The main differences between servo motors and stepper motors:
1. Different low-frequency characteristics
Stepper motors are prone to low-frequency vibration at low speeds. The vibration frequency is related to the load and driver performance, and is generally considered to be half of the motor's no-load starting frequency. This low-frequency vibration, determined by the working principle of stepper motors, is very detrimental to the normal operation of the machine. When stepper motors operate at low speeds, damping techniques should generally be used to overcome low-frequency vibration, such as adding dampers to the motor or using microstepping technology in the driver. AC servo motors operate very smoothly and do not exhibit vibration even at low speeds. AC servo systems have resonance suppression capabilities, which can cover insufficient mechanical rigidity, and the system has an internal frequency analysis function (FFT) that can detect mechanical resonance points, facilitating system adjustments.
2. Different torque-frequency characteristics
The output torque of a stepper motor decreases as the speed increases, and drops sharply at higher speeds. Therefore, its maximum operating speed is generally between 300 and 600 RPM. AC servo motors provide constant torque output, meaning they can output rated torque up to their rated speed (generally 2000 or 3000 RPM), and provide constant power output above the rated speed.
3. Different overload capacities
Stepper motors generally do not have overload capacity. AC servo motors, on the other hand, have strong overload capacity. Because stepper motors lack this overload capacity, a motor with a larger torque is often selected to overcome this inertial torque during selection. However, the machine does not need such a large torque during normal operation, resulting in wasted torque.
4. Different operating performance
Stepper motors are controlled in an open-loop manner. Excessive starting frequency or heavy load can easily lead to missed steps or stalling. Similarly, excessive stopping speed can cause overshoot. Therefore, to ensure control accuracy, the acceleration and deceleration issues must be carefully addressed. AC servo drive systems, on the other hand, use closed-loop control. The driver can directly sample the encoder feedback signal, internally forming position and speed loops. Generally, they do not exhibit the missed steps or overshoot issues common in stepper motors, resulting in more reliable control performance.
5. Different speed response performance
A stepper motor takes 200–400 milliseconds to accelerate from a standstill to its operating speed (typically several hundred revolutions per minute). AC servo systems offer better acceleration performance; therefore, in many aspects, AC servo systems outperform stepper motors. However, stepper motors are still frequently used as actuators in less demanding applications. Therefore, the design of a control system must comprehensively consider factors such as control requirements and cost to select an appropriate control motor.
