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Step motor 14 Basic knowledge questions and answers

Date: 2020-08-18
Clicks: 271
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Date: 2020-08-18
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Clicks: 271

After several years of development, stepper motor technology has been quite mature and widely used. The following questions and answers about stepper motor can further enable us to understand the theoretical basis of stepper motor.

1.What is a stepper motor?

 Stepper motor is an actuator that converts electric pulse into angular displacement. In layman's terms: when a stepper driver receives a pulse signal, it drives the stepper motor to rotate at a fixed Angle (and stepper Angle) in a set direction. You can control the angular displacement by controlling the number of pulses, so as to achieve the purpose of accurate positioning; At the same time, you can control the speed and acceleration of motor rotation by controlling the pulse frequency, so as to achieve the purpose of speed regulation.


2.Which kinds of stepper motors are there?

  There are three types of stepper motors: permanent magnet (PM), reactive (VR) and hybrid (HB).

  The permanent magnet stepper is generally two-phase, with small torque and volume. The stepper Angle is generally 7.5 degrees or 15 degrees.

  The reaction stepping is generally three-phase, which can achieve large torque output. The stepping Angle is generally 1.5 degrees, but the noise and vibration are very large. In Europe and the United States and other developed countries in the 1980s has been eliminated; Hybrid stepping is a mixture of the advantages of the permanent magnet type and the reactive type. It is also divided into two phases and five phases: the two-phase stepping Angle is generally 1.8 degrees and the five-phase stepping Angle is generally 0.72 degrees. This stepper motor is the most widely used.


3.What is HOLDING TORQUE?

  To hold TORQUE, or TORQUE, is the moment the stator locks the rotor when the stepper motor is energized but not rotating. It is one of the most important parameters of stepper motor, usually the moment of stepper motor at low speed is close to the holding torque. As the output torque of the stepping motor decreases with the increase of the speed, the output power also changes with the increase of the speed, so maintaining the torque has become one of the most important parameters to measure the stepping motor. For example, when people say a 2N.m stepper motor, they mean a stepper motor that maintains a torque of 2N.m without special instructions.


4.What is DETENT TORQUE?

  DETENT TORQUE is the TORQUE that the stator locks the rotor when the stepper motor is not energized.

  DETENT TORQUE has no unified translation method in China, which is easy to make people misunderstand it. Reactant stepper motors don't have DETENT TORQUE because their rotors aren't permanent magnet materials.


5.What is the precision of the stepping motor? Does it accumulate?

  Generally, the precision of stepping motor is 3-5% of stepping Angle, and it is not accumulated.


6.What is the external temperature of the stepper motor allowed?

  High temperature of stepping motor will first demagnetize the magnetic material of the motor, which will lead to a decline in torque and even step loss. Therefore, the maximum allowable temperature of the motor surface should depend on the demagnetization points of different magnetic materials of the motor. Generally speaking, the magnetic material demagnetization point is in the centigrade 130 degrees above, some even as high as the centigrade 200 degrees above, so the stepping motor surface temperature in centigrade 80-90 degrees completely normal.


7.Why does the torque of stepping motor decrease with the increase of speed?

  When the stepping motor rotates, the inductance of each phase winding of the motor will form a reverse electromotive force. The higher the frequency, the greater the reverse ELECTROmotive force. Under its action, the motor increases with frequency (or speed) and the phase current decreases, resulting in a decrease in torque.


8.Why the stepper motor can operate normally at a low speed, but cannot be started above a certain speed, accompanied by a loud noise?

  The stepper motor has a technical parameter: no-load starting frequency, that is, the pulse frequency that the stepper motor can start normally under no-load condition. If the pulse frequency is higher than this value, the motor cannot start normally, which may result in step loss or stalling. Under load, the startup frequency should be lower. If the motor is to reach high speed rotation, the pulse frequency should have an acceleration process, i.e. the starting frequency is low, and then the desired high frequency is increased according to a certain acceleration (motor speed is increased from low speed to high speed).


9.How to overcome the vibration and noise of two - phase hybrid stepping motor at low speed?

  The high vibration and noise of stepper motor at low speed rotation are its inherent disadvantages, which can be generally overcome by the following schemes:

  A. If the stepping motor works in the resonance region, it can avoid the resonance region by changing the deceleration ratio and other mechanical drives;

  B. Use the drive with subdivision function, which is the most commonly used and easiest method;  

  C. Replace it with a stepper motor with a smaller step Angle, such as a three-phase or five-phase stepper motor;

  D. Alternating current servo motor can almost completely overcome the vibration and noise, but the cost is high;

  E. Add a magnetic damper to the motor shaft. Such a product is available on the market, but the mechanical structure has changed greatly.


10.Does the fine fraction of the subdivision drive represent accuracy?

  The subdivision technology of stepper motor is essentially an electronic damping technology (please refer to relevant literature). Its main purpose is to reduce or eliminate the low-frequency vibration of stepper motor, and improving the running accuracy of motor is only an incidental function of the subdivision technology. For example, for a two-phase hybrid stepping motor with a stepping Angle of 1.8°, if the fine fraction of the subdivision driver is set as 4, the running resolution of the motor is 0.45° per pulse. Whether the precision of the motor can reach or approach 0.45° also depends on the control precision of the subdivision current of the subdivision driver and other factors. The precision of subdivision drives may vary greatly from manufacturer to manufacturer; The greater the fine fraction, the more difficult it is to control the accuracy.

11.What is the difference between the tandem connection and the parallel connection between the four - phase hybrid stepper motor and the driver?

  Four-phase hybrid stepping motor is generally driven by two-phase driver. Therefore, the four-phase motor can be connected into two phases by means of series connection or joint connection. The series connection method is generally used when the motor speed is relatively low. At this time, the output current of the driver is 0.7 times that of the phase current of the motor, so the motor generates little heat. And connection method is generally used in the case of high motor speed (also known as high-speed connection method), the required driver output current is 1.4 times of the motor phase current, so the motor heating.


12.How to determine the DC power supply of stepper motor driver?

  A. Determination of voltage: The power supply voltage of hybrid stepper motor driver is generally A wide range (for example, the power supply voltage of IM483 is 12 ~ 48VDC), and the power supply voltage is usually selected according to the working speed and response requirements of the motor. If the motor operates at a high speed or responds quickly to requirements, then the voltage value is also high, but be careful that the ripple of the power supply voltage does not exceed the maximum input voltage of the driver, otherwise the driver may be damaged.

  B. Determination of current: The power supply current is generally determined by the output phase current I of the driver. If linear power supply is used, the power supply current is generally 1.1 ~ 1.3 times of I; If switching power supply is used, the power current is generally 1.5 ~ 2.0 times of I.


13.When is the off-line signal FREE of hybrid stepper motor driver generally used?

  When the off-line signal FREE is low, the current output from the driver to the motor is cut off, and the motor rotor is in a FREE state (off-line state). In some automation equipment, if the driver is constantly on power and the motor shaft is required to be turned directly (manually), the FREE signal can be set low to make the motor off-line for manual operation or adjustment. After manual completion, set the FREE signal high to continue automatic control.


14.How to adjust the rotation direction of the two - phase stepping motor after energizing with a simple method?

  Just switch the A+ and A- (or B+ and B-) that the motor is wired to the driver.


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