The Comparison between Stepper Motor and AC Servo Motor

The application of stepper motor in production and life is very common, and the generation of all-digital AC servo motor makes it more and more important in the digital control research, so the comparison between the stepper motor and AC servo motor is very meaningful.

Performance of Comparative Analysis

1. The Feature Analysis

Stepper Motor

Theoperation of the stepper motor is mainly to convert electric pulse to angular displacement to achieve digital control. In this process, the driver will get pulse commands, which will make it run and turn to generate a fixed angle that is also called the step angle. The rotation angle of the stepper motor in the process of operation needs to complete step by step, and the number of displacement can be determined by controlling the number of pulses. The stepper motor can be used as a control motor and its operation has the feature of no cumulative error.

AC Servo Motor

(1) The control mode is a 32-bit CPU.

(2) By the self-set mode, it is convenient to operate the system. In this process, the user can conduct secondary development.

(3) Low speed and large torque output.

(4) Achieve a wide range of speed control.

(5) The AC servo motor has a commutation function and completes the host control by computer or PLC.

2. The Performance Analysis

The step angle of the stepper motor usually is a fixed value, the step angle of the two-phase stepper motor can reach up to 1.8 degrees. Also, the motor with better performance has a smaller degree.

The control accuracy of the AC servo motor is controlled by the rotary encoder of the motor shaft. The encoder in the standard motors mainly uses 4-fold frequency technology encoding, its the number of pulses can reach up to 0.036 degrees.

3. Difference analysis of low-frequency characteristic

The stepper motor is prone to low-frequency vibration at low speed, whose main reason is related to a load of equipment and the performance of the driver. The vibration frequency is generally considered to be half of the no-load starting frequency of the motor. This type of low-frequency vibration will cause damage. So, damping technology needs to use.

The AC servo motor has relatively stable properties in the process of operation, it will not cause the low-frequency vibration at low speed. However, this system may have poor mechanical rigidity characteristic, the position needs to be adjusted by the system.

4. Difference Analysis of Moment Frequency Properties

The output torque of the stepper motor will decrease due to the increase in speed. In this process, the torque will decrease rapidly when the speed arrives at a certain height. The AC servo motor generates a constant torque. In the rated speed, the torque is rated.

5. Difference Analysis of Over-load Capacity

Under normal circumstances, the stepper motor doesn’t have the overload capacity, while the AC servo motor can generate strong over-load capacity. So, in the selection process of the stepper motor, the stepper motor with larger torque should choose. But in normal working, the machine doesn’t need such a large. So, there is a waste of torque.

6. Difference Analysis of Operating

During the operation of the control system, the stepper motor uses an open-loop control system. When starting- frequency is too high or over-load too large, it is prone to lose step or stall. When the speed stops at the high speed, the servo operation will overshoot. Therefore, to improve the accuracy of the system, then the problem of controlling the speed should be solved.

The control system of the AC servo motor uses a closed-loop control mode, the driver can complete the sampling of feedback information of the motor encoder. The internal has a position and speed ring, it will not lose steps or overshoot. Compared with the stepper motor, the control performance is more reliable.

7. Comparative Analysis of Speed Response

For the stepper motor, the time from the static to working needs 200-400ms. Due to high acceleration performance, the AC servo motor usually takes a few milliseconds from the static to working. So, it is suitable in the application that requires rapid start and stops.

Attention

Stepper Motor

(1) The stepper motor system is mostly used in low-speed environments. When the rotation speed is less than 1000 rpm, it should be used in the 1000-3000pps.

(2) In the process of running a stepper motor, the full-step state should avoid as much as possible. Because the vibration frequency is relatively high.

(3) The stepper motor should not work in the vibration scope, if it is necessary, you can solve by changing voltage, current.

(4) During the operation of the stepper motor, the speed is increased by the gradual lifting mode. The reason why using this method is that it can prevent the motor from losing a step and reduce noise, and improve position accuracy.

AC Servo Motor

(1) The AC servo motor can be used in places where it is eroded by oil or water.

(2) If it is used in conjunction with gearbox, an oil seal should be added to preventing the oil from gearbox from entering the motor.

(3) During installation and operation, ensure that the radial and axial load applied to the shaft should control within the specified value.

Conclusion:

AC servo motor has many advantages in many fields, but in some specific fields, the stepper motor is more suitable to use. In conclusion, the selection of the motor should be based on the application.