Stepper motors , as actuators, are one of the key products in mechatronics and are widely used in various automated control systems. With the development of microelectronics and computer technology, the demand for stepper motors is increasing daily, and they are used in various sectors of the national economy.
A stepper motor is an actuator that converts electrical pulses into angular displacement. When a stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle (called the "step angle") in a set direction. Its rotation occurs step by step at fixed angles. The amount of angular displacement can be controlled by controlling the number of pulses, thus achieving accurate positioning; simultaneously, the speed and acceleration of the motor can be controlled by controlling the pulse frequency, thus achieving speed regulation. Stepper motors are widely used in various open-loop control systems due to their characteristic of having no accumulated error (100% accuracy).
Commonly used stepper motors include reactive stepper motors (VR), permanent magnet stepper motors (PM), hybrid stepper motors (HB), and single-phase stepper motors.
Permanent magnet stepper motors are generally two-phase, with smaller torque and size, and a step angle of 7.5 degrees or 15 degrees.
Reluctance stepper motors are typically three-phase, capable of high torque output, with a step angle generally around 1.5 degrees, but they also generate significant noise and vibration. The rotor windings of a reluctance stepper motor are made of soft magnetic material, while the stator has multi-phase excitation windings, utilizing changes in magnetic permeability to generate torque.
Hybrid stepper motors combine the advantages of permanent magnet and reactive stepper motors. They are further divided into two-phase and five-phase types: two-phase stepper motors typically have a step angle of 1.8 degrees, while five-phase stepper motors typically have a step angle of 0.72 degrees. This type of stepper motor is the most widely used and is the one selected for this microstepping drive solution.
Some basic parameters of a stepper motor:
Motor's inherent step angle:
It represents the angle the motor rotates for each step pulse signal sent by the control system. The motor is given a step angle value at the factory; for example, the 86BYG250A motor is given a value of 0.9°/1.8° (meaning 0.9° for half-step operation and 1.8° for full-step operation). This step angle can be called the 'motor's inherent step angle,' but it is not necessarily the actual step angle during motor operation. The actual step angle depends on the driver.
Number of phases in a stepper motor:
This refers to the number of coil groups inside the stepper motor. Commonly used stepper motors include two-phase, three-phase, four-phase, and five-phase motors. Different numbers of phases result in different step angles. Generally, two-phase motors have step angles of 0.9°/1.8°, three-phase motors 0.75°/1.5°, and five-phase motors 0.36°/0.72°. Without microstepping drivers, users primarily rely on selecting stepper motors with different numbers of phases to meet their step angle requirements. However, with microstepping drivers, the 'number of phases' becomes meaningless; users can simply change the microstepping setting on the driver to change the step angle.
Holding torque:
Holding torque refers to the torque with which the stator holds the rotor in place when the stepper motor is energized but not rotating. It is one of the most important parameters of a stepper motor, and typically, the holding torque at low speeds is close to the holding torque. Because the output torque of a stepper motor decreases with increasing speed, and the output power also varies with speed, holding torque becomes one of the most crucial parameters for evaluating a stepper motor. For example, when people say a 2 N·m stepper motor, unless otherwise specified, they mean a stepper motor with a holding torque of 2 N·m.
DETENTTORQUE:
This refers to the torque with which the stator locks the rotor in a stepper motor when it is not powered on. There is no standardized translation for "DETENTTORQUE" in China, which can easily lead to misunderstandings. Because the rotor of a reactive stepper motor is not made of permanent magnet material, it does not have a DETENTTORQUE.
Some characteristics of stepper motors:
1. The accuracy of a typical stepper motor is 3-5% of the step angle, and this accuracy does not accumulate.
2. The maximum permissible temperature of the stepper motor exterior.
Overheating of a stepper motor will first cause the magnetic material of the motor to demagnetize, resulting in a decrease in torque or even loss of steps. Therefore, the maximum allowable temperature of the motor surface depends on the demagnetization point of the magnetic material of different motors. Generally speaking, the demagnetization point of magnetic materials is above 130 degrees Celsius, and some are even as high as 200 degrees Celsius or more. Therefore, a stepper motor surface temperature of 80-90 degrees Celsius is perfectly normal.
3. The torque of a stepper motor decreases as the rotational speed increases.
When a stepper motor rotates, the inductance of each phase winding generates a back electromotive force (EMF); the higher the frequency, the greater the back EMF. Under its influence, the phase current of the motor decreases as the frequency (or speed) increases, resulting in a decrease in torque.
4. The stepper motor can operate normally at low speeds, but it cannot start if the speed exceeds a certain limit, and it is accompanied by a whistling sound.
A stepper motor has a technical parameter: the no-load starting frequency, which is the pulse frequency at which the stepper motor can start normally under no-load conditions. If the pulse frequency is higher than this value, the motor cannot start normally and may lose steps or stall. Under load, the starting frequency should be even lower. To make the motor reach high speed, the pulse frequency should have an acceleration process, that is, the starting frequency is low, and then it rises to the desired high frequency with a certain acceleration (the motor speed increases from low speed to high speed).
Stepper motors, with their distinctive characteristics, play a significant role in the era of digital manufacturing. With the development of various digital technologies and the improvement of stepper motor technology itself, stepper motors will be applied in even more fields.
