May I ask what excitation is?

How a magnetic field is generated in a rotating electrical machine. Most modern motors are based on electromagnetic induction, and a magnetic field is required in the motor. This magnetic field can be generated by a permanent magnet or by using an electromagnet to pass a current through a coil. The coil group in the motor specially designed to generate a magnetic field is called the field winding. Due to the limitations of the performance of permanent magnet materials, the magnetic field established by permanent magnets is relatively weak, and it is mainly used in small-capacity motors. However, with the emergence of new permanent magnet materials, especially rare earth materials with high magnetic energy product such as rare earth cobalt and neodymium iron boron, the development of permanent magnet motors with a capacity of hundreds of kilowatts has begun. 　　General motors mostly use current excitation. The excitation methods are divided into two categories: separate excitation and self-excitation. 　　Other motivations * * The power supply provides the required field current to the motor field winding. For example, use * * The DC power supply supplies the field winding of the DC generator. ； The AC power supply supplies power to the armature winding of the asynchronous motor to generate a rotating magnetic field, etc. The former is DC excitation and the latter is AC excitation. Depending on the situation of the power grid, the synchronous motor can be DC excited by the field winding of the rotor, or the stator can be provided with AC excitation by the power grid. Generally, DC excitation is the main method. If the DC excitation is insufficient, lagging reactive current is input from the power grid to supplement the motor's excitation. ； If the DC excitation is too strong, the motor will output lagging reactive current to the power grid, weakening the internal magnetic field of the motor. When DC excitation is used, there is only a voltage drop caused by resistance in the excitation circuit, the required excitation voltage is low, and the capacity of the excitation power supply is small. When AC excitation is used, since the excitation coil has a large inductive reactance, the required excitation voltage is much higher and the capacity of the excitation power supply is also much larger. 　　Separately excited excitation power supply, originally commonly used DC exciter. With the development of power electronics technology, AC exciters have been increasingly used to excite the excitation windings after being rectified by semiconductors. Excitation adjustment can be achieved by adjusting the excitation current of the AC exciter ； It is also possible to use controlled rectification adjustment while the output voltage of the AC exciter remains basically unchanged. The latter is relatively fast to adjust, and can also easily use the inverter working state of the controllable rectifier bridge to achieve rapid demagnetization and demagnetization, thereby eliminating the commonly used demagnetization switch. In the former method, the rectifier element is a diode. If it is installed on the rotor together with the armature winding of the AC exciter and the excitation winding of the synchronous motor, the excitation current can be directly input from the AC exciter to the excitation winding through the rectifier bridge. Collector rings and brushes are no longer needed, and a brushless excitation system can be formed, which brings a lot of convenience to the operation and maintenance of the motor. Of course, rectifier components, fast fuses and other devices rotate at high speed during operation and must withstand considerable centrifugal force, which must be considered during structural design. 　　Self-excitation uses part of the power generated by the motor to supply its own excitation needs. When the motor adopts self-excitation, it does not require a separate external excitation power supply and the equipment is relatively simple. But if there was no magnetic field inside the motor, it would be impossible to generate electromotive force and self-excitation would be impossible. Therefore, the condition for realizing self-excitation is that there must be residual magnetism inside the motor. 　　Self-excited systems can be divided into two types: parallel excitation and compound excitation. Shunt excitation refers to a self-excited system that obtains energy only from the voltage of a synchronous motor, and compound excitation refers to a self-excited system that obtains energy from both the voltage and current of a synchronous motor. The conditions and starting process of self-excitation of a shunt-excited generator are shown in Figures 1 and 2. Figure 1 is the schematic wiring diagram of a shunt DC generator. Figure 2 shows the excitation process. Among them, curve 1 is the magnetization curve Φ=f(If) of the generator. Since the induced electromotive force of the motor is proportional to the magnetic flux at a certain speed, curve 1 is also the no-load characteristic curve of the motor E 0 = f (If), that is, the relationship between the induced electromotive force of the motor and the excitation current If. Curve 2 is the resistance characteristic of the excitation circuit U=If·∑R, which represents the relationship between the excitation current and the motor voltage. It is actually a straight line with slope ΣR. Among them, ΣR is the total resistance of the excitation circuit, which includes the resistance of the excitation winding and the external regulating resistor Rr. 　　The process of motor self-excitation is as follows: When the motor rotates at a certain speed, electromotive force will be induced in the armature winding due to the residual magnetism in the motor. Under the action of this electromotive force, an excitation current If1 will be generated in the excitation circuit. If the excitation winding is connected correctly, the magnetomotive force generated by If1 will strengthen the magnetic field in the motor, and the induced electromotive force in the armature winding will further increase to E1, so that the excitation current will increase to If2. In this way, they promote each other until the intersection point A of the motor's no-load characteristics and resistance characteristics. At this point, the terminal voltage of the motor is U0, and the excitation current it generates is If1. Under this excitation current If1, the electromotive force generated by the motor is exactly U0, and the motor works stably at this point. If the resistance ΣR of the excitation circuit is increased, the slope of the resistance characteristic will increase, the intersection point with the no-load characteristic will move downward, and the output voltage of the generator will decrease. When the resistance increases to a certain critical value ΣRcr, the resistance characteristic 3 almost coincides with the generator no-load characteristic. At this point the motor voltage will be uncertain. If the motor temperature and operating conditions change even slightly, the voltage will change significantly. If the resistance is further increased, the generator will not be able to self-excite and establish voltage. In situations where a wide range of voltage adjustment is required, such as the exciter of a synchronous generator, a small slot can be opened in the magnetic pole steel piece to create a narrow area in the magnetic circuit. These areas begin to saturate under relatively small magnetic flux, making the no-load characteristics of the motor more curved (Figure 3). In this way, the excitation circuit resistance characteristics can intersect with the no-load characteristics within a larger range, thereby obtaining a wider voltage regulation range. 　　When the generator is loaded, the voltage drop across the internal resistance of the motor due to the load current will cause the terminal voltage to drop. For self-shunt-excited motors, the drop in terminal voltage reduces the excitation current and causes a further drop in the motor terminal voltage, as shown in curve 1 of Figure 4. In order to overcome this shortcoming, generators often use compound excitation, that is, in addition to the shunt winding, a series winding is added, and the series winding is connected in series with the load circuit. As the load increases, the magnetomotive force of the series winding increases, causing the induced electromotive force of the motor to increase accordingly to compensate for the voltage drop on the internal resistance of the load current, so that the terminal voltage of the motor can remain basically stable, as shown in curve 2 in Figure 4. 　　Self-excitation of asynchronous generators AC-excited asynchronous generators can also be self-excited. The AC excitation current must be supplied by a capacitor, and the voltage is established using the principle of LC parallel resonance. Like a DC generator, to achieve self-excitation, there must be residual magnetism in the motor core. The residual magnetism is used to generate electromotive force in the armature winding to supply power to the capacitive load and output capacitive current. Since the capacitive current with advanced output phase is equivalent to the inductive current with lagging input, it has a magnetizing effect, which strengthens the air gap magnetic field of the motor, thus increasing the induced electromotive force and capacitive current of the motor. Finally, due to the influence of magnetic circuit saturation, the voltage of the motor stabilizes at the intersection of no-load characteristics and capacitance characteristics (Figure 5). The process of building voltage is very similar to that of a self-excited DC generator. Just use capacitive characteristics instead of resistive characteristics. The slope of the capacitance characteristic is. In order to ensure that the asynchronous generator can self-excite and build voltage, it needs sufficient capacitance. When the capacitance reaches the critical value Ccr, the capacitance characteristics overlap with the no-load characteristics, and the motor cannot generate power stably. If the capacitance is further reduced, the motor will not be able to self-excite and establish voltage. 　　In addition to maintaining the motor voltage, the excitation excitation system of the synchronous motor also has a series of other requirements, such as regulating the reactive power of the system and being able to forcibly excite or demagnetize the motor when a sudden short circuit, sudden load or load shedding occurs in the power system to improve the stability and reliability of the power system operation. When a short circuit accident occurs inside the motor, it can quickly demagnetize the motor to prevent the accident from expanding and avoid further damage to the motor. Therefore, the excitation system of synchronous motor is relatively complex and comes in many types. Its classification is listed in the table. 　　The classification of synchronous motor excitation systems is as follows: 　　The excitation system of the synchronous motor consists of an excitation power supply, a manual adjustment device, an automatic excitation regulator and a demagnetization device. Excitation power supplies are also divided into two categories: self-excited and separately excited. Separately-excited equipment is relatively large but has better regulation performance, while self-excited power supplies are relatively simple. However, when the power system fails and the grid voltage drops seriously, its excitation current may actually decrease, worsening the grid voltage situation. The excitation voltage affects the stability of the motor operation, so appropriate equipment protection measures must be taken. 　　The self-excited excitation power supply is taken from the auxiliary winding inside the synchronous motor or directly from the outlet terminal of the synchronous motor itself. In the self-excited excitation system of synchronous motor, the automatic excitation regulator is an important component. Its function is that when the terminal voltage and reactive power of the synchronous motor change, it can automatically control the output current of the exciter or other excitation power supply based on the feedback signals sent back by the voltage measurement comparison unit and the reactive power compensation (difference) unit to achieve the purpose of automatically adjusting the terminal voltage and reactive power. In addition, there are some auxiliary regulating devices in the regulator, such as limiting units used to limit certain operating quantities of the generator (such as rotor current, stator current, etc.) ； Stabilization units and other compensation units that improve the dynamic performance of electronic systems by introducing additional signals such as rotational speed or frequency. In addition, there is a demagnetization device, which when a short circuit occurs inside the motor, the excitation current of the motor quickly decays to zero, thereby reducing the induced electromotive force of the motor to a very low level to avoid further damage.