Expert blog

Selecting a stepper motor.

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Stepper motors are inseparable from the development of industrial manufacturing technology. Selecting a stepper motor is very painstaking work. The engineer has to not only know the machines well, but must also possess a great deal of knowledge in electronics and computing. If the stepper motor is selected only according to the drive current, it will definitely not be suitable. Therefore, you need to understand the meaning of some basic parameters before making the selection, such as static torque, drive method, drive voltage, drive current, and so on.

Static torque

Static torque is the holding torque when the rated current passes through the motor's two-phase coil and the rotor does not turn. It also reflects the capacity of the stepper motor, which is similar to the “power” of a 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 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 according to the amount of torque required. Generally speaking, the torque is below 0.8 N.m, so a motor with the frame size NEMA8-NEMA17 can be selected. For torques of about 1 N.m, it is more suitable to select the NEMA23 motor. For motors with higher torque, choose NEMA34, NEMA42, and other stepper motors.

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Drive method

In addition to the motor body, the performance of a stepper motor is greatly affected by the drive method. When selecting a stepper motor and a driver, it is important to consider the drive method. The drive method is usually divided into constant voltage versus constant current drives, and unipolar versus bipolar drives.

For constant voltage drives, even if the drive voltage is 12 V or 24 V, the appropriate resistance value must be selected. Usually, the resistance value of the motor is in tens of ohms. Otherwise, the current will be too high and will burn the chip of the driver. For constant current drives, the motors used usually have 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.

In regard 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 these short circuits. 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 suited to unipolar drives.

Image 1 drivetech moons stepper

Drive voltage

Drive voltage refers to the input voltage of the driver. This is related to the operating environment of the machine. The drive voltage selected can be a DC voltage: 12 V, 24 V, 48 V, etc. It can also be an AC voltage: 24 V, 36 V, 50 V, 60 V, 110 V, 220 V, 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 speed performance of the motor.

Drive current

Drive current refers to the current that passes through 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 their resistance and rated current differ. Why? For example, MOONS' MS17HD series motors are available in a variety of resistance and current values. These motors with different current ratings are designed to match the customer's different functions and different cost drivers: when the current is low, the driver assembly is simple and low cost, and when the current is high, 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, and 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 higher the average current, the higher the output torque of the motor. The output torque of the motor is inversely proportional to the speed. That is to say, the stepper motor has a high output torque at a low speed and a low 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 high output torque at high speed.

Image 2 drivetech moons stepper


The so-called “phase number” is the number of coil groups of the motor. A two-phase stepper 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 arbitrarily. In fact, in different phases of the motor, the work effect is different. The two-phase motor provides a step angle of 0.9° and 1.8°, and the three-phase motor provides a step angle of 1.2°. The smaller the step angle, the smoother the motor runs. Two-phase motors are more commonly used. In a high-speed and high-torque working environment, it is more practical to select a three-phase stepper motor.

No-load starting frequency

Stepper motor no-load starting frequency, usually called “empty frequency,” is an important indicator when purchasing a motor. If the application requires starting and stopping 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, machines which have a relatively high no-load starting frequency.

Customized service

When you have special specifications for the parameters of the stepper motor, please contact a MOONS' engineer. We can provide customized services to the extent that the technology allows. This can include the diameter, length, and direction of the output shaft, and so on.

Author: maxon France

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