Working principle of induction stepper motor

(I) Principle of reactive stepper motor
Since the working principle of reactive stepper motor is relatively simple. The following is a description of the principle of three-phase reactive stepper motor.
1. Structure:
The motor rotor is evenly distributed with many small teeth, and the stator teeth have three excitation windings, whose geometric axes are staggered with the rotor tooth axes respectively.
0, 1/3て, 2/3て, (the distance between the axes of two adjacent rotor teeth is the tooth pitch, expressed in て), that is, A is aligned with tooth 1, B is staggered 1/3て to the right with tooth 2, C is staggered 2/3て to the right with tooth 3, A' is aligned with tooth 5, (A' is A, tooth 5 is tooth 1)
2. Rotation:
If phase A is energized, and phases B and C are not energized, due to the effect of the magnetic field, tooth 1 is aligned with A, (the rotor is not subjected to any force and the following are the same).
If the B phase is energized and the A and C phases are not energized, tooth 2 should be aligned with B. At this time, the rotor moves to the right by 1/3, and the offset between tooth 3 and C is 1/3, and the offset between tooth 4 and A is (1/3) = 2/3.
If the C phase is energized and the A and B phases are not energized, tooth 3 should be aligned with C. At this time, the rotor moves to the right by 1/3, and the offset between tooth 4 and A is 1/3.
If the A phase is energized and the B and C phases are not energized, tooth 4 is aligned with A, and the rotor moves to the right by 1/3. In
this way, after A, B, C, and A are energized respectively, tooth 4 (that is, the tooth before tooth 1) moves to the A phase, and the motor rotor rotates to the right by one tooth pitch. If A, B, C, A... are continuously pressed to energize, the motor rotates to the right by 1/3 per step (per pulse). If A, C, B, A... are pressed to energize, the motor will reverse.
It can be seen that the position and speed of the motor are in a one-to-one correspondence with the number of conduction times (number of pulses) and the frequency. The direction is determined by the conduction sequence.
However, for the sake of torque, stability, noise and angle reduction, the conduction state of A-AB-B-BC-C-CA-A is often used, so that the original 1/3 of each step is changed to 1/6. Even through different combinations of two-phase currents, 1/3 of each step is changed to 1/12, 1/24, which is the basic theoretical basis for motor subdivision drive.
It is not difficult to deduce that there are m-phase excitation windings on the motor stator, and their axes are offset from the rotor tooth axis by 1/m, 2/m......(m-1)/m, 1. And the conduction can be controlled in a certain phase sequence, which is the physical condition for the rotation of the stepper motor. As long as this condition is met, we can theoretically manufacture stepper motors of any phase. For cost and other considerations, the market generally has two, three, four, and five phases.
3. Torque:
Once the motor is powered on, a magnetic field (magnetic flux Ф) will be generated between the stator and the rotor. When the rotor and the stator are offset by a certain angle, a force will be generated.

S its magnetic flux

Br is the magnetic flux density, S is the magnetic conductive area,
F is proportional to L*D*Br
, L is the effective length of the core, D is the rotor diameter,
Br=N·I/R,
N·I is the ampere-turns of the excitation winding (current multiplied by the number of turns), and R is the magnetic resistance.
Torque = force*radius
Torque is proportional to the effective volume of the motor*ampere-turns*magnetic flux density (only considering the linear state).
Therefore, the larger the effective volume of the motor, the larger the excitation ampere-turns, the smaller the air gap between the stator and the rotor, and the greater the motor torque, and vice versa.

(II) Induction stepper motor
1. Features:
Compared with the traditional reaction stepper motor, the induction stepper motor has a permanent magnet added to the rotor to provide the working point of the soft magnetic material, and the stator excitation only needs to provide a changing magnetic field without providing the energy consumption of the working point of the magnetic material. Therefore, the motor has high efficiency, low current and low heat generation. Due to the presence of permanent magnets, the motor has a strong back electromotive force and its own damping effect is relatively good, making it relatively stable, low noise and low-frequency vibration during operation.
Induction stepper motors can be regarded as low-speed synchronous motors to some extent. A four-phase motor can be operated in four phases or in two phases. (Bipolar voltage drive must be used), but this is not the case with reaction motors. For example: four-phase, eight-phase operation (A-AB-B-BC-C-CD-D-DA-A) can completely adopt two-phase eight-beat operation. It is not difficult to find that the conditions are C=, D=.
The internal winding of a two-phase motor is exactly the same as that of a four-phase motor. Small-power motors are generally directly connected to two phases, while larger-power motors, in order to facilitate use and flexibly change the dynamic characteristics of the motor, often have their external wiring connected to eight leads (four phases). When used in this way, it can be used as a four-phase motor or as a two-phase motor winding in series or in parallel.
2. Classification
Induction stepper motors can be divided into two-phase motors, three-phase motors, four-phase motors, five-phase motors, etc. according to the number of phases. According to the frame size (motor outer diameter), they can be divided into: 42BYG (BYG is the code for induction stepper motors), 57BYG, 86BYG, 110BYG, (international standard), while 70BYG, 90BYG, 130BYG, etc. are all domestic standards.
3. Static indicator terms of stepper motors
Phase number: the number of pairs of exciting coils that generate N and S magnetic fields of different poles. It is usually represented by m.
Beat number: the number of pulses or conductive states required to complete a periodic change of a magnetic field is represented by n, or the number of pulses required for the motor to rotate through a tooth pitch angle. Taking a four-phase motor as an example, there are four-phase four-beat operation modes, namely AB-BC-CD-DA-AB, and four-phase eight-beat operation modes, namely A-AB-B-BC-C-CD-D-DA-A.
Step angle: corresponding to a pulse signal, the angular displacement of the motor rotor is represented by θ. θ=360 degrees (rotor teeth number J*operation beat number), taking a conventional two-phase, four-phase, 50-tooth motor as an example. When running with four beats, the step angle is θ=360 degrees/(50*4)=1.8 degrees (commonly known as full step), and when running with eight beats, the step angle is θ=360 degrees/(50*8)=0.9 degrees (commonly known as half step).
Positioning torque: The locking torque of the motor rotor itself when the motor is not powered (caused by the harmonics of the magnetic field tooth shape and mechanical errors)
Static torque: The locking torque of the motor shaft when the motor is not rotating under the rated static electrical action. This torque is a standard for measuring the volume (geometric dimensions) of the motor and has nothing to do with the drive voltage and drive power supply.
Although the static torque is proportional to the electromagnetic excitation ampere-turns and is related to the air gap between the stator and the rotor, it is not advisable to excessively reduce the air gap and increase the excitation ampere-turns to increase the static torque, which will cause heating and mechanical noise of the motor.
4. Dynamic indicators and terminology of stepper motors:
1. Step angle accuracy:
The error between the actual value and the theoretical value of each step angle of the stepper motor. Expressed as a percentage: error/step angle*100%. The value is different for different running beats. It should be within 5% when running with four beats and within 15% when running with eight beats.
2. Loss of step:
The number of steps when the motor is running is not equal to the theoretical number of steps. It is called loss of step.
3. Misalignment angle:
The angle at which the rotor tooth axis deviates from the stator tooth axis. There must be a misalignment angle when the motor is running. The error caused by the misalignment angle cannot be solved by using subdivided drive.
4. Maximum no-load starting frequency:
The maximum frequency at which the motor can be started directly without load under a certain drive form, voltage and rated current.
5. Maximum no-load operating frequency:
The maximum speed frequency of the motor without load under a certain drive form, voltage and rated current.
6. Operating torque-frequency characteristic:
The curve of the relationship between the output torque and frequency measured during the operation of the motor under certain test conditions is called the operating torque-frequency characteristic. This is the most important of the many dynamic curves of the motor and is also the fundamental basis for motor selection.
Other characteristics include inertia frequency characteristics, starting frequency characteristics, etc.
Once the motor is selected, the static torque of the motor is determined, but the dynamic torque is not. The dynamic torque of the motor depends on the average current (not the static current) when the motor is running. The larger the average current, the greater the output torque of the motor, that is, the harder the frequency characteristic of the motor.
Among them, curve 3 has the largest current or the highest voltage; curve 1 has the smallest current or the lowest voltage. The intersection of the curve and the load is the maximum speed point of the load.
To make the average current large, increase the driving voltage as much as possible, and use a motor with small inductance and large current.
7. Resonance point of the motor:
Stepper motors have a fixed resonance area. The resonance area of ​​two-phase and four-phase induction stepper motors is generally between 180-250pps (step angle 1.8 degrees) or around 400pps (step angle 0.9 degrees). The higher the motor drive voltage, the greater the motor current, the lighter the load, and the smaller the motor volume, the more the resonance area shifts upward, and vice versa. In order to make the motor output torque large, without losing steps and reduce the noise of the entire system, the general working point should shift more from the resonance area.
8. Motor forward and reverse control:
When the motor winding power-on sequence is AB-BC-CD-DA or (), it is forward rotation, and when the power-on sequence is DA-CA-BC-AB or (), it is reverse rotation.