The difference between servo motor and variable frequency motor

The basic concept of servo is accurate, precise and fast positioning. Frequency conversion is a necessary internal link of servo control, and there is also frequency conversion in the servo drive (stepless speed regulation is required). But the servo closes the current loop speed loop or position loop for control, which is a big difference. In addition, the structure of the servo motor is different from that of the ordinary motor, and it must meet the requirements of fast response and accurate positioning.

1. The common points of servo motor and variable frequency motor:

The AC servo technology itself is borrowed from and applied the technology of frequency conversion. On the basis of the servo control of the DC motor, the PWM method of frequency conversion is used to imitate the control method of the DC motor, which means that the AC servo motor must have the frequency conversion. Link: Frequency conversion is to first rectify the 50, 60HZ AC power of the industrial frequency into DC power, and then through various transistors (IGBT, IGCT, etc.) that can control the gate, the carrier frequency and PWM are adjusted to invert into a frequency adjustable waveform similar to The pulsating electricity of sine and cosine, because the frequency is adjustable, so the speed of the AC motor can be adjusted (n=60f/p, n speed, f frequency, p pole pair number)

2. Frequency converter:

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 V/F control method in the traditional sense. Nowadays, many inverters have been established through mathematical models to convert the AC motor's stator magnetic field UVW3 phase into two current components that can control the motor speed and torque.

Now most famous brand inverters that can perform torque control are To control the torque in this way, the output of each phase of UVW needs to add a Hall-effect current detection device. After sampling and feedback, the PID adjustment of the current loop of closed-loop negative feedback is formed; ABB's frequency conversion also proposes a direct torque control technology different from this method , Please refer to relevant information for details.

This can control both the speed of the motor and the torque of the motor, and the speed control accuracy is better than v/f control. The encoder feedback can also be added or not. The control accuracy and response characteristics are much better when added.

3. Servo motor:

Drive: Under the premise of the development of frequency conversion technology, the current loop, speed loop and position loop inside the drive (the frequency converter does not have this loop) have carried out more precise control technology and algorithm calculations than general frequency conversion.

It is also much stronger than the traditional frequency conversion, the main point can be 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 or directly set the position and speed parameters in the drive through bus communication), and the internal algorithm of the drive is faster More accurate calculations and better performance electronic devices make it more superior to inverters.

Motor: The material, structure and processing technology of servo motors are much higher than AC motors driven by inverters (generally AC motors or variable-frequency motors such as constant torque and constant power). That is to say, when the driver outputs current, voltage, When the frequency changes quickly, the servo motor can respond to changes in the power supply.

The response characteristics and overload resistance are much higher than the AC motors driven by inverters. The serious difference in the motor is also the fundamental reason for the difference in performance .