6 min read
19 Aug

The basic concept of the servo motor system is accurate, precise, and fast positioning. Frequency conversion is a necessary internal link of servo control, and there is also frequency conversion in servo drives (step-less speed regulation is required).

But the servo motor system closes the current loop speed loop or position loop for control, which is a big difference. Besides, the servo motor structure is different from that of the ordinary motor, and it must meet the requirements of fast response and accurate positioning.

Most of the AC servo motors circulating on the market are permanent magnet synchronous AC servos. Still, this kind of motor is limited by technology, and it isn't easy to achieve immense power. Synchronous servos of more than ten KW are very expensive to be used in the field. In high-end AC asynchronous servo, many drives are high-end frequency converters with encoder feedback closed-loop control.
The so-called servo is to satisfy accurate, precise, and fast positioning. As long as it is satisfied, there will be no competition for servo frequency conversion.

The Siemens Servo Motor System 1

Firstly, What the two have in common.

The AC servo technology itself is borrowed from and applied the technology of frequency conversion. Based on the DC motor's servo control, 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. 
Frequency conversion is to rectify the 50, 60HZ AC power of the industrial frequency into DC power. Then through various transistors (IGBT, IGCT, etc.) that can control the gate, adjust the carrier frequency, and PWM to invert it into an adjustable frequency waveform similar to sine and cosine. Due to the adjustable frequency, the speed of the AC motor can be adjusted (n=60f/p, n speed, f frequency, p pole pairs).

The Brief Introdution of Siemens Servo Motor 

Secondly, Talking about the Inverter.

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.
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 should be added with 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 could control both the motor's speed and the motor's torque, 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.

The Siemens Invert 1

Thirdly, Talking about 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, and the main point can be precise position control.
The upper controller's pulse sequence controls the speed and position (of course, some servos have integrated control units or directly set the position and speed parameters in the drive-through bus communication). The internal algorithm of the drive is faster accurate calculations. Better performance electronic devices make them superior to inverters.

The Siemens Driver 1

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 a 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 significant difference in the motor is also a fundamental reason for the difference in performance.
That is to say, and it is not that the inverter can not output the power signal that changes so quickly. Still, the motor itself cannot respond, so when setting the inverter's internal algorithm, the corresponding overload setting is made to protect the motor. Of course, even if the inverter's output capacity is not set, some inverters with good performance can directly drive the servo motor.

The Siemens Servo Motor 1

Fourth, Talking about AC Motors.

AC motors are generally divided into synchronous and asynchronous motors:
1. AC synchronous motor: the rotor is made of permanent magnet materials, so after rotation, as the stator rotating magnetic field of the motor changes, the rotor also changes the speed of the response frequency, and the rotor speed = the stator speed, so it is called "synchronization."
2. AC asynchronous motor: the rotor is composed of induction coils and materials. After rotation, the stator generates a rotating magnetic field, which cuts the stator's induction coil, and the rotor coil generates an induced current. Then the rotor generates an induced magnetic field. The induced magnetic field follows the change of the stator's rotating magnetic field.
Once it is equal, there is no changing magnetic field to cut the rotor's induction coil. There is no induced current in the rotor coil, the rotor magnetic field disappears, and the rotor stalls and the stator produces a speed difference, and the induced current is regained. So in AC asynchronous, A key parameter in the motor is the slip ratio, which is the ratio of the speed difference between the rotor and the stator.
3. Corresponding to AC synchronous and asynchronous motor inverters, corresponding synchronous inverters, and asynchronous inverters. Servo motors also have AC synchronous servos and AC asynchronous servos. Of course, AC asynchronous inverters are standard in inverters, and AC synchronous servos are typical for servos.

The Siemens Servo Motor 2

Finally, the Application of Servo Motor.

Due to the difference in performance and function between the inverter and the servo, the applications are also different:
1. In the speed control and torque control occasions, the general frequency converter is not very demanding. There is also the position control by adding the position feedback signal to the upper position to form a closed loop using frequency conversion. The accuracy and response are not high. Nowadays, some frequency converters also accept pulse sequence signals to control the speed, but it seems that they cannot directly control the position.
2. Servo Motor could only be used on occasions with strict position control requirements, and the response speed of servo is far greater than that of frequency conversion. Some occasions that require high-speed accuracy and response also use servo control and can use frequency conversion control. The servo can be used for almost all kinds of sports. The key is two points: one is that the servo price is much higher than that of frequency conversion; the other is the reason for power: the most extensive frequency conversion can achieve hundreds of KW or even higher, and the largest servo 10 KW.

The more Application of Servo Motor 1

Siemens S7 series PLC and SEW servo motor distributed Jinan LIJIANG Glass transmission control system; describe the system's network architecture and the control strategy of the underlying servo motor. Explain the selection method and parameter calculation formula of the servo motor, and design the servo controller's typical program. The system uses a single CPU module, a dual host computer management, and is based on WINCC and STEP7 software platforms to realize real-time monitoring of the production line drive system. Production practice results show that: based on the complete structure, rich functions, stable work, and excellent performance of the transmission control system, it meets the performance requirements of the Low-E engineering glass magnetron sputtering production process for the production line transmission system, which is beneficial to the realization of the automation of the production line. 

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