Introduction
Servo drivers are used to drive servo motors, which can be stepper motors or AC asynchronous motors. They are mainly used to achieve fast and accurate positioning, and are often used in applications where the movement is stop-and-go and high precision is required.
A frequency converter is used to convert industrial frequency AC power into a current suitable for adjusting the speed of a motor to drive it. Some frequency converters can now also achieve servo control, meaning they can drive servo motors. However, servo drives and frequency converters are still different! Let's explore the differences between servo drives and frequency converters.
Both definitions
A frequency converter is a control device that uses the switching action of power semiconductor devices to convert power frequency into electrical energy of another frequency. It can realize functions such as soft starting of AC asynchronous motor, variable frequency speed regulation, improved operating accuracy, and changing power factor.
Frequency converters can drive variable frequency motors and ordinary AC motors, and mainly play the role of regulating the motor speed.
A frequency converter typically consists of four parts: a rectifier unit, a high-capacity capacitor, an inverter, and a controller.
A servo system is an automatic control system that enables the output controlled variables, such as the position, orientation, and state of an object, to follow any changes in the input target (or given value). Its main task is to amplify, transform, and regulate power according to the requirements of control commands, so that the torque, speed, and position output of the drive device can be controlled very flexibly and conveniently.
A servo system is a feedback control system used to accurately follow or reproduce a process. It is also called a follow-up system. In many cases, a servo system specifically refers to a feedback control system where the controlled variable (the system's output) is mechanical displacement, velocity, or acceleration. Its function is to ensure that the output mechanical displacement (or angle) accurately tracks the input displacement (or angle). The structural composition of a servo system is not fundamentally different from other forms of feedback control systems.
Servo systems can be classified into electromechanical servo systems, hydraulic servo systems, and pneumatic servo systems based on the type of driving components used. The most basic servo system includes servo actuators (motors, hydraulic cylinders), feedback components, and servo drivers. For a servo system to operate smoothly, a host computer, such as a PLC and a dedicated motion control card (industrial PC + PCI card), is also required to send commands to the servo driver.
Working principles of both
The speed regulation principle of a frequency converter is mainly constrained by four factors: the speed (n) of the asynchronous motor, the frequency (f) of the asynchronous motor, the slip (s) of the motor, and the number of pole pairs (p) of the motor. The speed (n) is directly proportional to the frequency (f); changing the frequency (f) changes the motor speed. When the frequency (f) varies within the range of 0-50Hz, the motor speed adjustment range is very wide. Variable frequency speed regulation achieves speed regulation by changing the frequency of the motor's power supply. It mainly adopts an AC-DC-AC method, first converting the mains frequency AC power into DC power through a rectifier, and then converting the DC power into AC power with controllable frequency and voltage to supply the motor. The circuit of a frequency converter generally consists of four parts: rectification, intermediate DC link, inverter, and control. The rectification section is a three-phase bridge uncontrolled rectifier, the inverter section is an IGBT three-phase bridge inverter with a PWM waveform output, and the intermediate DC link is for filtering, DC energy storage, and reactive power buffering.
In simple terms, the working principle of a servo system is based on the open-loop control of an AC/DC motor. Speed and position signals are fed back to the driver via a rotary encoder, resolver, etc., for closed-loop negative feedback PID control. Combined with the driver's internal current closed-loop control, these three closed-loop adjustments significantly improve the accuracy of the motor's output in following the setpoint and its time response characteristics. A servo system is a dynamic follow-up system; the steady-state equilibrium it achieves is also a dynamic equilibrium.
Differences between the two
AC servo technology itself borrows from and applies frequency conversion technology. It is based on the servo control of DC motors and uses the frequency conversion PWM method to imitate the control method of DC motors. In other words, AC servo motors must have a frequency conversion step: frequency conversion is to first rectify the 50 or 60 Hz AC power into DC power, and then invert it into frequency-adjustable waveforms similar to sine and cosine pulsed electricity through various controllable gate transistors (IGBT, IGCT, etc.) by carrier frequency and PWM adjustment. Since the frequency is adjustable, the speed of AC motor is adjustable (n=60f/p, where n is the speed, f is the frequency, and p is the number of pole pairs).
Difference between the two
1. Different overload capacities.
Servo drives typically have a 3x overload capacity, which can be used to overcome the inertial torque of inertial loads at startup, while frequency converters generally allow a 1.5x overload.
2. Control precision.
The control precision of servo systems is far higher than that of frequency converters. Typically, the control precision of a servo motor is ensured by a rotary encoder at the rear end of the motor shaft. Some servo systems even achieve a control precision of 1:1000.
3. Different application scenarios.
Variable frequency control and servo control are two different categories of control. The former belongs to the field of drive control, while the latter belongs to the field of motion control. One is for general industrial applications that do not have high performance requirements and prioritize low cost. The other prioritizes high precision, high performance, and high responsiveness.
4. Different acceleration and deceleration performance.
Under no-load conditions, the servo motor can accelerate from a standstill to 2000 rpm in no more than 20 ms. The acceleration time of the motor is related to the inertia of the motor shaft and the load. Generally, the greater the inertia, the longer the acceleration time.
