The basic principle of stepper motor

As an actuator, stepper motor is one of the key products of mechatronics and is widely used in various automatic control systems. With the development of microelectronics and computer technology, the demand for stepper motors is increasing day by day and they are used in various fields of the national economy.

A stepper motor is an actuator that converts electrical pulses into angular displacement. When the stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle (called the "step angle") in the set direction. Its rotation is performed step by step at a fixed angle. The angular displacement can be controlled by controlling the number of pulses to achieve accurate positioning; at the same time, the speed and acceleration of the motor can be controlled by controlling the pulse frequency to achieve speed regulation. Stepper motors can be used as a special motor for control. They are widely used in various open-loop controls due to their lack of accumulated error (100% accuracy).

The more commonly used stepper motors now 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 small torque and volume, and the step angle is generally 7.5 degrees or 15 degrees;

Reactive stepper motors are generally three-phase, can achieve high torque output, and the step angle is generally 1.5 degrees, but the noise and vibration are very large. The rotor magnetic circuit of the reactive stepper motor is made of soft magnetic material, and there are multi-phase excitation windings on the stator, which uses the change of magnetic permeance to generate torque.

Hybrid stepper motors are those that combine the advantages of permanent magnet and reactive stepper motors. They are divided into two-phase and five-phase motors: the two-phase step angle is generally 1.8 degrees and the five-phase step angle is generally 0.72 degrees. This type of stepper motor is the most widely used and is also the stepper motor selected for this subdivision drive solution.

Some basic parameters of stepper motors:

Motor inherent step angle:

It indicates the angle that the motor rotates every time the control system sends a step pulse signal. The motor is given a step angle value when it leaves the factory. For example, the value given for the 86BYG250A motor is 0.9°/1.8° (0.9° for half-step operation and 1.8° for full-step operation). This step angle can be called the "motor inherent step angle". It is not necessarily the true step angle when the motor is actually working. The true step angle is related to the driver.

Number of phases of stepper motor:

Refers to the number of coil groups inside the motor. Currently, two-phase, three-phase, four-phase, and five-phase stepper motors are commonly used. The step angles of motors with different phases are different. Generally, the step angles of two-phase motors are 0.9°/1.8°, three-phase motors are 0.75°/1.5°, and five-phase motors are 0.36°/0.72°. When there is no subdivision driver, users mainly rely on selecting stepper motors with different numbers of phases to meet their step angle requirements. If a subdivision driver is used, the "number of phases" will become meaningless, and users only need to change the subdivision number on the driver to change the step angle.

Holding torque (HOLDING TORQUE):

It refers to the torque of the stator locking the rotor when the stepper motor is powered but not rotating. It is one of the most important parameters of the stepper motor. Usually, the torque of the stepper motor at low speed is close to the holding torque. Since the output torque of the stepper motor decays with the increase of speed, and the output power also changes with the increase of speed, the holding torque has become one of the most important parameters for measuring stepper motors. For example, when people say a 2N.m stepper motor, it means a stepper motor with a holding torque of 2N.m unless otherwise specified.

DETENT TORQUE:

It refers to the torque when the stator locks the rotor when the stepper motor is not powered. There is no unified translation method for DETENT TORQUE in China, which can easily lead to misunderstandings. Since the rotor of a reactive stepper motor is not made of permanent magnet material, it does not have DETENT TORQUE.

Some features of stepper motors:

1. The accuracy of a general stepper motor is 3-5% of the step angle and is not cumulative.

2. The maximum allowable temperature of the stepper motor surface.

If the temperature of a stepper motor is too high, the magnetic material of the motor will be demagnetized, which will cause a decrease in torque or even a loss of steps. Therefore, the maximum allowable temperature on the motor surface should depend on the demagnetization point of the magnetic material of the 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. Therefore, it is completely normal for the surface temperature of the stepper motor to be between 80 and 90 degrees Celsius.

3. The torque of a stepper motor decreases as the speed increases.

When the stepper 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 phase current decreases as the frequency (or speed) increases, resulting in a decrease in torque.

4. The stepper motor can run normally at low speed, but it cannot start if the speed is higher than a certain speed, and it will be accompanied by a howling sound.

Stepper motors have a technical parameter: 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 conditions, the starting frequency should be lower. If the motor is to rotate at high speed, the pulse frequency should have an acceleration process, that is, the starting frequency is low, and then it is increased to the desired high frequency at a certain acceleration (the motor speed increases from low speed to high speed).

With its remarkable characteristics, stepper motors play an important role in the era of digital manufacturing. With the development of different digital technologies and the improvement of stepper motor technology itself, stepper motors will be used in more fields.