I. What is a frequency converter?
A frequency converter is a control device that uses the switching action of power semiconductor devices to convert mains frequency power into electrical energy of another frequency. A frequency converter mainly consists of rectification (AC to DC), filtering, inversion (DC to AC), braking unit, drive unit, detection unit, and microprocessor unit.
A frequency converter is a device that transforms mains frequency power (50Hz or 60Hz) into AC power of various frequencies to achieve variable speed operation of a motor. The control circuit controls the main circuit, the rectifier circuit converts AC to DC, the DC intermediate circuit smooths and filters the output of the rectifier circuit, and the inverter circuit converts the DC back into AC. For frequency converters like vector control frequency converters that require a large amount of computation, a CPU for torque calculation and other corresponding circuits are sometimes also needed. Variable frequency speed control achieves speed regulation by changing the frequency of the power supply to the motor stator windings.
II. Principle of Frequency Converter
In embedded development, motor control is frequently involved. Currently, frequency converter control of AC motors is widely used. So let's take a look at the diagram and introduce the frequency converter. Assuming that you already understand the principle of motors.
The flowchart is as follows:
A frequency converter mainly consists of rectification (AC to DC), filtering, inversion (DC to AC), braking unit, drive unit, detection unit, and microprocessor unit. The frequency converter adjusts the output voltage and frequency by switching its internal IGBTs, providing the required power voltage according to the actual needs of the motor, thereby achieving energy saving and speed regulation. In addition, the frequency converter has many protection functions, such as overcurrent, overvoltage, and overload protection. With the continuous improvement of industrial automation, frequency converters have been widely used.
A typical inverter system diagram is shown below. It mainly includes an operation panel, VFD controller, motor, etc.
1. Typical structure:
It mainly includes: control platform, measurement circuit, power circuit, protection circuit, etc.
There are two common types of frequency converters: voltage-type and current-type. The power inverter section often uses power transistors such as IGBTs and IGCTs.
2. Typical Algorithm:
The control algorithms represented by Siemens are mainly based on coordinate transformation (vector control), while those interested in the algorithms represented by ABB can find relevant information on their own (direct torque control), which will not be elaborated here.
3. Vector control:
Many MCU and MPU manufacturers provide block diagrams and algorithm libraries for frequency conversion vector control, which you can study if you are interested. For example, the block diagram below is provided by Microchip Technology.
We can simulate the algorithm in Matlab.
III. The function of frequency converters
1. Variable frequency energy saving
The energy-saving benefits of frequency converters are mainly seen in the application of fans and pumps. To ensure production reliability, various production machines are designed with a certain margin of safety in their power drives. When a motor cannot operate at full load, the excess torque beyond meeting the power drive requirements increases active power consumption, resulting in energy waste. Traditional speed control methods for equipment such as fans and pumps involve adjusting the opening of inlet or outlet baffles or valves to regulate airflow and water flow. This method has high input power, and a significant amount of energy is consumed in the flow throttling process of the baffles and valves. When using frequency converter speed control, if the flow requirement decreases, the requirement can be met by reducing the speed of the pump or fan.
The purpose of using a frequency converter (VDC) for an electric motor is to regulate its speed and reduce starting current. To generate variable voltage and frequency, the device first converts the alternating current (AC) to direct current (DC), a process called rectification. The scientific term for the device that converts DC to AC is "inverter." Generally, an inverter converts DC power to a power supply with a fixed frequency and voltage. Inverters that convert DC to AC with adjustable frequency and voltage are called frequency converters. The output waveform of a frequency converter is a simulated sine wave, primarily used for speed control of three-phase asynchronous motors; it is also called a variable frequency drive (VFD). For variable frequency inverters used in instrumentation and testing equipment with higher waveform requirements, the waveform needs to be shaped to output a standard sine wave; this is called a variable frequency power supply. Generally, a variable frequency power supply costs 15-20 times more than a frequency converter. Because the main device in a frequency converter that generates changing voltage or frequency is called an "inverter," the product itself is named "inverter," i.e., a frequency converter.
Inverter technology doesn't always save electricity; in many situations, it doesn't necessarily save power. As an electronic circuit, the inverter itself consumes power (approximately 3-5% of its rated power). A 1.5 horsepower air conditioner consumes about 20-30W, equivalent to a light bulb left on continuously. It's true that inverters operate at the mains frequency and have energy-saving functions. However, this is conditional:
First, it must be a high-power load, specifically a fan/pump type load;
Second, the device itself has a power-saving function (software supported);
A variable-frequency drive (VFD) is a power control device that uses frequency conversion technology and microelectronics to control an AC motor by changing the frequency of its power supply. A VFD mainly consists of a rectification unit (AC to DC), a filter, an inverter (DC to AC), a braking unit, a drive unit, a detection unit, and a microprocessor unit. The VFD adjusts the voltage and frequency of the output power supply by switching its internal IGBTs, providing the required power voltage according to the actual needs of the motor, thereby achieving energy saving and speed regulation. In addition, VFDs have many protection functions, such as overcurrent, overvoltage, and overload protection. With the continuous improvement of industrial automation, VFDs have been widely used.
