Why should the transformer keep the magnetic potential balance under load?

In a broad sense, motor is a kind of electric energy conversion device, including rotating motor and static motor. Rotating electrical machine is an energy conversion device which realizes the mutual conversion of electrical energy and mechanical energy according to the principle of electromagnetic induction. Electrostatic motor is an electromagnetic device that realizes voltage change according to the law of electromagnetic induction and the principle of magnetic potential balance, also known as transformer. Here we mainly discuss the rotating electric machine. There are many kinds of rotating electric machines, which are widely used in modern industrial fields. It can be said that there will be rotating motors where electric energy is used. Compared with internal combustion engines and steam engines, the operating efficiency of rotating electric machines is much higher; Electric energy is more convenient and cheaper than other energy sources. In addition, the electric energy is clean and pollution-free and easy to control. Therefore, in real life and engineering practice, the application of rotating electrical machinery is more and more extensive. Different motors have different applications. With the continuous development of motor manufacturing technology and the in-depth study of motor working principle, many new motors have emerged, such as slotless brushless DC motor developed by EAD company in the United States, low-power hybrid stepping motor developed by SERVO company in Japan, and high-torque low-speed motor developed by China for industrial machine tools and electric bicycles. 1 Rotating electrical machines are classified as rotating electrical machines. Because generators are machines that generate electricity, there are far fewer types of generators than motors. Motor is a kind of industrial application machine, so compared with generator, people have much more research on motor and more detailed classification. In fact, what we usually call a rotating motor is a narrow sense, that is, a motor-commonly known as a "motor." As we all know, motor is an important part of transmission and control system. With the development of modern science and technology, the emphasis of motor in practical application has shifted from simple transmission to complex control. Especially the precise control of the speed, position and torque of the motor. It can be seen that it is very important for an electrical engineer to be familiar with the types and performances of various motors. Usually, people basically classify rotating electrical machines according to their uses. Starting with the control motor, this paper gradually introduces the most representative, commonly used and basic motors-control motor, power motor and signal motor. 2. Control motor 2. 1 Servo motor Servo motor is widely used in various control systems, which can convert the input voltage signal into the mechanical output on the motor shaft and drag the controlled components, thus achieving the control purpose. Servo motors can be divided into DC and AC; The earliest servo motor is a universal DC motor, which is used as a servo motor when the control accuracy is not high. At present, DC servo motor belongs to low-power DC motor in structure, and its excitation mostly adopts armature control and magnetic field control, but armature control is usually used. According to the classification of rotating motor, DC servo motor can meet the requirements of control system in mechanical characteristics, but there are many shortcomings due to the existence of commutator: sparks are easily generated between commutator and brush, which interferes with the work of the driver and cannot be used in situations with combustible gas; There is friction between the brush and commutator, which will produce a large dead zone; Complex structure and difficult maintenance. AC servo motor is essentially a two-phase asynchronous motor, and there are three main control modes: amplitude control, phase control and amplitude-phase control. Generally speaking, the servo motor requires that the speed of the motor should be controlled by the applied voltage signal; The rotating speed can change continuously with the change of applied voltage signal; The motor should have fast response, small volume and low control power. Servo motors are mainly used in various motion control systems, especially servo systems. 2.2 Stepping motor The so-called stepping motor is an actuator that converts electric pulses into angular displacement; More generally, when the stepping driver receives a pulse signal, it drives the stepping motor to rotate by a fixed angle in the set direction. We can control the angular displacement of the motor by controlling the number of pulses, so as to achieve the purpose of accurate positioning; At the same time, the speed and acceleration of motor rotation can be controlled by controlling the pulse frequency, so as to achieve the purpose of speed regulation. At present, the commonly used stepping motors are reactive stepping motor (VR), permanent magnet stepping motor (PM), hybrid stepping motor (HB) and single-phase stepping motor. The difference between stepping motor and ordinary motor mainly lies in its pulse driving form. It is this characteristic that makes stepping motor can be combined with modern digital control technology. However, the stepper motor is not as good as the traditional closed-loop DC servo motor in control accuracy, speed variation range and low-speed performance. Therefore, it is mainly used in occasions where accuracy requirements are not particularly high. Stepping motor is widely used in various fields of production practice because of its simple structure, high reliability and low cost. Especially in the field of NC machine tool manufacturing, stepping motor has been considered as the most ideal actuator of NC machine tool because it can directly convert digital pulse signal into angular displacement without A/D conversion. In addition to the application in CNC machine tools, stepping motors can also be used in other machines, such as motors in automatic feeders, motors in general floppy disk drives, printers and plotters. In addition, the stepping motor also has many defects; Because the stepping motor has no-load starting frequency, it can run normally at low speed, but it can't start when it is higher than a certain speed, accompanied by sharp howling; The subdivision driving accuracy of different manufacturers may vary greatly, and the greater the subdivision number, the more difficult it is to control the accuracy; Moreover, when the stepping motor rotates at low speed, it vibrates and makes noise. 2.3 torque motor the so-called torque motor is a flat multipole permanent magnet DC motor. Its armature has more slots, commutator and series conductor to reduce torque fluctuation and speed fluctuation. There are two kinds of torque motors: DC torque motors and AC torque motors. Among them, the self-inductance reactance of DC torque motor is very small, so the response is very good; Its output torque is proportional to the input current and has nothing to do with the speed and position of the rotor. It can be directly connected to the load, running at a low speed in a state close to locking rotation without gear deceleration, so it can produce a high moment-inertia ratio on the shaft of the load, eliminating the system error caused by using the deceleration gear. AC torque motor can be divided into synchronous and asynchronous, and squirrel-cage asynchronous torque motor is commonly used at present, which has the characteristics of low speed and large torque. Generally speaking, AC torque motor is commonly used in textile industry, and its working principle and structure are the same as that of single-phase asynchronous motor, but its mechanical characteristics are soft because of the large resistance of squirrel-cage rotor. 2.4 Switched Reluctance Motor Switched Reluctance Motor is a new type of speed regulating motor with extremely simple and firm structure, low cost and excellent speed regulating performance. It is a strong competitor of traditional control motor and has strong market potential. 2.5 Brushless DC Motor Brushless DC motor (BLDCM) is developed on the basis of brushless DC motor, but its driving current is out-and-out alternating current; Brushless DC motors can be divided into brushless speed motors and brushless torque motors. Generally, there are two kinds of driving currents for brushless motors, one is trapezoidal wave (generally "square wave") and the other is sine wave. Sometimes the former is called DC brushless motor, and the latter is called AC servo motor, which is a kind of AC servo motor. In order to reduce the moment of inertia, brushless DC motor usually adopts "slender" structure. Brushless DC motor is much smaller in weight and volume than brushless DC motor, and the corresponding moment of inertia can be reduced by about 40%-50%. Due to the processing problem of permanent magnet materials, the general capacity of brushless DC motor is below 100kW. This kind of motor has the advantages of good linearity of mechanical characteristics and adjustment characteristics, wide speed range, long life, convenient maintenance, low noise and so on, and there is no series of problems brought by brushes, so this kind of motor has great application potential in control system. 3 power motor 3. 1 DC motor DC motor is the earliest motor. At the end of 19, it can be roughly divided into commutator and non-commutator. DC motor has better control characteristics. DC motor is inferior to AC motor in structure, price and maintenance. However, the speed regulation problem of AC motor has not been well solved, and DC motor has the advantages of good speed regulation performance, easy starting and starting with load, so it is still widely used at present, especially after the emergence of SCR DC power supply. 3.2 asynchronous motor asynchronous motor is an AC motor that generates electromagnetic torque and realizes energy conversion based on the interaction between air gap rotating magnetic field and rotor winding induced current. Asynchronous motors are generally a series of products of various specifications, which are widely used in all motors and have the largest demand; At present, about 90% machines in electric drive use AC asynchronous motors, so their electricity consumption accounts for more than half of the total power load. Asynchronous motor has the advantages of simple structure, convenient manufacture, use and maintenance, reliable operation, small mass and low cost. Moreover, asynchronous motor has high running efficiency and good working characteristics, and it can run at a constant speed from no-load to full load, which can meet the transmission requirements of most industrial and agricultural production machinery. Asynchronous motors are widely used to drive machine tools, pumps, blowers, compressors, lifting equipment, mining machinery, light industrial machinery, agricultural and sideline products processing machinery, most agricultural production machinery, household appliances and medical devices. Among asynchronous motors, there are common single-phase asynchronous motors and three-phase asynchronous motors, among which three-phase asynchronous motors are the main body of asynchronous motors. Single-phase asynchronous motors are generally used in places where three-phase power supply is inconvenient, mostly miniature and small-capacity motors, and are widely used in household appliances, such as fans, refrigerators, air conditioners and vacuum cleaners. 3.3 Synchronous Motor The so-called synchronous motor is a motor driven by alternating current, and the rotating magnetic fields of the rotor and stator run synchronously. The stator of synchronous motor is exactly the same as that of asynchronous motor; However, there are two types of rotors: salient pole and hidden pole. Salient-pole rotor synchronous motor is simple in structure and convenient to manufacture, but its mechanical strength is low, so it is suitable for low-speed operation. The manufacturing process of hidden pole synchronous motor is complex, but its mechanical strength is high, which is suitable for high-speed operation. The working characteristics of synchronous motors are the same as all motors, and synchronous motors are also "retrograde", that is, they can run in generator mode or motor mode. Synchronous motors are mainly used in large machinery, such as blowers, pumps, ball mills, compressors, rolling mills, small and micro instruments and equipment or as control elements; Among them, the three-phase synchronous motor is its main body. In addition, it can also be used as a modulator to transmit inductive or capacitive reactive power to the power grid. 4 signal motors 4. 1 position signal motors At present, the most representative position signal motors are resolver, inductosyn and synchro. The resolver is essentially a transformer, which can change the coupling degree of the primary winding and the secondary winding at will. Its structure is the same as that of wound asynchronous motor. The stator and the rotor have two groups of windings vertically distributed with each other, and the rotor windings are connected with external circuits through slip rings and brushes. When the primary winding is excited, the output voltage of the secondary winding has a sine, cosine, linear or other functional relationship with the rotation angle of the rotor, which can be used for coordinate transformation and trigonometric operation in the calculation device, and can also be used as angle data transmission and phase shifter in the control system. Inductive synchronizer is a high-precision position or angle detection element, which has two types: disk type and linear type. Disc inductosyn is used to measure the angular position; Linear inductosyn is used to measure linear displacement. Synchro is an inductive electromechanical component, which is widely used in servo system as a device for angle transmission, transformation and indication. In the control system, two or more shafts are often used together, so that two or more shafts without mechanical connection automatically keep the same rotation angle change or rotate synchronously. 4.2 Speed Signal Motor The most representative speed signal motor is tachogenerator, which is essentially an electromechanical component that converts the rotating speed into an electrical signal, and its output voltage is proportional to the rotating speed. In terms of working principle, it belongs to the category of "generator". The tachogenerator is mainly used as damping element, differential element, integral element and tachometer element in the control system. Tachometer can be divided into DC and AC. DC tachogenerator can be divided into separately excited generator and permanent magnet generator. Its structure and working principle are the same as those of low-power DC generators, and its output power is usually small. As a computing element, the linear error and temperature error of its output voltage are required to be lower than the upper limit. Ac tachogenerator can be divided into synchronous and asynchronous. Synchronous tachogenerator includes: permanent magnet type, induction type and pulse type; Cup rotor asynchronous tachogenerator is the most widely used one. In order to improve the accuracy and reliability of tachogenerator, Hall effect DC tachogenerator with brushless structure has appeared at present. Because this Hall-effect brushless DC tachogenerator is a kind of motor without cogging and winding, it will not produce cogging harmonic potential, and its structure is simple and it is convenient for miniaturization. Conclusion Generally speaking, in a complete automatic control system, signal motor, power motor and control motor all have their own uses. Usually, the control motor is a very "accurate" motor, which acts as the "core executive device" in the control system; Power motor is a kind of "strong" high-power motor, which is often used to drag the mechanical equipment on site; Signal motor plays the role of "communicator" in the control system, which is essentially a "motor sensor". Of course, not all automatic control systems have these three kinds of motors. In the general automation field, such as motion control and process control, especially in motion control, the control motor is an essential "core device", so the control motor plays an important role in the automation field, which is one of the reasons why people study the control motor the most. In fact, with the continuous development and integration of motor manufacturing technology, the performance of various rotating motors is gradually "crossing" and "specialization". It is impossible to classify all kinds of rotating motors in great detail, because many new rotating motors are highly unified organisms, with many motor working principles and many motor manufacturing technologies. Therefore, it is enough for general electrical engineers and technicians who are not majoring in electrical machinery to master the characteristics and uses of various rotating electrical machines from the overall structure.