Frequency converter for a three-phase and single-phase electric motor: a frequency converter with your own hands, how to make

Today, asynchronous motors are the main traction drives for machine tools, conveyors, and other industrial units.

In order for the motors to function properly, they need a frequency converter. It allows you to optimize the operation of the unit and extend its service life. It is not necessary to buy a device - a frequency converter for a three-phase electric motor can be made by hand.

Purpose of the frequency converter

An asynchronous electric motor can work without a frequency converter, but in this case it will have a constant speed without the possibility of adjustment. In addition, the absence of a frequency converter will lead to an increase in the starting current by 5-7 times of the nominal, which will cause an increase in shock loads, increase energy losses and lead to a significant reduction in service life unit.

To eliminate all of the above negative factors, frequency converters were invented for asynchronous motors of three-phase and single-phase current.

The frequency converter makes it possible to regulate the speed of the electric motor within a wide range, provides a smooth start, allows adjust both starting speed and braking speed, connect a three-phase motor to a single-phase network and much other. All of these features depend on the microcontroller it is built on and may vary from model to model.

How the device works

The alternating current flows from the network to the diode bridge, where it is rectified and goes to a bank of smoothing capacitors, where finally turns into direct current, which flows to the drains of powerful IGBT transistors controlled by the main controller. The sources of the transistors, in turn, are connected to the motor.

Here is a simplified schematic of a frequency converter for a three-phase induction motor.

Now let's take a look at what happens to transistors and how they work.

A field-effect transistor (aka a key, a mosfet, etc.) is an electronic switch, its principle of operation is based on the occurrence conductivity between the two terminals (drain and source) of the mosfet, when a voltage appears on the control terminal (gate) that exceeds drain voltage.

Unlike conventional relays, the keys operate at very high frequencies (from several hertz to hundreds of kilohertz), so they cannot be replaced with a relay.

These fast switches enable the microcontroller to control the power circuits.

Apart from mosfets, the controller is also connected to current sensors, controls for the frequency converter, and other peripherals.

When the frequency converter is operating, the microcontroller measures the power consumption and, in accordance with the parameters set on the control panel, changes the duration and frequency of the periods when the transistor is open (on) or closed (off), thereby changing or maintaining the rotation speed electric motor.

Self-made device

Despite the many factory-made units, people make frequency converters independently, since today all its components can be bought at any radio store or order from China. Such a frequency converter will cost you much cheaper than the purchased one, moreover, you will not doubt the quality of its assembly and reliability.

Making a three-phase converter

We will assemble our converter on G4PH50UD mosfets, which will be controlled by the PIC16F628A controller using the HCPL3120 optodrivers.

The assembled frequency converter, when connected to a single-phase 220 V network, will have three full 220 V phases at the output, with a shift of 120 °, and a power of 3 kW.

The frequency converter circuit looks like this:

Since the frequency converter consists of parts operating at both high (power section) and low (control) voltage, it would be logical to split it into three boards (main board, control board, and low-voltage power supply for it) to exclude the possibility of breakdown between the tracks with high and low voltage and the output of the device from building.

This is how the control board layout looks like:

To power the control board, you can use any 24 V power supply with a ripple of no more than 1 V swing, with a delay in the interruption of the power supply for 2-3 seconds from the moment of the disappearance of the supply voltage 220 V.

The power supply can be assembled by yourself according to this scheme:

Please note that the ratings and names of all radio components on the diagrams have already been signed, so even a novice radio amateur can assemble a working device using them.

Before proceeding with the assembly of the converter, make sure:

1. You have all the necessary components in stock;
2. The correct layout of the board;
3. In the presence of all the necessary holes for installing radio components on the board;
4. The fact that they did not forget to fill the microcontroller with the firmware from this archive:

If you did everything correctly and did not forget anything, you can start assembling.

After assembly, you will end up with something similar:

Now you just have to check the device: for this we connect the motor to the frequency converter and apply voltage to it. After the readiness LED lights up, press the "Start" button. The motor should start to turn slowly. When you hold down the button, the engine starts to accelerate, when released, it maintains the speed at the level to which it managed to accelerate. When the Reset button is pressed, the motor will coast to a stop. The "Reverse" button is only activated when the engine is stopped.

If the test is successful, then you can start making the case and assembling the frequency converter in it. Do not forget to make holes in the case for the flow of cold and outflow of hot air from the heatsink of the IGBT transistors.

Frequency converter for single phase motor

A frequency converter for a single-phase motor differs from a three-phase one in that it has two phases at the output (there is no error, single-phase motor, when connected without a frequency converter, the working winding is connected directly to the network, and the starting winding is connected through capacitor; but when using a frequency converter, the starting winding is connected through the second phase) and one neutral - in contrast to the three phases in the latter, so that make a frequency converter for a single-phase electric motor, using a three-phase circuit as a basis, will not work, so you have to start all over again.

As the brain of this converter, we will use an ATmega328 MK with an arduina bootloader. In principle, this is the Arduino, only without its own strapping. So, if you have an arduinka with such a microcontroller lying around in your bins, you can safely solder it and use it for business, after filling it with a sketch (firmware) from this archive:

The IR2132 driver will be connected to the atmega, and the IRG4BC30 mosfets will be connected to it, to which we will connect a motor with a capacity of up to 1 KW inclusive.

Frequency converter circuit for a single-phase motor:

Also, to power the arduin (5v) and to power the power relay (12v), we need 2 stabilizers. Here are their diagrams:

12 volt stabilizer.

Stabilizer for 5 volts.

Attention! This scheme is not easy. You may have to tweak and debug the firmware to achieve full device performance, but this is not difficult, and there are a great many Arduino programming manuals on the Internet. In addition, the sketch itself contains rather detailed comments for each action. But if this is too difficult for you, then you can try to find such a frequency converter in the store. Although they are not as common as frequency drives for three-phase motors, you can buy them, albeit not in every store.

Also note that you cannot turn on the circuit without ballast - the output keys will burn out. The ballast must be connected through a diode facing the anode to the power filter capacitor. If you connect the ballast without a diode, the keys will fail again.

If everything suits you, you can start making the board, and then - to the assembly of the entire circuit. Before assembling, make sure that the board is wired correctly and that there are no defects in it, as well as that you have all the radio components indicated on the diagram. Also remember to install the IGBTs on the massive heat sink and insulate them from it by using thermal pads and insulating washers.

After assembling the frequency converter, you can start checking it. Ideally, you should have the following functionality: button "S1" - start, each subsequent press adds a certain (changed by editing the sketch) number of revolutions; "S2" is the same as "S1", only makes the motor rotate in the opposite direction; button "S3" - stop, when pressed, the engine stops with a coast.

Please note that the reverse is carried out through a complete stop of the engine, when trying to change the direction of rotation on a running engine will immediately stop it, and the power keys will burn out from overload. If you are not sorry for the money that will have to be spent on replacing mosfets, then you can use this feature as an emergency brake.

Possible verification problems

If, when checking the frequency converter, the circuit did not work or did not work correctly, then you made a mistake somewhere. Disconnect the frequency converter from the mains and check if installation of components, their serviceability and absence of breaks / short circuits of tracks where they should not be. After finding the fault, eliminate it and check the inverter again. If everything is in order with this, proceed to debugging the firmware.