In various situations it may be necessary to convert the frequency of the source current into a voltage of adjustable frequency. This is required, for example, when operating asynchronous motors to change their rotation speed. This article will discuss the purpose and principle of the frequency converter.
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What is a frequency converter
Frequency converter (FC) - is an electrical device, which converts and smoothly regulates single-phase or three-phase alternating current with a frequency of 50 Hz in a similar type of current with a frequency of 1 to 800 Hz. Such devices are widely used to control the operation of various asynchronous-type electrical machines, for example, to change their speed. There are also devices for use in industrial high-voltage networks.
Simple converters regulate frequency and voltage according to the V/f characteristic, complex devices use vector control.
A frequency converter is a technically complex device and consists not only of a frequency converter, but also has overcurrent, overvoltage and short-circuit protection. Such equipment may also have a choke to improve the signal shape and filters to reduce various electromagnetic interferences. A distinction is made between electronic converters as well as electromechanical devices.
Principle of operation of a frequency converter
Electronic converter consists of several main components: rectifier, filter, microprocessor and inverter.
The rectifier has a bundle of diodes or thyristors, which rectify the initial current at the input to the converter. Diode inverters are characterized by complete absence of ripple, are inexpensive, but reliable devices. Thyristor-based inverters allow current to flow in both directions and allow electrical energy to be returned to the grid when the motor is braked.
Filter is used in thyristor devices to reduce or eliminate voltage ripple. Smoothing is done with capacitive or inductive-capacitive filters.
The microprocessor - is the control and evaluation unit of the inverter. It receives and processes the signals from the sensors, allowing the output signal from the inverter to be controlled by the integrated PID controller. This component also records and stores event data, registers and protects the unit from overloads, short-circuits, analyzes the operating mode and shuts down the unit in case of emergency operation.
Inverter voltage and current is used to control electrical machines, i.e. for smooth regulation of current frequency. Such device outputs "pure sine", which allows you to use it in many industries.
The principle of operation of the electronic frequency converter (inverter) is as follows:
- The input sinusoidal AC single-phase or three-phase current is rectified by a diode bridge or thyristors;
- Special filters (capacitors) are used to filter the signal to reduce or eliminate voltage ripple;
- The voltage is converted into a three-phase wave with certain parameters by means of a microcircuit and a transistor bridge;
- At the output of the inverter, rectangular pulses are converted into a sinusoidal voltage with specified parameters.
Types of frequency converters
There are several types of frequency converters, which are currently the most common for production and use:
Electro-machine (electro-induction) converters: are used when it is impossible or impractical to use electronic frequency converters. Structurally, such devices are asynchronous motors with phase-wound rotor, which operate in generator-converter mode.
These devices are converters with scalar control. At the output of this device a voltage of a given amplitude and frequency is created to maintain a certain magnetic flux in the stator windings. They are used when it is not necessary to maintain the rotor speed depending on the load (pumps, fans and other equipment).
Electronic converters: Widely used in all operating conditions for a variety of equipment. Such devices are vector devices, they automatically calculate the interaction of stator and rotor magnetic fields and provide a constant rotor speed value regardless of the load.
- Cycloinverters;
- Cycloinverters;
- DC frequency inverters with a dc intermediate link:
- Current source frequency converter;
- Frequency converter of voltage source (with amplitude or pulse width modulation).
In terms of application, the equipment can be:
- For equipment up to 315 kW;
- vector converters for power up to 500 kW;
- explosion-proof devices for use in explosive and dusty conditions;
- frequency converters mounted on electric motors;
Each type of frequency converter has certain advantages and disadvantages and is applicable to different equipment and loads, as well as working conditions.
The frequency converter can be controlled manually or externally. Manual control is performed from the control panel of the frequency converter, which can adjust the speed or stop the operation. External control is performed by means of automatic control systems (ACS), which can control all parameters of the device and allow switching the circuit or mode of operation (via frequency converter or bypass). Also, external control allows to program the converter operation depending on operating conditions, load, time, which allows to work in automatic mode.
What the electric motor may need the frequency converter for
Application of frequency converters allows to reduce energy costs, depreciation costs of motors and equipment. It is possible to use them for cheap motors with squirrel cage rotor, which reduces production costs.
Many electric motors operate under conditions of frequently changing operating modes (frequent starts and stops, varying loads). Frequency converters allow a smooth start-up of the electric motor and reduce the maximum starting torque and heating of the equipment. This is important, for example, in hoisting machines and helps to reduce the negative effects of abrupt starts, as well as eliminating swinging of the load and jerks when stopping.
With the help of FC it is possible to smoothly regulate the work of blower fans, pumps and allows to automate technological processes (used in boilers, mines, oil production and refineries, in water supply stations and other plants).
The use of frequency converters in conveyors, conveyors, elevators can increase the service life of their units, because it reduces jerks, shocks and other negative factors when starting and stopping the equipment. They can smoothly increase and decrease frequency of motor rotation, perform reverse motion, which is important for a large number of high-precision industrial equipment.
Advantages of frequency converters:
- Reduced energy costs: by reducing inrush currents and adjusting motor power based on load;
- Increase equipment reliability and durability: allows you to extend service life and extend the period from one maintenance to the next;
- Allows the introduction of external control and management of equipment from remote computer devices and the ability to embed in automation systems;
- Frequency converters can operate with any load power (from one kilowatt to tens of megawatts);
- The presence of special components in frequency converters allows you to protect against overloads, phase loss and short circuits, as well as to ensure safe operation and shutdown of equipment in case of emergency.
Of course, looking at such a list of advantages, one may wonder why they are not used for all motors in the enterprise? The answer is obvious, alas, but it is the high cost of frequency converters, their installation and commissioning. Not every enterprise can afford these costs.
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