The fundamental concept of servo systems is accurate, precise, and rapid positioning. Frequency conversion is an essential internal component of servo control—servo drives also include frequency conversion (for stepless speed regulation).

However, a key difference is that servos close the current loop, speed loop, or position loop for control, which significantly distinguishes them from standard inverters. In addition, the structure of a servo motor differs from that of a regular motor, as it must support fast response and precise positioning.

Most AC servo motors currently on the market are permanent magnet synchronous AC servos. Due to manufacturing limitations, it is difficult for these motors to achieve very high power output; synchronous servos above a dozen kilowatts are extremely expensive. Therefore, in practical applications—where conditions allow—asynchronous (induction) AC servos are often used instead. In such cases, many drives are essentially high-end inverters with encoder feedback for closed-loop control.

The essence of a servo system is to fulfill the need for accurate, precise, and fast positioning. As long as these requirements are met, there is no need to argue over whether it is “servo” or “inverter.”


I. Commonalities Between the Two

AC servo technology has, in fact, borrowed and applied frequency conversion principles. Based on servo control of DC motors, AC servo control mimics DC motor behavior using PWM (Pulse Width Modulation) via frequency conversion. In other words, AC servo systems inherently include a frequency conversion stage.

Frequency conversion involves rectifying standard 50/60 Hz AC into DC power, and then using gate-controlled semiconductors (such as IGBTs, IGCTs, etc.) to invert the DC into a variable-frequency waveform (typically sinusoidal or near-sinusoidal) via carrier frequencies and PWM. Since frequency is adjustable, the motor speed becomes controllable (according to the formula: n = 60f/p, where n is speed, f is frequency, and p is the number of pole pairs).


II. Discussion on Inverters (VFDs)

A basic inverter can only control the speed of an AC motor. This can be done in open-loop or closed-loop, depending on the control strategy and inverter type. This is known as traditional V/F (voltage-to-frequency) control.

Nowadays, many inverters use mathematical models to convert the stator magnetic field (UVW 3-phase) into two current components that control motor speed and torque. Most modern torque-controlling inverters from reputable brands adopt this approach. Each phase output (UVW) is equipped with Hall effect current sensors to sample and feed back data, forming a closed-loop current loop with PID adjustment.

ABB, for example, proposes a different method called Direct Torque Control (DTC). For more technical details, you may refer to specialized documentation.

This method allows for control of both motor speed and torque, with greater speed control precision than V/F control. An encoder can be added to further enhance control accuracy and responsiveness, though it is not strictly required.