The basic concept of servo is accurate, accurate and fast positioning. Frequency conversion is a necessary internal link of servo control, and there is also frequency conversion (stepless speed regulation) in servo driver. But the servo will be current loop, speed loop or position loop are closed for control, this is a big difference. In addition, the structure of the servo motor is different from that of the ordinary motor, which needs fast response and accurate positioning. At present, most of the AC servo motors in circulation on the market are permanent magnet synchronous AC servo motors, but this kind of motor is limited by the technology, so it is difficult to achieve great power. The synchronous servo motor with more than 10 kW is very expensive, so AC asynchronous servo is often used when the field application allows. At this time, many drivers are high-end inverters with encoder feedback closed-loop control. The so-called servo is to meet the requirements of accurate, accurate and fast positioning. As long as it meets the requirements, there is no dispute about servo frequency conversion.

01 what they have in common

The AC servo technology itself is to learn from and apply the frequency conversion technology. Based on the servo control of DC motor, the DC motor is simulated by PWM mode of frequency conversion. That is to say, AC servo motor must have the frequency conversion link: frequency conversion is to first rectify 50 and 60Hz AC of power frequency into DC, Then, the frequency adjustable waveform is similar to the pulse current of the cosine through the carrier frequency and PWM regulation by various transistors (IGBT, IGCT, etc.) which can control gate pole. Because the frequency is adjustable, the speed of AC motor can be adjusted (n = 60F / P, n speed, f frequency, P-pole logarithm)

02 talking about frequency converter

A simple inverter can only adjust the speed of AC motor. At this time, it can be open-loop or closed-loop, depending on the control mode and inverter, which is the traditional V / F control mode. At present, many frequency converters have established mathematical models to convert the stator magnetic field uvw3 phase of AC motor into two current components that can control the motor speed and torque. Most famous brand converters that can control torque now use this method to control torque. The output of each phase of UVW needs Hall effect current detection device, The PID control of current loop is composed of sampling feedback and closed-loop negative feedback; ABB's frequency conversion also proposes direct torque control technology which is different from this way. Please refer to relevant information for details. In this way, not only the speed of the motor can be controlled, but also the torque of the motor can be controlled. Moreover, the speed control accuracy is better than V / F control. The encoder feedback can be added or not. When adding, the control accuracy and response characteristics are much better.

03 about servo

Driver: on the premise of the development of frequency conversion technology, servo driver has carried out more accurate control technology and algorithm calculation than general frequency conversion in the current loop, speed loop and position loop (the frequency converter does not have this loop) inside the driver, and it is much more powerful than traditional frequency conversion in function. The main point is that it can carry out accurate position control. The speed and position are controlled by the pulse sequence sent by the upper controller (of course, some servo integrated control unit or directly set the position and speed parameters in the driver through bus communication). The algorithm, faster and more accurate calculation and better performance electronic devices in the driver make it superior to the inverter.

Motor aspect: the material, structure and processing technology of servo motor are far higher than those driven by frequency converter (general AC motor or constant torque, constant power and other variable frequency motors). That is, when the driver outputs a power source with fast changes in current, voltage and frequency, servo motor can generate response action changes according to power supply change, The response characteristics and anti overload capacity are much higher than those of AC motor driven by frequency converter. The serious difference in motor is also the fundamental of the difference in performance. That is, not the frequency converter can not output such fast power signal, but the motor itself can not respond, so in the internal algorithm of frequency conversion, the corresponding overload setting is made to protect the motor. Of course, even if the output capacity of the converter is not set is limited, some excellent frequency converter can directly drive the servo motor!!!

04 Talking about AC motor

AC motor is generally divided into synchronous and asynchronous motor

1. AC synchronous motor: that is, the rotor is made of permanent magnet materials. Therefore, after rotation, with the change of the stator rotating magnetic field of the motor, the rotor also changes the speed of response frequency, and the rotor speed = stator speed, so it is called "synchronization".

2. AC asynchronous motor: rotor is composed of induction coil and material. After rotation, the stator produces a rotating magnetic field. The magnetic field cuts the induction coil of the stator, and the rotor coil generates induction current, and then the rotor generates the induction field. The induction field follows the change of the stator rotating magnetic field, but the change of the rotor magnetic field is always smaller than the change of the stator. Once the magnetic field is equal to the change, the induction coil of the rotor will be cut, There is no induction current in the rotor coil. The magnetic field of the rotor disappears, and the rotor stalls and the stator produce speed difference and obtains the induction current again... So in AC asynchronous motor, the key parameter is the ratio of rotor to stator speed difference.

3. Corresponding to AC synchronous and asynchronous motor converters, there are synchronous converters and asynchronous converters, and servo motors also have AC synchronous servo and AC asynchronous servo. Of course, AC asynchronous frequency conversion is common in frequency converter, and AC synchronous servo is common for servo.

05 application

Due to the difference in performance and function between inverter and servo, the application is not the same

1. In the situation of speed control and torque control, the requirements are not very high. Generally, frequency converter is used, and position feedback signal is added in the upper position to form a closed loop. Frequency converter is used for position control, so the accuracy and response are not high. Now some frequency conversion also accept the pulse sequence signal to control the speed, but it seems that it can not directly control the position.

2. In the case of strict position control requirements, only servo can be used. In addition, the response speed of servo is much faster than that of frequency conversion. In some cases with high speed accuracy and response requirements, servo control is also used. In almost all cases where the motion can be controlled by frequency conversion, servo can be used instead. The key is two points: first, the price of servo is much higher than that of frequency conversion, Second, the reason of power: the maximum frequency conversion can achieve hundreds of kW or even higher, and the maximum servo is dozens of kW.

As for the last point, now the servo can also achieve several hundred kW.