Autotransformers (latr). types and work

In the laboratory stands of my college, laboratory autotransformers (LATRs) regularly fail. It so happened that through trial and error I managed to master the technology for repairing them. At the moment, I have already managed to repair three laboratory autotransformers, and I rewound the LATRs in my dorm room. I would be glad if the technology for rewinding LATRs outlined here turns out to be useful to someone. Yes, this is my first article, so don't judge too harshly :-)

First, a short course on the LATR device (see picture).

LATR has two windings connected in series. The primary winding is supplied mains voltage(this must be taken into account when rewinding). The secondary winding is connected to the primary. It is designed for voltage from 0-240 V. Voltage is applied to terminals A and N in the magnetic circuit, creating a magnetic flux that induces current in the windings taken from terminals A1 and N.

Let's start by determining the diameter of the wire. This can be done using a caliper. To do this, you first need to measure the diameter of the original wire, and then, based on this, look for the wire that suits us. You can take a piece of old wire and then compare it with the desired sample.

Then you need to determine the length of the wire. This can be done using the usual mathematical expression: L=lturn×W 1.2 cm,

where L is the required wire length (in centimeters), lturn is the length of one turn; W 1.2 - number of turns of secondary and primary winding.

1) Calculation of the number of turns using formulas. This method is quite simple, but there is a high probability of errors, for example in calculations or in measurements of the area of ​​the magnetic circuit window. This method is given below:

Find the power of the autotransformer: P=U×I,

where U - output voltage, I - maximum current load (usually written in LATR).

The overall power is: Рг=1.9* Sc * S,

where 1.9 is the coefficient used for toroidal transformers.

Required number of turns per 1 volt:

K = 35/Sc, where 35 is the coefficient used for torroidal transformers.

Determine the number of turns; W1 = U1*K

We determine the dimensions of the core: Sc=((Dc-dc)/2)×h, So=πxd2/4,

where Sc is the area of ​​the transformer core; So is the window area.

2) The second option is quite labor-intensive, but reliable (when rewinding LATRs, I used this method). This method of determining the number of turns is that you need to unwind the old winding and at the same time count the number of turns. It requires: a leaf and a handle so as not to get lost, a reel or piece of wood to wind the old winding there, as well as nerves of steel and patience so as not to throw it out the window after a hundred counted turns.

After this, we rest and relax after the work done, because then we need maximum attentiveness and patience. When you've rested, let's start cooking. workplace. It is desirable that it be well lit and that all the necessary items can be placed, for example, a desk with a lamp or a chair in a room with good lighting.

For ease of rewinding, it is better to first wind the new wire on a wooden block as shown in the picture:

There is no fundamental difference in how the wire is laid out on the inner diameter of the window. But in order to lay the required number of turns, it is necessary to wind the first turn tightly to it, then wind the second turn, and lay the third turn on top between the first and second and repeat until we have wound the required number of turns at a voltage of 220V. After this, we make a terminal for the network terminal and wind the secondary winding from this terminal. On the outer diameter of the magnetic circuit window, all turns must be laid sequentially one by one as shown in the figure.

After rewinding is completed, the winding must be impregnated with varnish to improve the insulating properties and to secure the wound wire in its place. Since you don’t need a lot of varnish here, you can use any varnish that is resistant to temperatures up to 105 o C. After impregnation with varnish, leave the autotransformer to dry for a couple of hours. For best effect, you can place it in a warm place. Leave the room where the work was carried out and it is very advisable to open the window for ventilation.

After drying, it is necessary to make a path for stress relief. This can be done using a knife or sanding paper. We make a path from the outer window to the inner one about 3 cm long (shown in the figure below).

LATR - laboratory adjustable autotransformer - one of the types of autotransformers, which is an autotransformer of relatively low power, and is intended for regulation AC voltage (alternating current), supplied to the load from a single-phase or three-phase alternating current network.

The LATR, like any other network transformer, is based on an electrical steel core. But on the toroidal core of the LATR, unlike other types of network transformers, there is only one winding (primary), part of which can act as a secondary winding, and the number of turns of the secondary winding can be quickly adjusted by the user, this is the distinctive feature of the LATR from simple autotransformers .

To regulate the number of turns per secondary winding, the design of the autotransformer includes a rotary knob, to which a sliding carbon brush is connected. When you turn the handle, the brush slides from turn to turn along the winding, this is how it is adjusted.

One of the secondary terminals of the laboratory autotransformer is directly connected to the sliding brush. The second secondary pin is common to the network input side. Consumers are connected to the output terminals of the LATR, and its input terminals are connected to a single-phase or three-phase electrical network. In a single-phase LATR there is one core and one winding, and in a three-phase one there are three cores, and each has one winding.

The voltage at the output of the LATR can be either greater than the input or less, for example, for a single-phase network, the adjustable range is from 0 to 250 volts, and for a three-phase network, from 0 to 450 volts. It is noteworthy that the efficiency of LATR is higher, the closer the output voltage is to the input voltage, and can reach 99%. Output voltage form - .

On the front panel of the LATR there is a secondary circuit voltmeter for the ability to quickly monitor overload and more accurately set the output voltage. The LATR housing has ventilation holes through which natural air cooling of the magnetic core and winding occurs.

Laboratory autotransformers are used in laboratories for research purposes, for testing AC equipment, and simply for manual stabilization of the network voltage if it is currently below the required nominal value.

Of course, if the voltage in the network constantly fluctuates, then an autotransformer will not save you; you will need a full-fledged stabilizer. In other cases, LATR is just what you need to fine-tune the voltage for the task at hand. Such tasks may be: setting up industrial equipment, testing highly sensitive equipment, setting up radio-electronic devices, powering low-voltage equipment, charging batteries, etc.

Since LATR has only one winding, common to the primary and secondary circuits, the current of the secondary winding turns out to be common to the primary and secondary circuits. From this point of view, it is obvious that the secondary winding current and the primary current in the common turns are oppositely directed, therefore the total current is equal to the difference between the currents I1 and I2, that is, I2 – I1 = I12 is the current in the common turns. So it turns out that when the value of the secondary voltage is close to the input voltage, the common turns can be wound with a wire of a smaller cross-section than in the case of a two-winding transformer.

The design feature of the LATR forces us to separate the concepts of “throughput power” and “design power”. Design power is that which is transmitted from the primary winding to the secondary circuit by means of electromagnetic induction through the core, as in a conventional two-winding transformer, and throughput power is the sum of throughput power and that power that is transmitted only through the electrical component, that is, without the participation of the magnetic component induction in the core.

It turns out that in addition to the calculated power, purely electric power, equal to U2*I1. This is why autotransformers require a smaller magnetic core to transmit the same power compared to conventional two-winding transformers. This is the reason for the higher efficiency of autotransformers. In addition, less copper is required for the wire.

So, with a small transformation ratio, LATR can boast the following advantages: efficiency up to 99.8%, smaller magnetic core size, lower material consumption. And all this is due to the presence of an electrical connection between the primary and secondary circuits. On the other hand, the absence between the circuits leads to the danger of phase current from the output terminals of the LATR and even from one of the terminals, so you need to be extremely careful when working with a laboratory autotransformer.

Half a century ago, the laboratory autotransformer was very common. Today, the electronic LATR, the circuit of which every radio amateur should have, has many modifications. Old models had a current-collecting contact located on the secondary winding, which made it possible to smoothly change the value of the output voltage, made it possible to quickly change the voltage when connecting various laboratory instruments, changing the heating intensity of the soldering iron tip, adjusting electric lighting, changing the speed of the electric motor and much more. LATR is of particular importance as a voltage stabilization device, which is very important when setting up various devices.

Modern LATR is used in almost every home to stabilize voltage.

Today, when electronic consumer goods have flooded store shelves, purchase reliable regulator voltage became a problem for a simple radio amateur. Of course, you can also find an industrial design. But they are often too expensive and bulky, and this is not always suitable for home use. So many radio amateurs have to “reinvent the wheel” by creating an electronic LATR with their own hands.

Simple voltage regulation device

One of the simplest LATR models, the diagram of which is shown in Fig. 1, is also accessible to beginners. The voltage regulated by the device is from 0 to 220 volts. The power of this model is from 25 to 500 W. The regulator power can be increased to 1.5 kW; for this, thyristors VD1 and VD2 should be installed on radiators.

These thyristors (VD1 and VD2) are connected in parallel with the load R1. They pass current in opposite directions. When the device is connected to the network, these thyristors are closed, and capacitors C1 and C2 are charged through resistor R5. The magnitude of the voltage received at the load is changed as necessary using a variable resistor R5. It, together with capacitors (C1 and C2), creates a phase-shifting circuit.

Rice. 2. Scheme of LATR, which provides sinusoidal voltage without interference in the system.

The peculiarity of this technical solution is to use both half-cycles of alternating current, so the load uses full power rather than half.

The disadvantage of this circuit (the price to pay for simplicity) is that the shape of the alternating voltage at the load is not strictly sinusoidal, which is due to the specific operation of thyristors. This may cause interference on the network. To eliminate the problem, in addition to the circuit, you can install filters in series with the load (chokes), for example, take them from a faulty TV.

The electronic LATR circuit allows you to regulate the voltage from 0 to 220V. The load power can be in the range from 25 to 1000 W, if you install thyristors T1 and T2 on radiators, then output power can be increased to 1.5 kW.

The main elements of the circuit are thyristors; they alternately pass current in one direction or the other. When the regulator is connected to the network at the first moment, both thyristors are closed, and the capacitors are infected through R5.

The load voltage is set using variable resistor, which together with capacitors C1 and C2 form a phase-shifting chain. Thyristors are controlled by pulses generated by dinistors T3 and T4.

At some point, which is determined by the resistance of the part of resistor R5 connected to the circuit, one of the dinistors will open. The discharge current of the capacitor connected to it will flow through it, so the corresponding thyristor will open after the dinistor. Current will flow through the thyristor and, accordingly, through the load. At the moment the sign of the half-cycle changes, the thyristor closes and a new cycle of charging the capacitors begins, but in reverse polarity. Now the second dinistor and the second thyristor will open.

This circuit uses both half-cycles of the AC current, so full power is supplied to the load rather than half.

Literature - Bastanov V.G. 300 practical advice. Moscow: Moscow Worker Publishing House, 1982

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For smooth adjustment Autotransformers (LATR) serve as AC voltage in various works related to electrical engineering. They are most often used to change voltage in household appliances and construction.

An autotransformer is one of the types of transformers. The two windings in this device are directly connected to each other. As a result, two types of communication appear between them, one of which is electromagnetic and the other electrical. The coil has several terminals with different voltage output values. The difference from a conventional transformer is increased efficiency due to a partial change in power.

Design features

Transformers are electrical equipment with more than 2 or more windings, which have an inductive coupling that serves to change electrical energy by voltage.

There can be only one winding for an autotransformer, or several windings covered by a magnetic flux, wound on a core with ferromagnetic properties, for other transformers.

Today, 1-phase transformers (LATP) have gained popularity. This is a laboratory version of a transformer in which both windings are not isolated from each other, but have a direct connection, therefore, in addition to electromagnetic communication, they have an electrical connection. Such a common coil is equipped with several terminals. At their output you can get different voltages.

Principle of operation

Due to their design features, autotransformers can produce both low and high voltage. The figure shows circuits of autotransformers with voltage reduction and increase.

If you connect an alternating current source to X and “a,” a magnetic flux is created. At this moment, a potential difference is induced in the turns of the coil same value. As a result, between X and “a” an EMF appears equal to the value of the EMF of the 1st turn multiplied by the number of turns of the winding located in the interval between these points.

When the consumer load is connected to the coil to terminals X and “a”, the secondary coil current will flow through the section of the winding between these points. Keeping in mind that the primary and secondary currents overlap each other, an insignificant current will flow between X and “a”.

Due to this feature of the operation of the autotransformer, the main part of the winding is made of wire of small cross-section, which reduces its cost. If it is necessary to change the voltage within small limits, then it is advisable to use such autotransformers (LATR).

Types of autotransformers

Several types of autotransformers have found application:

• VU-25 - B, serves to smooth out secondary currents in the protective circuits of transformers.
• ATD— power 25 watts, long-lasting, has an old design and is little used.
• LATR - 1, suitable for 127 volt applications.
• LATR - 2, used with a voltage of 220 volts.
• DATR - 1, serves for weak consumers.
• RNO- for heavy loads.
• ATCN used in measuring television devices.

Autotransformers are also divided by power:

• Low power, up to 1000 volts;
• Medium power, over 1000 volts;
• Power.

Laboratory autotransformers

This design option is used in low voltage networks to regulate voltage in laboratories. Such single-phase LATRs are made of a ferromagnetic core in the form of a ring, on which one layer of insulated copper wire is wound.

In several places of the winding, conclusions are made in the form of branches. This makes it possible to use such devices as autotransformers with the ability to increase or decrease the voltage with a constant transformation ratio. On top of the winding there is a narrow path on which the insulation is cleaned. A roller or brush contact moves along it, allowing the secondary voltage to smoothly change.

Vitkov short circuits this does not happen in such laboratory autotransformers, since the load and network currents in the winding are directed towards each other and are close in value. LATR capacities range from 0.5 to 7.5 kVA.

Three-phase transformers

In addition to other design options, there are also three-phase versions of autotransformers. They have either three or two windings.

They are most often connected in the form of a star with a separate neutral point. A star connection makes it possible to reduce the voltage calculated for the insulation of the device. To reduce the voltage, power is supplied to terminals A, B, C, and the output is obtained at terminals a, b, c. To increase the voltage, everything is done the other way around. Such transformers are used to reduce the voltage level when starting powerful electric motors, as well as to regulate voltage in stages in electric furnaces.

High-voltage autotransformers are used in high-voltage network systems. The use of autotransformers optimizes the efficiency of energy systems, makes it possible to reduce the cost of energy transportation, but at the same time contributes to an increase in short circuit currents.

Operating modes

• Autotransformer.
• Combined.
• Transformer.

If the operating requirements of autotransformers are met, including compliance with oil temperature control, it can operate for a long time without overheating or breakdowns.

Advantages and disadvantages

The following advantages can be highlighted:

• The advantage can be called high efficiency, because only a small part of the transformer power is converted, and this matters when the output and input voltages differ by a small amount.
• Reduced consumption of copper in coils, as well as steel core.
• The reduced dimensions and weight of the autotransformer make it possible to create good conditions transportation to the installation site. If greater power of the transformer is required, then it can be manufactured within the permissible limitations of dimensions and weight for transportation by transport.
• Low cost.
• Smooth voltage removal from the movable current-collecting contact connected to the winding.

Disadvantages of autotransformers:

• Most often, the coils are connected with a star with a neutral, which is grounded. Connections according to other schemes are also possible, but their implementation causes inconvenience, as a result of which they are rarely used. The neutral must be grounded through a resistance or using a blind method. But we must not forget that the grounding resistance should not allow the potential difference across the phases to exceed the moment when any one phase is short-circuited to ground.
• The increased potential for overvoltage during a thunderstorm at the input of the autotransformer makes it necessary to install arresters that do not turn off when the line is turned off.
• Electrical circuits are not isolated from each other (primary and secondary).
• Dependence of low voltage on high voltage, resulting in failures and surges high voltage have an impact on low voltage stability.
• Low leakage flux between primary and secondary windings.
• Insulation of both windings has to be done for high voltage, since there is an electrical connection between the windings.
• Autotransformers of 6-10 kilovolts cannot be used as power ones with the voltage reduced to 380 volts, because people have access to such equipment, and due to an accident, the voltage from the primary winding can get to the secondary winding.

Application

Autotransformers have a wide range of uses in various fields of human activity:

• In low-power devices for setting up, powering and testing industrial and household electrical equipment and devices automatic control, in laboratory conditions on stands (LATRs), in communication devices and equipment, etc.
• Power versions of 3-phase autotransformers are used to reduce the starting current of electric motors.
• In the energy industry, powerful models of autotransformers are used to connect high-voltage networks with networks of similar voltages. The transformation ratio in such devices usually does not exceed 2 - 2.5. To change the voltage on an even larger scale, other devices are required, and the use of autotransformers becomes impractical.
• Metallurgy.
• Utilities.
• Production of equipment.
• Petroleum and chemical production.
• Educational institutions use LATRs to demonstrate experiments in physics and chemistry lessons.
• Surge Protectors.
• Auxiliary equipment for machines and recorders.

How to choose an autotransformer

First, determine where the autotransformer will be used. If for testing power equipment at an enterprise, then one model is needed, but for powering a car radio during repairs, then a completely different one.

• Power. It is necessary to calculate the load of all consumers. Their total power should not exceed the power of the autotransformer.
• Adjustment interval . This parameter depends on the action of the device, that is, to increase or decrease. Most often, devices are of the voltage-reducing type.
• Supply voltage . If you want to connect an autotransformer to your home network, then it is better to purchase a device for 220 volts, and if for a 3-phase network, then for 380 volts.

With such a device you can change the mains voltage values ​​and set the values ​​that are needed for specific type loads.