Inverters and servo drives are both commonly used electrical devices, but why use two different devices to control motor frequency? The following will introduce and compare them from three aspects: inverters, servo motor control, and motors.
Regarding frequency converters:
A simple frequency converter can only regulate the speed of an AC motor. Whether it can operate in open-loop or closed-loop mode depends on the control method and the frequency converter; this is the traditional V/F control method. Many modern frequency converters have established mathematical models that convert the three phases of the AC motor's stator magnetic field (UVW) into two current components that can control the motor's speed and torque. Most well-known brands of frequency converters capable of torque control use this method. Each phase of UVW requires a current sensing device to detect the Moore effect, and the sampled feedback forms a closed-loop negative feedback current loop PID control. AFSEN frequency converters have proposed a different direct torque control technology. This allows control of both motor speed and torque, with speed control accuracy superior to V/F control. Encoder feedback is optional; when added, control accuracy and responsiveness are significantly improved.
Regarding the motor:
The materials, structure, and manufacturing technology of servo motors are far superior to those of AC motors driven by frequency converters (typically AC motors or various types of frequency converters such as constant torque and constant power motors). This means that when the driver outputs a power supply with rapidly changing current, voltage, and frequency, the servo motor can respond accordingly, exhibiting significantly better response characteristics and overload resistance than frequency converter-driven AC motors. This difference in motor characteristics is the fundamental reason for the different functions of the two. It's not that frequency converters cannot output power signals that change so rapidly, but rather that the motor itself cannot react in time. Therefore, frequency converters incorporate overload protection mechanisms in their internal algorithms to safeguard the motor. Of course, even without these mechanisms, the output capacity of a frequency converter is still limited; some high-performance frequency converters can directly drive servo motors.
Regarding servo drivers:
Servo drives, while incorporating frequency conversion technology, employ more precise control techniques and algorithms in their internal current, speed, and position loops (which frequency converters lack). This results in significantly enhanced functionality compared to traditional servos, primarily due to their ability to achieve accurate position control. Speed and position are controlled by pulse sequences sent from a host controller (some servos integrate control units or directly set parameters like position and speed within the drive via bus communication). The drive's internal algorithms, faster and more accurate calculations, and superior electronic components make it superior to frequency converters. (PLC training course)
One important difference between servo and frequency converter is:
Variable frequency drives (VFDs) can operate without encoders, while servo drives must have encoders for electronic commutation. AC servo technology itself borrows from and applies VFD technology. It achieves this by using PWM (Pulse Width Modulation) to mimic the control method of a DC motor, building upon the servo control of a DC motor. In other words, AC servo motors inevitably involve a VFD step: PLC data shows that VFDs first rectify the 50-60Hz AC power into DC power, and then invert it into a frequency-adjustable waveform similar to a sine or cosine pulse through various gate-controllable transistors (IGBTs, IGCTs, etc.) via carrier frequency and PWM adjustment. Because the frequency is adjustable, the speed of the AC motor is also adjustable (n=60f/2p, where n is the speed, f is the frequency, and p is the number of pole pairs).
