Variable frequency drives (VFDs), a familiar face in industrial control, have been active in various industries for decades. They bear the heavy responsibility of motor speed regulation, playing an indispensable role in both improving production processes and saving energy. From a professional perspective, it is actually a power control device that uses variable frequency technology and microelectronics to control the speed of an AC motor by changing the frequency of the AC motor's power supply.
So, what exactly is the significance of frequency converters for motor speed control? Why are they indispensable for AC motor speed control?
To understand the importance of frequency converters, we can look at their birth and development process from the following aspects.
In industrial production, the accurate control of the speed and position of objects or components using electric motors is a necessary process, such as in various types of machinery and equipment like lifting equipment, looms, material conveyors, and winding and unwinding machines.
When motor speed control technology was not yet mature, people could only use some mechanical auxiliary parts to solve the problem of motion control of objects, such as gearboxes, clutches and other complex mechanical transmission devices. If there were situations where the motor could not be freely adjusted, in order to achieve a certain motion purpose, it was necessary to replace the gearbox, change the transmission ratio, or switch the clutch. This process was not only very time-consuming, but also caused great damage to the machinery.
In another type of fluid control application scenario, the motor drives the impeller to rotate, thereby promoting the flow of gas or liquid or generating corresponding air pressure or hydraulic pressure. In the early days, because the motor speed could not be freely controlled, the flow rate and pressure of the fluid could only be controlled by opening and closing valves in the pipeline. This control mode was very wasteful of electrical energy.
In the era before frequency converters, traditional machinery had to add many accessories to achieve certain motion purposes because the motor speed could not be freely adjusted. This not only increased the complexity and cost of the overall system, but also limited the performance and development space of the equipment. In order to solve these problems, the development of simple and efficient motor speed control technology has always been a hot topic and pain point in industrial transmission research.
To solve the problem, the focus of early motor speed control was always on DC motors. One of the main reasons was that people first mastered rectification technology, and the mechanical characteristics of DC motors were also very suitable for certain scenarios. The simplest way to adjust the armature voltage was to insert a resistor in series. The larger the resistance, the greater the voltage drop, and the slower the DC motor speed.
However, DC motors also have significant drawbacks, such as the need for regular maintenance of the slip rings and carbon brushes, and the complex manufacturing process and high cost. This means that DC motors are not suitable for a wide range of motor applications.
Compared to DC motors, AC motors have a much simpler internal structure, without commutators or other components. They are easier to manufacture, more robust and stable, and suitable for high-speed, high-voltage, and high-current applications. The only problem that needs to be solved is the speed regulation of AC motors.
Nikola Tesla invented the alternating current motor
Alternating current and AC motors were invented as early as 1888, but for a long time afterward, due to their structural limitations, AC motors could only operate at one or more fixed speeds. Their rotational speed was directly proportional to the frequency and inversely proportional to the number of pole pairs.
n = 60f（1 - s）/p
As can be seen from the above formula, slip s and number of pole pairs p are inherent characteristic parameters of the motor, which cannot be changed after the motor is manufactured. If you want to freely adjust the speed, you can only change the input frequency f of the power supply. Before the advent of frequency converters, there were basically no means to freely adjust the frequency of the grid voltage.
By the 1980s, with the development of semiconductor technology, especially the increasing maturity of microprocessors and thyristors, it became possible to use microprocessors to control the conduction state of thyristors. Thus, by using a microprocessor to control the switching of the upper and lower bridge switching elements, completing the actions continuously according to a specific timing sequence, direct current could be converted into alternating current—this is what we commonly refer to as inverter technology. Simultaneously, we could adjust the switching cycle of the power components, thereby regulating the inverter output frequency.
Finally, by combining rectification technology, we can quickly convert the standard frequency of the power grid into AC power of the corresponding frequency and voltage according to the amplitude and frequency of the required power supply, thereby changing the input frequency of the motor and realizing the regulation and control of the speed of the AC motor. Through long-term technological development and the unremitting efforts of scientists, frequency converters have been upgraded and evolved in repeated applications, gradually becoming what we see today.
Having learned about the development of frequency converters from scratch, let's now discuss the evolution of frequency converter technology, which can be broadly divided into three stages:
1. Upgrading and replacement of power electronic devices
With the continuous development of semiconductor devices, we have replaced semi-controlled thyristors (SCRs) with fully controlled devices, changing the output waveform to pulse width modulation (PWM) waveform, which greatly reduces harmonic components, increases the speed regulation range of asynchronous motors, and reduces torque fluctuations.
IGBTs typically operate at frequencies between 10 and 20 kHz, an order of magnitude higher than the below 2 kHz of BJTs. They also surpass BJTs in certain voltage and current specifications, such as surge current withstand and peak voltage blocking. Because IGBTs allow for higher carrier frequencies and even the generation of desired PWM waveforms, harmonic noise is significantly reduced. Therefore, IGBTs have largely replaced BJTs in current frequency converter applications.
IPM, or Intelligent Power Module, uses IGBTs as switching devices and integrates not only the power switching devices and drive circuits, but also internally integrates fault detection circuits for overvoltage, overcurrent, and overheating, and can send the detection signals to the CPU. Even in the event of a load accident or improper use, the IPM itself can be guaranteed to remain undamaged.
IGBT module
2. Development of Control Methods
Early frequency converters used a constant voltage-frequency ratio (V/f) control method. V refers to the effective value of the voltage, and changing V/f only adjusts the steady-state flux and torque of the motor. To improve torque at low frequencies, torque boosting is required, usually achieved by compensating for voltage drops. Some converters can also compensate for stator winding voltage drops as the load changes.
Later, a new control method emerged for frequency converters—vector control. Its basic principle is to establish an equivalent DC motor model and decompose the stator current of the asynchronous motor into excitation and torque components for separate control. Controlling the excitation vector is the most important, so vector control is also called field-oriented control, while torque control is indirect.
Vector control system structure diagram
Vector control requires coordinate transformation calculations and the detection of actual rotational speed signals, thus necessitating speed sensors for feedback—essentially closed-loop vector control. Subsequently, a sensorless vector control scheme was proposed. This scheme calculates the observed values of rotor flux linkage and torque current based on the actual phase voltage and current of the motor during operation, as well as the stator and rotor winding parameters, thereby achieving field-oriented vector control.
Another approach that has developed alongside vector control is called direct torque control (DTC), which emphasizes direct torque control. It works by calculating estimated values of motor flux and torque based on measured motor voltage and current. After controlling the torque, the motor speed can also be controlled.
Direct Torque Control System Structure Diagram
3. Diverse Functions
Modern frequency converters, equipped with powerful microprocessors, not only perform the basic task of variable frequency speed control of motors but also incorporate a variety of other functions. For example:
(1) Automatic acceleration and deceleration.
(2) Program execution
(3) Automatic energy-saving operation
(4) Motor parameter self-learning
(5) PID control operation
(6) Communication and feedback functions
In the 1970s, Siemens engineers first proposed the vector control theory for asynchronous motors to solve the torque control problem of AC motors. As for direct torque control, it is generally attributed in the literature to Professor M. Depenbrock of Ruhr University in Germany and I. Takahashi of Japan, who proposed it separately in 1985. It is evident that foreign research on motor control predates that of my country, and their frequency converter products also entered the public eye earlier than domestic brands.
Now, thanks to the continuous learning and research of numerous engineers in this field, the domestic frequency converter industry, in terms of brand, performance, and price, has largely met the needs of my country's industrial development. As a company specializing in the research, development, production, and service of frequency converters, Weichuang will continue to focus on research in this field, diligently producing each frequency converter product, and contributing to the future of Intelligent Manufacturing 2025.
