Servo drivers are used to drive servo motors, which can be stepper motors or AC asynchronous motors. They are mainly used to achieve fast and accurate positioning, and are often used in applications where the movement is stop-and-go and high precision is required.
A frequency converter is used to convert industrial frequency AC power into a current suitable for adjusting the speed of a motor, thereby driving the motor. Some frequency converters can now also achieve servo control, meaning they can drive servo motors. However, servo drives and frequency converters are still different! Let's break down the differences between servo drives and frequency converters.
Both definitions
A frequency converter is a control device that uses the switching action of power semiconductor devices to convert power frequency into electrical energy of another frequency. It can realize functions such as soft starting of AC asynchronous motor, variable frequency speed regulation, improved operating accuracy, and changing power factor.
Frequency converters can drive variable frequency motors and ordinary AC motors, and mainly play the role of regulating the motor speed.
A frequency converter typically consists of four parts: a rectifier unit, a high-capacity capacitor, an inverter, and a controller.
A servo system is an automatic control system that enables the output controlled variables, such as the position, orientation, and state of an object, to follow any changes in the input target (or given value). Its main task is to amplify, transform, and regulate power according to the requirements of control commands, so that the torque, speed, and position output of the drive device can be controlled very flexibly and conveniently.
A servo system is a feedback control system used to accurately follow or reproduce a process. It is also called a follow-up system. In many cases, a servo system specifically refers to a feedback control system where the controlled variable (the system's output) is mechanical displacement, velocity, or acceleration. Its function is to ensure that the output mechanical displacement (or angle) accurately tracks the input displacement (or angle). The structural composition of a servo system is not fundamentally different from other forms of feedback control systems.
Servo systems can be classified into electromechanical servo systems, hydraulic servo systems, and pneumatic servo systems based on the type of driving components used. The most basic servo system includes servo actuators (motors, hydraulic cylinders), feedback components, and servo drivers. For a servo system to operate smoothly, a host computer, such as a PLC and a dedicated motion control card (industrial PC + PCI card), is also required to send commands to the servo driver.
Working principles of both
The speed regulation principle of a frequency converter is mainly constrained by four factors: the speed (n) of the asynchronous motor, the frequency (f) of the asynchronous motor, the slip (s) of the motor, and the number of pole pairs (p) of the motor. The speed (n) is directly proportional to the frequency (f); changing the frequency (f) changes the motor speed. When the frequency (f) varies within the range of 0-50Hz, the motor speed adjustment range is very wide. Variable frequency speed regulation achieves speed regulation by changing the frequency of the motor's power supply. It mainly adopts an AC-DC-AC method, first converting the mains frequency AC power into DC power through a rectifier, and then converting the DC power into AC power with controllable frequency and voltage to supply the motor. The circuit of a frequency converter generally consists of four parts: rectification, intermediate DC link, inverter, and control. The rectification section is a three-phase bridge uncontrolled rectifier, the inverter section is an IGBT three-phase bridge inverter with a PWM waveform output, and the intermediate DC link is for filtering, DC energy storage, and reactive power buffering.
In simple terms, the working principle of a servo system is based on the open-loop control of an AC/DC motor. Speed and position signals are fed back to the driver via a rotary encoder, resolver, etc., for closed-loop negative feedback PID control. Combined with the driver's internal current closed-loop control, these three closed-loop adjustments significantly improve the accuracy of the motor's output in following the setpoint and its time response characteristics. A servo system is a dynamic follow-up system; the steady-state equilibrium it achieves is also a dynamic equilibrium.
Differences between the two:
AC servo technology itself borrows from and applies frequency conversion technology. It is based on the servo control of DC motors and uses the frequency conversion PWM method to imitate the control method of DC motors. In other words, AC servo motors must have a frequency conversion step: frequency conversion is to first rectify the 50 or 60 Hz AC power into DC power, and then invert it into frequency-adjustable waveforms similar to sine and cosine pulsed electricity through various controllable gate transistors (IGBT, IGCT, etc.) by carrier frequency and PWM adjustment. Since the frequency is adjustable, the speed of AC motor is adjustable (n=60f/p, where n is the speed, f is the frequency, and p is the number of pole pairs).
The difference between the two is:
1. Different overload capacities.
Servo drives typically have a 3x overload capacity, which can be used to overcome the inertial torque of inertial loads at startup, while frequency converters generally allow a 1.5x overload.
2. Control precision.
The control precision of servo systems is far higher than that of frequency converters. Typically, the control precision of a servo motor is ensured by a rotary encoder at the rear end of the motor shaft. Some servo systems even achieve a control precision of 1:1000.
3. Different application scenarios.
Variable frequency control and servo control are two different categories of control. The former belongs to the field of drive control, while the latter belongs to the field of motion control. One is for general industrial applications that do not have high performance requirements and prioritize low cost. The other prioritizes high precision, high performance, and high responsiveness.
4. Different acceleration and deceleration performance.
Under no-load conditions, the servo motor can accelerate from a standstill to 2000 rpm in no more than 20 ms. The acceleration time of the motor is related to the inertia of the motor shaft and the load. Generally, the greater the inertia, the longer the acceleration time.
Domestic servo and frequency converter market situation
Although my country's servo technology started relatively late—servo systems, composed of servo motors, feedback devices, and controllers, have only been around for about 50 years—it's undeniable that China's manufacturing industry is gradually recognizing the increasingly important role servo systems play in enhancing product competitiveness. Strong market demand for servo systems is beginning to emerge. It is believed that a new round of growth for servo systems will soon write another chapter in the development history of "Chinese frequency converters." The main reasons are analyzed as follows:
Firstly, with the overall positive development of China's economy, many key servo application industries, such as machine tools, electronic special equipment, medical devices, hybrid vehicles, and new energy, have recovered far beyond expectations due to economic and political reasons. This industry growth has directly led to strong demand in the servo market, resulting in the rise of numerous domestic servo brands. Furthermore, the acceleration of industrialization, industrial upgrading, and import substitution have also driven the widespread use of servo products, with increasingly evident energy-saving and production-increasing effects. It is worth mentioning that the initial maturity of servo application technology in the wind power industry suggests that the business opportunities brought to servos by energy conservation and carbon reduction are no less significant than those brought to high-voltage frequency converters by energy conservation and emission reduction.
Secondly, in the high-end market, the most important factors for users, such as stability, responsiveness, and accuracy, are all advantages of servo systems. In today's increasingly demanding technological landscape, whoever offers the highest performance will win user favor; price is no longer the decisive factor hindering the development of servo systems. The high-end market is undoubtedly dominated by servo systems, while frequency converters only play a role in some lower-end, simpler applications.
According to incomplete statistics, there are currently about twenty manufacturers in China offering servo products. The entry barrier for the servo field has always been higher than that for the low-voltage frequency converter field, and many manufacturers have developed their products based on frequency converter technology. Companies like Huichuan, a domestic manufacturer, have already begun to experience the practical benefits brought by the servo industry. Huichuan's existing R&D strength provides effective support for its foothold in the servo field, while increased brand influence, capacity expansion, and extension into niche industries have led to a gradual increase in its position in the servo market. In 2010, its servo sales reached over 100 million RMB. The ultimate reason for its shift towards servo products can be summarized by the favorable national policies for servo development. Among the seven strategic emerging industries, the machinery industry accounts for two: high-end equipment manufacturing and new energy vehicles, and the other five strategic emerging industries also require the support of the machinery industry. Therefore, the development of the manufacturing industry will also bring new opportunities for the development of servo products.
Market competition between servo drives and frequency converters
Due to the differences in performance and functionality between frequency converters and servo drives, their applications also differ, with the main competition focusing on:
Competition arises from technological content. In the same field, if the buyer has high and complex technical requirements for the machinery, they will choose a servo system. Conversely, they will choose a frequency converter. High-tech machinery such as CNC machine tools and specialized electronic equipment often prefer servo products.
Price competition. Most buyers are cost-conscious and often overlook technology, prioritizing lower-priced frequency converters. It is well known that servo systems cost several times more than frequency converters.
Although servo systems are not yet widely used, especially domestically produced ones, their applications are far fewer compared to foreign products. However, with the acceleration of industrialization, people will gradually realize the advantages of servo systems, and they will gain acceptance from buyers. Similarly, domestic servo technology will not stagnate. Whether driven by substantial profit margins or a sense of historical mission to revitalize the nation, it is believed that more and more manufacturers will invest in servo system research and development. This will usher in the golden age of China's "servo industry."
