Stepping motor is an open-loop control element that converts electrical pulse signals into angular displacement or linear displacement. In the case of non-overload, the speed and stop position of the motor only depend on the frequency and pulse number of the pulse signal, and are not affected by the load change, that is, when a pulse signal is added to the motor, the motor will rotate through a step angle.
The existence of this linear relationship, coupled with the characteristics of the stepper motor having only periodic errors and no cumulative errors. This makes it very simple to use a stepper motor to control the speed, position and other control fields.
Although stepper motors have been widely used, stepper motors are not like ordinary DC motors. AC motors are used routinely. It must be composed of a dual ring pulse signal, power drive circuit, etc. to form a control system before it can be used. Therefore, it is not easy to use a stepper motor well. It involves many professional knowledge such as machinery, electrical machinery, electronics and computers.
At present, there are indeed many manufacturers of stepping motors, but they have professional and technical personnel who can develop them on their own. However, there are very few manufacturers. Most of the manufacturers have only one or twenty people, and they do not even have the most basic equipment. Only in a blind imitation stage. This causes many troubles for users in product selection and use. In view of the above situation, we decided to take a wide range of induction stepper motors as an example. Describe its basic working principle. Hope it will be helpful to the majority of users in the selection, use, and improvement of the whole machine.
Induction stepper motor
The working principle of inductive stepping motor
(1) Principle of reactive stepper motor
The working principle of reactive stepper motor is relatively simple. The following first describes the three-phase reactive stepper
Schematic diagram
1. Structure: There are many small teeth evenly distributed in the rotor of the motor, and the stator teeth have three excitation winding resistors, and their geometric axes are staggered with the axis of the rotor teeth respectively. 0, 1/3て, 2/3て, (the distance between the tooth axes of two adjacent rotors is the tooth pitch, expressed in て), that is, A is aligned with tooth 1, and B is offset to the right by 1/3て, C and tooth 3 are offset by 2/3 to the right, A’is aligned with tooth 5, (A’ is A, tooth 5 is tooth 1)
2. Rotation: If phase A is energized, phase B and C are not energized, tooth 1 is aligned with A due to the action of the magnetic field (the same applies when the rotor is not subject to any force). If phase B is energized, when phase A and C are not energized, tooth 2 should be aligned with B. At this time, the rotor moves to the right by 1/3て, and the offset of tooth 3 and C is 1/3て, tooth 4 and A Offset (て-1/3て)=2/3て. If phase C is energized, phase A and B are not energized, tooth 3 should be aligned with C. At this time, the rotor moves to the right by 1/3て, and the offset of tooth 4 and A is 1/3て aligned. If phase A is energized, phases B and C are not energized, tooth 4 is aligned with A, and the rotor moves to the right by 1/3 so that after A, B, C, and A are energized, tooth 4 (that is, the tooth before tooth 1) ) Move to the A phase, the motor rotor rotates to the right by one pitch, if you keep pressing A, B, C, A… and energize, the motor will rotate to the right every step (per pulse) 1/3て. If you press A, C, B, A… to energize, the motor will reverse.
It can be seen that the position and speed of the motor have a one-to-one correspondence between the number of conduction (pulse number) and frequency. The direction is determined by the order of conduction. However, for consideration of torque, stability, noise and angle reduction. The conductivity 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て becomes 1/12て, 1/24て, which is the basic theoretical basis of motor subdivision driving. It is not difficult to deduce: There is m-phase excitation winding on the motor stator, and its axis is offset from the rotor tooth axis by 1/m, 2/m…(m-1)/m,1. And electric conduction according to a certain phase sequence, the motor can be controlled forward and reverse-this is the physical condition of the stepper motor rotation. As long as this condition is met, we can theoretically manufacture stepper motors of any phase. Due to cost and other considerations, there are generally more than two, three, four, and five phases in the market.
3. Torque: Once the motor is energized, a magnetic field (magnetic flux Ф) will be generated between the stator and the rotor. When the rotor and the stator are staggered by a certain angle, the force F is proportional to (dФ/dθ). The magnetic flux Ф=Br*S Br is the magnetic density, S Is the magnetic area F is proportional to L*D*Br, L is the effective length of the iron core, D is the rotor diameter Br=N·I/RN·I is the number of ampere turns of the excitation winding (current multiplied by the number of turns) R is the magnetic resistance . Torque = Force * Radius Torque is proportional to the effective volume of the motor * Ampere-turns * Magnetic density (only linear state is considered) Therefore, the larger the effective volume of the motor, the greater the number of excitation ampere-turns, the smaller the air gap between the stator and the rotor, and the motor torque The bigger, and vice versa.
(2) Induction stepping motor
1. Features: Compared with the traditional reactive stepping motor, the induction sub-type stepping motor has permanent magnets added to the structure to provide the working point of soft magnetic materials, while the stator excitation only needs to provide a changing magnetic field instead of providing The energy consumption of the working point of the magnetic material, so the motor has high efficiency, low current and low heat generation. Due to the presence of permanent magnets, the motor has a strong back EMF and its own damping effect is relatively good, making it relatively stable during operation, low noise, and low frequency vibration. Induction stepper motors can be regarded as low-speed synchronous motors to some extent.
A four-phase motor can be used for four-phase operation or two-phase operation. (It must be driven by a bipolar voltage), which is not the case with reactive 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=. A The internal windings of two-phase motors are exactly the same as those of four-phase motors. Small-power motors are generally directly connected to two-phase motors. For motors with higher power, to facilitate use and flexibly change the dynamic characteristics of the motor, the external wiring is often eight leads. (Four-phase), when used in this way, it can be used as a four-phase motor, and can be used 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 number (motor outer diameter), it can be divided into: 42BYG (BYG is the code of induction stepping motor), 57BYG, 86BYG, 110BYG, (international standard), and 70BYG, 90BYG, 130BYG, etc. are all domestic standards.
3. Terminology of the static index of the stepping motor Phase number: the number of pairs of excitation coils that produce N and S magnetic fields of different opposite poles. Commonly used m said. Number of beats: the number of pulses required to complete a periodic change of the magnetic field or the conduction state is represented by n, or the number of pulses required for the motor to rotate a pitch angle. Taking a four-phase motor as an example, there is a four-phase four-beat operation mode, namely AB -BC-CD-DA-AB, four-phase eight-beat operation mode is A-AB-B-BC-C-CD-D-DA-A. Step angle: corresponding to a pulse signal, the angular displacement of the motor rotor Expressed by θ. θ=360 degrees (number of rotor teeth J*number of running beats), taking a conventional two-phase and four-phase motor with 50-tooth rotor teeth as an example. The step angle in four-beat operation is θ=360 degrees/(50*4)=1.8 degrees (commonly known as full step), and the step angle in eight-beat operation is θ=360 degrees/(50*8)=0.9 degrees (commonly known as Half step).
Positioning torque: When the motor is not energized, the locking torque of the motor rotor itself (caused by the harmonics of the magnetic field tooth profile and mechanical errors) Static torque: When the motor is under rated static electricity, the motor is not rotating. The locking torque of the shaft. This torque is a measure of the motor volume (geometric size) and has nothing to do with the driving voltage and driving power. Although the static torque is proportional to the number of electromagnetic excitation ampere turns and is related to the air gap between the stator rotors, it is not advisable to reduce the air gap and increase the excitation ampere turns to increase the static torque. Heat and mechanical noise.
4. Dynamic indicators and terms of stepping motor:
(1) Step angle accuracy: The error between the actual value and the theoretical value of each step angle of the stepper motor. Expressed in percentage: error/step angle*100%. The value of different running beats is different, it should be within 5% when four beats are running, and within 15% when eight beats are running.
(2) Loss of step: The number of running steps when the motor is running is not equal to the theoretical number of steps. Call it out of step.
(3) Misalignment angle: the angle at which the rotor tooth axis deviates from the stator tooth axis. There must be an offset angle in the motor operation. The error caused by the offset angle cannot be solved by using the subdivision drive.
(4) Maximum no-load starting frequency: The maximum frequency at which the motor can be started directly without load under a certain driving mode, voltage and rated current.
(5) Maximum no-load operating frequency: The maximum speed frequency of the motor without load under a certain driving mode, voltage and rated current.
(6) Running torque-frequency characteristics: The curve of the relationship between output torque and frequency measured under certain test conditions is called the running torque-frequency characteristic. This is the most important of the motor’s many dynamic curves and the fundamental of motor selection.
Other characteristics include inertia frequency characteristics, starting frequency characteristics and so on. 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) of the motor during operation. The greater the average current, the greater the motor output torque, which is the The harder the frequency characteristic.
