What are the Types of Frequency Converters?

A frequency converter is a device used to control the speed of an electric motor and regulate the voltage and frequency of the power grid, which is widely used in industrial automation, building management, and home appliances. With the development of technology, the types and functions of frequency converters are becoming more and more diversified to meet the needs of different scenarios. Common types of frequency converters include Voltage Source Inverter (VSI), Current Source Inverter (CSI), etc. Different types have their own unique circuit design and application scenarios.

When choosing the right frequency converter, it is important to consider not only the application environment but also the load characteristics of the equipment, the speed range, and the energy consumption requirements to determine the appropriate type. Understanding the basic classification and characteristics of frequency converters can help us use them more efficiently to improve equipment performance and save energy. In the following, the GoHz shop will discuss in detail several common types of frequency converters and their applications.

Frequency converters are commonly used industrial equipment for motor control, mainly by changing the size and frequency of the motor's power output, and are generally used to achieve power control of AC motors. Using a frequency converter, it can achieve the controlled AC motor frequency speed regulation, power factor, overcurrent and overvoltage protection, overload protection, etc., to better control the operation and running of the whole motor, and to provide the utilization rate of electric power and the working efficiency of the whole system.

But what are the commonly used types of frequency converters (what is a frequency converter)? Their principle and can achieve the function of those? These questions are the basics of frequency converter, for automatic control direction of electronic engineers, is one of the technologies that need to know and master, the following we come to a comprehensive analysis of the types and principles of frequency converter, function in this regard.

Classified by the DC Power Source Nature

Voltage Type Frequency Converter: The energy storage element is a capacitor, and the controlled quantity is voltage, which is equivalent to the voltage source, it has a slow dynamic response, and it needs to set up an anti-parallel inverter on the power supply side to realize the energy feedback when braking and it can be adapted to multi-motor dragging. Its inverter output AC voltage is rectangular wave or step wave, while the waveform of current is close to sinusoidal wave after motor load filtering, but with larger harmonic components. Because it is a voltage source to the AC motor to provide AC power, the main advantage is that the operation is almost independent of the power factor of the load or the influence of the current conversion; the disadvantage is that when the load is short-circuited or put into the load under the operating state of the frequency converter, are prone to overcurrent, it is necessary to impose protective measures in a very short period.

Current Type Frequency Converter: The energy storage element is a reactor, DC internal resistance is large, which is equivalent to the current source provided, fast dynamic response, can directly achieve feedback braking, induction motor current type frequency conversion speed control system can be frequently and quickly achieve four-quadrant operation, more suitable for a single inverter on a motor power supply stand-alone mode of operation. The advantage is that it has the ability of four-quadrant operation and can easily achieve the braking function of the motor. The disadvantage is that the inverter bridge needs to be forced to change current, the device structure is complex, and it is more difficult to adjust. In addition, due to the grid side of the thyristor phase-shift rectification, the input current harmonics are large, and large capacity will have a certain impact on the power grid.

Classification According to the Working Principle

V/f Control

V/f control frequency converter is to ensure that the output voltage is proportional to the frequency of the control so that the motor flux maintains a certain, to avoid weak magnetism and magnetic saturation phenomenon, mostly used in fans, pumps, energy saving, with a voltage-controlled oscillator to achieve.

Asynchronous motor torque is the motor flux and rotor current flowing through the interaction between the resulting in the rated frequency, the voltage must reduce the frequency, the flux becomes larger, the magnetic circuit tends to saturation, and serious cases will burn the motor. When the frequency and voltage should be changed proportionally, so that the magnetic flux of the motor to maintain a certain, to avoid weak magnetism and magnetic saturation phenomenon.

Differential Frequency Control

Differential speed control that is to changes the slip of the asynchronous motor to speed control, the larger the slip the slower the speed, and the knee-winding is the motor rotor string resistance, the use of differential frequency control technology, so that the frequency control system to a certain extent to improve the static and dynamic performance of the system, and at the same time it is simpler than the vector control method, with a simple structure, easy to implement, control accuracy and other characteristics, widely used in the Vector control speed control system of asynchronous motor.

There is no need to carry out complex magnetic flux detection and cumbersome coordinate transformation, as long as the rotor magnetic chain size is unchanged under the premise of detecting the stator current and rotor angular velocity, and after the operation of the mathematical model can be indirectly magnetic field orientation control. To improve the dynamic performance of the speed control system, mainly relying on the control of the rate of change of rotational speed, controlling the rotor angular frequency can achieve the purpose of control.

Vector Control

According to the characteristics of DC motor speed control, the stator winding current of asynchronous motor is decomposed and formed into magnetic field current component and torque current component similar to DC motor by vector transformation method, as long as the size and phase of the stator winding current of asynchronous motor are controlled, the excitation current and torque current can be controlled, so that the control of the speed of AC asynchronous motor is like the control of DC motor, and good speed control can be obtained.

This can control the speed of AC asynchronous motor like a DC motor, and get a good speed control effect. Its main characteristics are low-frequency torque, fast dynamic response, flexible control, generally used in harsh working environments, the requirements of high-speed response high accuracy of the power drag system, and so on.

Classification According to Switching Mode

• PAM Control: Pulse amplitude modulation, i.e. changing the pulse amplitude of the pulse train according to a certain law to regulate the output value and waveform, and using these principles to realize the inverter function of the equipment.
• PWM Control: Pulse width modulation, i.e., a modulation method that changes the pulse width of the pulse train according to a certain law to regulate the output volume and waveform, and equipment that uses these principles to realize the inverter function.
• SPWM Control: Based on PWM, the modulation pulse mode is changed, and the pulse width time duty cycle is arranged according to the sinusoidal rate so that the output waveform can be sinusoidal output after appropriate filtering. It is widely used in tributary AC inverters, three-phase SPWM is the use of SPWM to simulate the three-phase output of the utility, which is widely used in the field of frequency converters.
• SVPWM Control: Space vector pulse width modulation, is a modulation method of PWM technology, his idea is that the PWM wave formed by PWM modulation in the access to the three-phase stator winding of the motor, so that the stator of the motor generates a circular rotating magnetic field, thus driving the motor to rotate, here the space vector refers to the synthetic vector of the three-phase stator voltage, which is a kind of inverter method is sinusoidal pulse width modulation ( SPWM) a special case.

Classified According to the Link of Conversion

AC-DC-AC Frequency Converter

With the rectifier, filtering system, and inverter three parts, the working principle is first industrial frequency AC through the rectifier into DC, and then DC converted into frequency and voltage adjustable AC, also known as indirect frequency converter, is now widely used general-purpose frequency converter, it is in the field of industrial automation frequency converter and IT, power supply in the field of uninterruptible power supply have applications.

The rectifier is a diode three-phase bridge-type uncontrolled rectifier or a full-control rectifier composed of high-power transistors, and the inverter is a three-phase bridge circuit composed of high-power transistors, whose role is exactly the opposite of the rectifier, which exchanges constant DC power for AC power with adjustable voltage and adjustable frequency. The intermediate filtering link is to filter the rectified voltage or current with capacitors or reactors.

AC-AC Frequency Converter

Industrial frequency AC is directly converted into AC with adjustable frequency and voltage, adopting thyristor natural current exchange method, which is stable and reliable, also known as direct frequency converter. The highest output frequency is 1/3-1/2 of the grid frequency, in the high-power low-frequency range has a great advantage, because there is no DC link, high-frequency efficiency, the main circuit is simple, does not contain a DC circuit and filtering part, and the power supply reactive power processing as well as active power feedback is easy, in the traditional high-power motor speed control system is more widely used.

With the development of AC transmission and control technology, AC speed control instead of DC speed control and computer digital control technology instead of analog control technology has become a development trend. Motor AC frequency conversion speed control technology is today's power saving, improves the process to improve product quality and the environment, and promotes technological progress of a major means. Frequency converter as an important speed control equipment, has been widely used in various types of motor drive and control systems.

From the voltage source type to the power supply type, different types of frequency converters have their own advantages and application characteristics, whether in improving industrial production efficiency or reducing the energy consumption of equipment, have played an important role. With the continuous progress of technology, inverters will also have more space for development in terms of intelligence and energy saving. Understanding the types and characteristics of inverters can help us make better choices in practical applications and achieve the dual goals of energy saving and improving equipment efficiency. In the future, the innovation and upgrading of inverters will continue to promote the development of power electronics technology, bringing more convenience to industry and life.