At present, the popular speed regulation methods of asynchronous motors can be divided into two types: variable frequency speed regulation and variable voltage speed regulation. Among them, variable frequency speed regulation of asynchronous motor is widely used, and its speed regulation methods can be divided into variable frequency speed regulation and vector control. The control mode of the former is relatively simple, with more than 20 years of development experience. Therefore, it is widely used. At present, most frequency converters sold in the market adopt this control mode.
Keywords: AC speed control system, asynchronous motor, PWM technology .....
catalogue
Abstract 1
Preface 3
1. 1 design purpose and significance 3
1.2 energy-saving principle of frequency converter speed regulation operation 3
Chapter 2 Frequency Converter 4
2. 1 frequency converter selection: 4
2.2 Design of frequency converter control schematic diagram: 4
2.3 inverter control cabinet design 6
2.4 inverter wiring specification 7
2.5 Operation of frequency converter and setting of relevant parameters 8
2.6 Common Fault Analysis 8
Chapter III Overview of AC Speed Regulation System 10
3. 1 Characteristics of AC speed regulation system 10
Chapter IV Characteristics of Variable Frequency Motor 14
4. 1 electromagnetic design 14
4.2 Structural Design 14
The fifth chapter is the main characteristics and construction principle of variable frequency motor 15.
5. 1 The special motor for frequency conversion has the following characteristics: 15.
5.2 Construction principle of variable frequency motor 15
Chapter 6 AC asynchronous motor 16
6. 1 Basic principle of variable frequency speed regulation of AC asynchronous motor 16
6.2 Mechanical characteristics of speed regulating motor 18
6.3 Mechanical characteristics analysis of variable voltage and frequency conversion operation 19
Chapter VII Principle of PWM Technology 24
7. 1 sine wave pulse width modulation 25
7.2 monopole SPWM method .................................................................................................................. 26
Conclusion 3 1
Thanks 32
Reference 33
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1. 1 Design purpose and significance
In recent years, with the rapid development of power electronics technology, computer technology and automatic control technology, AC drive and control technology has become one of the fastest developing technologies at present, and electrical drive technology is facing a historic revolution, that is, AC speed regulation replaces DC speed regulation, and computer digital control technology replaces analog control technology has become a development trend. AC frequency conversion speed regulation technology of motor is the main means to save electric energy, improve technological process, improve product quality, improve environment and promote technological progress. Frequency conversion speed regulation is regarded as the most promising speed regulation method at home and abroad because of its excellent speed regulation and braking performance, high efficiency, high power factor, energy saving effect and wide application range. It is of great positive significance to deeply understand the development trend of AC drive and control technology.
1.2 energy-saving principle of frequency converter speed regulation operation
The device to realize variable frequency speed regulation is called frequency converter. Inverter is generally composed of rectifier, filter, drive circuit, protection circuit and controller (MCU/DSP). Firstly, the single-phase or three-phase AC power supply is filtered by rectifier and capacitor, and the DC voltage with basically fixed amplitude is added to the inverter. A rectangular pulse waveform with a certain shape is obtained at the output end of the inverter by using the switch control of the inverter power element. Here, the voltage amplitude is controlled by changing the width of the rectangular pulse; By changing the modulation period to control its output frequency, the output voltage and frequency can be controlled simultaneously on the inverter, which meets the requirements of U/F coordinated control for variable frequency speed regulation. The advantage of PWM is that it can eliminate or suppress low-order harmonics, and make the load motor run under AC voltage close to sine wave, with small torque ripple and wide speed regulation range.
The motor speed in PWM control mode is limited by the upper limit speed. For example, the compressor generally does not exceed 7000 r/rain. The speed of compressor controlled by PAM can be increased by about 1.5 times, which greatly improves the ability of rapid growth and deceleration. At the same time, PAM can shape the current waveform when adjusting the voltage, so it can obtain higher efficiency than PWM. In addition, it has incomparable advantages in anti-interference, which can suppress the generation of higher harmonics and reduce the pollution to the power grid. After adopting this control mode of variable frequency speed regulation technology, the stator current of the motor decreased by 64%, the power frequency decreased by 30%, and the glue outlet pressure decreased by 57%. According to the motor theory, the rotating speed of asynchronous motor can be expressed as: n=60? f 8( 1—8)/p
Chapter II Frequency Converter
Frequency converter is an electric energy control device which uses the switching function of power semiconductor devices to convert power frequency power supply to another frequency. The frequency converter we use now mainly adopts AC -DC- AC mode (VVVF frequency conversion or vector control frequency conversion). Firstly, the power frequency AC power supply is converted into DC power supply through rectifier, and then the DC power supply is converted into AC power supply with controllable frequency and voltage for the motor. The circuit of frequency converter is generally composed of rectifier, intermediate DC link, inverter and control. The rectifier part is a three-phase bridge uncontrolled rectifier, the inverter part is an IGBT three-phase bridge inverter, and the output is PWM waveform. The middle DC link is filtering, DC energy storage and buffering reactive power.
2. 1 frequency converter selection:
When selecting a frequency converter, the following points should be determined:
1) adopts frequency conversion; Constant voltage control or constant current control, etc.
2) Load type of frequency converter; Such as vane pump or displacement pump, we should pay special attention to the performance curve of load, which determines the application mode and method.
3) Matching between inverter and load;
First, voltage matching; The rated voltage of inverter is consistent with the rated voltage of load.
Two. Current matching; For ordinary centrifugal pumps, the rated current of frequency converter is consistent with the rated current of motor. For special loads such as deep water well pumps, it is necessary to refer to the motor performance parameters to determine the inverter current and overload capacity with the maximum current.
Three. Torque matching; This may happen when there is a constant torque load or deceleration device.
4) When the inverter is used to drive the high-speed motor, the high-order harmonics increase and the output current increases due to the small reactance of the high-speed motor. Therefore, the capacity of high-speed motor is slightly larger than that of ordinary motor.
5) If the frequency converter runs with a long cable, measures should be taken to suppress the influence of the long cable on the coupling capacitance, so as to avoid insufficient output of the frequency converter. Therefore, in this case, the capacity of the frequency converter should be expanded by a step or an output reactor should be installed at the output end of the frequency converter.
6) For some special applications, such as high temperature and high altitude, the capacity of the inverter will be reduced, and the capacity of the inverter will be enlarged by one step.
2.2 frequency converter control schematic design:
1) First confirm the installation environment of the inverter;
First, the working temperature. Inverter is a high-power electronic component, which is easily affected by working temperature. Generally, the product is required to be 0 ~ 55℃, but in order to ensure safe and reliable operation, it should be considered to leave room when using, and it is best to control it below 40℃. In the control box, the frequency converter should generally be installed at the upper part of the box, and the installation requirements in the product manual should be strictly observed. It is absolutely not allowed to install heating elements or easily heating elements near the bottom of the frequency converter.
Two. Ambient temperature. When the temperature is too high and the temperature changes greatly, condensation is easy to occur inside the inverter, which greatly reduces its insulation performance and may even lead to short circuit accidents. When necessary, desiccant and heater must be added in the box. In the water treatment room, water vapor is generally heavy, and this problem will be more prominent if the temperature changes greatly.
Three. Corrosive gas. If the concentration of corrosive gas in the environment is high, it will not only corrode the lead wires of components, printed circuit boards, etc. It will also accelerate the aging of plastic devices and reduce the insulation performance.
Four. Vibration and shock. When the control cabinet equipped with frequency converter is subjected to mechanical vibration and impact, it will cause poor electrical contact. Huai 'an Thermal Power Company has such a problem. At this time, in addition to improving the mechanical strength of the control cabinet and keeping away from vibration sources and impact sources, anti-seismic rubber pads should also be used to fix the electromagnetic switches and other components that generate vibration outside and inside the control cabinet. After the equipment has been running for a period of time, it should be inspected and maintained.
Electromagnetic interference. Due to the rectification and frequency conversion during the operation of the inverter, a large number of interference electromagnetic waves are generated around it, and these high-frequency electromagnetic waves have certain interference to nearby instruments and meters. Therefore, the instrument and electronic system in the cabinet should adopt metal shell to shield the interference of frequency converter to the instrument. All components shall be reliably grounded. In addition, shielded control cables should be used for wiring between electrical components and instruments, and the shielding layer should be grounded. If the electromagnetic interference is not handled well, it will often make the whole system unable to work, resulting in the failure or damage of the control unit.
2) The distance between the inverter and the motor determines the cable and connection mode;
I the distance between inverter and motor should be as short as possible. This reduces the capacitance of the cable to the ground and reduces the interference source.
Two. Shielded cables shall be used for control cables, and shielded cables for power cables or all cables from frequency converters to motors shall be shielded through threading pipes.
Three. The motor cable shall be routed independently of other cables with a minimum distance of 500mm. At the same time, long-distance parallel wiring of motor cables and other cables should be avoided to reduce the electromagnetic interference caused by the rapid change of inverter output voltage. If the control cable and power cable cross, try to cross at 90 degrees. Analog signal lines related to the frequency converter are wired separately from the main circuit, even in the control cabinet.
Four. It is best to use shielded twisted pair for analog signal lines related to the frequency converter, and shielded three-core cable (whose specification is larger than that of ordinary motor cable) for power cables or follow the user manual of the frequency converter.
3) Control schematic diagram of frequency converter;
1. Main circuit: The function of the reactor is to prevent the high-order harmonics generated by the frequency converter from returning to the power grid through the input circuit of the power supply, thus affecting other power receiving equipment. It is necessary to decide whether to add reactors according to the capacity of the frequency converter; The filter is installed at the output end of the frequency converter to reduce the higher harmonics output by the frequency converter. When the frequency converter is far away from the motor, a filter should be installed. Although the inverter itself has various protection functions, the lack of phase protection is not perfect. Circuit breaker plays the role of overload and lack of equality protection in the main circuit, and can be selected according to the capacity of inverter. Thermal relay can be replaced by overload protection of frequency converter itself.
Two. Control loop: it has manual switching power frequency conversion, so as to manually switch power frequency when frequency conversion fails. Because no voltage can be applied to the output terminal, constant power frequency and frequency conversion should be interlocked.
4) grounding of frequency converter;
Correct grounding of frequency converter is an important means to improve system stability and suppress noise. The smaller the grounding resistance of the inverter grounding terminal, the better. The section of grounding conductor is not less than 4 mm and the length is not more than 5 m. The grounding of frequency converter should be separated from the grounding point of power supply equipment, not * * *. One end of the shielding layer of the signal line is connected to the grounding end of the frequency converter, and the other end is suspended. The frequency converter is electrically connected with the control cabinet.
2.3 inverter control cabinet design
The frequency converter should be installed in the control cabinet, and the following problems should be paid attention to when designing the control cabinet.
1) heat dissipation: the heat of the inverter is caused by internal loss. The main circuit accounts for about 98% and the control circuit accounts for about 2% of the loss of each part of the frequency converter. In order to ensure the normal and reliable operation of the inverter, the inverter must be cooled. We usually use a fan to cool down. The built-in fan of the inverter can take away the heat dissipation inside the inverter box. If the fan can't work normally, stop the inverter immediately. The high-power inverter also needs to add a fan to the control cabinet, and the air duct of the control cabinet should be designed reasonably. All air inlets shall be provided with dust screens, and the air exhaust shall be smooth, so as to avoid vortex formation in the cabinet and dust accumulation in a fixed position. Select the matching fan according to the ventilation volume in the frequency converter manual, and pay attention to the shockproof problem when installing the fan.
2) Electromagnetic interference:
1. During the operation of the frequency converter, due to rectification and frequency conversion, a large number of interfering electromagnetic waves are generated around it. These high-frequency electromagnetic waves have certain interference to nearby instruments and meters, and also produce higher harmonics, which enter the whole power supply network through the power supply loop, thus affecting other instruments and meters. If the power of the inverter accounts for more than 25% of the whole system, it is necessary to consider the anti-interference measures of the control power supply.
Two. When there are high-frequency impact loads such as welding machine and electroplating power supply in the system, the inverter itself will be protected by interference, so the power quality of the whole system should be considered.
3) Protection issues need to pay attention to the following points:
1. Waterproof and dewing prevention: When the inverter is placed on the site, it should be noted that there should be no pipeline flange or other leakage points above the inverter cabinet, and there should be no splashing water near the inverter. In short, the protection level of the cabinet on site should be above IP43.
Two. Dust-proof: All air inlets shall be equipped with dust-proof nets to prevent flocculent impurities from entering, and the dust-proof nets shall be designed to be detachable for easy cleaning and maintenance. The grid of the dust screen shall be determined according to the specific site conditions, and the joint between the dust screen and the control cabinet shall be strictly handled.
Three. Anticorrosive gas: This situation is common in the chemical industry. At this time, the frequency conversion cabinet can be placed in the control room.
2.4 inverter wiring specification
The signal wire and power wire must be routed separately: when using analog signals to remotely control the frequency converter, in order to reduce the interference of analog signals of frequency converter and other equipment, please route the signal wire for controlling the frequency converter separately from the strong current loop (main loop and sequence control loop). The distance should be more than 30cm. Even in the control cabinet, this wiring specification should be maintained. The longest control loop between the signal and the frequency converter shall not exceed 50m.
The signal wire and power wire must be placed in different metal tubes or metal hoses: if the signal wire connecting PLC and frequency converter is not placed in metal tubes, it will be easily interfered by frequency converter and external equipment; At the same time, because the inverter has no built-in reactor, the power lines of the input stage and output stage of the inverter will have strong interference to the outside world. Therefore, the metal pipe or hose for placing the signal wire should extend all the way to the control end of the frequency converter to ensure that the signal wire is completely separated from the power wire.
1) The analog control signal line should adopt double-stranded shielded wire with the wire specification of 0.75mm2 When wiring, it is necessary to pay attention to the cable stripping as short as possible (about 5-7mm), and at the same time, the stripped shielding layer should be wrapped with insulating tape to prevent the shielded wire from contacting other equipment and introducing interference.
2) In order to improve the simplicity and reliability of wiring, it is suggested to use the wire pressing rod terminal on the signal line.
2.5 Operation of frequency converter and setting of relevant parameters
There are many setting parameters of frequency converter, and each parameter has a certain selection range. In use, it is often encountered that the inverter cannot work normally due to improper setting of individual parameters.
Control mode: speed control, torque control, PID control or other modes. After the control mode is adopted, static or dynamic identification is generally needed according to the control accuracy.
Minimum operating frequency: the lowest rotating speed of the motor. When the motor is running at low speed, the heat dissipation performance is very poor. If the motor runs at low speed for a long time, it will burn out. Moreover, at low speed, the current in the cable will also increase, which will also lead to the cable heating.
Maximum working frequency: the maximum frequency of general frequency converters reaches 60Hz, and some even reach 400 Hz. High frequency will make the motor run at high speed. For ordinary motors, their bearings cannot run at a fixed speed for a long time. Can the rotor of the motor withstand such centrifugal force?
Carrier frequency: The higher the carrier frequency is set, the greater the harmonic component, which is closely related to cable length, motor heating, cable heating and inverter heating.
Motor parameters: The inverter sets the power, current, voltage, speed and maximum frequency of the motor in the parameters, which can be obtained directly from the motor nameplate.
Frequency hopping: at a certain frequency point, vibration may occur, especially when the whole equipment is relatively high; When controlling the compressor, the surge point of the compressor should be avoided.
2.6 Common Fault Analysis
1) overcurrent fault: overcurrent fault can be divided into acceleration, deceleration and constant-speed overcurrent. It may be caused by too short acceleration and deceleration time of frequency converter, sudden load change, uneven load distribution, output short circuit and other reasons. At this time, it is generally possible to extend the acceleration and deceleration time, reduce the sudden change of load, increase energy-consuming braking parts, carry out load distribution design and check the line. If the load inverter is disconnected or overcurrent, it means that the inverter circuit of the inverter has been looped and the inverter needs to be replaced.
2) Overload fault: Overload fault includes frequency conversion overload and motor overload. It may be caused by too short acceleration time, too low grid voltage and too heavy load. Generally, you can extend the acceleration time, extend the braking time and check the grid voltage. The load is too heavy, so the selected motor and frequency converter can't drag the load, or it may be caused by poor mechanical lubrication. If it is the former, the high-power motor and frequency converter must be replaced; In the latter case, the production machine should be overhauled.
3) Undervoltage: It means that there is something wrong with the power input part of the inverter, and it can only be operated after inspection.
Chapter III Overview of AC Speed Regulation System
3. 1 Characteristics of AC speed regulation system
For speed regulating electric drive system, it is often divided into DC speed regulating system and AC speed regulating system in engineering. This is mainly divided according to what type of motor is currently used to convert electrical energy and mechanical energy. The so-called AC speed regulation system is to use AC motor as the conversion device of electrical energy and mechanical energy to control the speed it needs.
Throughout the development of electric drive, AC and DC speed control systems have always coexisted in various industrial fields. Although their status is different due to the development of science and technology in different periods, they always compete with each other with the development of industrial technology, especially with the development of power electronic devices. For a long time in the past, due to the excellent speed regulation performance of DC motor, DC speed regulation system has been used in the technical field of reversible, speed regulation, high precision and wide speed regulation range. However, due to the fatal weakness of DC motor mechanical commutator, DC motor has high manufacturing cost, expensive price, troublesome maintenance and limited use environment. Its own structure also limits the upper limit of the speed and power of a single motor, which brings a series of restrictions to the application of DC drive. Compared with DC motor, AC motor, especially squirrel-cage asynchronous motor, has the advantages of simple structure, low manufacturing cost, durability, reliable operation, convenient maintenance, small inertia, good dynamic response, and easy development in the direction of high voltage, high speed and high power. Therefore, in recent decades, many countries have devoted themselves to the research of AC speed regulation system, using commutatorless AC motor instead of DC motor to realize speed regulation and break through its limitations.
With the rapid development of power electronic devices, large-scale integrated circuits and computer control technology, and the penetration of modern control theory into the field of AC electric drive, it has further created favorable conditions for the development and research of AC speed regulation system. For example, the cascade speed regulation of AC motor, various types of variable frequency speed regulation, especially the application of vector control technology, make the AC speed regulation system gradually have good technical performance such as wide speed regulation range, high speed stability accuracy, fast dynamic response and four-quadrant reversible operation. At present, from a servo system of several hundred watts to an ultra-high power and high-speed transmission system of several hundred kilowatts, from a small-range speed regulation drive that is generally required to a high-precision, fast-response and large-range speed regulation drive, from a single machine drive to a multi-machine coordinated operation, almost all AC speed regulation drives can be adopted. The objective development trend of AC variable speed drive shows that it can compete with DC drive and has a tendency to replace it.
3.2 Common AC speed regulation schemes and their performance comparison
According to motor mechanics, the speed formula of AC asynchronous motor is as follows:
n= 60？ 1( 1-s)pn( 1- 1)
Where Pn—— refers to the number of magnetic poles of the motor stator winding;
F 1- motor stator voltage supply frequency;
S-slip rate of motor.
It can be seen from the formula (1- 1) that there are three schemes for AC asynchronous motor speed regulation.
(1) Change the number of poles of the motor.
By the synchronous speed of asynchronous motor
No = 60? 1 pn
It can be seen that under the condition that the power supply frequency f 1 is constant, the synchronous speed n0 of asynchronous motor can be changed by changing the connection mode of stator winding to change the pole logarithm Pn, so as to achieve the purpose of speed regulation. This control method is relatively simple, only the stator winding of the motor has multiple taps, and then the number of magnetic poles of the motor is changed by making and breaking the contacts. With this control method, the change of motor speed is step by step, not continuous, and generally there are only three gears at most, which is suitable for occasions with low degree of automation and only step-by-step speed regulation.
(2) Variable frequency speed regulation
It can be seen from the formula (1-1) that when the number of poles Pn of asynchronous motor is fixed and the slip rate s-is fixed, the purpose of speed regulation can be achieved by changing the power frequency of stator winding f 1, and the motor speed n is basically proportional to the frequency of power supply f 1. Therefore, smoothly adjusting the frequency of the power supply can be smooth and stepless. Variable frequency speed regulation has the characteristics of wide range, hard and low speed. Below the fundamental frequency f=50Hz, it belongs to the constant torque speed regulation mode, and above the fundamental frequency, it belongs to the constant power speed regulation mode, which is very similar to the step-down weak magnetic speed regulation of DC motor. Moreover, adopting frequency conversion starting can significantly improve the starting performance of AC motor, greatly reduce the starting current of motor and increase the starting torque. Therefore, variable frequency speed regulation is an ideal speed regulation scheme for AC motors.
(3) Variable slip speed adjustment
There are many ways to change slip speed regulation, and the common schemes are stator voltage regulation of asynchronous motor, electromagnetic slip clutch speed regulation, rotor circuit series resistance speed regulation of wound asynchronous motor, cascade speed regulation and so on.
Stator voltage regulation and speed regulation system is to connect thyristor as AC voltage controller between constant AC power supply and AC motor. This voltage regulation and speed regulation system is only suitable for some short-term and repeated short-term deep speed regulation loads. In order to obtain good speed regulation accuracy and stable operation, speed negative feedback control mode is generally adopted. The motor used can be wound asynchronous motor or squirrel-cage asynchronous motor with high slip rate.
The speed control system of electromagnetic slip separator consists of squirrel-cage asynchronous motor, electromagnetic slip clutch and control device. As the prime Mover, squirrel-cage motor drives the armature of electromagnetic clutch to rotate at a uniform speed, and the speed of its magnetic pole is adjusted by controlling the excitation current of electromagnetic clutch. This kind of system also generally adopts speed closed-loop control.
The series resistance speed regulation of rotor circuit of wound asynchronous motor is to adjust the speed by changing the series resistance of rotor circuit. This speed regulation method is simple, but the speed regulation is step by step. After adding a large additional resistor in series, the mechanical characteristics of the motor are very soft, the low-speed operation loss is large and the stability is poor.
The cascade speed regulation system of wound asynchronous motor introduces the reverse potential Ef with the same frequency as the rotor potential in the rotor circuit of the motor. As long as the additional reverse potential Ef with the same frequency as the motor rotor voltage is changed, the wound asynchronous motor can be adjusted smoothly. The larger Ef, the lower the motor speed.
* * * The same characteristic of the above-mentioned speed regulation is that the synchronous speed n0 of the motor remains unchanged during the speed regulation, so at low speed, the slip rate S is larger.
In AC asynchronous motor, the electromagnetic power PM transmitted from stator to rotor can be divided into two parts: one part P2 = (1-s) PM is the effective power of dragging load, and the other part is slip power PS=sPM, which is proportional to slip rate S, and its destination is the symbol of the efficiency of speed regulation system. As far as the direction of slip power is concerned, AC asynchronous motor speed regulation system can be divided into three types:
1) slip power consumption type
The slip power of this speed control system is all consumed, and the increase of slip power consumption results in the decrease of rotational speed, the increase of slip rate S, and the increase of slip power PS=sPM, which is consumed in the rotor circuit in the form of heat, which reduces the efficiency of the system. Stator voltage regulation and speed regulation of wound asynchronous motor, electromagnetic slip clutch speed regulation and rotor series resistance speed regulation all belong to this category. This kind of speed regulation system has the problems of wide speed regulation range, large slip power PS and low system efficiency, so it is not worth popularizing.
2) slip power feedback type
Most of the slip power of this speed regulation system is fed back to the power grid or used through converter devices. The lower the speed, the more power is fed back, but the added devices also consume more power. Rotor cascade speed regulation of wound asynchronous motor belongs to this category, which feeds back slip power to AC power grid through rectifier and inverter, but does not consume energy in the form of heat. Even at low speed, the efficiency of cascade speed regulation system is very high.
3) the slip rate is unchanged
In this speed control system, slip power is still consumed in the rotor, but the slip power is basically unchanged, regardless of speed. Such as pole-changing logarithmic speed regulation, frequency conversion speed regulation belongs to this category. Because the synchronous speed n0 changes and the slip rate S is constant during speed regulation, the system efficiency will not be reduced due to speed regulation. Among the two speed regulation schemes to change n0, pole-changing logarithmic speed regulation is pole speed regulation, and the number of poles is limited, so frequency conversion speed regulation is the most ideal and promising AC speed regulation scheme at present.
Chapter IV Characteristics of Variable Frequency Motor
4. 1 electromagnetic design
For ordinary asynchronous motors, the main performance parameters to be considered in redesign are overload capacity, starting performance, efficiency and power factor. The variable frequency motor can be started directly when the critical slip is close to 1, because the critical slip is inversely proportional to the power frequency. Therefore, overload capacity and starting performance do not need to be considered too much, but the key problem to be solved is how to improve the adaptability of the motor to non-sinusoidal power supply. The way is generally as follows: