The rotor core of a wire-wound motor is not insulated, and the core of a doubly-fed motor is insulated, mainly because doubly-fed motors need to consider the rotor alternating current excitation of the working conditions, and wire-wound motors are generally operated in asynchronous state with a low rate of rotation.
The biggest problem of using wound motor instead of doubly-fed motor is that the rotor eddy current loss is large, the range of speed regulation work is very limited, too wide speed regulation will lead to high rotor excitation AC frequency, the loss is large.
In addition, the insulation of the wound rotor line of the wound motor is very low, and the reactive power of the motor must rely on the grid to supplement during normal operation. When used as a doubly-fed motor, if the motor needs to send reactive power to the grid, the voltage of the excitation will be higher, which may exceed the motor's limit and cause a breakdown accident.
Using a wire-wound motor instead of a doubly-fed motor is feasible, but the speed range is much smaller than that of a true doubly-fed motor.
The main reason for using a wire-wound motor as a substitute is that conventional asynchronous motors have a larger magnetic leakage than generators and a larger magnetic air gap. In addition wire-wound motor because the rotor is not insulated, equivalent to the existence of a damping winding, resulting in the rotor AC excitation field is partially offset by the eddy current (also affects the transient process), to be close to the state of the doubly-fed motor, you can only be tested near the rated speed, the speed of the speed adjustment will be very small, the winding excitation frequency is very low, there is no sense. The higher the excitation frequency, the greater the eddy current influence, the greater the deviation, affecting the experimental results. During speed regulation operation, there is a differential speed between the rotor and stator magnetic field, which is equivalent to a magnetic field sweeping from the rotor surface, which causes eddy currents in the rotor and also causes power loss in the stator. To modify the winding motor is almost equal to buy a few new, very uneconomical.
Double-fed generators are also known as AC-excited generators. Because the rotor side of the AC voltage excitation, so that it has a flexible mode of operation, in solving the power station continuous frequency overvoltage, variable speed and constant frequency power generation, pumped storage power station electric - generator set speed control and so on has a traditional synchronous generator can not be compared to the superiority of the problem. The main operation modes of the AC excitation generator are as follows: 1) running in the variable speed and constant frequency mode; 2) running in the mode of reactive power wide-range regulation; 3) running in the mode of power generation - electric.
The asynchronous generator is an asynchronous motor is in the working state of power generation, from which the excitation mode has two cases of grid power excitation power generation (he excitation) and shunt capacitor self-excitation power generation (self-excitation).
1, grid power excitation power generation: the asynchronous motor connected to the grid, the stator windings within the motor to generate synchronous speed rotation of the rotating magnetic field, and then drag the prime mover, so that the rotor speed is greater than the synchronous speed, the direction of the magnetic torque provided by the grid must be opposite to the direction of the rotational speed, while the direction of the mechanical torque with the rotational speed of the direction of the same direction, which will be converted to the prime mover of the mechanical energy into electrical energy. In this case, the asynchronous motor sends out active power to the grid; at the same time, it consumes the reactive power of the grid for excitation, and supplies the reactive power consumed by the stator and rotor leakage, so the asynchronous generator grid-connected power generation is generally required to add a reactive power compensation device, usually in the way of parallel capacitor compensation.
2, shunt capacitor self-excited power generation: shunt capacitor connection is divided into two star and triangle. Excitation capacitor access in the generator using its own residual magnetism in the process of generating electricity, the generator periodically to the capacitor charging; at the same time, the capacitor is also periodically discharged through the asynchronous motor stator winding. This capacitor and the winding composed of alternating charging and discharging process, constantly play the role of excitation, so that the generator normal power generation. Excitation capacitor is divided into the main excitation capacitor and auxiliary excitation capacitor, the main excitation capacitor is to ensure that the no-load condition to establish the voltage required capacitance, auxiliary capacitance is to ensure that access to the load after the voltage is constant, to prevent the voltage collapse and set.
Now commonly used wind turbines are generally divided into quasi-constant-speed wind turbines and variable-speed constant-frequency wind turbines, variable-speed constant-frequency wind turbines are more advanced. Each sub type of many, quasi-constant speed is the most stall-type wind turbine + speed gear box + cage asynchronous generator, in the current domestic capacity has been installed in the proportion of the largest. Variable speed constant frequency is currently the most common pitch wind turbine + boost gearbox + doubly-fed asynchronous generator, or remove the gearbox to add a power electronic converter, or trade-off.
Pitch torque regulation controls the power and speed absorbed by the wind turbine in the high wind section, and the electromagnetic torque of the generator can be adjusted to control the speed in the low wind section, but the stall-type wind turbine can only rely on the gear box. The purpose is to maximize the power absorbed by the wind turbine.
Theoretically n=60f/p , n is the generator speed, f is the frequency, our country is 50, p is the polar logarithm, for the motor, p must be, the speed is also fixed.
Classification of motors
0 Introduction
In a broad sense, a motor is a transforming device for electrical energy, including rotating motors and stationary motors. Rotary motor is a kind of energy conversion device that realizes the mutual conversion between electric energy and mechanical energy according to the principle of electromagnetic induction; stationary motor is a kind of electromagnetic device that realizes the change of voltage according to the law of electromagnetic induction and the principle of balance of magnetic potential, which is also called transformer.
Here we mainly discuss rotary motors, rotary motors are many kinds of rotary motors, in the modern industrial field is extremely wide range of applications, it can be said that there are electrical energy applications will be rotary motors. Compared with the internal combustion engine and steam engine, rotary motor operating efficiency is much higher; and electrical energy than other energy transmission is more convenient, cheaper, in addition to electrical energy also has a clean and pollution-free, easy to control and so on, so in real life and engineering practice, rotary motors are increasingly widely used.
Different motors have different applications, with the continuous development of motor manufacturing technology and the motor principle of operation of the study continues to deepen, there are many new motors, for example, the U.S. EAD developed by the company's grooveless brushless DC motors, the Japanese SERVO company developed a low-power hybrid stepping motors, China's self-development of low-torque motors suitable for use in industrial machine tools and e-bikes, such as large torque and low-speed motors. The large torque low-speed motors and so on.
1 rotary motor classification
In the rotary motor, because the generator is the production of electrical energy machine, so compared with the motor, it is much less diverse; while the motor is the application of the machine in the industry, so compared with the generator, the study of electric motors to be much more detailed classification. In fact, what we usually refer to as a rotating motor is narrowly defined as an electric motor - commonly known as a "motor". As we all know, the motor is an important part of the transmission and control system, with the development of modern science and technology, the motor in the practical application of the focus has begun to shift from the past simple transmission to the complex control; especially for the motor speed, position, torque, precise control.
It can be seen that, for an electrical engineer and technician, familiar with the various types of motors and their performance is a very important thing. Often people classify rotating motors basically according to their purpose. Here we will start from the control motor, step by step to introduce the most representative, most commonly used, the most basic motors in the motor - control motors and power motors and signal motors.
2 Control motors
2.1 Servo motors
Servo motors are widely used in a variety of control systems, which can convert the input voltage signal into a mechanical output on the motor shaft, dragging the controlled element, so as 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 precision 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.
Categorization of rotary motors, DC servo motor in the mechanical characteristics can be very good to meet the requirements of the control system, but due to the existence of the commutator, there are many shortcomings: the commutator and the brush is easy to produce sparks between the drive to interfere with the drive work, can not be applied in the presence of combustible gases; brushes 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, the servo motor requires the motor speed to be controlled by the added voltage signal; the speed can be continuously changed with the change of the added voltage signal; the motor's reflection should be fast, the volume should be small, and the control power should be small. Servo motors are mainly used in a variety of motion control systems, especially the follower system.
2.2 Stepping motor
The so-called stepping motor is an actuator that converts an electrical pulse into an angular displacement; to put it more plainly: when the stepping driver receives a pulse signal, it drives the stepping motor to rotate a fixed angle in a 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 stepper motors include reactive stepper motor (VR), permanent magnet stepper motor (PM), hybrid stepper motor (HB) and single-phase stepper motor.
The difference between a stepper motor and an ordinary motor lies mainly in the form of its pulse drive, and it is this feature that allows the stepper motor to be combined with modern digital control technology. However, the stepper 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 occasions where the accuracy requirements are not particularly high. Because of its simple structure, high reliability and low cost, the stepping motor is widely used in various fields of production practice; especially in the field of CNC machine tool manufacturing, because the stepping motor does not need A/D conversion, can directly convert digital pulse signals into angular displacement, so it has been considered the most ideal CNC machine tool actuating elements.
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 in printers and plotters.
In addition, stepper motors also have many defects; due to the existence of no-load starting frequency of stepper motors, so stepper motors can run normally at low speeds, but if higher than a certain speed can not be started, and accompanied by a sharp whistling sound; different manufacturers of subdivided drive precision may vary greatly, the larger the subdivision of the accuracy of the larger the harder it is to control; and, stepper motors rotate at low speeds when the larger vibration and noise
2.
2.3 Torque motor
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 motors and AC torque motors.
Of these, the DC torque motor has a small self-induced reactance, so it is very responsive; its output torque is proportional to the input current, independent of the speed and position of the rotor; it can be connected directly 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 in the load's shaft, and it can eliminate systematic errors due to the use of reduction gears
AC torque motors can be divided into synchronous and asynchronous two kinds, is currently commonly used is the squirrel cage asynchronous torque motors, it has a low speed and large torque characteristics. 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 Motor
Switched reluctance motor is a new type of speed motor, extremely simple and sturdy structure, low cost, excellent speed performance, is a strong competitor of the traditional control motor, with strong market potential.
2.5 Brushless DC motor
Brushless DC motor (BLDCM) is developed on the basis of brushed DC motor, but its drive current is uncompromisingly AC; 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 DC brushless motor, and the latter is called an AC servomotor, a type of AC servomotor to be exact.
Brushless DC motors are usually of "slender" construction in order to reduce 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 motor's mechanical characteristics and regulation characteristics of the linearity of good, wide speed range, long life, easy maintenance and low noise, there is no brush caused by a series of problems, so this motor in the control system has a lot of potential for application.
3 power motors
3.1 DC motor
DC motor is the earliest motor, about the end of the 19th century, which can be roughly divided into two categories of commutator and non-commutator. 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 silicone-controlled DC power supply appeared after.
3.2 Asynchronous motor
Asynchronous motor is based on the air gap rotating magnetic field and the 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 wide range of specifications, its 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.
The asynchronous motor has the advantages of simple structure, easy manufacture, use and maintenance, reliable operation and smaller quality, lower cost. Moreover, the asynchronous motor has higher operating efficiency and better operating 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 motors are 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 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 and so on.
3.3 Synchronous motor
The so-called synchronous motor is a motor that runs in synchronization with the rotating magnetic field of the rotor and stator under the drive of alternating current. The stator of the synchronous motor is exactly the same as that of the asynchronous motor; however, the rotor has two types: "convex pole type" and "hidden pole type". Convex pole type rotor synchronous motor structure is simple, easy to manufacture, but the mechanical strength is low, suitable for low-speed operation; hidden pole type synchronous motor manufacturing process is complex, but the mechanical strength is high, suitable for high-speed operation.
Synchronous motor operating characteristics and all the same motor, 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 a control element; three-phase synchronous motors are the main body. In addition, it can also be used as a regulator to deliver inductive or capacitive reactive power to the grid.
4 signaling motors
4.1 Position signaling motors
At present, the most representative position signaling motors: rotary transformers, induction synchronizers, and self-aligning angle machines.
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. After 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 the coordinate transformation and trigonometric operation in the calculation device, and also can be used as the angle data transmission and phase shifter in the control system.
The inductive synchronizer is a high-precision position or angle detection element, there are two kinds of disc 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. Often two or more are used jointly in control systems to enable two or more axes that are not connected to each other on the machinery to automatically maintain the same change in angle, or to rotate synchronously.
4.2 Speed signal motor
The most representative speed signal motor is the speed generator, in essence, it is a kind of rotational speed will be converted into an electrical signal machine electromagnetic components, its output voltage is proportional to the speed. From the working principle, it belongs to the category of "generator". Velocimetric generator in the control system mainly as a damping element, differential element, integral element and speed components to use.
The speed generator is divided into DC and AC; the DC speed generator is divided into he-excited and permanent magnet, and its structure and working principle is the same as the low-power DC generator, the output power is usually small, and the output voltage linearity error and temperature error are lower than an upper limit when it is used as a calculation element. And AC speed generator and synchronous and asynchronous; synchronous speed generator including: permanent magnet, induction and pulse type; asynchronous speed generator is the most widely used cup rotor asynchronous speed generator.
In order to improve the accuracy and reliability of the speed generator, at present, the DC speed generator appears brushless structure of the Hall effect DC speed generator. Because this Hall effect brushless DC speed generator is a cogwheel, no winding motor, so it does not produce due to the cogwheel and the existence of the "cogwheel harmonic potential", this motor structure is simple, easy to miniaturize.
5 Conclusion
Generally, in a complete automatic control system, signal motors, power motors and control motors will have their own place. Usually, the control motor is a very "precise" motor, which acts as the "core actuator" in the control system; while the power motor is a relatively "strong" high-power motor, which is often used to drag machines and equipments in the field. The power motor is a relatively "strong" high-power motor, commonly used to drag the field of machinery and equipment; the signal motor in the control system as a "communicator" role, essentially a "motor sensor".
Of course, not all automatic control systems have these three kinds of motors, in the general field of automation, such as motion control and process control, especially in motion control, control motors are essential to the "core device", so control motors in the field of automation in the status of the pivotal. This is also one of the reasons why control motors have been studied the most.
In fact, with the continuous development of motor manufacturing technology and the integration of each other, the performance of various rotary motors are gradually "cross-sectionalization" and "specialization". A variety of rotary motors for extremely detailed classification is impossible, because many new rotary motors are many motor operating principles and many motor manufacturing technology is highly unified organism. Therefore, for the general electrical engineering technicians of non-electrical machinery, can grasp the characteristics and uses of various rotary motors from the overall structure can be.
Synchronous generator
Synchronous generator in order to realize the conversion of energy, there is a need for a dc magnetic field and produce this field of dc current, known as the generator excitation current. According to the excitation current supply, where the excitation current from other sources of power to obtain the generator, known as the other excitation generator, from the generator itself to obtain the excitation power, known as self-excited generator.
A generator to obtain the excitation current of several ways
1, DC generator power supply excitation mode: this excitation mode of generator with a special DC generator, this special DC generator is called DC exciter, the exciter is generally coaxial with the generator, the generator's excitation winding through the large shaft mounted on the slip ring and the fixed brushes to obtain DC current from the exciter. This type of excitation has the advantages of independent excitation current, reliable operation and reduction of self-power consumption, etc. It is the main excitation method for generators in the past decades, and has more mature operation experience. The disadvantage is that the excitation regulation speed is slow, maintenance workload, so in more than 10MW unit is rarely used.
2, AC exciter power supply excitation, modern large-capacity generators, some use AC exciter to provide excitation current. AC exciter is also installed in the generator shaft, it outputs AC current rectified to supply the generator rotor excitation, at this time, the generator's excitation mode is he excitation mode, but also due to the use of static rectifier, so it is also known as he excitation static excitation, AC sub-exciter to provide the excitation current. The AC sub-exciter can be a permanent magnet machine or an alternator with a self-excitation constant voltage device. In order to improve the speed of excitation regulation, AC exciter usually adopts 100 - 200HZ medium frequency generator, while AC sub-exciter adopts 400 - 500HZ medium frequency generator. This generator DC excitation winding and three-phase AC winding are wound in the stator slot, the rotor only teeth and groove and no winding, like a gear, so it does not have brushes, slip rings and other rotating contact parts, has the advantages of reliable operation, simple structure, convenient manufacturing process. The disadvantage is that the noise is larger, the harmonic component of the AC potential is also larger.
3, no exciter excitation mode:
In the excitation mode does not set up a special exciter, but from the generator itself to obtain the excitation power supply, rectified and then supplied to the generator itself excitation, known as self-excited static excitation. Self-excited static excitation can be divided into self-parallel excitation and self-re-excitation two ways. Self-excitation mode it is connected to the generator outlet rectifier transformer to obtain the excitation current, rectified and supplied to the generator excitation, this
excitation mode has a simple knot, less equipment, investment and maintenance workload and other advantages. Self-recovery excitation mode in addition to the absence of rectifier transformer, but also has a series connected to the generator stator circuit of high-power current transformer. The role of this transformer is in the event of a short circuit, the generator to provide a larger excitation current to make up for the lack of rectifier transformer output. This type of excitation has two types of excitation sources, a voltage source obtained through the rectifier transformer and a current source obtained through the series transformer.
Second, the generator and the excitation current related characteristics
1, voltage regulation
Automatically regulated excitation system can be viewed as a negative feedback control system with voltage as the regulated quantity. Reactive load current is the main cause of generator terminal voltage drop, when the excitation current is unchanged, the generator terminal voltage will be reduced with the increase of reactive current. However, in order to meet the user's requirements for power quality, the generator's terminal voltage should be basically kept unchanged, and the way to realize this requirement is to adjust the excitation current of the generator with the change of the reactive current.
2, reactive power regulation:
When the generator and system parallel operation, can be regarded as the bus operation with infinite capacity power supply, to change the generator excitation current, the induced potential and stator current also follow the change, at this time the generator's reactive current also follow the change. When the generator is operated in parallel with an infinite capacity system, the generator excitation current must be adjusted in order to change the reactive power of the generator. At this time to change the generator excitation current is not usually referred to as "voltage regulation", but only to change the reactive power into the system.
3, the distribution of reactive load:
Parallel operation of generators according to their respective rated capacity, proportional to the distribution of reactive current. Large-capacity generators should be burdened with more reactive load, while smaller capacity is negative to provide less reactive load. In order to realize the automatic distribution of reactive load, can be adjusted through the automatic high-voltage regulation of the excitation device, change the generator excitation current to maintain its end voltage constant, but also on the generator voltage regulation characteristics of the tilt can be adjusted to realize the parallel operation of the generator reactive power load distribution is reasonable.
Three, automatic regulation of the excitation current method
In changing the excitation current of the generator, generally not directly in the rotor circuit, because the circuit current is very large, it is not easy to directly adjust the method usually used is to change the excitation current of the excitation machine, in order to regulate the rotor current of the generator purpose. Commonly used methods are to change the resistance of the exciter excitation circuit, change the additional excitation current of the exciter, change
Silicon controlled conduction angle and so on. Here mainly talk about changing the SCR conduction angle method, it is according to the generator voltage, current or power factor changes, accordingly change the SCR rectifier conduction angle, so the generator's excitation current will follow the change. This device is generally composed of transistors, silicon controlled electronic components, with sensitive, fast, no failure zone, high output power, small size and light weight and other advantages. In case of accident, it can effectively suppress the overvoltage of the generator and realize rapid demagnetization. Automatic regulation excitation device usually consists of measurement unit, synchronization unit, amplification unit, adjustment unit, stabilization unit, restriction unit and some auxiliary units. The measured signals (such as voltage, current, etc.) are compared with the given values after transformed by the measuring unit, and then the comparison results (deviation) are amplified by the preamplifier unit and power amplifier unit, and used to control the conduction angle of the thyristor to regulate the excitation current of the generator. The function of the synchronization unit is to synchronize the trigger pulse output from the phase-shifting part with the AC excitation power supply of the SCR rectifier to ensure the correct triggering of the SCR. The function of the regulation unit is to enable the generators running in parallel to stabilize and reasonably distribute the reactive power load. The stabilization unit is a unit introduced to improve the stability of the power system . Excitation system stabilization unit Used to improve the stability of the excitation system. The limiting unit is set up so that the generator does not operate under over-excitation or under-excitation conditions. It must be pointed out that not every automatic regulation of excitation device has the above units, a regulator device has a unit related to its specific tasks.
Four, automatic regulation of excitation components and auxiliary equipment
Automatic regulation of excitation components organic voltage transformer, machine current transformer, excitation transformer; excitation device needs to provide the following currents, factory AC380v, factory DC220v control power. Factory DC220v closing power supply; need to provide the following empty contact, automatic start. Automatic shutdown. Grid-connected (a normally open, a normally closed) increase, decrease; need to provide the following analog signals, generator machine voltage 100V, generator machine current 5A, bus voltage 100V, excitation device output the following relay contact signals; excitation variable overcurrent, demagnetization, excitation device abnormality and so on.
The excitation control, protection and signaling circuits are composed of demagnetizing switch, magnetizing circuit, fan, demagnetizing switch stealing jump, excitation overcurrent, regulator failure, abnormal generator operating conditions, power transmitter and so on. In addition to the synchronous generator must be unlisted when the internal fault occurs, but also must be demagnetized, the rotor magnetic field as soon as possible to minimize the extent of the rotor to ensure that the rotor does not however, so that the demagnetization time as short as possible, is the main function of demagnetization device. According to the size of the rated excitation voltage can be divided into linear resistance demagnetization and non-linear resistance demagnetization.
In the past decade, due to the emergence and use of new technologies, new techniques and new devices, the generator excitation method has been continuously developed and improved. In the automatic regulation of the excitation device, but also continue to develop and promote the use of many new regulators. Due to the use of microcomputer software to realize the automatic regulation of the excitation device has significant advantages, many countries are now developing and testing the microcomputer with the corresponding external equipment composed of digital automatic regulation of the excitation device, this regulation device will be able to realize the best adaptive regulation.
The method of obtaining the excitation current is called the excitation mode. Currently used in the excitation method is divided into two categories: one is the DC generator as the excitation power supply DC exciter excitation system; the other is the use of silicon rectifier to convert AC into DC to supply the excitation of the rectifier excitation system. It is described as follows:
1 DC exciter excitation DC exciter is usually coaxial with the synchronous generator, using parallel excitation or other excitation method. When he is connected, the excitation current of the exciter is supplied by another coaxial DC generator called the deputy exciter. As shown in Figure 15.5.
2 Static Rectifier Excitation There are three alternators on the same shaft, the main generator, the AC main exciter, and the AC vice exciter. The excitation current of the vice-exciter is initially supplied by an external DC power supply, and then switched to self-excitation (sometimes a permanent magnet generator is used) when the voltage is established. The output current of the subexciter is rectified by a stationary thyristor rectifier and supplied to the main exciter, while the AC output current of the main exciter is rectified by a stationary three-phase bridge silicon rectifier and supplied to the excitation winding of the main generator. (See Figure 15.6.)
3 Rotating Rectifier Excitation The DC output of the stationary rectifier must pass through the brushes and collector rings to be delivered to the rotating excitation windings, and for large-capacity synchronous generators, the excitation current reaches thousands of amperes, which makes the collector rings overheat severely. Therefore, in large-capacity synchronous generators, a rotating rectifier excitation system that does not require brushes and collector rings is often used, as shown in Figure 15.7. The main exciter is a rotating-armature three-phase synchronous generator, and the alternating current from the rotating armature is rectified by a silicon rectifier rotating with the main shaft and sent directly to the rotor excitation winding of the main generator. The excitation current of the AC main exciter is supplied by the coaxial AC subexciter after being rectified by a stationary thyristor rectifier. Because this excitation system eliminates the collector ring and brush device, it is also known as brushless excitation system
1 Basic equations, equivalent circuits and vector diagrams
Variable-speed, constant-frequency doubly-fed generators are similar in structure to the wire-wound induction
motor. Its stator and rotor are three-phase symmetrical windings, magnetic circuit,
circuit symmetry, and has a uniform air gap distribution. Assumptions: a- Neglect the
higher harmonics of the stator and rotor currents and the higher harmonics of the stator and rotor space potentials
wave components; b- Neglect the effects of hysteresis in the motor core, eddy-current losses, and saturation of the magnetic circuit
; c- The power source for rotor excitation can provide the required
rotor currents without regard to resistance and losses; d- The stator of the motor is connected to the
infinite grid. infinite power grid.
When the prime mover drags the rotor of the motor to rotate at a speed, and a three-phase symmetric power supply with a differential frequency fe-Sfl is applied to the rotor
winding, the rotor current produces a fundamental rotating magnetic potential that rotates with respect to the rotor
at a differential speed = rotation ( =60fl is the synchronous
speed of the motor). The It rotates at a synchronous speed with respect to the stator. This potential is stationary with respect to the stator fundamental potential generated by the three-phase current in the stator, and a synthetic potential is formed in the
air gap. According to the law of electromagnetic induction, the synthetic magnetic field generated in the air gap by this synthetic magnetic potential will induce potentials dah and dah in the stator and rotor windings, respectively.
The stator and rotor windings will have the potentials dah and dah, respectively, according to the law of electromagnetic induction. Similar to the induction motor, when the physical quantities on the rotor side are converted to the stator side, the converted basic equations can be written as follows