Frequency inverters for asynchronous motors: operating principle and operation

Today, in industry, frequency converters for asynchronous motors are often used. It is worth noting that such motors have in their design three windings, which are connected according to the "star" or "triangle" scheme. But they have one drawback - it is very difficult to regulate the rotation speed of the rotor. But that was before. Now, when micro and power electronics come to the rescue, this task is simplified. Turning the variable resistor can change the speed of rotation in a wide range.

What is the purpose of the frequency converter?

There are many functions for this device, but most often a small amount is used. In fact, to control an induction motor, you need to be able to adjust not only the speed of rotation, but also the acceleration and deceleration times. In addition, any system requires protection. It is necessary that the frequency converter takes into account the current consumed by the induction motor.

Frequent use of frequency in ventilation systems. Despite the apparent ease of the fan impeller, the loads on the rotor are very large. And instant acceleration is impossible. There are also situations in which it is necessary to increase the speed of rotation so that the airflow becomes larger or smaller. But this is just an example, the frequency converter is often used in other systems. With the help of the chastotnik you can synchronize the speed of the conveyor, consisting of several tapes.

How the inverter works

The basis is microprocessor control and several circuits for converting AC and DC voltages. Several processes occur with a voltage that is applied to the power input of the device. The operation of the frequency converter is not complicated, it is sufficient to consider three steps. First, straightening occurs. Secondly, filtration. Third, inverting is the conversion of DC to AC.

Only at the last stage is it possible to change the properties and parameters of the current. By varying the current characteristics, it is possible to adjust the rotational speed of the rotor of the induction motor. In the inverter cascade, powerful assemblies of transistors are used. These elements have three outputs - two power, and one manager. The magnitude of the signal applied to the latter depends on the current-voltage characteristic at the output of the chastotnik.

How can I replace the inverter?

Frequency inverters for asynchronous motors have been used relatively recently. But science went to them gradually, at first the speed of rotation of the rotor was changed by means of gears or a variator. True, this control was very cumbersome, and the power of the drive was wasted due to unnecessary mechanisms. The belt drive helped to increase the speed of rotation, but here it was very difficult to specify the final parameter. For these reasons, the use of a frequency converter is much more advantageous, since it avoids power losses. But most importantly - it makes it possible to change the parameters of the drive without making any changes to the mechanics.

Which drive should I choose to use at home?

It is worth noting that the connection can be made to a single- and three-phase current network . It all depends on the particular IF model, and more specifically, on what type of inverter inverter circuitry was used in production. To understand the principle of operation, it is enough to look at the structure of the device. The very first node is a rectifier, which is assembled on semiconductor diodes. It is a bridge circuit for converting a single- or three-phase alternating current into a constant one. To use the house you need to choose those models of chastotniki, whose input is connected to a single-phase alternating current network. The choice is connected with the fact that it is problematic to conduct a three-phase network in private houses, and it is disadvantageous, because it is necessary to use more sophisticated electricity meters.

The main IF nodes

Little was said about what the scheme of the frequency converter is. But for detailed study it is necessary to consider it in more detail. At the first stage, conversion is carried out - rectification of alternating current. Regardless of how many phases are input to the input (three or one), at the output of the rectifier you get a constant unipolar (one plus and one minus) voltage of 220 volts. It is so much between phase and zero.

Next comes the filter block, which helps to get rid of all the variables of the rectified current. And at the very last stage there is an inverting - from the DC current is made variable with the help of power transistors controlled by the microcontroller. As a rule, frequency converters for asynchronous motors have a monochrome LCD display, on which the necessary parameters are displayed.

Can I make the device myself?

The manufacture of this device is associated with many difficulties. You need to learn the basics of programming microcontrollers to expand the capabilities of the device. It is important to take into account all the basic requirements. For example, the possibility of automatic emergency shutdown when exceeding the maximum permissible current, which is consumed by the motor. To do this, it is necessary to install current transformers at the output, which will carry out constant monitoring. Also, active and passive cooling of all power elements of the system - diodes and transistors, as well as switching-off of the device with excessive heating should be provided. Only in this case, frequency converters for asynchronous motors can be safely operated.