This article will discuss the frequency converter for an electric motor, the principle of its operation and the main components. The main emphasis will be placed on theory, so that you understand the principle of operation of the frequency converter and be able to further design and manufacture with your own hands. But first, you need a small introductory course, which will tell you what a frequency converter is and for what purposes it is needed.
Inverter functions
The lion's share in the industry is occupied by asynchronous motors. And it has always been difficult to manage them, since they have a constant rotor speed, and changing the input voltage turns out to be very difficult, and sometimes even impossible. But the frequency converter completely changes the picture. And if earlier, for example, various gearboxes were used to change the speed of the conveyor, today it is enough to use one electronic device.
In addition, chastotniki allow you to get not only the ability to change the drive parameters, but also several additional degrees of protection. There is no need for electromagnetic starters, and sometimesit is not even necessary to have a three-phase network to ensure the normal operation of an induction motor. All these duties associated with switching and turning on the electric drive are transferred to the frequency converter. It allows you to change the phases at the output, the frequency of the current (and therefore the speed of the rotor changes), to adjust the start and brake, and you can also implement many other functions. It all depends on the microcontroller used in the control circuit.
Operation principle
Making a frequency converter for an electric motor with your own hands, the diagram of which is given in the article, is quite simple. It allows you to convert one phase into three. Therefore, it becomes possible to use an asynchronous electric motor in everyday life. At the same time, its efficiency and power will not be lost. After all, you know that when you turn on the motor in a network with one phase, these parameters decrease almost by half. And it's all about several transformations of the voltage supplied to the input of the device.
The rectifier unit is the first in the scheme. It will be discussed in more detail below. After the rectified voltage is filtered. And a clean direct current is supplied to the input of the inverter. It converts direct current to alternating current with the required number of phases. This cascade can be subjected to adjustments. It consists of semiconductors to which a control circuit on a microcontroller is connected. But now about all the nodes in more detail.
Rectifier unit
It can be of two types - one- and three-phase. The first type of rectifier can be used in any network. If you have a three-phase, then it is enough to connect to one. The chastotnik circuit for an electric motor is not complete without a rectifier unit. Since there is a difference in the number of phases, it means that a certain number of semiconductor diodes must be used. If we are talking about frequency converters that are powered by a single phase, then a four-diode rectifier is required. They are bridged.
It allows you to reduce the difference between the voltage value at the input and output. Of course, a half-wave circuit can also be used, but it is inefficient, and a large number of oscillations occur. But if we are talking about a three-phase connection, then it is necessary to use six semiconductors in the circuit. Exactly the same circuit in the rectifier of a car generator, there are no differences. The only thing that can be added here is three additional diodes for reverse voltage protection.
Filter elements
After the rectifier comes the filter. Its main purpose is to cut off the entire variable component of the rectified current. For a clearer picture, you need to draw up an equivalent circuit. So, plus goes through the coil. And then an electrolytic capacitor is connected between plus and minus. This is what is interesting in the replacement circuit. If the coil is replaced by reactance, then the capacitor, if present,different current can be either a conductor or a break.
As it was said, the output of the rectifier is direct current. And when it is applied to an electrolytic capacitor, nothing happens, since the latter is an open circuit. But there is a small variable in the current. And if an alternating current flows, then in the equivalent circuit, the capacitor becomes a conductor. Therefore, there is a closure of plus to minus. These conclusions are made according to Kirchhoff's laws, which are fundamental in electrical engineering.
Power Transistor Inverter
And now we have reached the most important node - the transistor cascade. They made an inverter - a DC-to-AC converter. If you are making a frequency converter for an electric motor with your own hands, then it is recommended to use assemblies of IGBT transistors, you can find them in any radio parts store. Moreover, the cost of all components for the manufacture of a frequency converter will be ten times less than the price of a finished product, even made in China.
Two transistors are used for each phase. They are included between plus and minus, as shown in the diagram in the article. But each transistor has a feature - a control output. Depending on which signal is applied to it, the properties of the semiconductor element change. Moreover, this can be done both with the help of manual switching (for example, apply voltage to the necessary control outputs with several microswitches), and automatic. That's aboutthe latter and will be discussed further.
Control scheme
And if the connection of the frequency converter to the electric motor is simple, you just need to connect the corresponding terminals, then everything is much more complicated with the control circuit. The thing is that there is a need to program the device in order to achieve the maximum possible adjustments from it. At the heart is a microcontroller, to which readers and actuators are connected. So, it is necessary to have current transformers that will constantly monitor the power consumed by the electric drive. And in case of exceeding, the frequency converter should be turned off.
Connecting the control circuit
In addition, overheating protection is provided. The control outputs of IGBT transistors are connected to the output of the microcontroller using a matching device (Darlington assembly). In addition, it is necessary to visually control the parameters, so you need to include an LED display in the circuit. Of the readers, you need to add buttons that will allow you to switch between programming modes, as well as a variable resistance, by rotating it, the speed of rotation of the rotor of the electric motor changes.
Conclusion
I would like to note that you can also make your own frequency converter for an electric motor, the price of the finished product starts from 5000 rubles. And this is for electric motors whose power does not exceed 0.75 kW. If you need to manage morepowerful drive, you will need a more expensive chastotnik. For use in everyday life, the scheme discussed below is sufficient. The reason is that there is no need for a large number of functions and settings, the most important thing is the ability to change the rotor speed.
