Advantages The use of inverter air conditioners is low frequency start (20 ~ 30Hz), fast operation after starting, heating or cooling with maximum power and maximum air volume, and quickly approach the set temperature. After that, the compressor will run at low speed and low energy consumption, and it will be in a non-stop state. It only maintains the set temperature with the required power, which makes people feel like a spring breeze, which not only stabilizes the temperature, but also avoids frequent compressors. The impact on the service life caused by the opening and closing of the ground, and the power consumption is greatly reduced, achieving high efficiency and energy saving, 20% to 30% <1> than ordinary air conditioning.
The core of the inverter air conditioner is the inverter, and the inverter air conditioner using the variable frequency control technology can automatically select the heating, cooling and dehumidifying operation modes according to the ambient temperature. It has the ability of ultra-low temperature heating and rapid defrosting. Generally, when the outdoor temperature is less than 0 °C, the heating capacity is greatly reduced. At -12 °C, the output is only about 35% of the rated value, but the inverter air conditioner can be changed according to the outdoor temperature. Automatic speed regulation, the output can reach about 70% of the rated value. In the case of low temperature, the general air conditioning outdoor defrost shutdown takes 5 to 10 minutes. During this period, the indoor unit stops heating, and the inverter air conditioner can use the microprocessor to control the electronic expansion valve. When the defrosting, the electronic expansion valve is fully open, and the compressor is high speed. Rotating, using the defrosting of the compressor exhaust to achieve uninterrupted heating defrosting. The maximum heating capacity can reach 115 times that of ordinary air conditioners of the same type, and it can maintain good heating effect at low temperatures. In addition, the general splitter only has four speeds for adjustment, while the indoor fan of the inverter air conditioner automatically runs, the speed will vary with the operating frequency of the compressor in the range of 12 speeds, due to the speed of the fan and the air conditioner. The ability to cooperate is more reasonable, achieving quiet operation with low noise, and the lowest noise is only about 30dB <2>.
In addition, a significant feature of inverter air conditioners is high voltage adaptability (up to 142 to 270V).
Disadvantages (1) The power preamplifier generally adopts diode full-bridge rectification mode, which causes harmonic pollution of the power grid, power factor decreases, and generates strong EMI, which poses potential harm to the safe operation of the power grid and other electrical equipment. (2) Because most of its running time is in the low frequency state, the harmonic components are high, electromagnetic radiation exists, the power factor is low, and the low speed performance is poor. (3) Since the power is not stopped, the power electronic device has a large heat loss, causing additional energy consumption. (4) The control is complicated, the price is high, and the maintenance is troublesome.
It is suitable for occasions with large voltage variation range, large single unit capacity, large outdoor temperature difference, poor room sealing and frequent switching.
Working principle of DC inverter air conditioner The unique advantage of high-efficiency digital DC inverter compressor is that it rejects the cyclic operation mode of the original AC frequency conversion "AC voltage → DC voltage → AC voltage → variable speed mode AC motor", using advanced "AC voltage" → DC voltage → variable speed mode digital motor “control technology, DC speed control only through a voltage conversion, reducing the number of current transitions in the work, so that the power conversion efficiency is improved.
It is the basic structure of the inverter part of the inverter air conditioner. The power supply 220V AC voltage is converted to DC by the converter. The main function of the inverter is to realize the commutation, convert the DC voltage into any frequency, and the effective value is equivalent to the pulse voltage signal of the three-phase AC. The basic structure of the most common frequency conversion part is composed of six semiconductor switching elements. Three-phase bridge circuit (high power module).
Inverter air conditioners are divided into two categories: AC frequency conversion and DC frequency conversion according to the type of compressor. The output voltage mode of the inverter in AC frequency conversion is generally unequal width PWM modulation mode, and the output voltage mode of the inverter in DC frequency conversion is generally an equal width PWM modulation mode. At present, PAM is used in the modulation of the input voltage of the compressor of the DC inverter air conditioner with its unique superiority <3>. Its modulation waveform is as shown. 2 The various modulation modes of frequency conversion are frequency conversion part circuits. The virtual frame shows an inverter composed of 6 triode modules. It is customarily called high power module, in which A+, B+ and C+ form a branch. A-, B-, and C- form the lower branch. The cycle is turned on and off in a certain order, and each time a triode of the upper branch is always turned on with a triode of the lower branch, and a square wave voltage is applied to the stator coil of the compressor. 3 Frequency conversion part of the circuit schematic In the compressor, since the cylinder is filled with Freon vapor, it is not possible to use a brushed DC motor that generates sparks. Therefore, it is necessary to use a brushless DC motor that is commutated by an electronic circuit.
Advantages In addition to the conventional advantages of AC frequency conversion, it also has the following advantages: (1) Reduce the number of times the current is converted during operation, greatly improve the conversion efficiency of electric energy, and improve the efficiency ratio. (2) The fan of DC inverter air conditioner adopts digital motor. The digital motor refers to the motor with very precise rotation speed controlled by digital pulse information. It can realize complete control of deceleration and acceleration during motor rotation and changing speed, which can ensure the most The best way to run smoothly and quietly. At the same time, the AC noise caused by the AC motor is avoided, and the running noise is greatly reduced. (3) It adopts DSP digital control chip, which is more than 20 times faster than the MCU chip used for AC frequency conversion, with more precise control and faster running speed. (4) Since the brushless DC motor is used in the DC frequency conversion, the rotor is a permanent magnet, and no external current is supplied, which reduces the loss, so the efficiency is high. If the DC inverter compressor uses rare earth permanent magnet material, no magnetic loss, no magnetization, so that the magnetic lines are concentrated, greatly reducing the energy loss during the conversion process, making the compressor run more smoothly and quietly, and saving up to 18 times compared with AC frequency conversion. %~20%<4>, and its low speed performance is better.
Discussion on the development direction of high-performance inverter air conditioner Energy conservation is the development theme of high-performance air conditioner. The energy saving depends mainly on the design and configuration of the refrigeration system, depending on the cooling coefficient of the system. The improvement of constant-speed air conditioners into speed-regulated air conditioners is mainly to improve the way of automatic adjustment to improve the quality of the machine, rather than to save energy. Speed ​​regulation is only a basis for improving the operation quality of air conditioners, and real energy conservation depends on the matching of refrigeration systems. The key component for adjusting the cooling capacity of the system is the throttling and decompression element. If it is not changed from a constant damping capillary to an adjustable damping electronic pressure reducing valve (expansion valve), then this The speed-regulated air conditioner does not make much sense, and even consumes more power for internal conversion.
In order to further improve the performance of the inverter air conditioner, the following methods can be used: (1) Based on the BOOST circuit topology, UC3854BN is used as the PFC control core, how to configure its voltage loop and current loop parameters, and the circuit is applied to the inverter air conditioner to realize Its power factor correction makes each harmonic current content meet the "3C certification" EMC standard. Based on the double closed-loop design principle to realize the active power factor correction of the inverter air conditioner, a good function can be achieved.
(2) Application of PAM modulation method on air conditioners. In order to keep the motor torque constant, the DC inverter compressor must make Ud/fd constant. When the speed increases, the compressor input voltage should increase proportionally.
The equal-width PWM inverter used has the characteristics of large torque and high sensitivity, but the output voltage capability is insufficient, which limits the maximum speed of the compressor. The PAM modulation method can obtain a higher inverter output voltage under the same grid input voltage, so if the equal-width PWM modulation method is used in the low-speed range of the compressor, the PAM is efficient and low-noise in the high-speed range. Mixed modulation is undoubtedly the best of both worlds.
Conclusion Air conditioning variable frequency power supply should be designed with the following points in mind: (1) small size; (2) strong voltage regulation capability; (3) excellent performance in low frequency range; (4) high reliability. In addition, there are some special requirements in terms of switching loss, feedback signal interface and cost. Some problems belong to the control level, and work on the software and control circuits; if there are some power levels, work on the selection of modules and components.
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