It is quite difficult to make frequency converters with your own hands, since you need to be very well versed in power electronics and semiconductor technology. But before you think about designing this device, you need to find out for what purposes they are used. You will also need to know what are the main components of these electronic systems.
What is a frequency converter?
Everyone knows that there is alternating current in the network, and it has a certain frequency. In Russia, the standard is 50 Hertz. In some Western countries, a slightly different standard is 60 Hertz. The operation of many devices depends on the frequency of the current. Converters are used to power asynchronous motors. There are many reasons for using electronic means. For example, in the industry, inverters have become widespread, since their useallows you to get rid of huge mechanisms.
In more detail, you can change the speed of rotation of the belt on the conveyor by using a gearbox, which is based on a kind of car gearbox. Moreover, it can be either mechanical (using several gears) or CVT. But it is much more effective to change the parameters of the current that feeds the motor. Turning the variable resistor changes the rotation speed of the conveyor. Moreover, the frequency can be changed in a wide range.
What other features do frequency converters have?
In addition, the inverter settings allow the electric motor to gradually gain speed over several seconds. The time is set by the user through frequency converter function programming. Similarly, you can do with the time of stopping the armature of the engine. This reduces the load on the drive, which directly affects its resource.
In addition, for small businesses that do not have the ability to provide themselves with a three-phase network, but there is a need for it, the use of frequency converters is a true panacea. There are many models of such devices that are connected to a single-phase alternating current network, and they produce three at the output. Therefore, it is possible to plug an electric motor into a regular outlet. And in this case, it will not lose power, its work will be correct.
Power components of converters
All frequency converters use powerful IGBT or MOSFET transistors. They are ideal for this type of work. They are mounted in separate modules. This installation method can improve the performance of an electronic device. These transistors work in key mode, control is carried out using a microprocessor system. The fact is that all control is low-current, switching high voltages is not required. Therefore, this can be achieved using the simplest microprocessor.
The most commonly used special assemblies of the IR2132 and IR2130 series. They consist of six drivers that control the keys. Three are used for the bottom and three for the top. This assembly allows you to implement a simple cascade of the frequency converter. In addition, it has several degrees of protection. For example, against short circuit and overload. More detailed characteristics of all elements can be found in the manuals. But all power elements have a huge drawback - the high cost of products.
Structural diagram of the converter
Any frequency converter for a motor has three main blocks - a rectifier, filters, an inverter. It turns out that the AC voltage is first converted to DC, then filtered. After all this, it is inverted into a variable. But there is a third block - microprocessor control of the inverter. And to be more precise, powerful IGBT transistors. If you have ever de alt withfrequency converters, then you know that on the front panel they have several buttons for programming.
Inverter manual will guide you how to setup all functions. This is a very complicated matter, since there are quite a lot of settings even in the simplest device. In addition to the fact that the electronic device allows you to change the frequency of rotation of the motor armature, adjust the acceleration and deceleration time, there are also several degrees of protection. For example, overcurrent. In the case of using such a device, there is no need to install automatic switches.
Rectifier unit
Depending on the purpose of the frequency converter, different rectifier stages are used. And the power supply option can be either from a three-phase network, or from a single-phase one. But at the output of the inverter, in any case, there is a three-phase alternating voltage. But in order to control the current, it must first be rectified. The thing is that it is quite difficult to control the variable - it is necessary to use large rheostats, which is not very convenient. Moreover, now is the time of microelectronics and automation, it is not only unreasonable to use outdated technologies, but also very unprofitable.
An electronic device consisting of six semiconductor diodes is used to rectify a three-phase alternating current. They are switched on in a bridge circuit, it turns out that each pair of diodes serves to rectify one phase. A constant voltage appears at the output of the rectifier unit, itsthe value is equal to that which flows to the input. At this stage, all transformations are completed, no control of this block is performed. In the event that power is supplied from a single-phase network, a rectifier stage is sufficient even from a single diode. But it is more efficient to use a bridge circuit of four.
Filter box
This module is used for DC voltage filtering. The simplest version of the block is an inductor included in the gap of the positive shoulder. An electrolytic capacitor is connected between the poles. It has one function - to get rid of the variable component. The thing is that the rectifier is not able to completely get rid of ripples. There remains a small amount of AC that can produce significant noise during operation.
To consider the principle of operation of the filter block, it is necessary to analyze by replacing the elements. When operating in DC conditions, the inductance is replaced by resistance, the capacitor is replaced by an open circuit. But when powered by alternating current, the capacitance is replaced by resistance. Therefore, the entire variable component disappears, since a short circuit occurs in this case. To understand this is quite difficult, it is necessary to understand the theoretical foundations of electrical engineering. But a 3-phase frequency converter cannot be made without this.
Inverter stage
And here the fun begins - the use of powerful IGBT transistors. They are controlled by a microprocessor system, fromthe quality of their functioning depends on the operation of the entire frequency converter. Such a voltage converter circuit is widely used. In fact, with the help of power transistors, any voltage can be inverted. In total, six elements are used in the simplest circuit - two for each phase. The frequency converter produces 220 volts on each phase with respect to zero.
To get rid of the occurrence of reverse voltage, it is necessary to use semiconductor diodes. They are connected between the collector and emitter of power transistors. Management is carried out at the entrance of the base. As mentioned earlier, do-it-yourself frequency converters have two transistors for each phase in the inverter cascade. Turn on their p-n junctions in series. The phase is removed from the midpoint of each shoulder. Ready-made modules are available for sale, they have leads for supplying direct voltage, as well as three contacts for removing three-phase alternating. In addition, there is a connector for connecting a microcontroller control system.
Microprocessor control
Used to change the motor shaft speed by the inverter, the voltage, whose default frequency is 50Hz, can be changed in amplitude over a wide range. And more specifically, from zero to the frequency that the microprocessor can provide. The main requirement for the latter is the ability to connect multiple devices. When youdesign a converter, the voltage, the frequency of which is changed by a variable resistance, must be controlled by the processor. It is chosen carefully, it must have a sufficient number of I / O ports.
You can complicate the system a little by connecting an LCD display to the microcontroller. High color rendering is not required from it, monochrome is enough, as in simple calculators. Buttons for programming are also connected to the input-output ports. This is how you can make a simple frequency converter. The price of all elements will be no more than two thousand rubles. But the cost of an inverter with a power of 200-750 watts ranges from 6500 to 12000 rubles. It all depends on the manufacturer and the capabilities of the device.
Device case
Do-it-yourself frequency converters must have a reliable housing. Not only ease of use depends on it, but also efficiency. The base is made of aluminium. The reason for using this material is the need for high-quality cooling. During operation, the IGBT module becomes very hot, and the temperature of the semiconductor diodes also rises. And it doesn’t matter at all whether you have a 380 or 220 Volt frequency converter.
The rest of the body is made of plastic. It is necessary that all power elements be hidden by it, so that there is no accidental contact with high-voltage terminals during operation. In the front part, it is necessary to provide a hole for the LCD display and buttons. Separately, ina convenient place, a variable resistor is installed. When programming the microcontroller, it is necessary to take into account that this resistance changes the frequency of the output current.
Heat exchange of system elements
Special attention should be paid to heat dissipation. The more powerful the device being developed, the more reliable the cooling system should be. As mentioned above, the base must be made of aluminum. The voltage converter circuit must provide protection against overheating. For this purpose, it is required to drill a hole in the housing, a temperature sensor is mounted in it. From it, the signal is fed through a matching device to the microcontroller. If the maximum temperature is exceeded, the load must be disconnected. Therefore, the power transistor module turns off.
To improve heat dissipation, fans must be used. Their location must be chosen so that the air flow cools the radiator fins of the case. To increase the efficiency of the cooling system, you must use thermal paste. It is more reasonable to turn on the fans at the time of starting the device. But it is also possible to program the controller using a signal from a temperature sensor. When the temperature reaches half of that at which the emergency shutdown of the device occurs, the fans turn on.
Circuit board
As a circuit board, it is best to use ready-made options. On sale there are boards of different sizes with holes around whichsmall tinned contacts. They are colloquially referred to as "fish". The only thing worth considering is the possibility of replacing the processor and microcircuits. For this purpose, you need to use connectors that are soldered to the board. Do-it-yourself frequency converters are best made with the expectation of quick replacement of elements. The IC or controller is simply plugged into this socket, like a plug into a socket.
Conclusions
You can also make your own frequency converter. The price of analogues, as we found out, is much higher. Although, of course, they have more opportunities. But in fact, if you take a closer look, it turns out that no more than five functions are actually used. When the drive is running, it is necessary to change the rotational speed, as well as adjust the acceleration and deceleration times. A little less often, the reverse function and change the maximum allowable current are used.
