1, the composition of the frequency converter, classification and functional characteristics
The frequency converter is a kind of static frequency converter, can change the constant frequency of 50Hz of the power distribution network power into the adjustable frequency alternating current and regard as the power supply device of the electric motor, it is very common to use at home and abroad at present. The performance of inverters depends to a large extent on the power semiconductor devices used in the inverter. Power semiconductor devices used in frequency converters go from thyristors (VTs) to turn-off thyristors (GTOs) to power transistors (GTRs) to insulated gate bipolar transistors (IGBTs) until today's best performing smart power modules (IPM). Only the appearance of IGBT and IPM has made a qualitative leap in the performance of the inverter.
1.1 The function of the component converter is to convert AC to DC. The role of the inverter is to change the DC to the specified frequency AC according to the command of the control loop (PWM pulse). In general, inverters use high-performance single-chip microcomputers to perform calculation and control functions.
1.2 Classification
A. According to the method of changing frequency, it can be divided into AC-DC-AC frequency converter and AC-AC frequency converter. The AC-DC-AC frequency converter firstly converts the 50Hz AC into DC by rectification, and then filters and then converts the smooth DC into an AC with adjustable frequency; the AC-AC frequency converter consists of three groups of anti-parallel thyristors reversible bridge-type transformers. The flow device consists of the principle of natural commutation of the power grid, which has the advantages of strong overload capacity, high efficiency, and good output waveform, but at the same time there is also a low output frequency (the maximum frequency is less than 1/2 of the grid frequency), the power factor of the power grid is low, and the side frequency Harmonic effects and other shortcomings.
B. According to the method of changing the inverter output voltage (or current) can be divided into PAM type and PWM type. The so-called PAM (Pulse Amplitude Modulation) is a method of changing the amplitude of the voltage or current source of the voltage source and controlling the output. In the inverter part, only the frequency is controlled, and the output voltage or current is controlled in the converter part; the PWM (Pulse Width Modulation) method generates multiple pulses in a half period of the output waveform so that the equivalent voltage of each pulse is sinusoidal The output voltage or current waveform is smooth and has low harmonics. Figure 2 shows the PWM inverter voltage regulation schematic. Triangular waveform modulation wave and sine waveform signal wave comparison, take its logic (if the signal wave ratio triangle wave is large when the positive logic), you can get the pulse width modulation voltage output waveform corresponding to the amplitude of the signal wave, it and sine wave Equivalent.
C. Classification by voltage level can be divided into low-voltage and high-voltage inverters. Low-voltage inverter refers to single-phase 220V ~ 240V, three-phase 220V or 380V ~ 460V, capacity from 0.2kW ~ 280kW ~ 500kW a type of inverter, generally called small and medium capacity inverter. High-voltage inverters generally refer to high-voltage and large-capacity inverters, and there are two structural forms. One is the use of a riser and step-down transformer called an indirect high-voltage inverter of the "high-low-high" type; the other uses a high-voltage inverter. Capacity GTO thyristor or thyristor power element series structure, no input and output transformer, direct rectification of high voltage power supply to direct current, and then inverter output high voltage, ie "high-high" type converter.
D. According to different purposes, it can be divided into frequency converters for subway vehicles, frequency converters for electric vehicles, frequency converters and fans for rolling mills, and frequency converters for pumps. At present, GTO thyristors have been successfully applied in the transmission systems of trolley buses, light rail vehicles, subway locomotives, mains electric locomotives and magnetic levitation test vehicles. Rolling mill is a large-capacity, high-overload and fast-response production machine. Its vector-controlled AC variable-frequency speed-regulating transmission mechanism not only has the same superior speed control performance as the DC drive, but also has many advantages over the DC drive. The power consumption of fans and pumps accounts for 40% of the electricity used in China. In order to save energy and reduce the price of inverters, energy-saving fans and pumps are used in various countries in the world.
1.3 Features
The current advanced frequency converter has the following functions: A. Speed ​​and torque characteristics. Using different control methods, speed and torque characteristics are not the same, such as the use of V / f control speed range is narrow, in the low-speed running slip rate, the characteristics of softer; and the use of speed sensorless vector control, Even driving a universal motor ensures accurate torque control over the entire frequency range, with more than 150% high starting torque at 1Hz.
B. Quick response function. The frequency converter is controlled by a single-chip microcomputer, especially a high-speed digital signal processor (DSP). The calculation speed is fast, the speed adjustment response is fast, and the torque response speed is about 0.1 second.
C. AVR function ensures the realization of high starting torque.
When the line voltage drops, the AVR (automatic voltage adjustment) function can maintain high starting torque.
D. Automatic adjustment of motor parameters. Inverter and motor parameter adjustments are performed automatically, making it easy to use and operate.
E. Fuzzy logic plus, deceleration function. This function automatically calculates the optimal acceleration/deceleration time based on the motor load and braking requirements. This eliminates the need for a test machine and avoids errors.
F. Reduce energy consumption, automatic energy-saving operation function.
The frequency converter will automatically select the operating parameters so that the motor will run at minimum current when the load torque requirements are met.
G. Reduce motor noise and achieve quiet operation. By using IGBT devices in power switching circuits, circuits can operate at high carrier frequencies, reducing noise compared to conventional frequency converters.
H. Built-in PI or PID adjustment function.
2, commonly used asynchronous motor variable frequency speed control method We know, asynchronous motor speed n = 60 × f / p, where f is the power frequency, p is the number of pole pairs of the motor.
So there are two ways to change the speed of the motor: First, change p, this method is stepped speed, and the speed range is limited; the other is to change f, as long as f "continuous" changes, the motor speed can be achieved without step Speed ​​control, wide speed range, frequency control is precisely based on this point.
The frequency conversion speed control mode can generally be divided into two kinds: Open loop control and closed loop control, the latter carries on motor speed feedback. As an open-loop control with V/f control, closed-loop control with slip frequency control, vector control and direct torque control.
From the perspective of the development process, various control methods are developed in the order of V/f control, slip frequency control, vector control, and direct torque control. Therefore, the more excellent the performance of later control methods is.
V/f control is to change the frequency while controlling the inverter output voltage, so that the motor flux to maintain a certain, within a wide range of speed, the motor efficiency, power factor does not fall. Because it is the control voltage and frequency ratio, it is called V / f control, this method is commonly used in general-purpose inverter. It is used for the energy-saving operation of fans, pump machinery, and the workbench drive of the production line.
Slip frequency control requires detection of the speed of the motor to constitute a speed closed loop, the output of the speed regulator is the slip frequency, and then the sum of the motor speed and the slip frequency as the frequency converter given output frequency. Since the torque and the current are controlled by controlling the slip frequency, the acceleration/deceleration characteristics and the limitation of the overcurrent capability are improved as compared with the V/f control.
Vector control is a high-performance asynchronous motor control method. It is based on the mathematical model of the motor and controls the torque current and the excitation current of the motor respectively, and has similar control performance as the DC motor. The main difference from scalar control is that it not only controls the size of the current but also controls the phase of the current. However, the premise of this method is the need to correctly estimate the motor parameters, how to improve the accuracy of the parameters has been the subject of research.
Direct torque control was proposed by Prof. Depenbrok of Ruhr University in 1985. The space vector analysis method was used to directly calculate and control the torque of the AC motor in the stator coordinate system. The stator magnetic field orientation was adopted, and the discrete two-point method was used. The PWM signal is adjusted to directly control the switching state of the inverter to obtain high dynamic performance of the torque. To a large extent, it solves some of the major problems in vector control where the computational control is complex and the characteristics are susceptible to motor parameters.
3, frequency changer application in energy conservation
In terms of power consumption, fan and pump load account for a large proportion. According to relevant information, the annual power consumption of this kind of load in China accounts for about one-third of the national electricity consumption, and accounts for 40% to 50% of the national industrial electricity consumption. In boiler room heating projects, fans and pumps use electricity. The quantity accounts for more than 80% of all electricity consumption; in the modern air-conditioned modern tourist hotels, high-level hotels and office buildings, the electricity consumption of fans and pump equipment accounts for 30% to 40% of the entire building electricity consumption, accounting for the entire Power consumption (that is, electricity consumption other than lighting) is 40% to 55%. In this type of system, it is very important to apply frequency conversion speed regulation for design or technological transformation.
3.1 Frequency Conversion Speed ​​Regulation and Energy Saving Principle
3.1.1 Operating Characteristics of Fans and Pumps
Since the power and operating characteristics of the fan and the pump are very similar, the pump is used here as an example to discuss its characteristics.
The water pump water supply system has a characteristic curve of the pipe network, that is, the relationship curve between the flow rate of the channel pipe network and the energy consumed, which can be determined by the resistance law K=RQ2. K - pipe network resistance (Pa); R - pipe resistance (kg/m3); Q - flow (m3 / s). KQ curve is a parabola, as shown in Figure 3, commonly known as the characteristics of the pipe network. R is related to the opening of the valve. The smaller the opening, the larger the value and the steeper the curve.
The intersection of the H (head)-Q (flow) curve of the pump and the resistance curve of the pipe network is the working point of the pump. The location of the pump operating point is related to the pump load. In the case where the pump load changes frequently, the pump cannot always work in an efficient area. In order to adapt the pump to the requirements of external load changes, we can use variable speed regulation, that is, change the pump's Q-H characteristic curve by changing the pump speed when the pipe network characteristic curve is basically constant. Thus changing its operating point, to achieve both the change in flow and to ensure the constant pump and input power reduction purposes.
3.1.2 Frequency conversion speed energy-saving principle The frequency conversion technology is used to change the speed of the motor to adjust the flow rate.
For a certain pipeline, when the flow needs Q1 for the head is H1, the operating point A1 can be obtained on the pump characteristic curve diagram, the pump speed n=2950r/min can meet the requirements, the corresponding power is P1; when the flow from When Q1 descends to Q2, it can be seen from the pipeline impedance characteristic R1 that the pipeline requires the head of the material flow to be H2. At this time, the impeller rotation speed is n=2475 r/min and the head is H2 according to the characteristic curve of the pump at different speeds. The working point A2 appears, and its corresponding power is P2. Obviously, the power consumption at this time is greatly reduced, that is, under the condition that the pipeline characteristics are not changed, the purpose of adjustment is achieved by changing the pump speed.
According to the similar law of the water pump, the relationship between flow, head, power and speed before and after variable speed is: Q2=Q1(n2/n1)H2=H1(n2/n1)2 P2=P1(n2/n1)3 where P1, H1, Q1 is the power, head, flow rate when the speed is n1; P2, H2, Q2 are the power, head, flow rate when the speed is n2. It can be seen that when the water pump is under variable load operation, when the frequency converter is used to adjust the speed of the water pump motor, the cubic power ratio of the shaft power to the speed is proportionally changed, and the power saving effect is obvious.
3.1.3 practical application of energy-saving effect
Example 1: The water pump house of Shilou Hotel in Shijiazhuang has three water supply pumps, the model number is 65DL-4, the rated head is 64m, the flow rate is 30m3/h, the rotating speed is 1450r/min, and the motor power is 11kW. The Shenyang Ma'er Water Pump Factory produces. Due to the extremely unstable water consumption in hotels, the morning and evening water peaks are often double the normal usage, while the water consumption in the latter half of the night is only one-third of the usual usage, and the flow rate of water changes rapidly, and the time for regulating is short. The original use of electrical contact pressure gauge to control the start and stop of the pump, but when the water load changes greatly, the pump motor start and stop frequently, causing frequent motor and control switch failure, and the water pressure is not stable.
Aiming at the problems existing in the hotel water supply pump group and the actual situation of the water load, a set of frequency conversion speed control and constant pressure water supply control device was designed. The 11kW Fuji inverter, pressure sensor, and microcomputer controller (including PID regulation) were used to form a closed loop regulator. Pressure control system, so that the water pump constant pressure water supply, its water supply pressure is adjustable, to solve the big problem of the hotel's water supply system instability.
Under the original control mode, the average input power of the three pumps was 20 kW, the water supply pressure was 3.7-5.8 kg/cm2, and the annual power consumption was 175200 kWh. Calculated at 0.5 yuan/kWh, the annual electricity cost was 88,000 yuan. After the constant pressure water supply control device is used, the average input power of the three pumps is 16kW, the water supply pressure is 4.5kg/cm2, the annual power consumption is 140160kWh, the electricity cost is 70,000 yuan, and the annual electricity saving expenditure is 18,000 yuan. The project's transformation investment is 32,000 yuan. It can be seen that after adopting the constant pressure water supply control device, the annual energy saving is 35040kWh, the energy saving rate is 20%, and the investment recovery period is 1.8 years.
Example 2: No. 2 boiler of Guangming Road Heating Boiler Station in Urumqi is a DHL-2500-16/150-A type hot water boiler, rated heat supply 29000kW, hot water outlet temperature 150°C, return water temperature 90°C, boiler efficiency 0.80, the total rated power consumption of the boiler is 560kW, Uzbekistan bituminous coal is used, the low calorific value is 29442kJ/kg, and the rated fuel consumption is 4440kg/h; the induced draft fan matching the boiler is Y4-73-11 type 18D, and the air volume is Q=19 ×104 m3 /h (3167m3/min), full pressure H=2646Pa, motor 220kW, fan efficiency ηF=0.70; blower G4-73-11 14D, Q=74600m3/h (1243m3/min), H=2757Pa , fan efficiency ηF = 0.80, motor 75kW. Drum induced fan power 295kW, accounting for 50.7% of the total power consumption of the boiler. The boiler drum induced draft fan using frequency converter control, frequency converter selection of Mitsubishi MT-140A-75K and MT Each one of the -140A-220K units has a total investment of 372,800 yuan, including 2 sets of frequency converters, 330,000 yuan. It was designed and constructed in 1992 and put into use at the end of 1993. The author conducted a comparative test in March 1994.
(1) test instrument, temperature. 0 ~ 150 °C mercury thermometer, scale 0.5 °C; Flow: Rotor-type water meter, 0.045 ~ 2800m3 / h, precision 2.0; current, power meter: conventional industrial, precision 2.0.
(2) Data Analysis
Boiler operation thermal load without frequency conversion speed regulation operation: running thermal load Q=MÏcpΔt=421.5×984.3×(84-51.7)×1≈15581kW with variable frequency speed regulation operation: running thermal load Q′= MÏcpΔt=432.5×966.2×(93) -63.4) × 1 = 14419kW Air blower and induced draft fan Unit hour power consumption: No frequency conversion speed regulation running N=58+140=198kWh, variable frequency speed regulation running N′=26+60=86kWh.
The power consumption of air blower and induced draft fan per 1 × 106 W boiler heat load: no frequency conversion = 198/15.581 = 12.71kWh, frequency conversion = 86/14.419 = 5.96kWh.
The thermal load of each 1×106 W boiler is saved by 12.71- 5.96=6.75kWh, which means that it saves 53.1% of electricity.
The above test results show that the use of variable frequency speed regulation is very significant.
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