Technical Knowledge

What is the difference between servo and inverter?
Release Date : 2020-04-22 10:44:57

The frequency converter is a power control device that converts the power frequency power supply to another frequency by using the on-off function of the power semiconductor device. It can realize the functions of soft start for the AC asynchronous motor, frequency conversion speed regulation, improved operation accuracy, and power factor change. Inverters can drive variable frequency motors and ordinary AC motors, and mainly serve to regulate the speed of the motor. The inverter is usually composed of four parts: rectifier unit, high-capacity capacitor, inverter and controller.


The servo system is an automatic control system that enables the output controlled quantity of the object's position, orientation, state, etc. to follow any change of the input target (or a given value). The main task is to amplify, transform and regulate the power according to the requirements of the control command, so that the torque, speed and position output of the drive device are very flexible and convenient to control.
A servo system is a feedback control system used to accurately follow or reproduce a process. Also called servo system. In many cases, the servo system refers specifically to the feedback control system where the controlled quantity (the output of the system) is mechanical displacement or displacement speed and acceleration. Its function is to make the output mechanical displacement (or rotation angle) accurately track the input displacement ( Or corner). There is no difference in principle between the structure of the servo system and other forms of feedback control system.



Servo system can be divided into electromechanical servo system, hydraulic servo system and pneumatic servo system according to the type of driving components used. The most basic servo system includes servo actuators (motors, hydraulic cylinders), feedback components and servo drives. If you want to make the servo system run smoothly, you need a higher-level mechanism, PLC, and a special motion control card, industrial computer + PCI card, in order to send commands to the servo drive.
The speed regulation principle of the frequency converter is mainly subject to the four factors of the speed n of the asynchronous motor, the frequency f of the asynchronous motor, the motor slip s, and the number of motor pole pairs p. The speed n is proportional to the frequency f. As long as the frequency f is changed, the speed of the motor can be changed. When the frequency f changes within the range of 0-50 Hz, the motor speed adjustment range is very wide. Variable frequency speed regulation is to achieve speed regulation by changing the motor power frequency.


Mainly adopts AC-DC-AC mode, first converts the power frequency AC power supply to DC power supply through the rectifier, and then converts the DC power supply into AC power supply whose frequency and voltage can be controlled to supply the motor. The circuit of the frequency converter is generally composed of four parts: rectification, intermediate DC link, inverter and control. The rectification part is a three-phase bridge-type uncontrollable rectifier, the inverter part is an IGBT three-phase bridge inverter, and the output is a PWM waveform, and the intermediate DC link is filtering, DC energy storage and buffering reactive power.
The working principle of the servo system is simply that the speed and position signals are fed back to the drive through a rotary encoder, rotary transformer, etc. to do closed-loop negative feedback PID adjustment control on the basis of an open-loop controlled AC and DC motor. Coupled with the current closed loop inside the driver, the accuracy and time response characteristics of the motor output to the set value are greatly improved through these three closed loop adjustments. The servo system is a dynamic servo system, and the steady state balance achieved is also a dynamic balance.



The AC servo technology itself is based on the application and application of frequency conversion technology. Based on the servo control of the DC motor, it is implemented by imitating the control method of the DC motor through the PWM method of frequency conversion. Link: Frequency conversion is to rectify the AC power of 50, 60 Hz of the power frequency into DC power first, and then adjust the inverter to the frequency adjustable waveform through carrier frequency and PWM through various transistors (IGBT, IGCT, etc.) that can control the gate The pulsating electricity of the sine and cosine, because the frequency is adjustable, so the speed of the AC motor is adjustable (n = 60f / p, n speed, f frequency, p pole logarithm).
Servo drives generally have three times the overload capacity and can be used to overcome the moment of inertia of the inertial load at the moment of starting, while the frequency converter generally allows double overload. The control accuracy of the servo system is much higher than that of frequency conversion. Usually the control accuracy of the servo motor is guaranteed by the rotary encoder at the rear end of the motor shaft. The control accuracy of some servo systems even reaches 1: 1000.


Frequency control and servo control are two categories of control. The former belongs to the field of transmission control, and the latter belongs to the field of motion control. One is to meet the requirements of general industrial applications, and applications that do not require high performance indicators, the pursuit of low cost. The other is to pursue high precision, high performance, and high response. In the case of no-load, the servo motor is processed from a standstill to 2000r / min, and the time will not exceed 20ms. 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.
Although China's servo technology started late, the servo system composed of servo motors, feedback devices and controllers has only gone through 50 years. But it is undeniable that China's manufacturing industry is beginning to realize that servo systems are playing an increasingly important role in improving product competitiveness. The strong market demand for servo systems is beginning to emerge. I believe that in the near future, the new history of the growth of the servo system will continue to write another "China inverter" development history.
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