Frequency converter (electric drive)

Frequency asynchronous frequency converter is used to convert mains three-phase or single-phase alternating current with a frequency of 50 (60) Hz into three-phase or single-phase current, frequency from 1 Hz to 800 Hz.

The industry manufactures frequency converters of electrical induction type, which is a design of an induction motor with a phase- rotor , operating in a generator-converter mode, and electronic type converters.

Electronic type frequency converters are often used to smoothly control the speed of an asynchronous electric motor or a synchronous motor by creating an electrical voltage at the output of a given frequency. In the simplest cases, the regulation of frequency and voltage occurs in accordance with a given V / f characteristic, in the most advanced converters, the so-called vector control is implemented.

An electronic type frequency converter is a device consisting of a rectifier (DC bridge) that converts industrial frequency alternating current to direct current, and an inverter (converter) (sometimes with PWM ) that converts direct current into alternating frequency and amplitude. Output thyristors ( GTO ) or transistors ( IGBT ) provide the necessary current to power the motor.

To improve the shape of the output voltage between the converter and the motor, a choke is sometimes placed, and to reduce electromagnetic interference, an EMC filter.

Electronic frequency converter consists of circuits, which include a thyristor or transistor , which operate in the mode of electronic switches. At the heart of the control part is a microprocessor , which provides control of power electronic keys, as well as solving a large number of auxiliary tasks (monitoring, diagnostics, protection).

Depending on the structure and principle of operation of the electric drive, there are two classes of frequency converters:

1. With direct communication.
2. With a pronounced intermediate DC link.

Each of the existing classes of transducers has its advantages and disadvantages, which define the area of ​​rational use of each of them.

In direct coupled converters, the electrical module is a controlled rectifier. The control system alternately unlocks the thyristor groups and connects the motor windings to the mains.

Thus, the output voltage of the converter is formed from the "cut out" sections of the sinusoids of the input voltage. The frequency of the output voltage of such converters can not be equal to or higher than the frequency of the supply network. It is in the range from 0 to 50 Hz, and as a result - a small range of engine speed control (no more than 1:10). This restriction does not allow the use of such converters in modern frequency-controlled drives with a wide range of control of technological parameters.

The use of unlocked thyristors requires relatively complex control systems that increase the cost of the converter. The “cut” sinusoid at the output of the converter with direct coupling is a source of higher harmonics, which cause additional losses in the electric motor, overheating of the electric machine, reduction in torque, very strong interference in the supply network. The use of compensating devices leads to an increase in the cost, mass, dimensions, decrease in the efficiency of the system as a whole.

The most widely used in modern frequency-controlled modules are converters with a pronounced intermediate DC link . In converters of this class, double electric energy conversion is used: input sinusoidal voltage with constant amplitude and frequency is rectified in a rectifier, filtered by a filter, smoothed, and then again converted by an inverter into an alternating voltage of variable frequency and amplitude. Double conversion of energy leads to a decrease in efficiency and to a certain deterioration of mass-dimensional parameters in relation to converters with direct coupling.

For the formation of a sinusoidal AC voltage, an autonomous inverter is used , which generates a voltage of a given shape on the motor windings (as a rule, using the pulse-width modulation method). As electronic switches in inverters, lockable thyristors GTO and their improved modifications of GCT, IGCT, SGCT, and bipolar transistors with an insulated gate IGBT are used .

The main advantage of thyristor frequency converters, as well as in the circuit with a direct connection, is the ability to work with large currents and voltages, while maintaining a continuous load and pulse effects. They have a higher efficiency (up to 88%) in relation to converters on IGBT transistors .

Frequency converters are nonlinear load, creating currents of higher harmonics in the supply network, which leads to a deterioration in the quality of electricity.

• Inverter (electrical engineering)
• Umformer
• Variable frequency drive

• WFD 39-1.10-052-2001 Guidelines for the selection and use of asynchronous variable frequency drives up to 500 kW