Making frequency converters with your own hands is quite difficult, since you need to be very good at understanding 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 the network has alternating current, 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 current frequency. Converters are used to power induction motors. The reasons for using electronic tools are many. For example, in the industry, inverters are widespread, since their use allows you to get rid of huge mechanisms.
If in more detail, then 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 both mechanical (using several gears) and CVT. But it is much more effective to change the current parameters that feed the motor. By turning the variable resistor, the conveyor speed changes. Moreover, the frequency can be changed over a wide range.
What other properties are inverters?
In addition, the inverter settings allow you to achieve that the electric motor will gradually gain momentum within a few seconds. The time is set by the user by programming the functions of the frequency converter. The same can be done with the stop time of the engine armature. This allows you to reduce the load on the drive, which directly affects its resource.
In addition, for small enterprises that do not have the opportunity to provide themselves with a three-phase network, but there is a need for it, the use of frequency converters is a real panacea. There are many models of such devices that are connected to a single-phase alternating current network, and three are produced at the output. Consequently, it is possible to turn on the motor in a conventional outlet. And in this case, he will not lose power, his 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 is able to improve the performance of the electronic device. These transistors work in key mode, control is performed using a microprocessor system. The fact is that all control is low-current, switching of high voltages is not required. Therefore, this can be achieved using the simplest microprocessor.
Most often, special assemblies of the IR2132 and IR2130 series are used. They consist of six drivers that manage 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, from 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.
Converter block diagram
Any frequency converter for the motor has three main units - a rectifier, filters, and an inverter. It turns out that the alternating voltage is first converted to direct, 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, with powerful IGBT transistors. If you happen to deal with chastotniki, then you know that on the front panel they have several buttons for programming.
The frequency converter instruction will tell you how to configure all the functions. This is a very complicated matter, since there are 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 braking times, there are also several degrees of protection. For example, over current. In the case of using such a device, there is no need to install circuit breakers.
Depending on the purpose of the frequency converter, various rectifier stages are used. And the power supply option can be either from a three-phase network, or from a single-phase. But in any case, there is a three-phase alternating voltage at the inverter output. But in order to conduct current control, you must first straighten it. The thing is that managing a variable is quite difficult - you need to use large rheostats, which is not very convenient. Moreover, now is the time of microelectronics and automation, using outdated technologies is not only unreasonable, but also very disadvantageous.
To rectify an alternating three-phase current , an electronic device is used, consisting of six semiconductor diodes. They are turned 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, its value is equal to that which flows to the input. At this stage, all conversions are completed; no control is performed on this unit. In the event that power is supplied from a single-phase network, a rectifier stage even from one diode is sufficient. But it’s more efficient to use a bridge circuit of four.
This module is used to filter DC voltage. 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. He has one function - to get rid of the variable component. The thing is that the rectifier is not able to completely get rid of the ripple. There remains a small component of alternating current, which can give significant interference during operation.
To consider the principle of operation of the filter block, it is necessary to conduct an analysis by performing the replacement of elements. When working in direct current conditions, the inductance is replaced by resistance, while the capacitor is replaced by an open circuit. But when powered by alternating current, the capacitance is replaced by resistance. Consequently, 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 the frequency converter 3 phases to do without this is impossible.
And here the fun begins - the use of powerful IGBTs. They are also controlled by a microprocessor system, the operation of the entire frequency converter depends on the quality of their functioning. Such a voltage converter circuit is widespread. In fact, using power transistors, you can invert any voltage. In total, six elements are used in the simplest scheme - two for each phase. A 220 volt frequency converter produces 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 included between the collector and the emitter of power transistors. Management is done at the entrance of the base. As mentioned earlier, self-made frequency converters have two transistors per phase in the inverter cascade. Include their pn junctions sequentially. The phase is removed from the midpoint of each shoulder. Ready-made modules are available for sale, they have terminals for supplying direct voltage, as well as three contacts for removing a three-phase variable. In addition, there is a connector for connecting a microcontroller control system.
Used to change the speed of the motor shaft, a converter whose voltage, the default frequency of which is 50 Hz, can be changed in amplitude over a wide range. And more specifically, from zero to the frequency that a microprocessor can provide. The main requirement for the latter is the ability to connect multiple devices. When you design a converter, the voltage, the frequency of which varies with variable resistance, must be controlled by the processor. It is selected carefully, it should have a sufficient number of input / output ports.
You can complicate the system a little by connecting an LCD display to the microcontroller. It does not require high color rendering, monochrome 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 the inverter with a power of 200-750 watts ranges from 6500 to 12000 rubles. It all depends on the manufacturer and device capabilities.
Case for device
Do-it-yourself frequency converters must have a reliable housing. Not only ease of use, but also effectiveness depends on it. The base is made of aluminum. The reason for using this material is the need for quality cooling. During operation, the IGBT module is very hot, the temperature rises for semiconductor diodes. And it doesn’t matter at all whether you have a frequency converter of 380 or 220 volts.
The rest of the body is made of plastic. It is necessary that all power elements be hidden by him, so that accidental contact of the high-voltage terminals does not occur during operation. A hole for the LCD and buttons must be provided in the front. Separately, in a convenient place, a variable resistor is installed. When programming the microcontroller, it is necessary to take into account that the output current frequency changes with this resistance.
Heat transfer system elements
Particular 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 circuitry 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 is turned off.
To improve heat transfer, fans must be used. Their location must be chosen so that the air flow cools the fins of the radiator of the casing. To increase the efficiency of the cooling system it is necessary to use thermal grease. It is more reasonable to turn on the fans at the time of starting the device. But you can also program the controller using the signal from the temperature sensor. Upon reaching a temperature equal to half that at which an emergency shutdown of the device occurs, the fans are turned on.
As a circuit board, it is best to use off-the-shelf options. On sale there are boards of various sizes with holes around which there are small tinned contacts. In common parlance, they are called "fish." The only thing worth considering is the ability to replace the processor and chips. For this purpose, you need to use connectors that are soldered to the board. Do-it-yourself frequency inverters are best made with the expectation of quick replacement of elements. The chip or controller is simply installed in this connector, like a plug in a power outlet.
You can make yourself a 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 look closely, it turns out that no more than five functions are actually used. When the drive is operating, it is necessary to change the speed of rotation, as well as adjust the acceleration and braking times. A little less often, the function of reverse and changing the maximum permissible current is used.