How to choose the fit stepper motor
The development of manufacturing industrial technology is inseparable from stepper motors. The selection of the stepper motor is a very tedious task. The engineer not only needs to know about machine knowledge but also needs to know a lot of electronic and computer knowledge. If the stepper motor is selected only by the drive current, it is definitely not suitable. Therefore, you must understand the meaning of some basic parameters before the selection, such as static torque, drive method, drive voltage, drive current and so on.
Static torque is the holding torque when the rated current passed the motor two-phase coil and the rotor does not turn. It also reflects the ability of the stepping motor, which is similar to the “power” of the traditional motor. Of course, there is also a fundamental difference between the two types of motors: the physical structure of the stepper motor is completely different from that of the AC and DC motors, and the output power of the stepper motor is variable. It is usually possible to choose which frame size to use depending on the amount of torque required. Generally speaking, the torque is below 0.8N.m, and the motor with the frame size NEMA8-NEMA17 can be selected. For the torque of about 1N.m, it is more suitable to select the NEMA23 motor. For larger torque motors, choose NEMA34, NEMA42 and other stepper motors.
In addition to the motor body, the performance of the stepper motor is greatly affected depending on the drive method. When selecting a stepper motor and a driver, it is important to consider the drive method. The driving method is usually divided into constant voltage and constant current drive, unipolar and bipolar drive.
For constant voltage drive, even if the drive voltage is 12V or 24V, the appropriate resistance value must be selected. Usually, the resistance value of the motor is tens of ohms. Otherwise, the current will be too large to burn the chip of the driver. For constant current drive, it is usually used for motors with small resistance and high rated current. The rated current is usually 1-3A. If you need more current, you need to choose some special driver chips, but the price will be slightly more expensive.
Compared to unipolar and bipolar drive circuits, a unipolar drive circuit requires four power tubes, and current flows in a single direction within the coil. The number of power tubes of the bipolar drive circuit is twice that of the unipolar, and the current flows alternately in the positive and negative directions in the coil. Short-term simultaneous conduction causes a short circuit of the power supply and generates a strong current, so it is necessary to prevent the short circuit. The bipolar drive circuit is more complicated than the unipolar case. Generally, low-speed, high-torque loads use bipolar drives, while high-speed drive applications are more suitable for unipolar drives.
The drive voltage refers to the input voltage of the driver. This is related to the operating environment of the machine. It can be selected from DC 12V, 24V, 48V, etc. It can also be AC 24V, 36V, 50V, 60V, 110V, 220V and so on. The effect of voltage selection on the performance of the motor is: the higher the voltage, the better the performance of the stepper motor, especially the higher speed performance of the motor.
The drive current refers to the current passes the single-phase winding of the stepper motor, also called the phase current. Some motors have the same dimensions and the same static torque, but the resistance and rated current are different. Why? For example, MOONS’ MS17HD series motors are available in a variety of resistant and current values. These motors with different current ratings are designed to match the customer’s different functions and different cost drivers: the current is small, the driver assembly is simple and low cost, and the current is large, so the cost of each component of the drive is high.
Torque Frequency Characteristic
The curve of the output torque obtained by the motor during operation is called the running torque frequency characteristic curve, which is one of the most important dynamic curves of the motor. The dynamic torque of the motor depends on the average current when the motor is running. The larger the average current, the larger the output torque of the motor. The output torque of the motor is inversely proportional to the speed. That is to say, the stepping motor has a large output torque at a low speed and a small torque at a high speed. If some operating conditions require a high-speed stepper motor, the drive voltage, resistance and inductance of the stepper motor must be adjusted. Increasing the input voltage of the driver and using a small inductor and small resistance motor can achieve a large output torque at high speed.
The so-called “phase number” is the number of coil groups of the motor, and the two-phase stepping motor has two sets of coils. Many customers pay little attention to the number of phases when purchasing motors, and most of them are purchased at will. In fact, in different phases of the motor, the work effect is different. The 2-phase motor provides a step angle of 0.9° and 1.8°, and the 3-phase motor provides a step angle of 1.2°. The smaller the step angle, the smoother the motor runs. In most cases, there are more two-phase motors. In the high-speed and high-torque working environment, it is more practical to select a three-phase stepping motor.
No-load Starting Frequency
Stepper motor no-load starting frequency, usually called “empty frequency”. This is an important indicator of purchasing a motor. If it is required to start and stop frequently in an instant, and the rotation speed is around 1000 rpm (or higher), “accelerated start” is usually required. If you need to start directly to achieve high-speed operation, it is best to choose reactive or permanent magnet motors, these machines have a relatively high no-load starting frequency.
When you have special specifications for the parameters of the stepper motor, please contact the engineer of MOONS’. We can provide customized services to the extent that technology allows. For example, the diameter, length, and direction of the output shaft， and so on.
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