To convert direct current into alternating current, special electronic power devices called inverters are used. Most often, an inverter converts DC voltage of one value into AC voltage of another value.
Thus, an inverter is a generator of periodically varying voltage, and the voltage shape can be sinusoidal, close to sinusoidal or pulsed. Inverters are used both as independent devices and as part of uninterruptible power supply (UPS) systems.
As part of uninterruptible power supplies (UPS), inverters allow, for example, to obtain continuous power supply to computer systems, and if the network voltage suddenly disappears, the inverter will instantly begin to power the computer with energy received from the backup battery. At least the user will have time to shut down correctly and turn off the computer.
Larger uninterruptible power supply devices use more powerful inverters with batteries of significant capacity, capable of autonomously powering consumers for hours, regardless of the network, and when the network returns to normal again, the UPS will automatically switch consumers directly to the network, and the batteries will begin to charge.
Technical side
In modern technologies for converting electricity, an inverter can only act as an intermediate link, where its function is to convert voltage by transforming at a high frequency (tens and hundreds of kilohertz). Fortunately, today this problem can be easily solved, because for the development and construction of inverters, both semiconductor switches are available that can withstand currents of hundreds of amperes, as well as magnetic circuits with the required parameters, and electronic microcontrollers specially designed for inverters (including resonant ones).
Requirements for inverters, as for other power devices, include: high efficiency, reliability, and the smallest possible dimensions and weight. It is also necessary that the inverter maintains the permissible level of higher harmonics in the input voltage, and does not create unacceptably strong impulse noise for consumers.
In systems with “green” electricity sources (solar panels, wind turbines), Grid-tie inverters are used to supply electricity directly to the general network - inverters that can operate synchronously with the industrial network.
During operation of the voltage inverter, a constant voltage source is periodically connected to the load circuit with alternating polarity, while the frequency of connections and their duration are formed by a control signal that comes from the controller.
The controller in the inverter usually performs several functions: adjusting the output voltage, synchronizing the operation of semiconductor switches, and protecting the circuit from overload. In principle, inverters are divided into: autonomous inverters (current inverters and voltage inverters) and dependent inverters (grid-driven, Grid-tie, etc.)
Inverter circuit design
The semiconductor switches of the inverter are controlled by a controller and have reverse shunt diodes. The voltage at the inverter output, depending on the current load power, is regulated by automatically changing the pulse width in the high-frequency converter unit, in the simplest case it is.
The half-waves of the output low-frequency voltage must be symmetrical so that the load circuits in no case receive a significant constant component (for transformers this is especially dangerous); for this, the pulse width of the low-frequency block (in the simplest case) is made constant.
In controlling the output switches of the inverter, an algorithm is used that ensures a sequential change in the power circuit structures: direct, short-circuited, inverse.
One way or another, the magnitude of the instantaneous load power at the inverter output has a pulsating character with double the frequency, so the primary source must allow such an operating mode when pulsating currents flow through it, and withstand the corresponding level of interference (at the inverter input).
If the first inverters were exclusively mechanical, today there are many options for semiconductor-based inverter circuits, and there are only three typical circuits: bridge without a transformer, push-pull with zero terminal of the transformer, bridge with a transformer.
A bridge circuit without a transformer is found in uninterruptible power supply devices with a power of 500 VA or more and in automotive inverters. A push-pull circuit with a zero transformer terminal is used in low-power UPSs (for computers) with a power of up to 500 VA, where the voltage on the backup battery is 12 or 24 volts. A bridge circuit with a transformer is used in powerful uninterruptible power supplies (for units and tens of kVA).
In voltage inverters with a rectangular output, a group of switches with freewheeling diodes is switched so as to obtain an alternating voltage across the load and provide a controlled circulation mode in the circuit.
The proportionality of the output voltage is determined by: the relative duration of control pulses or the phase shift between the control signals of groups of keys. In an uncontrolled reactive energy circulation mode, the consumer influences the shape and magnitude of the voltage at the inverter output.
In voltage inverters with a step output, the high-frequency pre-converter generates a unipolar step voltage curve, roughly approximating a sinusoid in shape, the period of which is equal to half the period of the output voltage. The LF bridge circuit then turns the unipolar step curve into two halves of a multipolar curve, roughly resembling a sine wave in shape.
In voltage inverters with a sinusoidal (or almost sinusoidal) output waveform, the high-frequency preliminary converter generates a constant voltage close in magnitude to the amplitude of the future sinusoidal output.
After this, the bridge circuit forms a low-frequency alternating voltage from a direct voltage, using multiple PWM, when each pair of transistors at each half-cycle of the output sinusoid is opened several times for a time varying according to a harmonic law. The low-pass filter then extracts a sine wave from the resulting shape.
The simplest circuits for preliminary high-frequency conversion in inverters are self-generating. They are quite simple in terms of technical implementation and are quite effective at low powers (up to 10-20 W) for powering loads that are not critical to the energy supply process. The frequency of self-oscillators is no more than 10 kHz.
Positive feedback in such devices is obtained from saturation of the transformer magnetic circuit. But for powerful inverters such schemes are not acceptable, since losses in the switches increase, and the efficiency ends up being low. Moreover, any short circuit at the output disrupts self-oscillations.
Better circuits for preliminary high-frequency converters are flyback (up to 150 W), push-pull (up to 500 W), half-bridge and bridge (more than 500 W) on PWM controllers, where the conversion frequency reaches hundreds of kilohertz.
Types of inverters, operating modes
Single-phase voltage inverters are divided into two groups: with pure sine wave output and with modified sine wave. Most modern devices allow a simplified form of the network signal (modified sine wave).
A pure sine wave is important for devices that have an electric motor or transformer at the input, or if it is a special device that works only with a pure sine wave at the input.
Three-phase inverters are typically used to create three-phase current for electric motors, such as power supply. In this case, the motor windings are directly connected to the inverter output. In terms of power, the inverter is selected based on its peak value for the consumer.
In general, there are three operating modes of the inverter: starting, continuous and overload mode. In the starting mode (charging the capacity, starting the refrigerator), the power can for a split second exceed twice the inverter rating; this is acceptable for most models. Long-term mode - corresponding to the inverter rating. Overload mode - when the consumer's power is 1.3 times higher than the nominal - in this mode, the average inverter can operate for about half an hour.
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Inverter– converts direct current into alternating current.
Converter– DC to DC voltage converter, but of a different level (with intermediate conversion of input voltage to AC and transformation to the desired level).
The central link is the DC-AC voltage converter.
Various schemes of such devices are used:
Transistor and electronic tubes;
Built on transistors with saturable cores;
Relaxation generators, triggers, multivibrators;
Single-cycle, push-pull and bridge circuits;
Thyristor simple and bridge circuits (in powerful devices).
6.1 Simple circuit of a push-pull thyristor inverter.
Rice. 6.1 - simple circuit of a push-pull thyristor inverter
From T2 control pulses are sent to the thyristor circuit.
From a constant source, voltage is supplied to the input of the circuit. It goes through 
to VD anodes.
charges to double the input voltage. If you now apply pulses to VD2, VD1 immediately closes, 
recharges, all signs in T1 will change to the opposite and current will flow through VD2.
As can be seen from the operation of the circuit, on the switching capacitance 
at the moment the thyristor closes, a voltage equal to twice the supply voltage is applied, which is a disadvantage for the circuit.
It is eliminated by the bridge circuit of the thyristor inverter.
6.2 Bridge circuit of a thyristor inverter.
Rice. 6.2 - Bridge circuit of a thyristor inverter
The control circuit opens VD1 and VD4 first, and then, when the capacity is charged to 
, at this moment, if you open other thyristors, VD1 and VD4 will instantly close.
In this circuit, only the voltage of the power source acts on the closed thyristors.
Thyristor rectifiers are effective promising inverters. They are used at significant power and are currently used to replace electric machine units that convert DC energy from backup batteries into alternating current in guaranteed power supply devices (GPD) of equipment at communications enterprises.
DC-DC converters.
Often, when powering electronic devices, power supplies are low-voltage, and significant voltages are required to power consumption circuits. In this case, they resort to voltage conversion. For this, inverters and converters are used. Electromagnetic transducers, vibration transducers and static transducers are used on semiconductor devices.
Electromagnetic converters produce a sinusoidal voltage, while semiconductor and vibration converters produce a rectangular voltage. Currently, there are static converters with an output voltage that is close to sinusoidal in shape. Disadvantage of the electromagnetic converter: large dimensions and weight. Vibration converters are low-power and unreliable. Therefore, semiconductor converters with small dimensions and weight, high efficiency and operational reliability are most widely used.
The construction of converters based on thyristors and transistors should be related to the magnitude of the supply voltages, the required power, and the nature of the load change.
The energy supplied through power lines is not always used in its pure form. To perform specific tasks, it is transformed by electrical devices that change one or more parameters - type of voltage, frequency and others.
Electric power converters: classification
These devices are classified according to several criteria:
- Type of transformation.
- Type of construction.
- Controllability.
Parameters that change
The following parameters are subject to transformation:
- Voltage type – from alternating to direct and vice versa.
- Amplitude values of current and voltage.
- Frequency.
Types of structures
These devices are divided into electrical and semiconductor devices.
Electric machine (rotary) machines consist of two machines, one is the drive, and the other is the actuator. For example, to convert alternating current into direct current, an AC induction motor (driver) and a DC generator (performer) are used. Their disadvantage is their large dimensions and weight. In addition, the total efficiency of the technological combination is lower than that of a single electric machine.
Semiconductor (static) converters are built on the basis of electrical circuits consisting of semiconductor or lamp elements. Their efficiency is higher, their size and weight are small, but the quality of the output electricity is low.
Managed and unmanaged
If the amount of change in the electrical energy parameter is fixed, then an uncontrolled converter is used. Such devices are used in the first stages of power supplies. An example is a power transformer that reduces the mains voltage from 220 to 12 volts.
Converters with variable parameters are actuators in controlled electrical circuits. For example, by changing the frequency of the supply voltage, the rotation speed of asynchronous motors is controlled.
Electric power converters: examples of devices
Converters can perform either one function or several.
Changing voltage type
Those devices that convert alternating current into direct current are called rectifiers. Those that act the other way around are inverters.
If this is an electrical machine device, then the rectifier consists of an asynchronous AC motor that rotates the rotor of a DC generator. The input and output lines have no electrical contact.
The most common type of static rectifier circuit is a diode bridge. It contains four elements (diodes) with one-way conductivity, connected back to back. After it, an electrolytic capacitor must be installed, which smoothes out the pulsating voltage.
There is a hybrid design that combines electric machine and static rectifiers. This is a car generator, which is an alternating current machine, the stator windings of which are connected to a rectifier bridge with a capacitor.
Inverter circuits are used to start a continuous oscillation generator (multivibrator), built on thyristors or transistors. They are the basis of frequency converters.
Changing amplitude values
These are all types of transformers - step-down, step-up, ballast.
Controlled transformers are called rheostats. If they are connected in parallel with a source of electricity, they change the voltage. In series - current.
To absorb the heat generated during the operation of powerful high-voltage network transformers, liquid (oil) cooling systems are used.
Frequency change
Frequency converters can be either electric machine (rotary) or static.
The actuator of rotary frequency converters is a high-frequency asynchronous three-phase generator. Its rotor rotates a direct or alternating current electric motor. Like a rotary rectifier, its input and output lines do not have electrical contact.
Inverter circuits used in static frequency converters are either controlled or uncontrolled. Increasing the frequency makes it possible to reduce the size of devices. A transformer with an operating frequency of 400 Hz is eight times smaller than one operating at 50 Hz. This property is used to build compact welding inverters.
Voltage converters, widely used in everyday practice, are specialized devices designed to adjust the range and frequency of the output supply voltage. Electronic systems of this type allow you to adjust the output parameters (including the frequency of the output voltage).
The need for their use arises when it is necessary to connect devices with non-standard input characteristics. Conversion circuits can be designed as a separate unit or integrated into an existing uninterruptible power supply system. These devices are in high user demand and are also widely used to solve individual production problems.
Design
To change the level of the current supply voltage, specialized pulse converters with inductive circuits built into them are most often used. In accordance with the task facing them, all known models of converter devices are divided into the following classes:
- Inverting circuits;
- Boosting electronic units;
- Buck converters.
Regardless of the type of these devices, they all work on the same principle, providing the required functionality and quality of the generated signals. The similarity of devices of this class is most often revealed by the following characteristic features:
- Availability of its own power module;
- The switching elements included in the circuit are represented by powerful semiconductor transistors;
- Energy storage devices in the form of a separate choke or coil;
- Filter capacitors connected in parallel with the load resistance;
- Special diodes used as a blocking element.
The use of all the elements listed above in the required combinations makes it possible to obtain any of the known categories of pulse devices.
Operating principle
The operation of pulse converters is based on the principle of adjusting the signal level by changing the width of the pulses that control the operation of the switching element.
Note! This method of electronic control of signal parameters is found in various types of modern equipment and is called pulse width.
To stabilize the operating mode, feedback is introduced into the electrical circuit, due to which, when the output voltage fluctuates, the parameters of the operating pulses also change.
The simplest voltage converters are based on a conventional transformer, the output of which generates a voltage with an amplitude different from the input value.
Other types of converter devices are known that operate on a principle similar to the previously described samples, but are somewhat different in their design. They are, as a rule, based on semiconductors and allow obtaining high conversion efficiency (high efficiency).
Classification of pulse converters
Pulse converters produced by the domestic industry, in accordance with current parameters, are divided into the following classes:
- Electronic converters that provide conversion of alternating level (AC) to a constant output signal (DC). They are designed for industrial use and are used in systems where reduced supply voltages of 380/220 Volts are required;
- Inverters that perform inverse conversion: input (DC) signal to output (AC). These devices are in demand in uninterruptible power supply systems, as well as in electronic welding units, in which, as a result of inversion, it is possible to reduce the dimensions and weight of the device;
- Converter devices of constant voltage or current, allowing you to convert one value of the supply parameter to another.
These devices are often used to organize power supply to batteries when it is necessary to connect loads with different voltage ratings to them.
Converter composition
The design of pulse devices usually includes the following functional units:
- Built-in pulse signal generator, powered by its own power supply unit (PSU);
- A pulse transformer that converts signals of a given periodicity into output pulses of a higher frequency;
- Built-in stabilizers that ensure constant parameters of the signals received at the output of the devices;
- Electronic switches based on powerful transistor elements, operating in a pulsed mode close to the saturation state.
To this list should be added storage inductances used in the construction of generator circuits. They are usually included in such widely used devices as current converters.
A typical representative of component elements is a transformer, which provides voltage conversion with minimal power losses. They are widely used in the construction of a wide variety of radio-electronic and electrical circuits.
Advantages and disadvantages of converter devices
The advantages of most well-known models of converting devices include:
- High efficiency of converting standard network voltages into a user-friendly form with simultaneous control of their main parameters;
- Compactness and mobility of individual samples of inverter devices, allowing their use as automotive converters;
- Good efficiency indicators with efficiency approaching 90%;
- Versatility and reliability of converter devices, providing the ability to connect any types of consumers;
- Possibility of compensating for electricity losses by increasing the output voltage.
Important! The listed advantages of converting devices allow them to be installed in the most critical components of security and lighting systems, as well as in control modules for the operation of heating boilers, pumping stations and other special equipment.
The advantages of these devices also include the presence of such additional options as the ability to switch indicators of measured quantities from input to output voltage. Let's add to this the admissibility of adjustment within certain limits of controlled output parameters.
Quite removable disadvantages of converters of this class include sensitivity to operation in conditions of high humidity (this does not apply to models produced in a moisture-proof design). Let's add to this the high cost of conversion systems.
Use of converters in everyday life
Universal models belong to the category of the most complex devices that are capable of adjusting several parameters (current, voltage and frequency) at once. But in everyday practice, simpler samples of converters in which only one of the input indicators is regulated are quite sufficient.
Additional Information. A voltage and current control circuit designed to limit one of these parameters (usually current) is widely used in battery charging circuits. More complex devices of this class can use modern microcontrollers.
In conclusion of the review, it should be noted that there are many design options for pulse converter modules. But, regardless of the type and complexity of the electronic device, the underlying operating principles do not change. Having mastered the basic technical techniques for constructing these devices, you can learn how to handle equipment of any complexity, as well as successfully repair it in the event of a breakdown.
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