I. Overview
The development of power electronics began with the invention of mercury arc rectifiers in the early 20th century. In 1956, Bell Labs invented thyristors. In 1958, General Electric Company introduced commercial products. After more than 50 years of development, power electronics technology has become a multidisciplinary edge technology, including converter circuits, power electronics, computer-aided design, analog electronics and digital electronics, microcomputer control theory, and ultra-large-scale integration. Circuits and high frequency technology, electromagnetic compatibility, etc. The application of power electronics technology has also made great progress. The rectified AC-DC adjustable power supply from the beginning has been applied to electrochemical production, lighting control, electric welding technology, grid reactive and harmonic compensation, high-voltage direct current transmission system. , Photovoltaic cells and fuel cells and renewable energy, variable speed constant frequency systems, solid state circuit breaker induction heating, motor drive, green traffic electric motor vehicles, electric vehicles, electric warships and maglev trains, medium frequency power supplies and ultrasonic power supplies, also have resonance transformation technology . The development center of these applications is a frequency conversion technology problem, that is, inverting direct current into alternating current of different frequencies, or converting alternating current into direct current and then inverting into alternating current of different frequencies, all of which are electric energy does not change to Other energy, but only Frequency changes.
Second, the basic concept of frequency conversion speed regulation and its working principle
(1) The purpose of frequency conversion speed control transmission
The low efficiency of China's energy use is mainly due to the extensive economic growth mode, unreasonable structure, backward technical equipment and low management level. The use of frequency converters to control the speed of mechanical equipment is of great significance for saving energy and improving economic efficiency. The application of frequency converters in terms of labor saving, automation, product quality improvement, productivity improvement, and product qualification rate are equally important in the field of energy conservation. The mechanical equipment adopts the frequency conversion speed regulation transmission, which can increase the service life of the equipment, reduce the noise of the mechanical equipment, make the equipment miniaturized, and the work environment and living environment are more comfortable.
(2) Industry production range of frequency conversion speed application
After years of development, the frequency conversion technology has become more and more mature. The frequency conversion technology is in steel, copper wire, chemical, fiber, automobile, electrical machinery, machine tools, food, paper, cement, mining, gas, transportation, loading and unloading, factory building, Agriculture, life services, electricity, experimental research, petroleum and other industries have been widely developed and applied. Frequency conversion technology has been closely related to our production and life. At present, household appliances such as refrigerators, air conditioners, and electric vehicles have adopted related frequency conversion technologies.
(3) Characteristics of frequency conversion speed control transmission
The frequency converter is used to control the speed of the mechanical equipment. The characteristic of the variable frequency speed control transmission is that the standard motor can be adjusted in speed, the speed can be continuously adjusted, the starting current of the motor is small, the maximum speed is not affected by the power supply, and the motor can be speeded, miniaturized and explosion-proof. It is easy to set the torque output at low speed, and the acceleration and deceleration can be adjusted. The cage motor can be used without maintenance motor.
(4) Working principle of the inverter
Inverter circuit mode has two circuits: one AC and one frequency conversion and one AC and one frequency conversion. The basic working principle is that the rectifier turns the alternating current into direct current, the smoothing wave circuit smoothes the direct current, and the inverter inverts the direct current into the alternating current with adjustable frequency, the basic composition of the frequency converter.
1. AC-AC Inverter refers to a converter that has no DC intermediate link and directly converts the voltage of the grid frequency into a variable output voltage with a lower frequency than the grid frequency.
2. The working principle of AC-DC-AC inverter. At present, the most widely used is the AC-DC-AC inverter. The working principle is that the three-phase or single-phase unadjustable power frequency power supply is first rectified into a direct current through a rectifier bridge, and then the direct current power is inverted into an alternating current whose frequency is arbitrarily adjusted by the inverter bridge to realize stepless speed regulation. The main circuit of the AC-DC-AC inverter has voltage type frequency conversion and current type frequency conversion. The voltage type is a frequency converter that converts the direct current of the voltage source into an alternating current; the current type refers to a frequency conversion method that converts the direct current of the current source into an alternating current. Control methods are also divided into voltage control and current control. These two methods can be applied regardless of whether the main circuit mode is voltage type or current type.
(5) Quality performance indicators of the inverter
1. How much starting torque can be output at 0.5HZ? The better inverter can output 200% high starting torque at 0.5HZ and below 22KW, and can output 180% starting torque above 30KW.
2. How long can the speed adjustment range control precision be, and how much torque control accuracy can reach? The high-performance inverter speed control accuracy can reach ±0.005%, and the torque control accuracy can reach ±3%.
3. Pulse condition at low speed. The high-quality inverter has a speed pulse of only 1.5r/min at 1HZ.
4. Noise and harmonic interference. Noise and harmonic interference are important criteria for measuring a frequency converter. The modulation frequency of IGBT and IPM devices is higher than that of GTO and GTR. Although higher harmonics are always present, if the control method is better, some harmonics can be reduced.
5. Heat. The inverter generates a large amount of heat, which proves that the power consumption is large, the efficiency is low, the ventilation and heat dissipation system is not good, and the life of the inverter is affected accordingly.
Third, the application effect of the inverter and its development direction
(1) Application effect of the inverter
The use of frequency converters for variable frequency speed control of mechanical equipment can achieve energy-saving effect. The use of multiple motors to operate at proportional speed can achieve the effect of labor-saving automation. The effect of increasing the speed of the motor can increase the output. High-frequency motors can be used for high-speed operation. To achieve the effect of increasing production, it can replace the DC motor in the harsh environment to reduce maintenance, choose the stepless speed to improve the quality of the product, and use the air conditioner compressor speed control for continuous temperature control, which can improve comfort. Effect.
(2) The core and technical development direction of the frequency conversion speed control device
1. The core of the frequency is the power electronic device and control method. A power electronic device is a device that functions as an on-off function in a circuit and realizes various current changes. A frequency converter is such a device for changing current, which develops with the development of an inverter device. The quality of the inverter device depends on its on-off capability, on-off current and rated voltage. The loss in the on-off process determines the efficiency and size of the inverter. The speed regulation performance characteristics and uses obtained by the inverter with different control methods are different. The control method is roughly divided into open loop and closed loop control. The open loop control has a proportional control mode of u/f (voltage and frequency); the closed loop has slip frequency control and various vector controls.
2. The application field of the frequency converter is expanding continuously, and the technology used is also continuously widened. The main circuit adopts the gate-control new technology, and the saturation voltage Uce(sat) between the collector and the emitter is greatly reduced, thereby developing the first Four generations of IGBTs, even ordinary general-purpose inverters have entered a new era of vector control. As long as the user selects a spare board, the general-purpose inverter can be turned into a vector-controlled inverter with a speed sensor PG (pulse generator). The sensor device mostly uses a high-resolution 16-bit digital encoder, and the parallel transmission mode of the signal is changed to serial transmission. In terms of intelligence, the newly developed inverter can measure some components of the inverter, such as the capacitance, total running time and ambient humidity of the capacitor in advance, and comprehensively evaluate the life of the component, etc. It has standard functions with RS-485 and operates via a dedicated open bus.
Fourth, the selection method of frequency converter
(1) Select the inverter according to the different physical and load characteristics of the control
1. Speed ​​control inverter selection: (1) According to the system requirements, it is necessary to select the inverter that can cover the required speed control range; (2) In order to avoid continuous operation under dangerous speed, the inverter with frequency jump function should be selected.
2. Position control Inverter selection: (1) The inverter with sufficient capacity should be selected to obtain large acceleration and deceleration torque; (2) The equipment with extremely low load inertia should be selected for use with the inverter.
3. Selection of tension control inverter: (1) Inverter must be selected with torque current to control the tension; (2) Inverter control tension must use inverter with speed feedback control; (3) Adopt The tension control of the adjusting roller should use u/f to control the general-purpose inverter; (4) The tension control using the tension detector should use the vector control mode inverter.
4. Flow control Inverter selection: For those occasions that may be reversed due to external factors, a large-capacity inverter must be used in order to fully withstand the inrush current initiated from the reverse state.
5. Temperature control inverter selection: (1) IGBT, IPM modulation frequency high frequency converter should be preferred; (2) The inverter should be selected according to the starting current and running time of the equipment.
6. Selection of pressure control inverter: (1) should choose inverter with no water supply protection and mains power saving function; (2) should choose to install check valve and have instantaneous stop countermeasure and start interlock function Inverter.
7. Load characteristics require fast response. Selection of the inverter: (1) When the system requires acceleration and deceleration in a short time, the inverter with large overload capacity and current limit function with limited current function or vector control should be preferentially selected; (2) In the case where the cross-angle frequency Wc is relatively large and the response speed is relatively fast, the inverter with high switching frequency of the main circuit, large overload capacity, and high resonance frequency of the system should be preferred; (3) The switching frequency required for the inverter controlled by PWM is 1-3KHZ, can meet the needs of machine tools.
8. Load characteristics require high adjustment accuracy. Selection of the inverter: (1) When the system requires ±0.05% high accuracy, the inverter with PLG analog control and low drift control circuit should be selected; (2) When the system requires ±0.01 When the speed control accuracy is %, the inverter with PLG feedback full digital control should be selected.
9. Negative load inverter selection: (1) For cranes, elevators, production lines, inverters with regenerative rectifiers should be preferred; (2) Over-voltage trips should be effectively prevented when deceleration braking torque is required. Transistor inverter with diode rectifier; (3) When using the brake unit, you must pay full attention to heat dissipation, and do not adversely affect other equipment.
10. Selection of shock load inverter: (1) The inverter with sufficient capacity should be selected to withstand the impact overcurrent; (2) Adding the flywheel can reduce the impact load directly added to the motor, and the slip frequency control should be preferred. Frequency converter.
(2) Select the inverter according to the type of different motors
Selection of standard cage motor inverter
(1) Select the inverter according to the motor current
1) In the case of continuous operation, the rated output current of the inverter should be ≥ (0.5-1.1) rated current of the motor, that is: I variable ≥ (0.5 ~ 1.1) I electricity.
2) Selection of inverter capacity during acceleration and deceleration: Under normal circumstances, the inverter is allowed to reach the rated output current of 130%~150% for short-time acceleration/deceleration (depending on the inverter capacity).
3) When the motor is directly started, the inverter capacity is selected. The rated current of the inverter can be selected as follows: ie: I variable tI blocked / kg
I variable - rated current of the inverter (A)
I stalled - the stall current at the start of the motor at the rated voltage rated frequency (A)
The allowable overload multiple of K-Guangdong frequency converter is Kg=1.3~1.5.
Concerned about surprises
Label: The application effect of the inverter and the basic method of its selection
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