Discussion on the Key Technology of DC Inverter Air Conditioner Control

Discussion on the Key Technology of DC Inverter Air Conditioner Control

Inverter air conditioners have become a hot spot for the development of air-conditioning companies because of their energy-saving, comfortable, and high-energy-efficiency advantages and conform to the national policy of "energy saving and emission reduction". my country's inverter air conditioners have experienced three stages: AC inverter, DC inverter based on 120° square wave drive, and DC inverter based on 180° sine wave drive. At present, the mainstream DC inverter air conditioner in the market is driven by 180° sine wave. The high energy efficiency of inverter air conditioners, the comfort and energy saving of high and low frequency operation, and low vibration will also apply multiple key technologies.

Current control mode improves air conditioning energy efficiency ratio

Energy efficiency ratio is one of the most important indicators to measure the performance of air conditioners. The inverter drive adopts compressor current control mode to maximize the output capacity of the motor and improve the energy efficiency of the air conditioner. At present, the DC compressor used in inverter air conditioners can easily change the reluctance torque by adjusting the direct axis current id through vector control, that is, through the coordinated control of the direct axis current id and the quadrature axis current iq to optimize the control of the system Performance, such as maximum power output, optimal efficiency, maximum torque/current ratio (MTPA), etc.

The experimental data in Table 1 is the control effect under the MTPA current control mode of the compressor with air-conditioning load. The comparison is carried out at the same speed and power. By comparing the output current of the "id=0" control mode and the MTPA control mode , It can be clearly seen that the current under MTPA is significantly lower.

PPFC harmonic current control technology is low cost

Inverter air conditioners must meet the national EMC requirements for home appliances. Harmonic current is one of the important EMC indicators. The current harmonic current control methods include passive power factor correction (also known as passive PFC) and active power factor correction (also known as active Type PFC, AFPC for short) and partial chopping power factor correction (also known as semi-active PFC, PPFC for short).

Passive PFC is composed of passive inductors, capacitors and diodes. The advantage is that the circuit is simple, but the disadvantage is that it is difficult to correct the harmonic control, the output voltage changes greatly with the load, the cost is high, and it is difficult to promote.

APFC generally adopts continuous current mode control due to its large power. The advantage is that it has the characteristics of stable DC voltage under wide input voltage fluctuations. The disadvantage is that the circuit performs chopping control during the entire input voltage cycle, and the power device has high working stress and high cost. At the same time, under high current, chopping will bring about the problems of reduced efficiency and reduced reliability.

The PPFC control method only performs chopping control during a low voltage period after the input voltage crosses zero. The high input voltage is still natural rectification, and the stress of the power device is relatively small, generally only about one-third of APFC . With proper control of PPFC, the input current waveform is as close to a sine wave as possible.

The PPFC solution has low cost and excellent performance, meets the requirements of the national standard GB17625.6 (IEC61000-3-4), and is suitable for application in 1~3HP inverter air conditioners.

Torque compensation control to solve the low frequency vibration of the press

At present, ordinary household inverter air conditioners usually use relatively low-cost single-rotor compressors. When the single-rotor compressor is running at low frequency, the unbalance of the rotor can easily cause the coil connected to the compressor to vibrate greatly. Periodic low-frequency vibration will cause resonance between the pipeline and the system of the whole machine, which will not only cause noise, but also serious It may also cause pipe breaks. The long-term low-frequency operation of inverter air conditioners makes the vibration problem more prominent. Now we use torque compensation control technology to solve the problem of low frequency vibration of single rotor compressor.

Figure 1 is the measured vibration displacement curve of the 1.5HP window machine with or without vibration control. It can be seen from the figure that the vibration of the entire air conditioning system appears at 25Hz for the 1.5HP compressor without vibration control. After the vibration control is performed by torque compensation, the vibration The amplitude is greatly reduced. Good vibration control effect even makes it possible to use single-rotor compressors for integrated air conditioners (window machines and mobile air conditioners). Shanghai Xinyuan Inverter has already provided batch products to window air conditioners for drive control of single rotor compressors.

Field weakening control technology ensures high frequency operation of compressor

The current inverter air conditioner compressor usually uses a high back EMF motor to improve the energy efficiency ratio of the air conditioner. As far as drive control is concerned, ensuring the high frequency operation of the compressor becomes the key technology. The original method is to use a general boost power factor correction (Boost-APFC) circuit to increase the bus voltage to ensure high-frequency operation of the compressor, but this method will complicate the system structure, high controller costs, and low air conditioning efficiency. . We think that we can use the weak magnetic control method to cooperate with the PPFC harmonic control to increase the compressor operating frequency. This has the advantages of low cost and stable performance. Through experiments, under ultra-low temperature heating conditions, the air conditioner compressor has a lighter load, and the use of weak magnetic control to increase the frequency can fully meet the product requirements. In short, the research and development of inverter air conditioners needs to be designed according to the concept of integrated air conditioning system and inverter electronic control. A comprehensive analysis of the technical characteristics of each program and the application of existing mature technologies can reduce product costs while improving product performance and quality.