The rotor core of wound motor is not insulated, and the core of doubly-fed motor is insulated. The main reason is that doubly-fed motor needs to consider the working condition of rotor AC excitation, while wound-rotor motor generally works in asynchronous state with low slip.
The biggest problem of replacing doubly-fed motor with wound motor is that the rotor eddy current loss is large and the speed regulation range is very limited. Too wide speed regulation will lead to high AC frequency of rotor excitation and large loss.
In addition, the insulation of the wound rotor of the wound motor is very low, and the reactive power of the motor must be supplemented by the power grid during normal work. When used as a doubly-fed motor, if the motor needs to transmit reactive power to the power grid, the excitation voltage will be relatively large, which may exceed the limit of the motor and cause breakdown accidents.
It is feasible to replace doubly-fed motor with wound motor, but the speed regulation range is much smaller than that of real doubly-fed motor.
Using wound motor instead of conventional asynchronous motor has larger magnetic leakage and air gap than generator. In addition, because the rotor of wound motor is not insulated, which is equivalent to the existence of damping winding, the AC excitation magnetic field of the rotor is partially offset by eddy current (which will also affect the transient process). In order to approach the state of doubly-fed machine, it can only be tested near the rated speed, the speed regulation range is very small, and the winding excitation frequency is very low, which is meaningless. The higher the excitation frequency, the greater the influence of eddy current and the greater the deviation, which will affect the experimental results. During speed regulation operation, there is a speed difference between the magnetic fields of the rotor and the stator, which is equivalent to a magnetic field sweeping from the rotor surface, which will lead to eddy current in the rotor and power loss in the stator. Reforming the winding motor is almost equivalent to buying several new ones, which is very uneconomical.
Doubly-fed generator is also called AC excitation generator. Because the rotor is excited by AC voltage, its operation mode is flexible, and it has incomparable advantages in solving the problems of continuous power frequency overvoltage, variable speed constant frequency power generation, speed regulation of motor-generator set and so on. There are three main operation modes of AC excited generators: 1) variable speed constant frequency operation; 2) operate in a wide range of reactive power regulation mode; 3) Run in the power generation-electric mode.
Asynchronous generator refers to the working state of asynchronous motor in power generation, and its excitation modes include grid power supply excitation power generation (separate excitation) and shunt capacitor self-excitation power generation (self-excitation).
1, grid power supply and excitation power generation: the asynchronous motor is connected to the grid, the stator winding in the motor generates a rotating magnetic field rotating at synchronous speed, and then the prime mover drags the rotor speed, so that the direction of magnetic torque provided by the grid is necessarily opposite to that of the speed, while the direction of mechanical torque is the same as that of the speed. At this time, the mechanical energy of the prime mover is converted into electrical energy. In this case, the active power generated by asynchronous motor is transmitted to the power grid; At the same time, the reactive power of the power grid is consumed for excitation, and the reactive power consumed by magnetic leakage of the stator and rotor is supplied. Therefore, when asynchronous generators are connected to the grid to generate electricity, it is generally required to install reactive power compensation devices, and the parallel capacitor compensation method is usually adopted.
2. Self-excitation of parallel capacitors: The connection modes of parallel capacitors are divided into star and triangle. Connection of excitation capacitor In the process of generating electricity by using its own remanence, the generator periodically charges the capacitor; At the same time, the capacitor also periodically discharges through the stator winding of the asynchronous motor. This alternating charging and discharging process of capacitor and winding constantly plays the role of excitation, making the generator generate electricity normally. Excitation capacitors are divided into main excitation capacitors and auxiliary excitation capacitors. The main excitation capacitor needs to ensure the voltage establishment under no-load condition, and the auxiliary capacitor is designed to ensure the voltage constant after the load is connected to prevent voltage collapse.
At present, commonly used wind turbines are generally divided into constant-speed wind turbines and variable-speed constant-frequency wind turbines, and variable-speed constant-frequency wind turbines are more advanced. There are many types, among which stall wind turbine+speed-increasing gearbox+cage asynchronous generator is the most quasi-constant speed, accounting for the largest proportion of installed capacity in China at present. At present, the most common variable-speed constant-frequency wind turbine+speed-increasing gearbox+doubly-fed asynchronous generator, or removing the gearbox and adding a power electronic converter, or weighing.
The power and speed absorbed by the wind turbine are controlled by adjusting the propeller torque, and the speed can be controlled by adjusting the electromagnetic torque of the generator at low wind speed, but the stall fan can only rely on the gearbox. The aim is to maximize the power absorbed by the wind turbine.
Theoretically speaking, n=60f/p, where n is the generator speed, f is the frequency, 50 in China, and p is the logarithm. For the motor, if P is constant, the speed will be fixed.
Motor]
Introduction to 0
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 electric machines 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 Classification of rotating electrical machines
In the rotating electric machine, because the generator is the production machine of electric energy, compared with the motor, its types are much less; 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. It can convert the input voltage signal into the mechanical output on the motor shaft and drag the controlled components, so as to achieve the purpose of control.
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 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 the actuator that converts electric pulse 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 (SRM) is a new type of speed regulating motor, which is extremely simple and firm in structure, low in cost and excellent in 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 is developed on the basis of brushless DC motor, but its driving current is out-and-out AC. Brushless DC motors can be divided into brushless speed motors and brushless torque motors. Generally speaking, 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, which appeared 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 to realize 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 driven by alternating current, and the rotating magnetic fields of rotor and stator run synchronously. The stator of synchronous motor is exactly the same as that of asynchronous motor; However, there are two kinds 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.
Synchronous motors have the same working characteristics as all motors. Synchronous motors are also "retrograde", that is, they can operate 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 motor
4. 1 position signal motor
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 of the Hall effect, the brushless DC tachometer generator is a kind of motor without cogging and winding, and it will not produce "cogging harmonic potential" because of cogging. This kind of motor is simple in structure and convenient for miniaturization.
5 conclusion
Generally speaking, in a complete automatic control system, signal motor, power motor and control motor will 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 engineering to master the characteristics and uses of various rotating electrical machines from the overall structure.
synchronous generator
In order to realize energy conversion, synchronous generator needs a DC magnetic field to generate DC current of the magnetic field, which is called generator excitation current. According to the supply mode of excitation current, generators that get excitation current from other power sources are called separately excited generators, and generators that get excitation power from themselves are called self-excited generators.
First of all, several ways for generators to obtain excitation current.
1.DC generator excitation mode: The generator with this excitation mode has a special DC generator called DC exciter. The exciter is generally coaxial with the generator, and the excitation winding of the generator obtains DC current from the exciter through the slip ring and fixed brush installed on the main shaft. This excitation method has the advantages of independent excitation current, reliable operation and reduced self-use electricity. In the past few decades, it has been the main excitation mode of generators and has mature operating experience. The disadvantage is that the excitation adjustment speed is slow and the maintenance workload is heavy, so it is rarely used for units above 10MW.
2, AC exciter power supply excitation mode, some modern large-capacity generators use AC exciter to provide excitation current. The AC exciter is also installed on the main shaft of the generator, and its output AC is supplied to the generator rotor for excitation after rectification. At this time, the excitation mode of the generator belongs to other excitation modes. Because of the static rectifier device, it is also called other excitation static excitation, and the excitation current is provided by the AC auxiliary exciter. The AC auxiliary exciter can be a permanent magnet or an alternator with a self-excited constant voltage device. In order to improve the speed of excitation regulation, AC exciter usually adopts 100-200 Hz intermediate frequency generator, while AC auxiliary exciter adopts 400-500 Hz intermediate frequency generator. The DC excitation winding and three-phase AC winding of this generator are wound in stator slots, and the rotor has only teeth and slots without windings, just like gears. Therefore, it has no rotating contact parts such as brushes and slip rings, and is reliable in operation, simple in structure and convenient in manufacturing process. The disadvantage is that the noise is large, and the harmonic component of AC potential is also large.
3. Excitation mode without exciter:
In the excitation mode, there is no special exciter, but the excitation power supply is obtained from the generator itself, which is supplied to the generator itself after rectification. This mode is called self-excited static excitation. Self-excited static excitation can be divided into self-shunt excitation and self-compound excitation. In the self-shunt excitation mode, the excitation current is obtained through the rectifier transformer connected to the generator outlet, and then it is supplied to the generator for excitation after rectification.
Excitation mode has the advantages of simple structure, less equipment, less investment and less maintenance workload. In the self-compound excitation mode, in addition to rectification and transformation, a high-power current transformer is connected in series in the stator circuit of the generator. The function of this transformer is to provide large excitation current to the generator in case of short circuit, so as to make up for the shortage of rectifier transformer output. There are two kinds of excitation power sources in this excitation mode, voltage power source obtained by rectifier transformer and current power source obtained by series transformer.
Second, the characteristics of generator and excitation current
1, voltage adjustment
Self-regulating excitation system can be regarded as a negative feedback control system with voltage as the regulated quantity. Reactive load current is the main reason for the decrease of generator terminal voltage. When the excitation current is constant, the generator terminal voltage will decrease with the increase of reactive current. However, in order to meet the user's requirements for power quality, the terminal voltage of the generator should remain basically unchanged, and the way to achieve this requirement is to adjust the excitation current of the generator with the change of reactive current.
2. Adjustment of reactive power:
When the generator runs in parallel with the system, it can be considered as running together with the bus of infinite large-capacity power supply. To change the excitation current of the generator, the induced potential and stator current will also change, and the reactive current of the generator will also change. When the generator runs in parallel with the infinite capacity system, in order to change the reactive power of the generator, the excitation current of the generator must be adjusted. At this time, the changed generator excitation current is not so-called "voltage regulation", but only changes the reactive power sent to the system.
3. Distribution of reactive load:
Generators running in parallel distribute reactive current in proportion to their rated capacity. Large-capacity generators should bear more reactive load, while small-capacity generators should provide less reactive load. In order to realize the automatic distribution of reactive load, the excitation current of the generator can be changed by the excitation device with automatic high voltage regulation to keep its terminal voltage unchanged, and the gradient of generator voltage regulation characteristics can be adjusted to realize the reasonable distribution of reactive load of generators running in parallel.
Third, the method of automatically adjusting the excitation current
When changing the excitation current of the generator, it is generally not directly carried out in its rotor circuit, because the current in this circuit is very large, which is not convenient for direct adjustment. The usual method is to change the excitation current of the exciter to adjust the rotor current of the generator. The commonly used methods include changing the resistance of the excitation circuit of the exciter, changing the additional excitation current of the exciter, and changing
The conduction angle of SCR, etc. This paper mainly talks about the method of changing the conduction angle of thyristor. According to the change of generator voltage, current or power factor, the conduction angle of thyristor is changed, so that the excitation current of generator changes accordingly. This device is generally composed of transistors and thyristor electronic components, and has the advantages of sensitivity, rapidity, no fault zone, large output power, small volume and light weight. When an accident occurs, it can effectively suppress the generator overvoltage and realize rapid demagnetization. The automatic regulating excitation device usually consists of measuring unit, synchronizing unit, amplifying unit, regulating unit, stabilizing unit, limiting unit and some auxiliary units. Measured signal (such as voltage, current, etc. ) after being converted by the measuring unit, it is compared with the given value, and then the comparison result (deviation) is amplified by the pre-amplifier unit and the power amplifier unit to control the conduction angle of the thyristor, so as to achieve the purpose of adjusting the excitation current of the generator. The function of the synchronization unit is to synchronize the trigger pulse output by the phase-shifting part with the AC excitation power supply of the thyristor to ensure the correct trigger of the thyristor. The function of differential regulating device is to enable generators running in parallel to distribute reactive load stably and reasonably. The introduction of stability unit is to improve the stability of power system. The excitation system stabilization unit is used to improve the stability of the excitation system. The limiting device is set to prevent the generator from operating under over-excitation or under-excitation. It must be pointed out that not every self-regulating excitation device has the above-mentioned various units, but the units of a regulating device are related to its specific tasks.
Four, automatic adjustment of excitation components and auxiliary equipment
Machine terminal voltage transformer, machine side current transformer and excitation transformer are components that automatically adjust excitation; The excitation device needs to provide the following currents: auxiliary AC380v, auxiliary DC220v control power supply and auxiliary DC220v closing power supply; Need to provide the following empty contacts, automatic start, automatic close, grid connection (normally open and normally closed) increase or decrease; The following analog signals need to be provided: generator terminal voltage 100V, generator terminal current 5A, bus voltage 100V, and the excitation device outputs the following relay contact signals; Overcurrent of excitation transformer, loss of excitation, abnormal excitation device, etc.
Excitation control, protection and signal loop are composed of field-extinguishing switch, auxiliary magnetic circuit, fan, tripping of field-extinguishing switch, over-current of excitation transformer, failure of regulator, abnormal working condition of generator, electric quantity transmitter, etc. When the synchronous generator has an internal fault, it must be demagnetized to reduce the rotor magnetic field to the minimum as soon as possible and shorten the demagnetization time as much as possible. This is the main function of the demagnetization device. According to the rated excitation voltage, it can be divided into linear resistance demagnetization and nonlinear resistance demagnetization.
In recent ten years, due to the emergence and use of new technologies, new processes and new equipment, the excitation mode of generators has been continuously developed and improved. In the aspect of automatic regulating excitation device, many new regulating devices have been continuously developed and popularized. Because of the obvious advantages of using microcomputer software to realize automatic adjustment of excitation device, many countries are developing and testing digital automatic adjustment excitation device composed of microcomputer and corresponding external equipment, which will realize adaptive optimal adjustment.
The method of obtaining excitation current is called excitation mode. At present, there are two kinds of excitation methods: one is DC exciter excitation system with DC generator as excitation power supply; The other is rectifier excitation system, which uses silicon rectifier to convert alternating current into DC for excitation. The explanation is as follows:
1 DC exciter DC exciter is usually coaxial with synchronous generator and adopts parallel excitation or other excitation methods. When another excitation mode is adopted, the excitation current of the exciter is provided by another coaxial DC generator called auxiliary exciter. As shown in figure 15.5.
2 Static rectifier excitation There are three alternators on the same shaft, namely the main generator, the AC main exciter and the AC auxiliary exciter. The excitation current of the auxiliary exciter is initially provided by an external DC power supply, and then converted into self-excitation after the voltage is established (sometimes using a permanent magnet generator). The output current of the auxiliary exciter is rectified by the static thyristor rectifier and supplied to the main exciter, while the AC output current of the main exciter is rectified by the static three-phase bridge silicon rectifier and supplied to the excitation winding of the main generator. (see figure 15.6)
3 DC output of rotating rectifier excitation static rectifier can only be transmitted to rotating excitation winding through brush and slip ring. Large-capacity synchronous generator, the excitation current reaches several thousand amperes, which makes the slip ring overheat seriously. Therefore, in large-capacity synchronous generators, brushless and slip-ring rotary rectifier excitation system is often used, as shown in figure 15.7. The main exciter is a rotating armature three-phase synchronous generator. The alternating current of the rotating armature flows through the silicon rectifier rotating with the main shaft and is rectified, and then directly sent to the rotor excitation winding of the main generator. The excitation current of AC main exciter is provided by coaxial AC auxiliary exciter after rectification by static thyristor rectifier. Because this excitation system cancels the slip ring and brush device, it is also called brushless excitation system.
The basic equation, equivalent circuit and vector diagram of 1
Variable speed constant frequency doubly-fed generator adopts winding induction in structure.
The motive is similar. Its stator and rotor are three-phase symmetrical windings,
The circuit is symmetrical and has a uniform air gap distribution. Suppose. Ignore.
Higher harmonics of stator and rotor current and higher harmonics of stator and rotor space magnetic potential.
Wave component; B. Ignore the motor core hysteresis, eddy current loss and magnetic circuit.
The influence of saturation; C. the rotor excitation power supply can meet the requirements.
Rotor current, regardless of its resistance and loss; D. the motor stator is connected to
Infinite power grid.
When the prime mover drags the motor rotor to rotate at a certain speed and rotates
Three-phase symmetrical power supply with slip frequency SFL of sub-winding.
The fundamental rotating magnetic potential generated by rotor current is relative to the rotor.
Take slip speed = rotational speed (=60fl for motor synchronization).
Rotational speed). Rotate at synchronous speed relative to the stator. The magnetic potential is the same as that of the stator.
The fundamental magnetic potential of the stator generated by three-phase current is relatively static, and it lies in
A synthetic magnetic potential is formed in the air gap. According to the law of electromagnetic induction, this
The resultant magnetic field generated by the resultant magnetic potential in the air gap will be
The potentials zh 1 and zh 1 are induced in the stator and rotor windings respectively. . And induced current.
Similar to a machine, when each physical quantity on the rotor side is converted to the stator side, it can be written as
The basic equation after conversion is as follows