Since the beginning of the 21st century, with the rise of Industry 4.0 and Made in China 2025, a common question in industrial control is: "What are the differences between servo motors and frequency converters?" This article will compare them from several aspects. Please forgive any shortcomings in the explanation.
From the definition:
First, by definition , a frequency converter is a power 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 start, variable frequency speed regulation, improved operating accuracy, and change of power factor for AC asynchronous motors.
Frequency converters can drive variable frequency motors and ordinary AC motors, and mainly play the role of regulating the motor speed.
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.
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.
In summary, the main difference between the two in terms of definition can be summarized in one sentence: frequency converters are for speed control, while servo drives are for position control.
From the perspective of the motor :
While more complex than induction motors, synchronous AC servo motors are simpler than DC motors. Like induction motors, their stators have symmetrical three-phase windings. However, the rotors differ, falling into two main categories based on their structure: electromagnetic and non-electromagnetic. Non-electromagnetic rotors are further divided into hysteresis, permanent magnet, and reactive types. Hysteresis and reactive synchronous motors suffer from low efficiency, poor power factor, and limited manufacturing capacity. Permanent magnet synchronous motors are commonly used in CNC machine tools. Compared to electromagnetic motors, permanent magnet motors offer advantages such as simpler structure, more reliable operation, and higher efficiency; however, they are larger and have less desirable starting characteristics.
However, by using rare-earth magnets with high remanence and high coercivity, permanent magnet synchronous motors can be about half the size of DC motors, with a 60% reduction in weight and a rotor inertia reduced to 1/5 of that of DC motors.
Compared to asynchronous motors, permanent magnet synchronous motors (PMSMs) are more efficient because they use permanent magnet excitation, eliminating excitation losses and related stray losses. Furthermore, because they lack the slip rings and brushes required by electromagnetic synchronous motors, their mechanical reliability is the same as that of induction (asynchronous) motors, while their power factor is significantly higher. This allows PMSMs to be smaller than asynchronous motors. This is because at low speeds, induction (asynchronous) motors, due to their low power factor, have a much larger apparent power output for the same active power, and the main dimensions of a motor are determined by its apparent power.
To create a circular rotating magnetic field within the motor, a 90-degree phase difference is required between the excitation voltage Uf and the control voltage UK. Common methods include:
1) Utilize the phase voltage and line voltage of the three-phase power supply to form a 90-degree phase shift.
2) Utilizing any line voltage of a three-phase power supply
3) Employing a phase-shifting network
4) Connect a capacitor in series in the excitation phase.
Three-phase asynchronous motors are a type of motor that is powered by simultaneously connecting to a 380V three-phase AC power supply (with a phase difference of 120 degrees). Since the rotor and stator rotating magnetic fields of a three-phase asynchronous motor rotate in the same direction but at different speeds, there is slip.
From a technical perspective:
Regarding the comparison of technical specifications , we will mainly elaborate on the following six points:
1. Different overload capacities
Servo drives typically have a 3x overload capacity (a few servo manufacturers can now achieve 3.5x), which can be used to overcome the inertial torque of inertial loads at startup, while frequency converters generally allow 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 an encoder located at the rear of the motor shaft. Commonly used encoders on the market are incremental photoelectric encoders and absolute photoelectric encoders.
3. Different application scenarios
Variable frequency drives (VFDs) and servo drives 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.
5. Dynamic characteristics
In automation applications, servo systems often require high control precision and rapid response to minute errors. Therefore, their response adjustment time must be much shorter, typically in the millisecond or even microsecond range. Many servo products boast speed frequency response bandwidths in the kHz range. In contrast, typical variable frequency drive products often only offer this bandwidth in the hundreds of Hz range.
6. Power range
Since servo drives are often used in applications that require high precision and high dynamic response, their overall load is relatively light, and their overall output power is generally within tens of kilowatts, which is much smaller than that of variable frequency drive systems that are known for their power transmission. In contrast, motion control applications with heavier loads usually do not have high requirements for response characteristics, and asynchronous frequency conversion can generally meet the requirements.
From the perspective of the market situation
Through the above explanation , you should now have a better understanding of the technical differences between frequency converters and servo drives . Next, I'd like to discuss the differences between the two from a market perspective.
Although China'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's believed that a new round of growth for servo systems will soon write another chapter in the development history of "Chinese frequency converters." Why is this ? The main reasons are 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 dozens of manufacturers in China offering servo products. The entry barrier for the servo field has always been higher than that for low-voltage frequency converters, and many manufacturers have developed their products based on frequency converter technology. Domestic companies like Huichuan, Invt, and Weichuang have successfully extended their frequency converter technology to servo control technology and applied it skillfully. Among the seven strategic emerging industries, the machinery industry accounts for two: high-end equipment manufacturing and new energy vehicles. Furthermore, the other five strategic emerging industries also require the support of the machinery industry. Therefore, the development of the manufacturing sector will bring new opportunities for the development of servo products.
From the perspective of market competition between servo drives and frequency converters
Since we've discussed the differences in market conditions, there's bound to be some competition between the two. In some respects , due to the differences in performance and functionality between frequency converters and servo drives, their applications also differ. The main competition focuses on the following two points:
Competition based on technological content . In the same field, if the buyer has high and complex technical requirements for the machinery, they will choose a servo system. Otherwise, they will choose a frequency converter.
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.
The above comparison of servo and frequency converter technologies is from a product perspective, examining the differences between the two. To summarize briefly:
The inverter has higher power, while the servo drive has lower power.
· Frequency converters are generally expressed in power (kW), while servo drives typically emphasize speed and torque;
· Variable frequency drives (VFDs) are designed for speed control, while servo drives are designed for position control; they are used in different scenarios.
In summary, in industrial applications, frequency converters are generally used for situations where speed and torque control requirements are not very high. In situations where strict position control is required, AC servo drives are used. Furthermore, the response speed of servos is much faster than that of frequency converters. In some situations where high speed accuracy and response are required, AC servo drives are also used for control. In other words, almost any motion control that can be achieved with frequency converters can be replaced by AC servo drives.
As one of the supporting technologies for modern industrial automation and motion control, AC servo drives are widely used in machine tools, printing equipment, packaging equipment, textile equipment, rubber and plastic equipment, electronic semiconductors, new energy sources such as wind power/solar power, as well as robotics and automated production lines due to their high-speed and precise control, wide speed range, dynamic characteristics and high efficiency.
