To adjust the output voltage in AC circuits with rectification, controlled rectifiers are used. Along with other methods of controlling the output voltage after the rectifier, such as LATR or a rheostat, a controlled rectifier allows you to achieve greater efficiency with high reliability of the circuit, which cannot be said about either regulation using LATR or rheostat regulation.
The use of controlled valves is more advanced and much less cumbersome. Thyristors are best suited for the role of controlled valves.
In the initial state, the thyristor is locked, and it has two possible stable states: closed and open (conductive). If the source voltage is higher than the lower operating point of the thyristor, then when a current pulse is applied to the control electrode, the thyristor will go into a conducting state, and the following pulses applied to the control electrode will not affect the anode current in any way, that is, the control circuit is only responsible for opening the thyristor, but not for locking it up. It can be argued that thyristors have a significant power gain.
To turn off the thyristor, it is necessary to reduce its anode current so that it becomes less than the holding current, which is achieved by lowering the supply voltage or increasing the load resistance.
Thyristors in the open state are capable of conducting currents up to several hundred amperes, but at the same time, thyristors are rather inertial. The turn-on time of the thyristor is from 100 ns to 10 µs, and the turn-off time is ten times longer - from 1 µs to 100 µs.
In order for the thyristor to work reliably, the rate of rise of the anode voltage should not exceed 10 - 500 V / µs, depending on the model of the component, otherwise false switching may occur due to the action of capacitive current through p-n junctions.
To avoid false inclusions, the control electrode of the thyristor is always shunted with a resistor, the resistance of which usually lies in the range from 51 to 1500 ohms.
In addition to thyristors, others are used to regulate the output voltage in rectifiers: triacs, dinistors and lockable thyristors. Dinistors are switched on by the voltage applied to the anode, and they have two electrodes, like diodes.
Triacs are distinguished by the ability to turn on control pulses, even relative to the anode, even relative to the cathode, however, all these devices, like thyristors, are turned off by reducing the anode current to a value below the holding current. As for the lockable thyristors, they can be locked by applying a current of reverse polarity to the control electrode, but the gain when turned off is ten times lower than when turned on.
Thyristors, triacs, dinistors, controlled thyristors - all these devices are used in power supplies and in automation circuits for regulating and stabilizing voltage and power, as well as for protection purposes.
As a rule, thyristors are placed in controlled rectification circuits instead of diodes. In single-phase bridges, the switching point of the diode and the switching point of the thyristor are different, there is a phase difference between them, which can be reflected by considering the angle.
The DC component of the load voltage is non-linearly related to this angle, since the supply voltage is initially sinusoidal. The constant component of the voltage at the load connected after the regulated rectifier can be found by the formula:
The control characteristic of a thyristor controlled rectifier shows the dependence of the output voltage on the load on the phase (angle) of the bridge connection:
On an inductive load, the current through the thyristors will have a rectangular shape, and at an angle greater than zero, the current will be pulled due to the action of the self-induction EMF from the load inductance.
In this case, the main harmonic of the mains current will be shifted relative to the voltage by a certain angle. To exclude tightening, a zero diode is used, through which the current can close and give a shift less than two times with respect to the angle of the bridge.
The level and frequency of the output voltage, and besides this, the stabilization of the average voltage over a certain period of time, can be coordinated by a single device, known as a rectifier of a certain type and specifics of operation. Such electrical devices are quite common and familiar to many - these are controlled AC rectifiers. The basis of such devices most often become semiconductors: diodes, thyristors, transistors, and so on.
A half-wave thyristor controlled rectifier, otherwise called a quarter-bridge, operates according to the simplest scheme (Fig. 1, a). The converter, which acts as a controlled key, is controlled by an output pulse that turns on the semiconductor. The main condition for the operation of this circuit is the switching angle, i.e., the pulse must be shifted relative to Uin = 0. As soon as the angle value exceeds zero, the thyristor rectifier comes into operation.
The VD converter stops working when the potential difference is close to zero under load Rn. The duration of the working period of the rectifier can be expressed by the following equation, focusing on the specified data, conventional units and principles of operation:
When active, the thyristor controlled rectifier automatically turns off at the moment when its voltage approaches zero. Thus, in the presence of a control signal, the duration of the on state of the thyristor rectifier is determined by the equation:
where T is the oscillation time of the input voltage Uin.
Based on this, the average value of the momentum difference can be expressed as follows:
Let's say the angle = 0°, and the period tu 1 = T/2. Then the controlled semiconductor is in working condition at a positive half-wave value of the supply potential difference.
Rice. Fig. 1. Scheme of a controlled quarter-bridge on a thyristor rectifier VD (a) and a time graph explaining the operation at various values of the switching angle a (b, c, d).
Rice. 2. Thyristor rectifier control circuit (a) and timing diagram explaining the operation (b)
For example, a \u003d l / 4 tu 2 \u003d (T / 2) (3/4) \u003d 3 T / 8, which corresponds to a reduction in the period tu 1 of switching on the semiconductor by a quarter, i.e., by 25%, etc.
The operation of a controlled thyristor rectifier VD is shown in the graphs shown in fig. 1, b-d.
With the minimum value of the angle of switching on the converter a = 0 (Fig. 1, b), the average voltage at the load Un, cp is expressed by the maximum value equal to Un, сро = Um/l. At a = l/2 (Fig. 1, d) the potential difference (Un, sr) l/2 = 0.5(Un, sr)o = Um/2 l.
Let us assume that at minimum load values, the angle a = l (Fig. 1, d), but with a gradual increase in these indicators, the angle a decreases (Fig. 1, c), then due to the extension of tu, the voltage drop at the rectifier output is compensated to a constant Un , cf. This allows you to smooth out the current ripple - to straighten it. This type of control is called vertical, or pulse-phase. Most often, this control principle is found in low-power amplifiers and measuring instruments.
The thyristor rectifier control circuit must generate turn-on pulses at certain time points specified by the value of the angle a. Stability is achieved by generating pulses with peak performance. The simplest solution is to use low voltage dynistor peak generators.
The simplest circuit for controlling a thyristor rectifier using a peak generator is shown in fig. 2, a. This circuit includes an oscillator of relaxation oscillations on a dinistor (capacitor Su and dinistor VD 2 connected in parallel), which also generates short pulses to control the rectifier VD 1.
With the passage of half-waves of positive values \u200b\u200bin the supply voltage Uin, the capacitor Su begins to charge. The charge accumulates until the moment when the voltage Uс on the capacitor does not reach Uvd 2, on, which is enough to switch the dinistor VD 2. At this time t = t1 (Fig. 2, b), the dinistor opens and becomes a conductor with a very low indicator output resistance. Due to this, a discharge occurs in the capacitor Su, passing through the dinistor VD 2 to the resistor Ru and the control transition of the rectifier VD 1 (Fig. 2, 6). The discharge period is determined by lowering the current in the dinistor to the value I off. At this time, the dinistor again comes into a closed state - a cut-off, and the capacitor Su can again begin to accumulate charge iypr.
As soon as the current icontrol changes (Fig. 2, b), the period of accumulation of the charge of the capacitor Su up to the voltage Uvd 2, incl. also changes, which is expressed in the shift of control pulses relative to time values (Fig. 2, b). Thus, the angle a can be changed, and after it, the magnitude of the output voltage can be influenced. This is the phase-pulse method of controlling the thyristor rectifier type converter.
This method is suitable for single-phase and multi-phase converters.
Rice. 3. Scheme of a single-phase controlled full-wave thyristor rectifier with a CLC filter (a) and a timing diagram explaining the operation (b)
The operation of a full-wave thyristor controlled converter, also called a stabilizer or electric valve, is shown in fig. 3, a.
The principle of control is carried out by supplying an unlocking voltage. With a stable voltage, the circuit acts as a full-wave mid-point rectifier (a pair of quarter-bridges). The average potential difference at the output is determined by the equation:
When the angle a, which controls the voltage in a half-cycle, is rotated, a delay occurs, and only a part of the voltage is supplied to the filter at the input (Fig. 3, 6). The dependence in this case is expressed as Ui, сp = F (a):
It follows that the average potential difference at the output of the rectifiers depends on the change in the angle a: Uav, max. = 2Um/l - Usr, min. = 0.
Converters that use input transformers are controlled by thyristors in the circuit. Such a scheme is especially good for the use of step-down transformers, because with U1\u003e U2 \u003d I1< I2. При таких показателях можно разработать устройство, основанное на тиристорном звене VD 1, работающем при низком вольтаже на выпрямители, и оно будет весьма компактным. Диодное же звено VD 2 можно построить на диодах Шотки. Это наиболее продуктивная схема с высокой эффективностью работы источников вторичного питания.
Functional diagram thyristor rectifiers for arc welding in a generalized and simplified form is shown in rice. 19.13. A distinctive element in the above diagram is the presence of a thyristor rectifier unit. This makes it possible to use it as a current regulator RT. Due to the time shift of the control pulse ( see fig. 19.3, b) supplied to the thyristor unit, form the current-voltage characteristic of the rectifier and carry out its adjustment to the specified mode of continuous or pulsed operation. For these purposes, the BFIU phase-pulse control block is provided in the source circuit. Through the same block, the feedbacks from the arc to the current regulator are also closed.
Thyristor rectifiers, as a rule, are characterized by high stabilization of voltage and current of the arc with changes in the supply voltage, arc length and ambient temperature.
Rice. 19.13. Functional diagram of arc rectifiers with thyristor current controllers
Rectifiers of the VSVU-VSP and VDU-VDG types have become widespread. At present, these are the main rectifiers for arc welding.
In rectifiers of the VSVU - VSP type, the phase control principle consists in the formation of a sawtooth voltage U c , comparing it with the control voltage U y and the subsequent formation of rectangular pulses. On rice. 19.14 the voltage map of the control pulse generation block is shown. Low values of control voltage U y = min (option a) ensure the opening of thyristors in the power unit at α = max. In this case, the minimum output parameters of the source are realized. The maximum values of the control voltage U y = max (option b) correspond to the minimum thyristor opening angles α = min and, accordingly, the maximum output parameters.
Rice. 19.14. Voltage map of the pulse forming unit: Uc - sawtooth voltage; Uу - control voltage; U0 - voltage ns thyristors
According to the principle of “vertical control” of thyristors, well-known, large-scale rectifiers for arc welding with steeply dipping (VSVU series) and gently dipping (VSP series) current-voltage characteristics have been developed. A single circuit diagram of these sources is implemented in the form of unified blocks.
A simplified schematic diagram of power supplies of the VSVU type is shown on rice. 19.15, a. A three-phase transformer T has one primary winding W 1 and two secondary windings W 2 and W 2v. Winding W 2 is connected to a thyristor rectifier V (RT), which acts as a current regulator and has a low-voltage characteristic. From the secondary winding W 2v, the voltage is supplied to the diode rectifier unit V in, which forms an auxiliary power source with a steeply falling current-voltage characteristic using linear chokes L B. The auxiliary source is designed for arc ignition, welding at low currents, provides feedback signals, etc. During welding, the arc is powered simultaneously from both sources. The combination of two sources made it possible to significantly reduce the open-circuit voltage of the main source and form steeply falling external characteristics in the region of operating currents ( rice. 19.15, b).
Rice. 19.15. Sources of the VSVU series: a - circuit diagram; b - current-voltage characteristics
VSP type power sources are designed for mechanized consumable electrode welding. In this regard, the pulse formation unit receives signals from the current and voltage control unit. Typical current-voltage characteristics of VSP series sources are given in rice. 19.16. In the range of 30-60 V, the voltage is smoothly regulated. To improve the dynamic properties of the characteristics, the angle of its inclination is changed.
Rice. 19.16. Volt-ampere characteristics of VSP series sources
In rectifiers of the VDU type, the thyristor phase-pulse control unit consists of three main elements ( rice. 19.17, a):
· Node for generating six-phase sinusoidal voltage (7);
· unit for generating a constant control voltage (2);
· unit for generating and amplifying control signals (3).
Rice. 19.17. Thyristor control circuits: a - electrical; b - positive signal formation
The control voltage Uу is the sum of two back-to-back DC voltages: the bias voltage Ucm and the adjustable reference voltage U3.
The bias voltage is used to stabilize the output parameters of the rectifier when the mains voltage fluctuates. The regulated reference voltage is part of the stabilized voltage and is varied by a resistor. On rice. 19.17, b shows the formation of a positive signal applied to the input of the amplification unit, and the formation of a thyristor control signal at two different setting voltages U 3l and U 32 . With a change in U 3, the phase and duration of the positive gsh nal at the input of the amplification unit (α 1 and α 2) change, which leads to a change in the opening angle of the thyristors and control of the source operation mode.
The circuit diagram of rectifiers for arc welding of the VDU type is shown on rice. 19.18, a. Transformer T has two secondary windings connected in two reverse stars through an equalizing reactor L yp . Thyristors V 1 - V 6 are included in each phase of the secondary windings. Linear inductor L smoothes the ripple of the rectified current and forms the dynamic properties of the source. A magnetic amplifier MU was used as a current sensor. A feedback signal proportional to the welding current is taken from the resistor R oc . External typical current-voltage characteristics of the considered rectifiers are given in rice. 19.18, b.
Rice. 19.18. VDU type rectifiers: a - electrical circuit diagram; b - current-voltage characteristics.
8. Thyristor amplifiers with pulse-phase control
With this control method, pulses are used as a control signal, the duration of which, as a rule, does not exceed the half-cycle of the supply voltage. Considering that the turn-on time of the thyristor is short, short-term pulses with a duration from several units to hundreds of microseconds are usually used to control it. The amplitude of the control current pulses must exceed the rectification control current I U.S.
By changing the phase of the control pulses within 0<α<π, регулируют напряжение в нагрузке от максимального значения до нуля. При этом методе управления полностью исключается влияние разброса входных параметров тиристора, температуры окружающей среды и p-n переходов, а также формы питающего напряжения на характеристики вход-выход усилителя. К достоинствам фазового метода управления следует отнести также малые потери в управляющем переходе тиристора благодаря кратковременности управляющего импульса. Этот метод получил наибольшее распространение в тиристорных усилителях любой мощности.
In order to control the voltage in the network, electronic rectifiers are used. These devices work by changing the frequency. Many modifications are allowed to be used in the AC mains.
The main parameters of rectifiers include conductivity. It is also worth considering the indicator of permissible overvoltage. In order to understand the issue in more detail, it is necessary to consider the rectifier circuit.
Device modifications
The rectifier circuit involves the use of a contact thyristor. The stabilizer, as a rule, is used as a transition type. In some cases, it is installed with a protection system. There are also many modifications on triodes. These devices operate at a frequency of 30 Hz. They are good for collectors. The rectifier circuit also includes low conductance comparators. Their sensitivity corresponds to an indicator of at least 10 mV. A certain class of devices is equipped with a varicap. Due to this modification, it can be connected to a single-phase circuit.
How it works?
As mentioned earlier, the rectifier works by changing the frequency. Initially, the voltage falls on the power thyristors. The current conversion process is carried out using a triode. To avoid overheating of the device, there is a stabilizer. When wave interference appears, the comparator is activated.
Scope of devices
Most often, devices are installed in transformers. There are also modifications for drive modules. Do not forget about the automated devices that are used in production. In modulators, rectifiers play a role. However, in this case, much depends on the type of device.
Existing types of modifications
By design, semiconductor, thyristor and bridge modifications are distinguished. A separate category includes power devices that can operate at increased frequency. Full-wave models are not suitable for these purposes. Additionally, rectifiers are distinguished by phase. Today you can find one-, two- and three-phase devices.
Semiconductor Models
Semiconductor rectifiers are great for many modifications are available on the basis of connector capacitors. Their input conductivity does not exceed 10 microns. It is also worth noting that semiconductor rectifiers differ in sensitivity. Devices up to 5 mV can be used at 12 V.
Their protection systems are of class P30. Adapters are used to connect modifications. At a voltage of 12 V, the reload parameter is on average 10 A. Modifications with plates are distinguished by a high operating temperature parameter. Many devices are capable of running on transistors. Filters are used to reduce distortion.
Features of thyristor devices
The thyristor rectifier is designed to regulate the voltage in the DC network. If we talk about modifications of low conductivity, then they use only one triode. when loaded at 2 A, it is at least 10 V. The protection system for the presented rectifiers is used, as a rule, class P44. It is also worth noting that the models are well suited for power conductors. How does a thyristor rectifier transformer work? First of all, the voltage goes to the relay.
DC conversion occurs due to the transistor. Capacitor blocks are used to control the output voltage. Many models have multiple filters. If we talk about the shortcomings of rectifiers, it is worth noting that they have high heat losses. When the output voltage is over 30V, the overload rate is greatly reduced. Additionally, it is worth considering the high price of a thyristor rectifier.
Bridge modifications
Bridge rectifiers operate at a frequency of not more than 30 Hz. The control angle depends on the triodes. Comparators are mainly mounted through diode conductors. Models are not suitable for power equipment in the best way. For modules, stabilizers with a low-resistance adapter are used. If we talk about the minuses, then one should take into account the low conductivity at high voltage. Protection systems, as a rule, are applied in class P33.
Many modifications are connected through a dipole triode. How does the transformer work on these rectifiers? Initially, voltage is applied to the primary winding. At a voltage of more than 10 V, the converter is switched on. The frequency change is carried out using a conventional comparator. In order to reduce heat losses, a varicap is installed on the bridge controlled rectifier.
Power devices
Power rectifiers have recently been considered very common. The overload indicator at low voltage does not exceed 15 A. The protection system is mainly used by the P37 series. Models are used for step-down transformers. If we talk about design features, it is important to note that the devices are produced with pentodes. They stand out for their good sensitivity, but they have a low operating temperature setting.
Capacitor blocks are allowed to be used at 4 microns. An output voltage above 10 V activates the converter. Filters are usually used on two insulators. It is also worth noting that there are many rectifiers with controllers on the market. Their main difference lies in the ability to operate at frequencies above 33 Hz. In this case, the average overload corresponds to 10 A.
Full-wave modifications
A full-wave single-phase rectifier is capable of operating at different frequencies. The main advantage of the modifications lies in the high operating temperature parameter. If we talk about design features, it is important to note that power thyristors are used of an integral type, and their conductivity does not exceed 4 microns. At a voltage of 10 V, the system outputs an average of 5 A.
Protection systems are quite often used in the P48 series. Modifications are connected via adapters. It is also worth noting the disadvantages of rectifiers of this class. First of all, this is a low susceptibility to magnetic vibrations. The overload parameter can sometimes change quickly. At frequencies below 40 Hz, current drops are felt. Experts also note that the models are not able to work on one filter. Additionally not suitable for devices
Single phase devices
A single-phase controlled rectifier is capable of performing many functions. Models are most often installed on power transformers. At a frequency of 20 Hz, the overload parameter does not exceed 50 A on average. The protection system for rectifiers is of class P48. Many experts say that the models are not afraid of wave interference and do an excellent job with impulse surges. Are there any disadvantages of this type of models? First of all, they relate to low current at high load. To solve this problem, comparators are installed. However, it should be borne in mind that they cannot work in an alternating current circuit.
In addition, periodically there are problems with current conductivity. On average, this parameter is 5 microns. Reducing the sensitivity greatly affects the performance of the triode. If we consider single-phase uncontrolled rectifiers, then their linings are used with an adapter. Many models have multiple insulators. It is also worth noting that rectifiers of this type are not suitable for step-down transformers. Stabilizers are most often used for three outputs, and their maximum voltage should not exceed 50 V.
Parameters of two-phase devices
Two-phase rectifiers are produced for DC and AC circuits. Many modifications are operated on contact-type triodes. If we talk about the parameters of modifications, then it is worth noting the low voltage at high overloads. Thus, the devices are not well suited for power transformers. However, the advantage of the devices is considered to be good conductivity.
The sensitivity of the models starts from 55 mV. At the same time, heat losses are insignificant. Comparators are used on two plates. Quite often, modifications are connected through one adapter. In this case, the insulators are preliminarily checked for output resistance.
Three-phase modifications
Three-phase rectifiers are actively used in power transformers. They have a very high overload parameter and are able to operate in high frequency conditions. If we talk about design features, it is important to note that the models are assembled with capacitor units. Due to this modification, it is allowed to connect to the DC circuit and not be afraid of wave interference. Impulse jumps are blocked by filters. Connection through an adapter is carried out using a converter. Many models have three insulators. The output voltage at 3 A should not exceed 5 V.
Additionally, it should be noted that rectifiers of this type are used for large network overloads. Many modifications are equipped with blockers. The frequency reduction occurs with the help of comparators, which are installed above the capacitor box. If we consider relay transformers, then an additional adapter is required to connect modifications.
Models with contact comparator
Controlled rectifiers with a contact comparator have recently been in great demand. Among the features of the modifications, it is worth noting the high degree of overload. Protection systems are mainly applied class P55. Devices with one capacitor box work. At a voltage of 12 V, the output current is at least 3 A. Many models boast high conductivity at a frequency of 5 Hz.
Stabilizers are often used low-resistance type. They perform well in AC circuits. In production, rectifiers are used for work. The permissible conductivity level for them is no more than 50 microns. The operating temperature in this case depends on the type of dinistor. As a rule, they are installed with several plates.
Devices with two comparators
Electronic rectifiers with two comparators are valued for their high output voltage parameter. With an overload of 5 A, modifications are able to work without heat loss. The smoothing factor for rectifiers does not exceed 60%. Many modifications have a high-quality protection system of the P58 series. First of all, it is designed to cope with wave interference. At a frequency of 40 Hz, devices give out an average of 50 microns. Tetrodes for modifications are used of a variable type, and their sensitivity is no more than 10 mV.
Are there any disadvantages of this type of rectifiers? First of all, it should be noted that they must not be connected to step-down transformers. In the DC network, the models have a small conductivity parameter. The operating frequency corresponds on average to 55 Hz. Modifications are not suitable for single-pole stabilizers. To use the devices on power transformers, two adapters are used.
The difference between modifications with an electrode triode
Controlled rectifiers with electrode triodes are valued for their high output voltage setting. At low frequencies, they operate without heat loss. However, it should be borne in mind that the overload parameter is on average 4 A. All this suggests that rectifiers are not capable of operating in a DC network. Filters may only be used on two covers. The output voltage is generally 50V, and the protection system is P58 class. In order to connect the device, an adapter is used. The smoothing factor for rectifiers of this type is at least 60%.
Models with capacitive triode
Capacitive triode controlled rectifiers are capable of operating in a DC network. If we consider the parameters of the modifications, we can note the high input voltage. In this case, the overload during operation will not exceed 5 A. The protection system uses class A45. Some modifications are suitable for power transformers.
In this case, much depends on the capacitor unit, which is installed in the rectifier. According to experts, the nominal voltage of many modifications is 55 V. The output current in the system is 4 A. Filters for modifications are suitable for alternating current. The smoothing factor for rectifiers is 70%.
Channel triode devices
Controlled rectifiers with channel triodes are characterized by a high degree of conductivity. Models of this type are great for step-down transformers. If we talk about the design, it is worth noting that the models are always made with two connectors, and their filters are used on insulators. According to experts, the conductivity at a frequency of 40 Hz does not change much.
Are there any disadvantages of these rectifiers? Heat losses are the weak side of modifications. Many experts note the low conductivity of the connectors that are installed on the rectifiers. To solve the problem, kenotrons are used. However, they must not be used on a DC network.
Difference of modifications
12V rectifiers are only used for step down transformers. Comparators in devices are installed with filters. The maximum overload of modifications is no more than 5 A. Protection systems are quite often used class P48. They are great for overcoming wave interference. Still often used converter stabilizers, which have a high smoothing factor. If we talk about the disadvantages of modifications, then it is worth noting that the output current in the devices is no more than 15 A.
Thyristor AC voltage regulator.
Thyristor regulators are widely used to regulate the alternating voltage in the load. The simplest diagram of such a regulator is shown in Fig. 5.17 a. Two thyristors VS1 and VS2 are connected in anti-parallel to the load circuit Z H . Each thyristor operates on its own half-cycle (positive or negative). Moreover, they open with a control angle α (Fig. 5.17, b), and close at the moment the load current passes through zero. By adjusting the angle α, you can adjust the voltage U H over a wide range from U H max \u003d U c to U H min \u003d 0.
Rice. 5.17. Scheme (a) and timing diagrams (b) of the AC voltage regulator
However, this method of regulation greatly distorts the shape of the voltage curve and changes the phase of its first harmonic.
There is a growing group of energy consumers who need a regulated output
tension. Thyristor rectifiers are used to power such consumers:
single-phase at low current consumption and three-phase high power.
On fig. 2.12, but a diagram of a single-phase controlled rectifier with an output
zero point of the transformer. Thyristors VS1 and VS2 are used as valves in the rectifier.
When indicated in Fig. 2.12, and the polarity of the secondary voltage u2 of the transformer Tr, the thyristor VS1 can pass current in ", provided that a control signal Iy1 is received at its control electrode. This signal is supplied with a phase shift with respect to the moment of natural unlocking by an angle α, called the control angle ( Fig. 2.12, b) The moment of natural unlocking of the thyristor is the moment when a positive voltage appears between the anode and cathode of the thyristor (at α = 0).
When the thyristor is turned on with an active load Rn at the time ωt = α
the voltage at the load un increases abruptly to the value un "= u2" (with an ideal thyristor and an ideal transformer). When ωt = π, the valve current and the load current become equal to zero, the thyristor VS1 is locked. Before the thyristor VS2 is unlocked, a currentless pause appears in the load, energy is not transferred to the load. At the moment ωt = π + α, a control pulse is applied to the thyristor VS2, the thyristor opens, voltage un "" is applied to the load. The current flows through the lower half-winding of the transformer, thyristor VS2 and the load, keeping the same direction. At the moment ωt = 2 π thyristor VS2 is turned off.
Average load voltage
The decrease in the average voltage Uav with an increase in the angle α is shown in fig.
2.12, c. Dependence Uср(α) is called the regulating characteristic of the rectifier.
The phase delay of the control signals applied to the thyristors is carried out using pulsed phase control systems.