How to distinguish servo motors and variable frequency motors

The basic concept of a servo motor is accurate, fast positioning, and a variable frequency motor is an internal link that must be carried out in servo control. Frequency conversion also exists in the servo driver (stepless speed regulation is required). So how to distinguish between servo motor and variable frequency motor? I believe this problem has been bothering everyone. The editor will introduce this problem as follows!

1. What they have in common:

The technology of AC servo motor itself draws on and applies frequency conversion technology. Based on the servo control of DC motor, it is realized by imitating the control method of DC motor through frequency conversion PWM method. In other words, AC servo motor must have frequency conversion. First step: Frequency conversion is to rectify the AC power of 50 and 60HZ at the industrial frequency into DC power, and then invert it into a frequency-adjustable waveform through various transistors (IGBT, IGCT, etc.) that can control the gate through carrier frequency and PWM adjustment. Based on the pulsating electricity of sine and cosine, since the frequency is adjustable, the speed of the AC motor is adjustable (n=60f/p, n speed, f frequency, p pole pair number).

2. Let’s talk about frequency converters:

A simple frequency converter can only adjust the speed of the AC motor. At this time, it can be open loop or closed loop, but it depends on the control method and the frequency converter. This is the V/F control method in the traditional sense. Nowadays, many frequency converters have established mathematical models to convert the UVW3 phase of the stator magnetic field of the AC motor into two current components that can control the motor speed and torque. Nowadays, brands of frequency converters that can perform torque control use To control torque in this way, the output of each phase of UVW needs to add a Hall effect current detection device. After sampling feedback, it forms a closed-loop negative feedback current loop PID adjustment; ABB's frequency conversion also proposes a direct torque control that is different from this way. Technology, please check the relevant information for details. In this way, both the speed and torque of the motor can be controlled, and the speed control accuracy is better than v/f control. The encoder feedback can be added or not. When added, the control accuracy and response characteristics are much better.

3. Let’s talk about servo motors:

In terms of driver: Under the premise of the development of frequency conversion technology, the servo driver has implemented more advanced control technology and algorithm calculations than ordinary frequency conversion in the current loop, speed loop and position loop inside the driver (the frequency converter does not have this loop). In terms of function, It is also much more powerful than traditional frequency conversion, and the main point is that it can perform position control. The speed and position are controlled through the pulse sequence sent by the upper controller (of course, some servo integrated control units or directly set parameters such as position and speed in the driver through bus communication), the algorithm inside the driver and faster More sophisticated calculations and better-performing electronic devices make it to inverters.

Motor: The material, structure and processing technology of servo motors are much higher than those of AC motors driven by frequency converters (general AC motors or various variable frequency motors such as constant torque and constant power). That is to say, when the driver outputs current, voltage, When the frequency changes rapidly, the servo motor can respond to changes in the power supply. The response characteristics and overload resistance are much higher than those of the AC motor driven by the frequency converter. The serious difference in the motor is also the fundamental reason for the different performance of the two. . That is to say, it is not that the frequency converter cannot output a power signal that changes so quickly, but that the motor itself cannot respond. Therefore, when setting the internal algorithm of the frequency converter, corresponding overload settings are made to protect the motor. Of course, the output capability of the inverter is still limited even if it is not set. Some inverters with performance can directly drive the servo motor!