The difference between servo and frequency conversion motors - Xinjie

The usual inverter motor (hereinafter referred to as VFD) can only regulate the speed of the communication motor, and it can be open-loop or closed-loop, depending on the control method and VFD. This is the traditional V/F mode.

The basic concept of servo is accurate, precise and fast positioning. Frequency conversion is a necessary internal link of servo control, and frequency conversion (stepless speed regulation) also exists in servo drives. However, the servo controls the current ring, and the speed loop is still the azimuth ring, which is a big difference. In addition, the structure of the servo motor is different from that of ordinary motors, which needs to meet the requirements of fast response and precise positioning.

At present, most of the communication servo motors circulating in the market are permanent magnet synchronous communication servo, but this kind of motor is limited by the process and is difficult to achieve high power. Synchronous servos of more than 10 KW are extremely expensive, so in the case of on-site use, communication asynchronous servos are often selected. At this time, many of the drivers are high-end VFDs with encoder reaction closed-loop operation. The so-called servo is to meet the requirements of accurate, precise and fast positioning, as long as it is satisfied, there is no controversy over servo frequency conversion.

The difference between servo and frequency conversion motors - Xinjie

The difference between servo and frequency conversion motors - Xinjie

First of all, the two have something in common:

The skill of communication servo itself is the learning and use of frequency conversion skills. It is based on the servo control of the DC motor, and the control method of the DC motor is imitated by the frequency conversion PWM method. That is to say, the communication servo motor must have a frequency conversion link: frequency conversion is to first rectify the 50 and 60HZ communication power supply frequency into DC power supply, and then pass through various transistors (IGBT, IGCT, etc.). Pulsating electricity that is converted into a frequency-adjustable waveform similar to that of sine and cosine by adjusting the carrier frequency and PWM. Since the frequency is adjustable, the speed of the AC motor can also be adjusted (n = 60 f/p, n speed, f frequency, p number of poles)

2. Let's talk about VFD:

A simple VFD can only regulate the speed of the communication motor, and it can be either open-loop or closed-loop, depending on the control method and VFD. This is the traditional V/F mode. At present, many frequency converters have been mathematically modeled to convert the stator magnetic field UVW3 phase of the communication motor into two current components that control the speed and torque of the motor. At present, most well-known brands of VFDs that can control torque are using this method, and the output of UVW each phase participates in the Hall effect current detection device, and after the sampling reaction, it forms a closed-loop negative reaction current loop PID conditioning. ABB's inverter also proposes direct torque manipulation skills that are different from this method. Please refer to the related materials for details. In this way, the speed and torque of the motor can be controlled, and the speed control accuracy is better than that of V/F control. The encoder response can be added or not, and the control accuracy and echo characteristics are much better.

3. Talk about servo:

Driver: Under the premise of carrying out frequency conversion skills, the servo drive has carried out more accurate control skills and algorithm calculations in the current loop, speed loop and azimuth ring inside the driver (VFD does not have this loop), and it is much stronger than the traditional frequency conversion in terms of function. The key is to be able to perform precise orientation manipulation. The speed and azimuth are controlled by a pulse train sent by the upper controller (of course, some servos have an integrated control unit and may directly communicate with the bus to set parameters such as azimuth and speed in the drive). The driver's internal algorithms, faster and more accurate calculations and better functioning electronics make it superior to VFD.

Motor: The data, structure and processing technology of servo motor are much higher than that of VFD-driven communication motor (general communication motor or constant torque and constant power variable frequency motor). That is to say, when the driver outputs a power supply with rapid changes in current, voltage and frequency, the servo motor can respond to the change of the power supply, and its echo characteristics and anti-overload can be much higher than that of the VFD-driven communication motor. The serious difference in the function of the motor is also the source of the difference between the two. That is to say, it is not that the VFD output cannot change the power signal so quickly, but that the motor itself cannot react. Therefore, in order to protect the motor, the corresponding overload setting was made when setting the internal algorithm of frequency conversion. Of course, even if you don't set the output talent of the VFD, it's still limited. There are some excellent VFDs that can drive servo motors directly!!

Fourth, talk about the communication motor:

Communication motors are generally divided into synchronous motors and asynchronous motors.

1. Communication synchronous motor: that is, the rotor is made of permanent magnet material, so after rotation, with the change of the rotating magnetic field of the motor stator, the speed of the rotor also changes in response to the frequency, and the rotor speed = stator speed, so it is called "synchronization".

2. Communication asynchronous motor: The rotor is composed of an induction coil and data. After the stator rotates, a rotating magnetic field is generated, and the induction coil of the stator is cut, and the rotor coil induces a current, and then the rotor induces a magnetic field. The induced magnetic field follows the change in the rotating magnetic field of the stator, but the change in the magnetic field of the rotor is always less than that of the stator. Once equal to the constant magnetic field, the rotor coil loses the induced current, the rotor magnetic field disappears, the rotor stalls and the stator speed difference occurs, and the corresponding current is obtained from the beginning. Therefore, one of the key parameters in the communication asynchronous motor is the slip rate, which is the ratio of the speed difference between the rotor and the stator.

3. Corresponding to the communication synchronous and asynchronous motor VFD, there is the corresponding synchronous VFD and asynchronous VFD. Servo motors also have communication synchronous servo and communication asynchronous servo. Of course, communication asynchronous frequency conversion is common in VFD, and communication synchronous servo is common in servo.

5. Use

Because VFD and servo are different in function and function, they are used differently:

1. In the case of speed control and torque control, the requirements are not very high. Generally, VFD is used, and the upper response signal forms a closed loop, and frequency conversion is used for azimuth control, so the accuracy and echo are not high. Some VFDs nowadays also accept pulse train signals to manipulate speed, but it seems that they cannot directly control the bearing.

2. In the case of severe azimuth control requirements, it can only be achieved by servo, and the response speed of servo is much faster than that of frequency conversion. Some occasions with high requirements for speed accuracy and response are also used for servo control, and servo can be used instead of all occasions that can be controlled by frequency conversion. There are two key points: one is that the price servo is much higher than the frequency conversion; Second, the power reason: the maximum frequency conversion can be hundreds of KW, or even higher, the maximum servo is tens of KW.

The difference between servo and frequency conversion motors - Xinjie

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