In this article, we will consider the frequency response for an electric motor, the principle of its operation and the main components. The main emphasis will be placed on the theory, so that you understand the principle of operation of the frequency converter and are able to carry out design and manufacture yourself. But first, you need a small introductory course, which will talk about what a frequency converter is and for what purpose it is needed.
Frequency converter functions
The lion's share in the industry is asynchronous motors. And it was always difficult to control them, since they have a constant rotor speed, and changing the input voltage is very difficult, and sometimes even impossible. But the chastotnik completely changes the picture. And if earlier, to change the speed of the conveyor, for example, various gearboxes were used, today it is enough to use one electronic device.
In addition, the chastotniks allow you to get not only the ability to change drive parameters, but also several additional degrees of protection. There is no need for electromagnetic starters, and sometimes it is not even necessary to have a three-phase network to ensure the normal operation of an induction motor. All these responsibilities associated with switching and turning on the electric drive pass to the frequency converter. It allows you to change the phase at the output, the frequency of the current (therefore, the rotor speed also changes), adjust the start and brake, and you can also implement many other functions. It all depends on the microcontroller used in the control circuit.
Operating principle
Making a chastotnik for an electric motor with your own hands, the circuit of which is given in the article, is quite simple. It allows the conversion of one phase to three. Therefore, it becomes possible to use an induction 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 are almost halved. And the thing is in several transformations of the voltage supplied to the input of the device.
The first in the scheme is the rectifier block. More details about it will be described below. After the rectified voltage is filtered. And pure direct current flows to the inverter input. It converts direct current to alternating current with the required number of phases. This cascade can be adjusted. It consists of semiconductors to which a control circuit is connected on a microcontroller. But now about all the nodes in more detail.
Rectifier unit
It can be of two types - single and three phase. The first type of rectifier can be used on any network. If you have a three-phase, then just connect to one. The circuit of the chastotnik for the electric motor is not complete without a rectifier block. Since there is a difference in the number of phases, it means that it is necessary to use a certain number of semiconductor diodes. If we are talking about frequency converters that are powered from one phase, then a rectifier of four diodes is required. They are included in the bridge circuit.
It allows you to reduce the difference between the voltage value at the input and output. Of course, you can use a half-wave circuit, but it is inefficient, there is a large number of oscillations. 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 designed to protect against reverse voltage.
Filter elements
After the rectifier is a filter. Its main purpose is to cut off the entire variable component of the rectified current. For a clearer picture, an equivalent circuit needs to be drawn up. So, the plus goes through the coil. And then, between the plus and minus, the electrolytic capacitor is turned on. So he is interesting in the equivalent circuit. If the coil is replaced by reactance, then the capacitor in the presence of a different current can be either a conductor or a gap.
As was said, in the rectifier output direct current. And when it is fed to the electrolytic capacitor, nothing happens, since the latter is an open circuit. But there is a small variable in the current. And if 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 the laws of Kirchhoff, which are basic in electrical engineering.
Power Transistor Inverter
And now we got to the most important node - the cascade of transistors. They made an inverter - a DC to AC converter. If you make a frequency inverter for an electric motor with your own hands, then it is recommended to use IGBT transistor assemblies, you can find them in any radio parts store. Moreover, the cost of all components for the manufacture of the chastotnik will be ten times less than the price of the finished product, even Chinese-made.
Two transistors are used for each phase. They are included between plus and minus, as shown in the diagram shown 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 either by manual switching (for example, by applying several microswitches to supply voltage to the necessary control terminals), or automatically. That's about the latter and we will go further.
Control circuit
And if connecting the frequency converter to the electric motor is simple, just connect the corresponding conclusions, then with the control circuit everything is much more complicated. The thing is that there is a need for programming the device in order to achieve the maximum possible adjustments from it. It is based on a microcontroller, connected to it are readers and actuators. So, it is necessary to have current transformers, which will constantly monitor the power consumed by the electric drive. And in case of exceeding, the chastotnik should be turned off.
Control circuit connection
In addition, overheating protection is provided. The output of the microcontroller using a matching device (Darlington assembly) is used to connect the control terminals of IGBT transistors. In addition, it is necessary to visually control the parameters, so you need to include an LED display in the circuit. Of the readers, it is required to add buttons that allow you to switch between programming modes, as well as a variable resistance, its rotation changes the speed of rotation of the rotor of the electric motor.
Conclusion
I would like to note that it is possible to make a freewheel for an electric motor on its own, the price of the finished product starts at 5,000 rubles. And this is for electric motors whose power does not exceed 0.75 kW. If you need to manage a more powerful drive, you will need a more expensive frequency counter. For use in everyday life, the scheme discussed below is sufficient. The reason - there is no need for a large number of functions and settings, the most important thing is the ability to change the rotor speed.