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The difference between servo motor and variable frequency motor

Views: 4     Author: Site Editor     Publish Time: 2023-05-05      Origin: Site Inquire

The basic concept of servo is accurate, precise and fast positioning. Frequency conversion is a necessary internal link of servo motor, and frequency conversion also exists in servo drives (stepless speed regulation is required). However, the servo controls both the current loop speed loop and the position loop closed, which is a big difference. In addition, the structure of the servo motor is different from the ordinary motor, and it must meet the requirements of fast response and accurate positioning. Most of the AC servo motors currently on the market are permanent magnet synchronous AC servos, but this kind of motor is limited by the process, and it is difficult to achieve a large power. In most cases, AC asynchronous servo is mostly used. At this time, many drives are high-end frequency converters with encoder feedback closed-loop control. The so-called servo is to meet accurate, precise and fast positioning, as long as it is satisfied, there will be no dispute over servo frequency conversion.

Common features of servo motor and inverters

The technology of AC servo itself borrows and applies the technology of frequency conversion. On the basis of the servo motor, it imitates the control mode of DC motor through frequency conversion PWM mode. That is to say, AC servo motor must have the function of frequency conversion. Link: frequency conversion is to rectify the AC power of 50 and 60HZ at power frequency into DC power first, and then through various gate-controllable transistors (IGBT, IGCT, etc.) Sine and cosine pulsating electricity, since the frequency is adjustable, the speed of the AC motor can be adjusted (n=60f/p, n speed, f frequency, p pole pairs)

Inverter Introduction

A simple inverter can only adjust the speed of the AC motor. At this time, it can be open-loop or closed-loop, depending on the control method and the inverter. This is the traditional V/F control method. Now many frequency conversions have established mathematical models to convert the stator magnetic field UVW3 phase of the AC motor into two current components that can control the motor speed and torque. Now most of the famous brand inverters that can perform torque control are Using this method to control the torque, the output of each phase of UVW needs to be equipped with a Hall effect current detection device, and after sampling and feedback, the PID adjustment of the current loop that forms a closed-loop negative feedback; ABB's frequency conversion also proposes a direct torque control technology that is different from this method. , please refer to the relevant information for details. In this way, both the speed and the torque of the motor can be controlled, and the speed control accuracy is better than v/f control, and the encoder feedback can be added or not, and the control accuracy and response characteristics are much better when added.

Servo Motor Introduction

Drive: On the premise of the development of frequency conversion technology, the servo drive has carried out more accurate control technology and algorithm calculations than ordinary frequency conversion in the current loop, speed loop and position loop (the frequency converter does not have this loop) inside the drive. It is also much more powerful than traditional frequency conversion, and the main point is that it can perform precise position control. The speed and position are controlled by the pulse sequence sent by the upper controller (of course, some servos have integrated control units inside or directly set parameters such as position and speed in the driver through bus communication), and the algorithm inside the driver is faster. More accurate calculations and better performance of electronic devices make it superior to frequency converters.

Motor: The material, structure and processing technology of the servo motor are much higher than those of the AC motor driven by the frequency converter (general AC motor or various frequency conversion motors such as constant torque and constant power), that is to say, when the driver outputs current, voltage, When the frequency of the power supply changes rapidly, the servo motor can respond to the change of the power supply. The response characteristics and anti-overload capacity are much higher than the AC motor driven by the frequency converter. The serious difference in the motor is also the root of the difference in performance between the two. . That is to say, it is not that the frequency converter cannot output the power signal that changes so fast, but that the motor itself cannot respond, so the corresponding overload setting is made to protect the motor when setting the internal algorithm of the frequency conversion. Of course, even if the output capacity of the inverter is not set, it is still limited, and some inverters with excellent performance can directly drive the servo motor! ! !


Due to the difference in performance and function between frequency converter and servo, the applications are also quite different:

1. In the occasions of speed control and torque control, the requirements are not very high. Generally, frequency converters are used. There are also position control systems that use frequency conversion to form a closed loop with position feedback signals added to the host. The accuracy and response are not high. Some existing frequency converters also accept pulse train signals to control the speed, but it seems that they cannot directly control the position.

2. In the occasions with strict position control requirements, it can only be realized by servo, and the response speed of servo is much faster than that of frequency conversion. Some occasions that require high speed accuracy and response also use servo control, and frequency conversion control can be used. Almost all sports occasions can be replaced by servo. The key points are two points: one is that the price of servo is much higher than that of frequency conversion. Ten KW.

As for the last point, the servo can now reach several hundred KW.

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