An electric motor is a device that converts electrical energy into mechanical energy. In machinery, metallurgy, petroleum, coal, chemical, aviation, transportation, agriculture and various other industries, electric motors are widely used. With the increasing degree of industrial automation, there is a need to use a variety of control motors as components of the automation system, and motors are indispensable in the automatic control system of artificial satellites. In addition, in the national defense, culture and education, medical and daily life (modern home appliance industry) motor is also more and more widely used.
The general motor is mainly composed of two parts: the fixed part is called stator, the rotating part is called rotor. There are also end caps, fans, cowlings, chassis, junction boxes and so on.
The working principle of an electric motor is based on the law of electromagnetic induction, Ohm's law for full circuits, and the law of electromagnetic force. When the magnetic poles rotate in the clockwise direction, the magnetic lines of force of the magnetic poles cut the rotor guide bars, and an electric potential is induced in the guide bars. The direction of the electromotive force is determined by the right-hand rule. Because the movement is relative, if the magnetic pole does not move, the rotor guide strip rotates in the counterclockwise direction, the guide strip can also be induced electric potential. Under the action of the electric potential, the closed guide strip in the current. The current interacts with the magnetic field of the rotating poles, and the rotor guide bar is subjected to electromagnetic force (Ampere force), the direction of the electromagnetic force can be determined by the left hand rule. The direction of the electromagnetic force can be determined by the left hand rule. From the electromagnetic force, an electromagnetic torque is generated, and the rotor rotates. Servomotors are widely used in a variety of control systems to convert the input voltage signal into a mechanical output on the motor shaft, dragging the controlled element to achieve the control purpose. Servo motors are divided into DC and AC; the earliest servo motors were general DC motors, and general DC motors were used as servo motors only when the control accuracy was not high. The current DC servo motor from the structure, is a small power DC motor, its excitation more armature control and magnetic field control, but usually use the armature control. Classification of rotary motors, DC servo motor in mechanical characteristics can well meet the requirements of the control system, but due to the existence of the commutator, there are many shortcomings: easy to generate sparks between the commutator and the brush, interfering with the drive work, can not be applied in the presence of combustible gases; brush and commutator friction exists, which will produce a large dead zone; complex structure, maintenance is more difficult. AC servo motor is essentially a two-phase asynchronous motor, there are three main control methods: amplitude control, phase control and amplitude-phase control. Generally speaking, servo motor requires that the speed of the motor should be controlled by the added voltage signal; the speed can be changed continuously with the change of the added voltage signal; the reflection of the motor should be fast, the volume should be small, and the control power should be small. Servo motors are mainly used in various motion control systems, especially follower systems.2.2 Stepping motorThe so-called stepping motor is an actuator that converts electrical pulses into angular displacement; more commonly known as: when a stepping driver receives a pulse signal, it drives the stepping motor to rotate a fixed angle in the set direction. We can control the number of pulses to control the angular displacement of the motor, so as to achieve the purpose of precise positioning; at the same time, you can also control the frequency of pulses to control the motor rotation speed and acceleration, so as to achieve the purpose of speed regulation. At present, the more commonly used stepping motors include reactive stepping motor (VR), permanent magnet stepping motor (PM), hybrid stepping motor (HB) and single-phase stepping motor. The difference between a stepper motor and an ordinary motor lies mainly in its pulse-driven form, and it is this feature that allows the stepper motor to be combined with modern digital control technology. However, the stepping motor is inferior to the traditional closed-loop control DC servo motor in terms of control accuracy, speed variation range, and low-speed performance; therefore, it is mainly used in the occasions where the accuracy requirement is not particularly high. Because of its simple structure, high reliability and low cost, stepping motor is widely used in various fields of production practice; especially in the field of CNC machine tool manufacturing, because stepping motor does not need A/D conversion, and it can directly convert digital pulse signals into angular displacements, so it has been regarded as the most ideal actuating element of CNC machine tools. In addition to their use in CNC machine tools, stepper motors can also be used in other machinery, such as as motors in automatic feeders, as motors in general-purpose floppy disk drives, and also in printers and plotters. In addition, there are many defects of stepping motor; because of the existence of no-load starting frequency of the stepping motor, the stepping motor can run normally at low speed, but if it is higher than a certain speed, it can not be started and accompanied by sharp whistling sound; the precision of subdivided driver of different manufacturers may vary greatly, and the larger the subdivided fraction the more difficult it is to control the precision; moreover, there is a large vibration and noise when the stepping motor is rotating at a low speed.2.3 Torque The so-called torque motor is a flat multi-pole permanent magnet DC motor. Its armature has a higher number of slots, commutator plates and series conductors to reduce torque pulsations and speed pulsations. There are two types of torque motors, DC torque motor and AC torque motor. Among them, the DC torque motor has a small self-induced reactance, so the responsiveness is very good; its output torque is proportional to the input current, independent of the speed and position of the rotor; it can be directly connected to the load at low speeds in a nearly blocked state without gear reduction, so it can produce a very high torque-to-inertia ratio on the load's shaft, and eliminate the system error due to the use of reduction gears. AC torque motors can be divided into synchronous and asynchronous types, and the commonly used type is the squirrel cage asynchronous torque motor, which is characterized by low speed and large torque. Generally, AC torque motors are often used in the textile industry, and their working principle and structure are the same as those of single-phase asynchronous motors, but their mechanical characteristics are softer due to the higher resistance of the squirrel-cage rotor.2.4 Switched Reluctance MotorSwitched reluctance motor is a new type of speed-regulating motor, with an extremely simple and sturdy structure, low cost, and excellent speed-regulating performance, and it is a strong contender for the traditional control motor, with strong market potential. 2.5 Brushless DC motor Brushless DC motor (BLDCM) is developed on the basis of brush DC motor, but its driving current is AC without compromise; brushless DC motor can be divided into brushless rate motor and brushless torque motor. Generally, brushless motors have two types of drive currents, one trapezoidal (usually "square") and the other sinusoidal. The former is sometimes called a brushless DC motor, and the latter is called an AC servomotor, or rather a type of AC servomotor. Brushless DC motors are usually of "slender" construction in order to minimize the moment of inertia. Brushless DC motors are much smaller in weight and volume than brushed DC motors, and the corresponding moment of inertia can be reduced by about 40%-50%. Due to the processing problems of permanent magnet materials, brushless DC motors generally have a capacity of less than 100kW. This kind of motor's mechanical characteristics and regulation characteristics of good linearity, wide speed range, long life, easy maintenance and low noise, there is no brush caused by a series of problems, so this kind of motor in the control system has a great potential for application.3 Power motors3.1 DC motorsDC motors are the earliest electric motors appeared, about the end of the nineteenth century, which can be divided into two categories: commutator and non-commutator. DC motors are the first motors that appeared around the end of the 19th century. DC motors have better control characteristics DC motors in the structure, price, maintenance are not as good as AC motors, but due to the AC motor speed control problems have not been a good solution, and DC motors have good speed performance, easy to start, able to load starting and other advantages, so the current DC motors are still widely used, especially in the silicon-controlled DC power supply appeared after. 3.2 Asynchronous motor asynchronous motor is based on the air gap rotating magnetic field and rotor winding induced current interaction to produce electromagnetic torque and realize the energy conversion of a kind of AC motor. Asynchronous motor is generally a series of products, a variety of specifications, which is the most widely used in all the motor, the largest demand; at present, about 90% of the machinery in the power transmission using AC asynchronous motor, so its power consumption accounts for more than half of the total electrical load. Asynchronous motor has the advantages of simple structure, easy manufacture, use and maintenance, reliable operation and smaller quality and lower cost. Moreover, asynchronous motor has high operating efficiency and good working characteristics, from no-load to full-load range close to constant speed operation, can meet most of the industrial and agricultural production machinery transmission requirements. Asynchronous motor is widely used to drive machine tools, pumps, blowers, compressors, lifting and winching equipment, mining machinery, light industrial machinery, agricultural and sideline product processing machinery and most of the industrial and agricultural production machinery, as well as household appliances and medical equipment. In the asynchronous motor in the more common single-phase asynchronous motor and three-phase asynchronous motor, which three-phase asynchronous motor is the main body of the asynchronous motor. While single-phase asynchronous motors are generally used in places where three-phase power supply is inconvenient, most of them are miniature and small-capacity motors, which are more commonly used in household appliances, such as fans, refrigerators, air-conditioners, vacuum cleaners, etc.3.3 Synchronous MotorsThe so-called synchronous motors are the ones that run synchronously between the rotor and stator's rotating magnetic field under the drive of alternating current (AC). The stator of synchronous motor is exactly the same as that of asynchronous motor; however, there are two kinds of rotor: "convex pole type" and "hidden pole type". Convex-pole rotor synchronous motor structure is simple, easy to manufacture, but the mechanical strength is low, suitable for low-speed operation; hidden-pole synchronous motor manufacturing process is complex, but the mechanical strength is high, suitable for high-speed operation. Synchronous motor operating characteristics and all electric motors, synchronous motor also has a "reversible", that is, it can be operated according to the generator mode, can also be operated according to the motor mode. Synchronous motors are mainly used in large-scale machinery, such as blowers, pumps, ball mills, compressors, rolling mills, as well as small, micro-instrumentation or as control elements; three-phase synchronous motors are the mainstay. In addition, it can also be used as a regulator to deliver inductive or capacitive reactive power to the grid.4 Signal motors 4.1 Position signal motors At present, the most representative position signal motors: rotary transformers, induction synchronizers and self-aligning angle machine. The resolver is essentially a transformer that can arbitrarily change the degree of coupling between the primary and secondary windings. Its structure and wire-wound asynchronous motor is the same, the stator and rotor each have two sets of mutually perpendicular distribution of windings, rotor windings using slip rings and brushes with the external circuit connection. When the primary winding is excited, the output voltage of the secondary winding and the rotor angle become sine, cosine, linear or other functions, which can be used for coordinate conversion and trigonometric operation in computing devices, and also can be used as angle data transmission and phase shifter in control systems. The inductive synchronizer is a high-precision position or angle detection element, and there are two types: disk type and linear type. Disc inductive synchronizers are used to measure angular position, while linear inductive synchronizers are used to measure line displacement. Self-aligning angle machines are inductive electromechanical components that are widely used in follower systems as angle transmission, transformation and indication devices. It is often used in control systems in combination with two or more units to enable two or more axes that are not connected to each other on a machine to automatically maintain the same change in angle or to rotate in synchronization.4.2 Speed Signal MotorsThe most representative speed signal motor is the tachogenerator, which is essentially an electromechanical element that converts rotational speed into an electrical signal, and whose output voltage is directly proportional to the rotational speed. From the working principle, it belongs to the category of "generator". Tachogenerators are mainly used in control systems as damping elements, differential elements, integral elements and tacho elements. Velocimetric generators are divided into DC and AC; and DC velocimetric generators are divided into he-excited and permanent magnet, whose structure and working principle are the same as that of low-power DC generators, usually with smaller output power, and when used as a calculation element, it is required that its output voltage linearity error and temperature error are lower than an upper limit. The AC tacho generators are synchronous and asynchronous; synchronous tacho generators include: permanent magnet type, induction type and pulse type; asynchronous tacho generators are most widely used is the cup-type rotor asynchronous tacho generators. In order to improve the accuracy and reliability of the tachymeter generator, at present, the DC tachymeter generator appears brushless structure of Hall effect DC tachymeter generator. Because this Hall effect brushless DC tacho generator is a kind of cogging, no winding motor, so it will not produce due to cogging and the existence of the "cogging harmonic potential", this motor structure is simple, easy to miniaturize.