Difference Between Stepper Motor and Servo Motor

Stepper motor and Servo motor are two most common driving motor of CNC router machine, wood lathe machine and plasma cutting machine. When buy a CNC machine, many client is very concerned about the difference between them, ask choose which way is better. Below is some difference between stepper motor and servo motor.

1. Different control accuracy

The step angle of the two-phase hybrid stepping motor is generally 3.6°, 1.8°, and the control accuracy of the AC servo motor is guaranteed by the rotary encoder at the back of the motor shaft.

2. Different low frequency characteristics

Stepping motors are prone to low-frequency vibration at low speeds. The AC servo motor runs very smoothly, and there is no vibration even at low speeds. The AC servo system has a resonance suppression function, which can cover the lack of mechanical rigidity.

3. Different moment frequency characteristics 

The output torque of the stepping motor decreases with the increase of the speed, and will drop sharply at higher speeds, so its maximum working speed is generally 300-600RPM. The AC servo motor has a constant torque output, that is, it can output a rated torque within its rated speed (generally 2000RPM or 3000RPM), and a constant power output above the rated speed.

4. Different overload capacity

Stepper motors generally do not have overload capacity. AC servo motors have strong overload capacity. Take the Panasonic AC servo system as an example, it has speed overload and torque overload capabilities.

5. Different operating performance

The stepper motor is controlled by open loop control. If the starting frequency is too high or the load is too large, it is easy to lose steps or stall, and when the speed is too high, it is easy to overshoot. The AC servo drive system is a closed-loop control. The drive can directly sample the feedback signal of the motor encoder. The position loop and speed loop are formed inside. Generally, the stepper motor will not lose steps or overshoot, and the control performance is more reliable.

6. Different speed response performance

It takes 200 to 400 milliseconds for a stepping motor to accelerate from a standstill to a working speed (generally several hundred revolutions per minute). The acceleration performance of the AC servo system is better. Taking the Panasonic MSMA 400W AC servo motor as an example, it only takes a few milliseconds to accelerate from a standstill to its rated speed of 3000RPM, which can be used in control situations that require fast start and stop.

7. Different noise

AC servo has lower noise comparing with stepper motor. 

8. Maintenance

Stepper motors do not require routine maintenance. AC servo is a more precise close loop system and there is feedback from encoder, alarm code will appear on servo pack sometimes when there is any error been detected even it is a tiny error. Meantime servo system instructs machine to stop working as error detected. Under this case, we have to find out the reason and solve it, then error code will disappear and machine works again.

9. Cost

AC servo motor costs is higher than stepper motor. Both China servo motor and Japan servo motor option. It could be choose according to different client's request or budgets.

In summary, AC servo motor is superior to stepper motor in many aspects of performance. Even though that, stepper motor still used for many industries of basic cnc router machine, cnc plasma cutter and wood turning lathe. 

Therefore, different production requirement, control performance, maintenance and cost should be considered comprehensively, and choose the proper driving motor.

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