The concept of stepping motor and its working principle

A stepping motor is an electromagnetic device that transforms a pulse signal into a corresponding angular displacement (or line displacement) and is a special motor. The general motor is continuous rotation, and the stepper motor has two basic states of positioning and operation. When the pulse input elbow stepper motor rotates step by step, it will turn a certain angle each time it is given a pulse signal. The angular displacement of the stepping motor is strictly proportional to the number of input pulses, and is synchronized in time with the input pulse. Therefore, as long as the number of input pulses, the frequency, and the phase sequence of the motor windings are controlled, the desired rotation angle can be obtained. Speed ​​and direction of rotation. When there is no pulse input, the air gap magnetic field can keep the rotor in the original position under the excitation of the winding power supply.

Stepping motors are divided according to the magnitude of their output torque and can be divided into fast stepping motors and power stepping motors. Fast stepping motor has high continuous operating frequency and low output torque, generally in N·cm class. It can be used as a workbench (linear cutting machine tool) for controlling small precision machine tools and can also be composed of electrohydraulic pulsed motors with hydraulic torque amplifiers. Drives the workbench of the numerically-controlled machine tool, and the output torque of the power stepper motor is relatively large and is N·m class, and can directly drive the moving parts of the machine tool. Stepping motor can be divided into three-phase, four-phase, five-phase, six-phase and even eight-phase according to the number of its excitation phases. Generally speaking, with the increase of the number of phases, under the same frequency, the on-time of each phase increases, the average current of each phase will be higher, so that the speed-torque characteristics of the motor will be better, and the step angle small. However, as the number of phases increases, the size of the motor increases, and the structure is also complicated. At present, 3 to 6-phase stepping motors are frequently used. Since the speed of the stepping motor varies with the input pulse frequency, the speed range is wide, the sensitivity is high, the output rotation angle can be controlled, and the output accuracy is high, and synchronous control can be realized. Therefore, it is widely used in an open loop system. It can also be used on general-purpose machine tools to increase the automation level of the feed mechanism. The stepping motor is divided according to its working principle. There are mainly two types of magnetoelectric and reactive type. Here only the working principle of the commonly used reactive stepping motor is introduced. Now the simplified drawing of the stepping motor in the following figure will be used to illustrate. .

There are three pairs of poles A, B, and C on the stator of the motor. The coils are wound on the poles. They are called phase A, phase B, and phase C respectively. The rotor is a core with teeth. This type of stepper motor is called For three-phase stepping motor. If a direct current is applied to the coil, a magnetic field will be generated. When the coils of the three magnetic poles A, B and C flow in turn, then the three pairs of magnetic poles A, B and C in turn generate a magnetic field to attract the rotor to rotate. First, when a phase coil (set as A phase) is energized, both teeth of the rotors 1 and 3 are attracted by the magnetic pole A, and the rotor stays in the position of FIG. 5-5a. Then, when phase A is powered off and phase 6 is energized, the magnetic field of magnetic pole A disappears and magnetic pole B generates a magnetic field. The magnetic pole's magnetic field attracts the nearest two and four teeth and stops at position b in Figure b. The rotor turned 30° counterclockwise. Then, the B phase is cut off and the C phase is energized. By the same token, the rotor turned 30° counterclockwise again and stopped at position c. If the A-phase is energized and the C-phase is disconnected, then the rotor will be reversed by 30° so that the magnetic field of magnetic pole A will attract both the 2 and 4 teeth. Each phase of the stator rotates one rotor and rotates one tooth. In this way, electric power flows in the order of A→B→C→A→B→C→A→... and the stepping motor rotates counterclockwise step by step. Each time the energizing coil is switched, the stepping motor rotates by 30°. We call the stepping motor the angle at which each step is rotated. If the order in which the stepping motor energizing coils are reversed is changed in the order of A→C→B→A→C→B→..., the stepping motor will rotate in the clockwise direction, so the rotation direction of the stepping motor must be changed. It can be done when any phase is energized.

Stepping motors have become the third category of motors except for DC motors and AC motors. The traditional motor as an electromechanical energy conversion device plays a key role in the process of human production and living into the electrification process. However, today's human society has entered the era of automation, the functions of traditional motors can no longer meet the requirements of various motion control systems such as factory automation and office automation. In order to meet these requirements, a series of new motor systems with control functions have been developed, of which the ones that have their own characteristics and are widely used are the stepping motors.

The development of stepper motors is closely related to the computer industry. Since stepper motors have replaced small DC motors in computer peripheral equipment, the performance of their equipment has been improved, and the development of stepping motors has been rapidly promoted. On the other hand, the development of microcomputers and digital control technology will in turn promote the application of stepper motors as the executing parts of numerical control systems to other fields, such as electric machining machines, low-power machining machines, measuring instruments, optical and medical instruments, and packaging. Machinery and so on.

The process of maturing any product is basically the process of gradual unification and simplification of specifications. Nowadays, the development of stepper motors has been attributed to the three types of magneto-resistive, hybrid, and claw-pole permanent magnets with single-segment structure. The claw pole motor is cheap, and the performance index is not high. The hybrid and magnetic resistance type are mainly used as high resolution motors. Because the hybrid stepping motor has small control power and good running stability, it gradually takes a dominant position. The most typical product is a two-phase 8-pole 50-teeth motor with a step angle of 1.8°/0.9° (full step/half step); there are also five-phase 10 poles and 50 teeth and some rotors with 100 teeth for two-phase and five-phase steps. Into the motor, five-phase motor is mainly used for high performance applications. Until now, the magnetoresistive stepper motors in industrially developed countries have been rare.

The largest producers of stepper motors are Japan, such as Japan Servo Corp., Dongfang Corporation, SANYO DENKI, MINEBEA, and NPM Corporation, and in particular, Japan Oriental Corporation. Whether it is the performance and appearance quality of motors, or the means of production, it can be called the world. The best. Now Japan's annual output of stepper motors (including wholly foreign-owned companies) is nearly 200 million units.

Germany is also a major producer of stepper motors in the world. After the expiry of the patent of the German five-phase hybrid stepping motor of BL in 1994, a new three-phase hybrid stepping motor series was introduced. It is a 60-pole rotor with a 50-teeth structure, supporting a current-type driver, and the number of steps per revolution is 200. , 400, 1000, 2000, 4000, 10000, and 20000, it has the resolution of the usual two-phase and five-phase stepping motors, can also be further subdivided on this basis, the resolution increased by 10 times, which is a A good solution fully utilizes the function of current-driven technology, allowing three-phase motors to have the performance of two-phase and five-phase motors at the same time.

At the same time, the Japanese servo company also introduced their three-phase hybrid stepping motor. The company's Dr. Han Zemin developed three different types of permanent magnet three-phase stepping motors, namely HB (hybrid), RM (similar to the stator and hybrid, the rotor is similar to the permanent magnet ring magnet), and claw poles. PM type. Compare the three-phase stepping motor with the two-phase stepping motor and display:

(1) Three-phase motors are better than two-phase motors in obtaining small step angles.

(2) The maximum holding torque of the two-phase excitation of the three-phase motor is √3T1 (T1 is the single-phase excitation torque), and the two-phase motor is √2T1, so the resultant torque of the three-phase motor is large.

(3) The torque ripple of a three-phase motor is smaller than that of a two-phase motor.

(4) Three-phase motor Two-step continuous two-step torque difference is smaller than that of two-phase motor.

(5) The three-phase motor windings can be star-connected, three terminals are driven, and the excitation circuit transistors are six; two-phase motors are eight.

(6) In the continuous operation, the three-phase harmonics of the magnetic flux and current are eliminated due to the three-phase stepper motor structure, so the three-phase motor has smaller vibration torque than the two-phase motor.

The conclusion is obvious. Another conclusion is that the HB type motor is more suitable for low speed and large torque applications; the RM type is suitable for smooth operation and applications with speeds greater than 1000 r/min; while the PM type has low cost, vibration at low speeds and high speed at high speeds. In terms of moments, three-phase PM motors have better performance than two-phase motors.

Therefore, the current most promising is a hybrid stepping motor, and the hybrid motor is developing in the following four directions:

One of the development trends is to continue to develop in the direction of miniaturization. With the widening of the application field of the motor itself and the continuous miniaturization of various types of complete machines, the motors required to match them must also be smaller and smaller. After the motors of the 57 and 42 frame sizes have been used for many years, their frame numbers are now available. Extends downward in directions 39, 35, 30, and 25. Swiss ESCAP also recently developed a stepper motor with an outer diameter of only 10mm.

The second trend is to change the circular motor to a square motor. Because of the rectangular structure of the motor, the rotor may be designed to be larger than a circle, and its torque volume ratio will be greatly improved. Motors with the same frame number will have a square torque increase of 30% to 40% compared with a round one.

The third trend of development is to comprehensively design the motor. That is, the rotor position sensor, reduction gear, and the like are integrally designed together with the motor body, so that it can easily form a closed-loop system, and thus has superior control performance.

The fourth trend is the development of five-phase and three-phase motors. Currently widely used two-phase and four-phase motors have large vibration and noise, and five-phase and three-phase motors have advantages. In the case of these two kinds of motors, the drive circuit of the five-phase motor is more complicated than the three-phase motor, so the three-phase motor system has a better performance-cost ratio than the five-phase motor.

The situation in China is different. Until the 1980s, the magneto-resistive stepper motor dominated. The hybrid stepper motor began to develop in the late 1980s. It is still the same time that the two types of structures coexist. Although the new hybrid stepping motor may completely replace the reluctance motor, the whole machine of the reluctance motor has been used for a long time and is familiar with its technology, especially the step angle of a typical hybrid stepping motor. (0.9°/1.8°) Unlike the step angle (0.75°/1.5°) of a typical reluctance motor, it is not easy for users to change the product structure, which makes it difficult to coexist in both models. Change in a short period of time. This situation is detrimental to the development of stepper motors.