happens if you combine both windings? The result is an autotransformer circuit (Fig. 1).

Depending on the inclusion of the windings of the autotransformer, you can get an increase or decrease in voltage.

Rice. 1 Schemes of single-phase autotransformers: a - step-down, b - step-up.

If you attach some load to the winding at points a and X, then the secondary current I2 will pass through part of the winding and precisely between points a and X. But since the primary current I1 also passes through the same turns, then both currents will add up geometrically, and a very small current will flow through section a X, determined by the difference between these currents. This allows part of the winding to be made from thin wire to save copper. If we take into account that this section makes up the majority of all turns, then the copper savings are quite noticeable.

Transformer and autotransformer

Autotransformers successfully compete with two-winding transformers when their transformation ratio differs little from unity and but more than 1.5 - 2. With a transformation ratio over 3, autotransformers do not justify themselves.

In terms of design, autotransformers are practically no different from transformers. There are two windings located on the cores of the magnetic core. The conclusions are taken from two windings and a common point. Most autotransformer parts are structurally no different from transformer parts.

Laboratory autotransformers (LATRs)

Autotransformers are also used in low-voltage networks as laboratory low-power voltage regulators (LAVR). In such autotransformers, voltage regulation is carried out by moving the sliding contact along the turns of the winding.

Laboratory adjustable single-phase autotransformers consist of a ring-shaped ferromagnetic magnetic circuit wound with one layer of insulated copper wire (Fig. 2).

Several permanent branches are made from this winding, which allows these devices to be used as step-down or step-up autotransformers with a certain constant transformation ratio. In addition, on the surface of the winding, cleared of insulation, there is a narrow path along which a brush or roller contact is moved to obtain a continuously adjustable secondary voltage ranging from zero to 250 V.

When adjacent turns are closed in the LATR, no turn short circuits occur, since the network and load currents in the combined winding of the autotransformer are close to each other and directed in the opposite direction.

Laboratory autotransformers are manufactured with a rated power of 0.5; 1; 2; 5; 7.5 kVA.

Laboratory autotransformer (LATR)

Three-phase autotransformers

Along with single-phase two-winding autotransformers, three-phase two-winding and three-phase three-winding autotransformers are often used.

In three-phase autotransformers, the phases are usually connected in a star with a lead-out neutral point (Fig. 3). If it is necessary to reduce the voltage, electrical energy is supplied to terminals A, B, C and removed from terminals a, b, c, and when the voltage increases, vice versa. They are used as devices for reducing voltage when starting powerful engines, as well as for stepwise voltage regulation at the terminals of electric furnaces.

Rice. 3. Diagram of a three-phase autotransformer with a star connection of the winding phases with a removed neutral point

Three-phase high-voltage three-winding transformers are also used in high-voltage electrical networks.

Three-phase autotransformers, as a rule, are connected in a star with a neutral wire on the high voltage side. The star connection provides a voltage reduction for which the autotransformer insulation is designed.

The use of autotransformers improves the efficiency of power systems, reduces the cost of energy transmission, but leads to an increase in short circuit currents.

Disadvantages of autotransformers

The disadvantage of an autotransformer is the need to insulate both windings at a higher voltage, since the windings are electrically connected.

A significant drawback of autotransformers is the galvanic connection between the primary and secondary circuits, which does not allow their use as power in networks of 6 - 10 kV when the voltage is reduced to 0.38 kV, since a voltage of 380 V is supplied to the equipment on which people work.

In case of accidents, due to the presence of an electrical connection between the windings in an autotransformer, the higher voltage may be applied to the lower winding. In this case, all parts of the operating installation will be connected to the high-voltage part, which is not allowed due to service safety conditions and due to the possibility of breakdown of the insulation of conductive parts of the connected electrical equipment.

Depending on the inclusion of the windings of the autotransformer, you can get an increase or decrease in voltage.

Rice. 1 Schemes of single-phase autotransformers: a - step-down, b - step-up.

An autotransformer is a transformer in which the low voltage winding is part of the high voltage winding (Fig. 7.6).

A single-phase autotransformer has only one winding. In idle mode, the autotransformer is no different from a conventional transformer. In load mode, a current flows through the common part of the turns, which is equal to the current difference ( i 1 - i 2 ), since the secondary current weakens the magnetic flux in the core (i.e., the corresponding magnetic flux has a sign opposite to that of the flux created by the primary winding current).

Most often, autotransformers are made with a sliding contact, which allows you to smoothly regulate the output voltage over a wide range. An example is a laboratory autotransformer (LATR) (Fig. 7.7, a).

The winding of this transformer is made of round wire on a toroidal steel core. On one end side, the insulation is removed along with part of the wire itself, but the turns remain isolated from each other (Fig. 7.7, b). A small brush slides along the bare surface of the turns, connecting the load to a different number of turns and thereby changing the output voltage. Since the moving brush short-circuits 1 - 2 turns, then if there is good contact between them, these turns can burn out. To prevent this from happening, the brush is made of graphite, the resistance of which is high enough to weaken the currents in short-circuited turns.

Copper cut

.Insulation (enamel)

If part of the autotransformer winding is made primary, and the entire winding is secondary, then the autotransformer will be a step-up one.

15. Current and voltage transformers.

In high current and high voltage technology, electrical quantities are measured only through measuring transformers - current transformers And voltage transformers, since direct measurements using shunts and additional resistors are very difficult. Thus, the highest current that can still be measured by directly turning on the device is 600 A, and the voltage is 2000 V. In addition, shunts and additional resistances are bulky and expensive, and touching such devices in high-voltage networks is life-threatening.

Current transformer consists of a core and two windings - primary and secondary (Fig. 7.8).

The primary winding, which contains a small number of turns, is connected in series with the load in the circuit of which the current must be measured, and an ammeter is connected to the secondary winding, with a large number of turns. Since the resistance of the ammeter is small, we can assume that the current transformer operates in short circuit mode, in which the total magnetic flux is equal to the difference in the fluxes created by the primary and secondary windings.

The measured current, flowing through the low-resistance primary winding, creates a very small voltage drop across it, which is transformed into the secondary winding. Since the number of turns of the secondary winding is much greater than that of the primary, it produces a significantly higher voltage at a lower current.

A current transformer is used not only to determine current strength, but also to switch on the current windings of wattmeters and some other devices. The terminals of the current transformer windings are marked as follows: primary winding - L1 and L 2 (line), secondary - I1 and I 2 (meter). In Fig. 7.8 also shows a schematic symbol for a current transformer.

The same current transformer can be used to simultaneously turn on several measuring instruments (Fig. 7.9), but it is desirable that there are no more than two of them. This is explained by the fact that as the number of devices increases, their total resistance increases, and the operating mode of the current transformer increasingly moves away from the short circuit mode (the secondary winding current decreases).

The current transformer not only expands the measurement limits of devices, but also galvanically separates the secondary circuit from the primary, thereby isolating the device from high network voltages. Therefore, measuring instruments are mounted in the usual way on distribution boards. At the same time, for safety, one terminal of the secondary winding is grounded so that if the insulation between the windings breaks down, the wire with a high potential is shorted to ground. Current transformers are manufactured in such a way that the rated current of the secondary winding is 5 A.

The secondary winding of an operating current transformer must not be opened or left open. It should always be shorted or shorted to the device. This should be done because when the secondary winding is open, the magnetic flux in the core is determined only by the large primary current, and not by the difference in the fluxes of the primary and secondary currents This big magnetic thread will create There is a high voltage on the secondary winding that is dangerous to life. In addition, a high magnetic flux can cause the core to overheat.

Structurally, current transformers are designed differently. All of them, as a rule, have several transformation ratios. The most convenient portable current transformer is a clamp meter (Fig. 7.10).

This is a split-core transformer mounted in the same housing with an ammeter. When you press the handle, the core opens and it wraps around the wire with the measured current. After releasing the handle, a special spring tightly closes the core, and the ammeter shows the current strength in the wire. In this case, the wire with the measured current acts as the primary winding. Clamp meters are very convenient because they allow you to measure current anywhere in the line without breaking the wire, although the accuracy of such measurements is low.

A voltage transformer consists of a core and two windings - primary and secondary (Fig. 7.11).

The primary winding contains significantly more turns than the secondary. The measured voltage U1 is applied to the primary winding, and a voltmeter is connected to the secondary winding. Since the resistance of the voltmeter is high, a small current flows through the secondary winding, and we can assume that the voltage transformer is operating in no-load mode, that is, changes in the secondary voltage are proportional to changes in the primary at a constant transformation ratio. The phase of the secondary voltage is opposite to the phase of the primary. The terminals of the voltage transformer are designated as follows: terminals of the primary winding - A, X, secondary terminals - a, x. All voltage transformers

manufactured in such a way that the rated voltage of the secondary winding is 100 V.

For the safety of operating personnel, one terminal of the secondary winding and the steel casing of the voltage transformer must be grounded so that if the insulation between the windings breaks down, the wire with a high potential is shorted to ground. Structurally, voltage transformers are very similar to low-power power transformers.