The fundamental concept of a servo motor is accurate and rapid positioning. Frequency conversion is an essential internal component of servo control, and servo drives also incorporate frequency conversion. However, a significant difference lies in how servo motors control the system by closing both the current loop, speed loop, and position loop. Furthermore, the construction of servo motors differs from that of ordinary motors, designed to meet the requirements of rapid response and accurate positioning. Currently , most commercially available AC servo motors are permanent magnet synchronous AC servos. However, due to manufacturing limitations, it's difficult to achieve high power outputs with these motors ; synchronous servos exceeding tens of kilowatts are extremely expensive. Therefore, when field applications allow, asynchronous AC servos are often used. In these cases, many drives are high- end frequency converters with encoder feedback closed-loop control. Ultimately, the essence of servo control is accurate and rapid positioning; once this is achieved, the debate between servo and frequency converter becomes irrelevant.
I. Let's talk about frequency converters:
A simple frequency converter can only regulate the speed of an AC motor. In this case, it can be either open-loop or closed-loop, depending on the control method and the frequency converter. This is the traditional V/F control method.
II. Similarities between the two:
AC servo technology itself borrows from and applies frequency conversion technology. It achieves servo control of DC motors by using frequency conversion PWM to mimic the control method of DC motors. In other words, AC servo motors inevitably involve a frequency conversion stage: frequency conversion first rectifies the 50-60Hz AC power into DC power, then uses various gate-controlled transistors to invert it into a frequency-adjustable waveform similar to sine and cosine pulses through carrier frequency and PWM adjustment. Because the frequency is adjustable, the speed of the AC motor is also adjustable.
III. Let's talk about servers:
Regarding the drive: With the development of frequency conversion technology, the servo drive has implemented more precise control technology and algorithm calculations in the current loop, speed loop and position loop inside the drive than ordinary frequency conversion. It is also much more powerful in function than traditional frequency conversion. The main point is that it can perform precise position control.
Regarding the motor: the materials, structure, and manufacturing process of servo motors are far superior to those of AC motors driven by frequency converters. This means that when the driver outputs a power supply with rapidly changing current, voltage, and frequency, the servo motor can respond accordingly. Its response characteristics and overload resistance are far superior to those of AC motors driven by frequency converters. This significant difference in motor characteristics is the fundamental reason for the performance difference between the two. In other words, it's not that frequency converters cannot output such rapidly changing power signals, but rather that the motor itself cannot react in time. Therefore, the internal algorithm of the frequency converter includes overload protection settings to protect the motor. Of course, even without these settings, the output capacity of the frequency converter is still limited; some high-performance frequency converters can directly drive servo motors!
IV. Application
Because frequency converters and servos differ in performance and functionality, their applications also differ:
1. In applications where speed and torque control requirements are not very high, frequency converters are generally used. Some frequency converters also use a closed-loop control system with position feedback signals added to the host computer for position control, but the accuracy and response are not high. Some frequency converters now also accept pulse sequence signals to control speed, but they don't seem to be able to directly control position.
V. Let's talk about AC motors:
Servo motor manufacturers generally categorize their motors into synchronous and asynchronous motors.
1. AC synchronous motor: The rotor is made of permanent magnet material. So when it rotates, the rotor also changes its speed in response to the change of the rotating magnetic field of the stator. Moreover, the rotor speed is equal to the stator speed, so it is called "synchronous".
2. Corresponding to AC synchronous and asynchronous motor frequency converters, there are corresponding synchronous frequency converters and asynchronous frequency converters. Servo motors also have AC synchronous servos and AC asynchronous servos. Of course, AC asynchronous frequency converters are more common in frequency converters, while AC synchronous servos are more common in servos.
3. AC asynchronous motor: The rotor is composed of induction coils and materials. After rotation, the stator generates a rotating magnetic field. This magnetic field cuts the stator's induction coils, inducing a current in the rotor coils. This, in turn, generates an induced magnetic field in the rotor. The induced magnetic field follows the changes in the stator's rotating magnetic field, but the change in the rotor's magnetic field is always less than the change in the stator's. Once they equal, there is no longer a changing magnetic field cutting the rotor's induction coils, and no induced current appears in the rotor coils. The rotor's magnetic field disappears, the rotor stalls, and a speed difference with the stator is created, allowing it to regain induced current. Therefore, a key parameter in AC asynchronous motors is the slip rate, which is the ratio of the speed difference between the rotor and stator.
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