How does an autotransformer work? Autotransformers (latr). types and work. application. Operating principle of an autotransformer
Autotransformer- this is a device for changing the alternating current voltage while maintaining its frequency, based on the effect of electromagnetic induction, which has one common winding on the magnetic core and at least three terminals from it.
In simple words, autotransformers are a type of conventional voltage transformers in which there is only one winding, some of the turns of which serve as the primary winding, and some as the secondary.
For a better understanding, let's look at the design of the most common type of autotransformer.
Autotransformer device
Most often, a standard autotransformer is a toroidal magnetic circuit - a core made of electrical steel in the form of a ring, on which copper wire is wound - called a winding.
In addition, in order for this design to serve as an autotransformer, it has an additional “tap” - a tap from this winding; there are at least three contacts in total.
The structure of the autotransformer is quite clearly shown in the image below:
In this example, you can see an autotransformer, to the extreme contacts of which an AC voltage source is connected, to A - phase, to X - zero. All turns of wire between these points are considered the primary winding.
The load, some electrical device that requires less voltage to operate than supplied from the network, is connected to terminals a2 and X - the turns between these contacts are already the secondary winding.
As you can see, the autotransformer has only one winding, but the voltage, if measured at different connection points, will be different, why it changes and how to determine how much (transformation ratio) we will consider below.
Autotransformer designation on diagrams
By the way, you can quite easily identify an autotransformer in any diagram and distinguish it from a regular transformer, most often it is designated like this:
As you can see, the autotransformer schematically shows all its main elements: a straight line is a steel core, on one side of which there is a single winding - in the form of a wavy line, from which there are several taps.
It will not be confused with a regular transformer, because in its diagram there will be at least two windings on either side of the core.
I will talk in more detail about the fundamental differences between an autotransformer and a conventional voltage transformer in the second part of this article.
Operating principle of an autotransformer
And now, for a better understanding of the basic operating principle of autotransformers, let’s consider the processes that occur in them.
As an example, we will take an autotransformer, which can either increase the output voltage or decrease it relative to the initial one. For ease of calculation, the total number of turns of copper wire is 20; it looks like this:
As you can see, this model already has four connection points to the common winding: A1, a2, a3 and X.
A source of alternating electric current is connected to contacts A1 and N, for example, the power supply from a standard city electrical network, with voltage (U1), in our case it is standard 220V. There are a total of 18 turns of copper wire between these points; this section of the spiral is designated as W1; it is considered the primary winding of the autotransformer.
What happens when voltage is applied to an autotransformer?
When alternating current flows through the winding, in the core (magnetic circuit) of the autotransformer, an alternating magnetic flux is formed, which circulates through a closed magnetic core, penetrating ALL turns of the winding.
Simply put, when current is connected to the primary winding - in our example, to 18 turns, the magnetic flux flowing through the core permeates the entire winding, all 20 turns. The voltage on the primary winding (at connection points A1 and X) remains 220V or, if distributed over each turn 220/18 = 12.222... Volts for each.
Now, to find out what voltage is generated on all 20 turns, to points a2 and X, we connect a load, some electrical appliance - this will be the secondary winding of the autotransformer. In the diagram we will conditionally denote the load, a certain electrical appliance connected to this winding, the voltage is U2, and the number of turns between the contacts is W2 = 20.
The relationship between the windings of an autotransformer is expressed by the following formula:
U1/w1 = U2/w2 , where U1 is the voltage on the first winding, U2 is the voltage on the second winding, w1 is the number of turns of the first winding, w2 is the number of turns of the second winding.
From this formula it follows that the voltage on the secondary winding changes relative to the voltage of the primary winding, proportional to the difference in turns. In our example, one turn of the primary winding has 12.22..Volts, while the secondary winding has 2 more turns, respectively, the total winding voltage is 24.44..Volts higher.
This is proven by a simple calculation:
U1/w1 = U2/w2,
220 Volt/18 Turns=U2/20 Turns,
U2 = 220*20/18 = 244.44V
An autotransformer in which the voltage on the secondary winding increases is called a step-up one.
Knowing the relationship between the windings, we can calculate the transformation ratio, a value that makes it easy to determine the change in input parameters (voltage, resistance, current) on the secondary winding.
TOThe transformation coefficient is calculated using the following formula: U1/U2=w1/w2
In our case it turns out 220/244.44=18/20=0.9
Now let's see how the voltages on the remaining contacts change.
We connect the load to contacts a3 and X of our autotransformer, the number of turns w3 in this winding is 16, we denote the voltage as U3.
Following the same formula, we calculate the voltage:
U1/w1 = U3/w3 = 220/18=U3/16, from this it follows that U3 =220*16/18 = 195.55.. Volts, and the transformation ratio U1/U3=w1/w3=220/195 .55=18/16=1.125, this winding is a step-down winding.
An autotransformer in which the voltage on the secondary winding decreases is called a step-down.
Now, knowing the transformation ratios at all terminals of the autotransformer, we can easily determine, for example, what the voltage will be on the secondary winding if the voltage of the electric current source changes:
For example, when the AC source voltage on the primary winding is 200V, this transformer has:
On contacts a2 and X, with a transformation ratio k1=0.9, the voltage will be U2=200V/0.9= 222.22 V
On contacts a3 and X, with a transformation ratio k2=1.125, the voltage is U3=200/1.125=177.77 V
RULE: If the transformation ratio k>1 - then the transformer is step-down, but if k<1, то повышающий.
Most often, a standard autotransformer has a larger number of terminals than in our example, a larger number of stages for adjusting the incoming voltage or current.
The logical development of autotransformers was the emergence of the so-called ADJUSTABLE AUTOTRANSFORMERS, which do not have many additional taps with different transformation ratios, and the number of turns of the secondary winding changes by moving the moving contact along it - read more about this.
Changing the current strength in an autotransformer
There is a simple rule for current strength - The current in the higher voltage winding is less than the current in the lower voltage winding.
In other words, if a step-down tap is used from the primary winding of an autotransformer, then the current on the secondary winding will be higher and the voltage lower, and vice versa, if a step-up tap is used, then the current on the secondary winding will be lower and the voltage higher.
The powers on both windings are approximately the same, therefore, according to the OMA law:
I1U1 = I2U2, where I1 is the current in the primary winding, I2 is the current in the secondary winding, U1 is the voltage in the primary winding, U2 is the voltage in the secondary winding.
Accordingly, the current, for example, in the primary winding is calculated as follows: I1 = U2*I2/U1
Knowing how the current changes, you can select the right power cables and automatic protective equipment in advance.
Now that you are familiar with the operating principle of an autotransformer and know its design, let's look at what they are, their purpose and places of application, what their pros and cons are, and how they fundamentally differ from conventional transformers. Read all this and much more in the second part of this article. Subscribe to our VKontakte group, stay tuned for new materials!
Compared with conventional transformers, autotransformers have a number of advantages. Among the advantages we can highlight the fact that the efficiency autotransformers much higher than that of conventional transformers, the number of turns, dimensions and weight of the magnetic core are smaller, which significantly saves material and, accordingly, price autotransformers. The disadvantage is that the device using autotransformer connected to the electrical network, that is, none of the points in the circuit of such a device can be grounded. This may cause a short circuit or damage the device.
IN autotransformers there is an electrical connection in addition to the magnetic one. Thus, the design power is a part of the throughput. In conventional transformers, the entire throughput power is calculated (depending on the dimensions and weight of the transformer) due to the existence of exclusively magnetic coupling. It is most advisable to use autotransformers with a transformation coefficient having a value less than 2. If the coefficient has a greater value, autotransformers some disadvantages appear.
Nowadays, autotransformers with a power value of up to 1 kVA are widely used in household appliances and automatic devices. Autotransformers with greater power are usually used in devices with powerful AC motors - the so-called power autotransformers. Their power reaches several hundred MVA.
49 ) how is power transferred from the primary network to the secondary network in autotransfer?
In this case, the transfer of power from the primary network to the secondary network occurs, in addition to magnetic communication, also due to electricity.
51 ) Why is the cosine in the x.x mode significantly less than in the nominal mode? Explain the dependence cosf=f(U1)
52 ) What is the danger of an emergency short circuit of an autotransformer (compared to a transfer)?
Short circuit current on... V U`/U 2 =1/1-n times higher than the short-circuit current of a conventional pipeline
53) How will the mutual induction flux, leakage flux, and induced emf change when the current in the secondary winding increases?
I 2 and F - F - unchanged ... creates a magnetic leakage flux, unchanged F 2, it meshes with the screws of its own winding, inducing a leakage emf in them
54) What is a trans-ra compound group? How can it be determined from a vector diagram?
To enable a transformer for parallel operation with other transformers, the phase shift between the emf of the primary and secondary windings is important. To characterize this shift, the concept of a group of winding connections is introduced. The group of connections of transformer windings is determined by the shift angle between the vectors of the same linear EMF (for example, EAB and Eab or EBA and Eba) of the high and low voltage windings.
55) What schemes and groups of transfer connections are standard?
Star, triangle and zigzag connection patterns
Groups - 0.11
According to GOST, one standard group of connections is established for single-phase transformers - 0.
With three-phase transformers, all twelve different connection groups are possible, but it is desirable to have a minimum number of different groups, so for three-phase transformers there are only two standard groups: 11 and 0.
Group 11 corresponds to two connection methods: star/delta (Y/D) and star with neutral point/delta (Y/D).
Group 0 corresponds to one connection method: star/star with neutral point removed (Y/Y). The special sign (Y) in the second and third cases indicates that for this winding connection, the neutral point has a terminal. The numerator of the designation always indicates the method of connecting the higher voltage winding.
Group 0-Y/Y is used for transformers with higher voltage up to 35 kV inclusive with a low voltage of 230 V and a power of up to 560 kVA or at the same higher voltage limit with a low voltage of 400 V and a power of up to 1800 kV A. Both connection methods are according to group 11 are intended for more powerful transformers and higher voltages.
As an example in Fig. 108 shows how, with a Y/D connection, the vector of the lowest (secondary) line voltage U ab forms with the vector of the highest (primary) linear voltage U AB an angle of 330°, which is equal to the angle between the hands at 11 o’clock; therefore, this connection method should be classified in group 11.
56 )Draw the equivalent circuit of the transformer under load, explain the parameters and explain the quantitative relationships of the parameters?
Z n =(w 1 /w 2) 2 (r n + -jx n) - load resistance
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Uconstructionautotransformer
In general, any transformers used in electrical networks to change the voltage. So, when transmitting electricity over long distances, increasing the voltage reduces energy losses on the active transmission resistance in proportion to the square of the operating voltage.
Therefore, the voltage of the power plant generator is increased 10-15 times, transmitted via power lines, and then reduced on site in stages to power local distribution networks of various voltages. All such voltage conversions from one value to another are carried out using transformers and their varieties - autotransformers.
Main difference autotransformer from usual transformer consists in the fact that its two windings necessarily have an electrical connection with each other, they are wound on one rod, power is transferred between the windings in a combined way - by electromagnetic induction and electrical connection.
This reduces the size and cost of the machine (the reasons and calculation of this fact are given below).
An autotransformer can be made two-winding and multi-winding; each of these modifications of autotransformers necessarily contains high-voltage windings ( high voltage -- input) and CH ( medium voltage -- output), electrically connected to each other. In multi-winding models there is one or more LV windings ( low voltage), which has only inductive electromagnetic coupling with the first two.
In a three-phase autotransformer, the HV and MV windings are connected in a star with a solidly grounded neutral U 0 (point 0 in Fig. 1), and the LV windings are necessarily connected in a triangle N.
From Figure 1 it can be seen that the HV winding includes a common winding OA m , which, in fact, makes up the CH winding, and the series winding A m A .
Rice. 1. Autotransformer windings: 1-- three-phase; 2-- single-phase
The distribution of currents in an operating autotransformer in rated load mode between the windings is not the same.
In the series winding A m A, the load current HV - I A passes. According to the law of electromagnetic induction, a magnetic flux is created in the core of the autotransformer, which induces a current I Am in the MV winding.
Thus, the current of the common winding CH is formed by the sum of the currents of the series winding I A with electrical connection (HV and CH), and the current I Am, along the magnetic connection of these same windings -
I CH=I A+I Am.
The power value at the output of the autotransformer is equal to the power at its input. In the absence of a LV winding, the HV power is equal to the MV power, this is the rated power S nom of the autotransformer via electrical connection. It is equal to the product of the rated voltage of the HV winding U HV and the rated current I HV of the series winding.
The typical power of the autotransformer is also calculated, which is part of the rated power transmitted electromagnetically.
S T=S nom*A V ,
Where A V=1-U CH/U VN-- coefficient of profitability of the autotransformer.
It determines the share of typical power in the nominal power; the smaller it is, the smaller the dimensions and cross-sections of the core (magnetic core) and windings of the autotransformer, which are calculated based not on the full nominal power, but only on its part - the typical power. Therefore, the production of autotransformers is much cheaper than conventional transformers of the same power.
The power on the common winding is one of the main parameters that need to be controlled when operating an autotransformer; exceeding it in long-term mode is unacceptable.
Figure 1 shows options for connecting an ammeter to measure the load on a common winding at three-phase And single-phase autotransformer version.
The lower the transformation ratio (the closer the values of U CH and U HV), the more profitable the use of autotransformers and the cheaper their production.
Another great advantage of autotransformers is the ability to regulate voltage under load without interrupting the power supply to consumers.
Most autotransformers use a method of switching taps of the control winding. These adjusting taps are taken from the less loaded HV winding; special devices - tap switches change the number of turns included in the operation, thereby increasing or decreasing the transformation ratio and output voltage.
Such regulation is possible in manual and automatic modes (using tracking systems with feedback, this makes the autotransformer a voltage stabilizer). Requirements for the quality of output voltage for powering consumers determine the use and importance of such devices.
electricity autotransformer magnetic
Figure 2 shows the circuits for regulating the voltage of the output A m on the autotransformer on the HV side (1) and on the MV side (2). These are the design and operating principles of autotransformers.
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Main difference autotransformer from usual transformer consists in the fact that its two windings necessarily have an electrical connection with each other, they are wound on one rod, power is transferred between the windings in a combined way - by electromagnetic induction and electrical connection. This reduces the size and cost of the machine (the reasons and calculation of this fact are given below). An autotransformer can be made two-winding and multi-winding; each of these modifications of autotransformers necessarily contains high-voltage windings ( high voltage - input) and CH ( medium voltage - output), electrically connected to each other. In multi-winding models there is one or more LV windings ( low voltage), which has only inductive electromagnetic coupling with the first two. In a three-phase autotransformer, the HV and MV windings are connected in a star with a solidly grounded neutral U 0 (point 0 in Fig. 1), and the LV windings are necessarily connected in a triangle C. From Figure 1 it can be seen that the HV winding includes a common winding OA m , which, in fact, makes up the CH winding, and the series winding A m A .
The distribution of currents in an operating autotransformer in rated load mode between the windings is not the same. In the series winding A m A, the load current HV - I A passes. According to the law of electromagnetic induction, a magnetic flux is created in the core of the autotransformer, which induces a current I Am in the MV winding. Thus, the current of the common winding CH is formed by the sum of the currents of the series winding I A with electrical connection (HV and CH), and the current I Am, along the magnetic connection of these same windings - I CH =I A +I Am.
Rice. 1. Autotransformer windings: 1 - three-phase; 2 - single-phase
The power value at the output of the autotransformer is equal to the power at its input. In the absence of a LV winding, the HV power is equal to the MV power, this is the rated power S nom of the autotransformer via electrical connection. It is equal to the product of the rated voltage of the HV winding U HV and the rated current I HV of the series winding.
The typical power of the autotransformer is also calculated, which is part of the rated power transmitted electromagnetically.
S t =S nom* a in, Where and in =1-U CH /U VN- coefficient of profitability of the autotransformer. It determines the share of typical power in the rated power; the smaller it is, the smaller the dimensions and cross-sections of the core (magnetic core) and windings of the autotransformer, which are calculated based not on the full rated power, but only on its part - the typical power. Therefore, the production of autotransformers is much cheaper than conventional transformers of the same power.
The power on the common winding is one of the main parameters that need to be controlled when operating an autotransformer; exceeding it in long-term mode is unacceptable. Figure 1 shows options for connecting an ammeter to measure the load on a common winding with and option for an autotransformer.
The lower the transformation ratio (the closer the values of U CH and U HV), the more profitable the use of autotransformers and the cheaper their production.
Another great advantage of autotransformers is the ability to regulate voltage under load without interrupting the power supply to consumers. Most autotransformers use a method of switching taps of the control winding. These adjusting taps are taken from the less loaded HV winding; special devices - tap switches change the number of turns included in the operation, thereby increasing or decreasing the transformation ratio and output voltage. Such regulation is possible in manual and automatic modes (using tracking systems with feedback, this makes the autotransformer a voltage stabilizer). Requirements for the quality of output voltage for powering consumers determine the use and importance of such devices.
Figure 2 shows the circuits for regulating the voltage of the output A m on the autotransformer on the HV side (1) and on the MV side (2). These are the design and operating principles of autotransformers.
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 winding of the autotransformer 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.
