Since the electric drive is one of the main methods of mechanizing production and household tasks, in some cases it becomes necessary to adjust the speed of electric motors. Various technical solutions are used depending on their type and principle of operation. One of them is a frequency converter. What it is and where the frequency converter is used, we will tell you in this article.
Content:
- Definition
- Device
- Types of chastotniki and scope
- Control methods
- Number of phases
- Connection diagram
Definition
By definition, a frequency converter is an electronic power converter for changing the frequency of an alternating current. But depending on the version, both the voltage level and the number of phases change. It may not be entirely clear to you why such a device is needed, but we will try to tell you about it in simple words.
The shaft rotation frequency of synchronous and asynchronous motors (AM) depends on the frequency of rotation of the stator magnetic flux and is determined by the formula:
n = (60 * F / p) * (1-S),
where n is the number of revolutions of the IM shaft, p is the number of pole pairs, s is the slip, f is the frequency of the alternating current (for RF - 50 Hz).
In simple terms, the rotor speed depends on the frequency and the number of pole pairs. The number of pole pairs is determined by the design of the stator coils, and the frequency of the current in the network is constant. Therefore, in order to regulate the speed, we can only regulate the frequency with the help of converters.
Device
Taking into account the above, we will formulate anew the answer to the question of what it is:
A frequency converter is an electronic device for changing the frequency of an alternating current, therefore, the number of revolutions of the rotor of an induction (and synchronous) electric machine.
You can see the conventional graphic designation according to GOST 2.737-68 below:
It is called electronic because it is based on a semiconductor switch circuit. Depending on the functional features and the type of control, both the electrical circuit diagram and the operation algorithm will be modified.
In the diagram below you can see how the frequency converter works:
The principle of operation of the frequency converter is as follows:
- The mains voltage is applied to rectifier 1 and becomes rectified pulsating.
- In block 2, pulsations are smoothed out and the reactive component is partially compensated.
- Block 3 is a group of power switches controlled by the control system (4) using the pulse-width modulation (PWM) method. This design makes it possible to obtain a two-level PWM-regulated voltage at the output, which, after smoothing, approaches a sinusoidal form. In expensive models, a three-level scheme has found application, where more keys are used. It allows you to achieve a more sinusoidal waveform. Thyristors, field-effect or IGBT transistors can be used as semiconductor switches. Recently, the last two types are most in demand and popular due to their efficiency, low losses and ease of management.
- With the help of PWM, the desired voltage level is formed, in simple words - this is how a sinusoid is modulated, alternately turning on pairs of keys, forming line voltage.
So we briefly described how a frequency converter for an electric motor works and what it consists of. It is used as a secondary power source and does not just control the shape of the supply network current, but converts its magnitude and frequency in accordance with the specified parameters.
Types of chastotniki and scope
Control methods
The speed control can be carried out in different ways, both by the method of setting the required frequency and by the method of regulation. Frequency tuners are divided into two types according to the control method:
- Scalar control.
- With vector control.
Devices of the first type regulate the frequency according to a given U / F function, that is, along with the frequency, the voltage also changes. An example of such a dependence of voltage on frequency can be seen below.
It can be different and programmed for a specific load, for example, on fans it is not linear, but resembles a parabola branch. This principle of operation keeps the magnetic flux in the gap between the rotor and stator almost constant.
A feature of scalar control is its prevalence and relative ease of implementation. Most often used for pumps, fans and compressors. Such frequency meters are often used if it is necessary to maintain a stable pressure (or other parameter), it can be submersible pumps for wells, if we consider domestic use.
In production, the scope of application is wide, for example, the regulation of pressure in the same pipelines and the performance of automatic ventilation systems. The control range is usually 1:10, in simple terms, the maximum speed can differ from the minimum by 10 times. Due to the peculiarities of the implementation of algorithms and circuitry, such devices are usually cheaper, which is the main advantage.
Disadvantages:
- Not too accurate RPM support.
- Slower response to regime change.
- Most often, there is no way to control the torque on the shaft.
- With an increase in speed above the nominal, the torque on the motor shaft drops (that is, when we raise the frequency above the nominal 50 Hz).
The latter is due to the fact that the output voltage depends on the frequency, at the rated frequency, the voltage is equal to the network, and above the frequency converter "does not know how", on the graph you could see an even part of the diagram after 50 Hz. It should be noted that the dependence of the torque on the frequency, it falls according to the 1 / f law, is shown in the graph below in red, and the dependence of the power on the frequency in blue.
Vector controlled frequency converters have a different principle of operation, here not only the voltage corresponds to the U / f curve. The characteristics of the output voltage are changed in accordance with the signals from the sensors, so that a certain torque is maintained on the shaft. But why do we need this kind of control? More accurate and faster adjustment are the hallmarks of a vector controlled frequency converter. This is important in such mechanisms, where the principle of action is associated with a sharp change in the load and torque on the executive body.
Such a load is typical for lathes and other types of machine tools, including CNC. Regulation accuracy up to 1.5%, adjustment range - 1: 100, for greater accuracy with speed sensors, etc. - 0.2% and 1: 10000, respectively.
There is an opinion on the forums that today the price difference between vector and scalar frequency drives is less than was earlier (15-35% depending on the manufacturer), and the main difference is more firmware than circuitry. Also note that most vector models support scalar control.
Advantages:
- great stability and accuracy;
- faster response to load changes and high torque at low speed;
- wider regulation range.
The main disadvantage is that it is more expensive than scalar ones.
In both cases, the frequency can be set manually or by sensors, for example, a pressure sensor or a flow meter (in the case of pumps), a potentiometer or an encoder.
All or almost all frequency converters have a soft start function for the motor, which makes it easier to start motors from emergency generators with little or no risk of overloading.
Number of phases
In addition to the methods of response, frequency converters also differ in the number of phases at the input and output. This is how frequency converters with single-phase and three-phase input are distinguished.
At the same time, most three-phase models can be powered from one phase, but with this application, their power is reduced to 30-50%. This is due to the permissible current load on diodes and other power elements of the circuit. Single-phase models are available in a power range of up to 3 kW.
Important! Please note that with a single-phase connection with a voltage to the input of 220V, there will be an output of 3 phases at 220V, and not at 380V. That is, the linear output will be exactly 220V, in short. In this connection, common motors with windings designed for voltages of 380 / 220V must be connected in a triangle, and those that are at 127 / 220V - in a star.
On the network you can find many offers of the type "220 to 380 frequency converter" - this is in most cases marketing, sellers call any three phases "380V".
To get real 380V from one phase, you need to either use a 220/380 single-phase transformer (if the input of the frequency converter designed for such a voltage), or use a specialized frequency converter with a single-phase input and 380V three-phase exit.
A separate and more rare type of frequency converters are single-phase frequency converters with a single-phase 220 output. They are designed to control single-phase capacitor-starting motors. Examples of such devices are:
- ERMAN ER-G-220-01
- INNOVERT IDD
Connection diagram
In reality, in order to get a 3-phase output from a 380V frequency converter, you need to connect 3 phases of 380V to the input:
The connection of the frequency converter to one phase is the same, except for the connection of the supply wires:
A single-phase frequency converter for a motor with a capacitor (pump or low-power fan) is connected as follows:
As you could see in the diagrams, in addition to the supply wires and wires to the motor, the frequency converter has other terminals, to them sensors, buttons of the remote control panel, buses for connecting to a computer (more often of the RS-485 standard) and other. This makes it possible to control the motor through thin signal wires, which allows the frequency converter to be removed to the electrical panel.
Frequency drives are universal devices, the purpose of which is not only to adjust the speed, but also to protect the electric motor from incorrect operating modes and power supply, as well as from overload. In addition to the main function, the devices implement a smooth start-up of drives, which reduces wear on equipment and the load on the power grid. The principle of operation and the depth of setting the parameters of most frequency converters allows you to save energy when control of pumps (previously, control was carried out not at the expense of pump performance, but with the help of valves) and other equipment.
This is where we conclude our consideration of the issue. We hope that after reading the article it became clear to you what a frequency converter is and what it is for. Finally, we recommend watching a useful video on the topic:
You probably don't know:
- How to measure AC frequency
- How does a magnetic starter work
- How to choose a frequency converter for power and current
