A frequency converter (VFD) is an electric drive control device that uses modern power electronics, computer control, and communication technologies to convert the fixed frequency of the power supply to a motor into a continuously adjustable frequency, thereby controlling the operation of an AC motor. VFDs offer excellent, rapid, and continuous speed regulation and starting/braking performance, significant energy savings, a wide range of applications, and many other advantages. They are an important means of energy conservation and emission reduction, process improvement, product performance enhancement, and technological advancement, and are widely used in various industries such as metallurgy, municipal engineering, machinery, mining, and textiles.
Understanding the classification of frequency converters is an important basic step in learning how to use them. There are many types of frequency converters, which are generally classified into the following categories:
I. Classification according to frequency conversion method
Frequency converters are mainly classified into two categories according to their frequency conversion method: AC-AC converters and AC-DC-AC converters. 1. AC-AC Converters: Also known as direct-drive converters, AC-AC converters directly convert industrial frequency AC power into AC power with adjustable frequency and voltage, which is then supplied to the motor. Because there are no intermediate steps, AC-AC converters have high conversion efficiency and strong overload capacity. However, due to their narrow continuously adjustable frequency range (generally less than half the rated frequency), they are mainly used in low-speed, high-capacity drive systems.
2. AC-DC-AC Inverter: Also known as an indirect inverter, the AC-DC-AC inverter first converts the mains frequency AC power into pulsating DC power through a rectifier circuit. Then, it uses an inverter circuit to convert the DC power into three-phase AC power with an arbitrary and continuously adjustable frequency, which is then supplied to the motor. Because the DC-to-AC conversion process is relatively easy to control, the AC-DC-AC inverter has significant advantages in terms of a wider frequency adjustment range and improved motor characteristics after frequency conversion. Currently, this type of inverter is the most widely used and is commonly found in general-purpose inverters.
3. Comparison of the structure and main characteristics of AC-DC-AC frequency converters: See the table below for a comparison of the characteristics of AC-DC frequency converters and AC-DC-AC frequency converters.
II. Classification by DC Link Energy Storage Method in Main Circuit
In AC-DC-AC frequency converters, based on the different energy storage methods of the DC link in the main circuit, they can be divided into two main categories: voltage-source frequency converters and current-source frequency converters. 1. Voltage-source frequency converters: In voltage-source frequency converters, the energy storage element used in the DC link of the main circuit is a large electrolytic capacitor. Because electrolytic capacitors are used for filtering and buffering the reactive power of the load, the DC voltage waveform output by the DC link is relatively flat, and the internal resistance of the power supply is small. Ideally, it can be considered a voltage source with zero internal resistance. The output voltage waveform of a voltage-source frequency converter is a rectangular wave or a stepped wave, and the output current waveform is approximately a sine wave. It is commonly used in applications with large load voltage variations.
Structure of voltage source frequency converter
2. Current-source frequency converters: In current-source frequency converters, the energy storage element used in the DC link of the main circuit is a series inductor. Due to the use of inductors for filtering, the output DC current waveform is relatively flat. The power supply has a high internal impedance, which can buffer the reactive power of the load, i.e., suppress current changes, making the voltage close to a sine wave. For the load, it is essentially a current source, hence the name current-source frequency converter. The output current waveform of a current-source frequency converter is a rectangular wave, and the output voltage waveform is approximately a sine wave. In current-source frequency converters, the stator voltage of the motor is controlled by detecting the voltage and then controlling the current. A major advantage of current-source frequency converters is that they can operate in four quadrants, feeding energy back to the grid; they can suppress frequent and rapid changes in load current, especially adapting to large load currents. This method is suitable for applications with large load current variations and for production machinery requiring regenerative braking and frequent reversible operation.
3. Comparison of the structure and main characteristics of current-source frequency converters: A comparison of the characteristics of voltage-source frequency converters and current-source frequency converters is shown in the table below:
III. Classification according to the modulation method of output voltage
In AC-DC-AC frequency converters, based on the different modulation methods of the output voltage, the frequency converters can be divided into sinusoidal pulse width modulation (PWM) frequency converters and pulse amplitude modulation (PAM) frequency converters.
1. Sine Wave Pulse Width Modulation (PWM) Inverter: A PWM inverter controls both the amplitude and frequency of the output voltage in the inverter circuit. In this method, the PWM inverter controls the switching of semiconductor devices in the inverter circuit at a relatively high frequency and controls the output voltage by changing the duty cycle of the output pulse. Features of PWM inverters: high power factor, fast adjustment speed, and output voltage and current waveforms close to a sine wave, improving motor performance issues such as overheating and torque reduction caused by rectangular wave inverters. Currently, the most commonly used inverter method is PWM, where the duty cycle varies according to a sine wave pattern. PWM inverters are suitable for speed control systems with single or multiple motors operating in parallel and requiring high dynamic performance.
2. Pulse Amplitude Modulation (PAM) Inverter: PAM inverters separate the "voltage transformation" and "frequency conversion" processes. The output voltage amplitude is controlled in the rectifier circuit, while the output frequency is controlled in the inverter circuit. Because the switching frequency of the inverter's commutator is the inverter's output frequency, this is a synchronous speed control method. In this method, harmonics and noise are relatively high when the system operates at low speeds. The difference between these two types of inverters is that PAM inverters require a controllable rectifier for speed control, and the conduction angle of the controllable rectifier must be controlled; while PWM inverters use an uncontrollable rectifier for speed control and do not require rectifier control during operation.
IV. Classification by Control Method
1. U/f Controlled Frequency Inverter: The U/f controlled frequency inverter is a relatively simple control method. Its basic characteristic is that the output voltage and frequency of the frequency inverter are controlled simultaneously in a certain ratio to obtain the required torque. Frequency inverters using the U/f control method have lower control circuit costs and are mostly used in applications where high precision requirements are not required.
2. Voltage Space Vector Control (Fluorescence Trajectory Method): Also known as SVPWM control, this method generates a three-phase modulated waveform and controls the motor by approximating a circle with an inscribed polygon. Frequency compensation eliminates speed control errors; additionally, closed-loop output voltage and current control improves dynamic accuracy and stability. However, the control circuit has many components and lacks torque regulation, thus not fundamentally improving system performance. 3. Vector Control (Field-Oriented Method): Also known as VC control, this method essentially treats the AC motor as an equivalent DC motor, independently controlling the speed and magnetic field components. Its advantages include continuous and smooth torque adjustment and a wide speed range. However, selecting control parameters is difficult and requires online adjustment. 4. Direct Torque Control (DTC): This method directly analyzes the mathematical model of the AC motor in the stator coordinate system, controlling the motor's magnetic field and torque, thus largely overcoming the shortcomings of vector control.
5. Comparison of Main Features: The characteristics of the frequency converters with the above four different control methods are compared in the table below:
V. Classification by Purpose
Currently, frequency converters used in industry can be broadly classified into two categories: general-purpose frequency converters and special-purpose frequency converters. The main performance indicators for general-purpose frequency converters include control method, starting torque, torque control accuracy, speed control accuracy, control signal type, speed control method, multi-speed setting, carrier frequency, frequency skipping function, and communication interface. Newer general-purpose frequency converters generally possess these functions, and their functions, operation, maintenance, and applications are largely similar.
1. General-purpose frequency converter
General-purpose frequency converters can be further divided into low-cost, simple general-purpose frequency converters and high-performance, multi-functional general-purpose frequency converters.
a. Simple General-Purpose Frequency Inverter: This type of frequency inverter is designed for energy saving and is mainly used in loads such as fans, pumps, and blowers. Its torque control performance is designed for loads with reduced torque. The starting torque at zero speed is lower than other types of frequency inverters, resulting in higher energy efficiency. It also boasts advantages such as small size and low price.
B. High-Performance Multifunctional General-Purpose Frequency Inverter This high-performance multifunctional general-purpose frequency inverter employs vector control or direct torque control methods and fully considers various needs that may arise during general-purpose frequency inverter applications. Special functions are also available as optional components. Corresponding functional settings have been made in both the system's software and hardware to meet application requirements. High-performance general-purpose frequency inverters are widely used in various fields with high speed regulation performance requirements.
2. Dedicated frequency converter
Specialized frequency converters are frequency converters specifically designed and developed for a particular aspect or field. These converters are highly targeted, possessing unique functions and advantages applicable to their specific field, thus enabling them to better fulfill the function of frequency conversion speed regulation.
a. High-performance dedicated frequency converters are designed and manufactured to meet the needs of specific applications, primarily used in specialized mechanical systems requiring high control performance of asynchronous motors. With the development of control theory, AC speed regulation theory, and power electronics technology, vector control of asynchronous motors has evolved, and dedicated frequency converters have largely adopted vector control. AC servo systems composed of vector control frequency converters and their dedicated motors have reached and surpassed the performance of DC servo systems. Furthermore, due to the advantages of asynchronous motors, such as strong environmental adaptability and simple maintenance, which DC servo motors lack, these high-performance AC servo frequency converters are gradually replacing DC servo systems in applications requiring high-speed and high-precision control.
b. High-frequency inverters are often used in ultra-precision machining where high-speed motors are required. General-purpose inverters typically have a maximum output frequency of 400Hz; anything exceeding this is considered a high-frequency inverter. To meet the needs of driving high-speed motors, high-frequency inverters using PAM control have emerged, with output frequencies reaching 3kHz. When driving a two-pole asynchronous motor, the maximum motor speed is 18000 r/min. c. High-voltage inverters are used for speed control of high-voltage motors. These inverters typically use high-voltage resistant GTOs as the switch in the inverter circuit. Output frequencies can reach 120Hz, and they are mainly used in mining machinery, power equipment, and other fields. High-voltage inverters are available in voltage levels of 3kV, 6kV, and 10kV, and include medium- and small-capacity high-voltage inverters as well as large-capacity high-voltage inverters.
d. Single-phase frequency converter: Single-phase frequency converters are mainly used for speed control of three-phase AC motors by inputting a single-phase AC power supply.
VI. Categorization and Summary
Finally, a mind map will be used to summarize the classification of frequency converters, making it easier to remember and master.
