Working principles and differences between inverter and servo system

1. Definition of the two

The frequency converter is an electric energy control device that uses the on-off function of power semiconductor devices to convert the industrial frequency power supply into another frequency. It can realize the functions of soft starting, variable frequency speed regulation, improving operation accuracy, changing power factor, etc. for AC asynchronous motors. The frequency converter can drive variable frequency motors and ordinary AC motors, and mainly plays the role of regulating motor speed. The frequency converter usually consists of four parts: rectifier unit, intermediate circuit, inverter and controller.

The servo system is an automatic control system that enables the output controlled quantities such as the position, orientation, and state of an object to follow any changes in the input target (or 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 control of the drive device output are very flexible and convenient.

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 in which the controlled quantity (the output of the system) is a mechanical displacement or displacement velocity or acceleration. Its function is to make the output mechanical displacement (or rotation angle) accurately track the input displacement (or rotation angle). The structural composition of a servo system is not fundamentally different from other forms of feedback control systems.

Servo systems can be divided into electromechanical servo systems, hydraulic servo systems and pneumatic servo systems according to the type of driving components used. The most basic servo system includes servo actuators (motors, hydraulic cylinders), feedback components and servo drivers. If you want the servo system to run smoothly, you also need a host mechanism, PLC, and a special motion control card, industrial computer + PCI card, to send instructions to the servo driver.

2. Working Principles of the Two

The speed regulation principle of the frequency converter is mainly subject to four factors: the speed n of the asynchronous motor, the frequency f of the asynchronous motor, the motor slip rate s, and the number of motor pole pairs p. The speed n is proportional to the frequency f. The speed of the motor can be changed by changing the frequency f. When the frequency f changes within the range of 0-50Hz, the motor speed adjustment range is very wide.

Frequency conversion speed regulation is to achieve speed regulation by changing the power frequency of the motor. It mainly adopts the AC-DC-AC method. The industrial frequency AC power is first converted into a DC power supply through a rectifier, and then the DC power supply is converted into an AC power supply with controllable frequency and voltage to supply the motor. The circuit of the inverter generally consists of four parts: rectification, intermediate DC link, inversion and control. The rectification part is a three-phase bridge uncontrolled rectifier, the inversion part is an IGBT three-phase bridge inverter, and the output is a PWM waveform. The intermediate DC link is filtering, DC energy storage and buffering reactive power.

The working principle of the servo system is to feed back the speed and position signals to the driver through rotary encoders, resolvers, etc. for closed-loop negative feedback PID control based on the open-loop controlled AC and DC motors. In addition, the current closed loop inside the driver is adjusted through these three closed loops to greatly improve the accuracy and time response characteristics of the motor output to the set value. The servo system is a dynamic follow-up system, and the steady-state balance achieved is also a dynamic balance.

3. Common characteristics of the two The technology of AC servo itself is based on and applies the technology of frequency conversion. It is realized by imitating the control method of DC motor through frequency conversion PWM method on the basis of servo control of DC motor. That is to say, AC servo motor must have this link of frequency conversion: frequency conversion is to rectify the industrial frequency 50, 60HZ AC power into DC power first, and then invert it into frequency-adjustable waveform similar to sine and cosine pulsating electricity through various transistors with controllable gates (IGBT, IGCT, etc.) through carrier frequency and PWM adjustment. Since the frequency is adjustable, the speed of AC motor can be adjusted (n=60f/p, n speed, f frequency, p pole pair number).

4. The difference between the two

1. Different overload capacity. Servo drives generally have a 3-fold overload capacity, which can be used to overcome the inertia moment of the inertial load at the moment of startup, while frequency converters generally allow 1.5-fold overload.

2. Control accuracy. The control accuracy of the servo system is much higher than that of the 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 can even reach 1:1000.

3. Different application occasions. Frequency conversion 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 the application occasions with low requirements for performance indicators pursue low cost. The other pursues high precision, high performance, and high response.

4. Different acceleration and deceleration performance. Under no-load conditions, the servo motor takes less than 20ms to accelerate from a stationary state to 2000r/min. 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. Market competition between the two

Due to the differences in performance and functions between inverters and servos, their applications are also quite different. The main competition focuses on:

Competition in technical content. In the same field, if the purchaser has high and complex technical requirements for machinery, they will choose servo systems. Otherwise, they will choose inverter products. For example, some high-tech machinery such as CNC machine tools and electronic special equipment will first choose servo products.

Price competition. Most buyers are concerned about cost and often ignore technology and prefer lower-priced inverters. As we all know, the price of a servo system is several times that of an inverter product.