The Difference Between AC Servo Motor And Stepper Motor

- Oct 23, 2017-

The difference between AC Servo Motor and Stepper motor

It is well known that the main role of the motor is to produce mechanical drive torque,as the power source of a variety of electrical and mechanicaloperation ,is an important part of CNC cutting machine,butthere are two types motor are widely used in the current market

,one is stepping Motor,the other is the AC servo motor,the following we have a closer look at the difference between the two motor performance

Stepper motor is a discrete motion device,it and modern digital control technology has an essential link.In the current domestic digital control system,the application of stepper motor is very extensive.With the advent of all-digital AC servo systems,AC servo motors are also increasingly being used in digital control systems.In order to meet the development trend of digital control,most of the motion control system using stepper motor or all-digital AC servo motor as the implementation motor.Although both are similar in control mode (pulse series and direction signal),there are significant differences in performance and application.Now compare the performance of the two.

First,the control accuracy is different

Two-phase hybrid stepper motor step angle is generally 3.6 °,1.8 °and 1.2°,five-phase hybrid stepper motor step angle is generally

 0.72 °, 0.36 °.There are also some high-performance stepper motors with smaller step angles.Such as the four-way company

for the use of EDM machine stepper motor,the step angle is 0.09 °;Germany BERGER LAHR company production of three-phase 

hybrid stepper motor, its step angle through DIP switch can set to 1.8 °, 0.9 °, 0.72 °, 0.36 °, 0.18 °, 0.09 °, 0.072 °, 0.036 °,compatible with the two-phase and five-phase hybrid stepper motor step angle.Shenzhen ECON Technology normally produces bipolar and three phase stepping motor,and step angle are 1.8°,1.2°,can be used in most CNC machines.

The accuracy of AC servo motor is guaranteed by the rotary encoder at the rear of the motor shaft.Take ECON all-digital AC servo motor for example,with a standard 2500-wire encoder motor,because the drive internal use of the quadruple frequency technology,the pulse equivalent of 360 ° / 10000 = 0.036 °.For motors with 17-bit encoders,the driver receives 217 = 131072 pulse, motors turn one round,ie,the pulse equivalent is 360 ° / 131072 = 9.89 seconds,is 1/655 of the pulse equivalent of the stepper motor with step angle 1.8 °.

Second,different low-frequency characteristics

Stepper motor is easy to have low frequency vibration phenomenon at low speed.The vibration frequency is related to the load 

condition and the drive performance.It is generally believed that the vibration frequency is half of the motor no-load take-off frequency.This low-frequency vibration phenomenon,which is determined by the working principle of the stepper motor,is very unfavorable to the normal operation of the machine.When the stepper motor is operating at low speed,damping technology should generally be used to overcome the low frequency vibration phenomenon,such as add damper on the motor or drive using micro-step technology.

AC servo motor runs very smoothly,even at low speed will not occur vibration phenomenon.AC servo system with resonance suppression function,can cover the lack of mechanical rigidity,and system internal has a frequency resolution function(FFT),can detect the mechanical resonance point,easy to adjust the system.

Third,different torque-frequency characteristics

The output torque of the stepper motor decreases as the speed increases and decreases drastically at higher speeds,so the maximum operating speed is typically between 300 and 600 RPM.AC servo motor is constant torque output,that is,within its rated speed (usually 2000RPM or 3000RPM),can output rated torque,above the rated speed is constant power output.

Fourth,overload capacity is different

Stepper motors generally do not have overload capacity.AC servo motor has a strong overload capacity.Take ECON AC servo

system as an example,it has a speed overload and torque overload capacity.The maximum torque is three times of the rated torque and can be used to overcome inertia torque of the inertia load at the moment of activation.Stepping motor since do not have such overload capacity,in order to overcome this inertia moment in the selection,often need to select a larger torque of the motor,while the machine do not need such a large torque during normal work,then waste of torque.

Fifth,the running performance is different

Stepper motor control is open-loop control,the starting frequency is too high or the load is too large tend to step lost or stall phenomenon,stop at high speed is prone to overshoot phenomenon,so to ensure its control accuracy, should be handled increase and reduce speed problem.AC servo drive system is closed-loop control,the driver can be directly sampled from the motor encoder feedback signal,the internal configuration of the location loop and speed loop,generally do not appear stepping motor step lost or overshoot phenomenon,control performance is more reliable.

Six,the speed response performance is different

Stepping motor from the static acceleration to the working speed (usually hundreds of turns per minute) need 200 to 400 

milliseconds.AC servo system acceleration performance is better,take ECON 400W AC servo motor for example,from the static acceleration to its rated speed 3000RPM only a few milliseconds,can be used for fast start and stop control occasions.

In summary,the AC servo system is superior to stepper motors on many performance aspects.But in some low requirement occasions

often use stepper motor to do the implementation motor.Therefore,ECON reminds consumers in the purchase driving control system process should consider control requirements,costs and other factors,choose the suitable control motor.

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