• Main Functions and Advantages of Servo Motors

    5 days ago 0 comments

    The main purpose of servo motors is to control the displacement and operation of the motor equipment. As the commonly used motors, servo motors enjoy the advantages of precise positioning, which results in a wide application.

    1.The Main Functions



    (1)The servo motor itself also has function of generating a pulse current. Each rotation of an angle will emit a corresponding number of pulses that can be used by servo motors to form a response and a closed loop. In this way, the system can know the number of pulses it has already sent to the servo motor and how many pulses it has already received at the same time, through which it can control the rotation precisely and the positioning precision can reach 0.001mm.

    (2)DC servo motor can be divided into brushed motor and brushless motor. In terms of brushed motor, it has edges in low cost, simple structure, large starting torque, wide speed range and easy control. But it needs to change carbon brushes. Although the change has no difficulty, it will cause electromagnetic interference and is burdened with environmental requirements. Therefore, it can be applied in common industry and civil applications that are sensitive to the costs.

    (3)While brushless motors are featured by small size, light weight, great output, fast responding speed, small inertial, and stable rotation and torque. It is complex to control, but it is easy to be intelligent. Commutated by square wave and sine wave, the electronic commutation is flexible. The motor is free from maintenance and enjoys a high efficiency, low working temperature, subtle electromagnetic radiation, and a long service time. Besides, it can be applied in any environment.

    (4)The AC servo motor can be divided into a synchronous motor and an asynchronous motor. Currently, the former is often applied in motion control. It has a large power range and can work with large power. With low maximum RPM and inertial which will rapidly decreases with the increase of power. Therefore, it is used in what requires low speed and stable operation.

    2.The Advantages

    (1)Having extraordinary response speed, the servo motor provides a large torque in the point-to-point fast positioning motion occasion, which makes the system have a extremely high dynamic response and greatly exceeds the limit of the traditional stepping system.

    (2) The servo motor remains in the range of its rated speed range and belongs to constant torque output. And it has a variety of feedback adjustments to ensure the running accuracy of the servo motor and output torque.

    (3)The Main Applications

    *Machinery industry:machining centers, CNC punching machines, bending machines, shearing machines, PLC program controllers, etc.;

    *Printing industry:drying board machine; printing machine, offset printing machine, rotary machine, laser phototypesetting, proofing equipment, automatic punching and inkjet printing machine, etc.;

    *Medical industry:monitor, B-ultrasound machine, CT control box, electrocardiogram ( EEG) tester, X-ray machine, etc.;

    *Food industry: sterilizer, homogenizer, labeling machine, can making machine, flow control instrument, sealing machine, etc.;

    *Rubber tire industry: steel wire belt ply production line, inner liner extrusion production line, steel wire calender, etc.;

    *Elevator industry: door control, floor control, etc.

  • Differences between Stepper Motors and Servo Motors

    5 days ago 0 comments

    Both stepper motors and servo motors are servo units for motion control purposes, which can precisely control the location and speed of motion mechanisms.To adapt to the development trend of digital control, stepper motors or all-digital motors are mostly hired as executive motors in motion control systems, which are widely used in the era of labor saving. Despite the similarity in position controlling, namely pulse train and direction signal, both two motors greatly differ in performance and application.

    1.Different Control Precision

    The step angle of the two-phase hybrid stepper motor is generally 1.8°, and the step angle of the three-phase hybrid stepper motor is 1.2°.There are also some high-performance stepper motors with smaller step angles. After using subdivision drive technology, the control precision get enhanced so that the number of steps the two-phase hybrid stepper motor can distinguish is about 51200 steps/rev. There are also some motors with higher precision, but their cost will be more.

               

    The control precision of AC servo motors is guaranteed by the rotary encoder at the rear end of the motor shaft. For a servo motor with a standard 2500 line encoder, its pulse equivalent should be 360°/10000=0.036°due to the adoption of quadruple frequency technology.

    It is worth noting that a considerable number of people think highly of the precision of AC servo motors, which is actually a misinterpretation. Even with 100 times electronic gear or 30000 line encoder, it will make no difference if the servo motor cannot distinguish that small step. The feedback of encoder is nothing more than making the servo driver align it teeth repeatedly.

    Compare with the vast majority of users, the stepper servo motors possess the highest physical precision in terms of the mechanical transmission precision or positioning precision of photoelectric sensor. It is unnecessary to pursue the highest precision in single aspect.

    2.Different Overload Capacity

    Stepper motors are not able to address overload, while AC servo motors own abilities to address overload and torque overload at 2 to 3 times, which can be used to overcome the inertial torque when starting the inertial overload. Without this ability, the torque of overload and movement characteristics should be taken into consideration when you select the type of motors.

    3.Different Operating Performance

          

    The stepper motors control in a open-loop way, where situations including step loss or stalling will easily occur when its starting frequency is too high or its load is too large. Therefore, acceleration and deceleration control program need to be handled in order to ensure its control precision.However, the AC servo motors adopts a closed-loop way. The driver can directly sample the feedback signal of the motor encoder and form position loop and speed loop inside, which makes its control more reliable. Generally, there will be no step loss or overshooting.Besides, the servo driver can automatically rectify lost pulse and give feedback on time to the controller when it is blocked. However, the stepper motor must use sufficient torque margin to avoid stalling.

    4.Different Speed Response Performance

    It takes 100-2000 milliseconds for a stepping motor to accelerate from a standstill to a working speed (generally several hundred revolutions per minute). With better acceleration performance, it only takes a few milliseconds for the AC servo motors to accelerate from a standstill to its rated speed of 3000RPM, which can be applied in occasions requiring fast start and stop.

    In one word, the AC servo motor is superior to stepper motor in terms of some aspects. However the latter will not be replaced wholly due to it unique characteristics.

  • The Working Principle and Application of Variable-frequency Drive

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    Adopting frequency conversion technology and microelectronics technology, Variable-frequency Drive, a kind of power control, changes its working power frequency in order to control AC motor.

    The VFD is mainly composed of rectification (AC to DC), filter, inverter (DC to AC), braking unit, drive unit, detection unit, micro-processing unit, etc.


    Dependent on switching on or off internal IGBT, the VFD can adjust the voltage and frequency of output power supply. For the purpose of energy saving and speed adjustment, it can provide the power and voltage needed in accordance with practical demand of motors. Besides, it also supports many protection functions to avoid terrible consequences caused by overcurrent, overvoltage, overload and so on.

    The working principle of VFD is to adopt circuit control to control main circuit, whose rectifier converts AC power into DC power that will be filtered and smoothed in DC intermediate circuit. Then the inverter finally converts the DC power into AC power of the required frequency and voltage. Some VFD will also add CPU and other components to perform necessary torque calculations.

    The VFD is an electrical device that can convert power frequency into any frequency and any voltage, which is used to adjust the power and speed of motors. It can be divided into 2 parts: the control circuit and main circuit, and the latter is composed of rectifiers and inverters.

    The power frequency of VFD is generally 50Hz or 60Hz, and its frequency and voltage remains the same regardless of being applied in domestic use or production use. Those motors which work at power frequency will cause power loss when adjusting speed. While after the adjustment of VFD, the motor can reduce loss.

    As the requirement of industrial automation becomes enhanced and advocacy of energy conservation is prevailing , the VFD get an increasing application, such as in frequency conversion energy saving, automation system, improvement of crafting products qualities and motor soft starting.