C 24 - 4 Research and Development of Variable - Speed Scroll

The adjustment and control mode of frequency conversion for an air-conditioner can make the refrigeration capacity adapt the variety of its load very well. Thus, it has many advantages such as low variety in in-door temperature, short time to reach the wished temperature, low energy consumption, low start electric current and low strike to electric source etc. This mode is widely used in room air-conditioners. Up to now, household air-conditioners with frequency conversion control are very popular in China. This paper presented the research and the development of a variable-speed scroll compressor used in commercial airconditioners with large input power. Different type of compressors were compared for this purpose, the results showed that scroll compressor is the most reasonable type to realize frequency conversion control. The electric control system was analyzed. Some engineering applications and technical requirements were also given in this paper for the reasonable, reliable and effective use of variable-speed scroll compressors. NOMENCLATURE f: frequency (Hz) I: Electric current of compressor motor (Ampere) K: Coefficient Ne: Input power of compressor (Watt) p: Number of motor poles Qo: Refrigeration capacity (Watt) RS: Rotating speed of compressor (Rev/min) s: slip ratio T: Torque (Nm) Tc: Condensing temperature (°C) Te: Evaporating temperature (°C) Tr: Room temperature (°C) V: Input electric voltage of motor (Volt) INTRODUCTION With the largest challenge of energy shortage in the following several decades, there is a very important task for the refrigeration and air-conditioning industries all over the world is to improve the performances of refrigeration system to reduce the energy consumption. Taking air-conditioners as example, more than 10 millions of air-conditioners were produced and sold in China. The production capacity is keeping increase and new manufacturers are also increasing year after year. A great amount of electric power is needed to support the operation of these air-conditioners. The traditional air-conditioner with constant rotating speed is usually designed with its rated working conditions or its maximum thermal loads. It means that this kind of air-conditioner has a good performance only at its designing point. In fact, the working point at which an air-conditioner works is dependent on its real thermal load. But the thermal load usually varies with the variety of air temperature while an air-conditioner works. In order to match the refrigeration capacity with the variety of thermal load, the air-conditioner must be turn on and off frequently according to the temperature in the adjusted room. This situation means that the air-conditioner often works at non-designed points. Its performance is decreased and energy consumption is increased. In the other hand, the frequent on/off of the air-conditioner will cause a large pulsation of room temperature and influence the comfortableness. As mentioned above, one of the most important ways to improve the performance of an air-conditioner is to set up its ability to match the variety of thermal load. The best way to do so is to use a compressor with variable refrigeration capacity. Up to now, the air-conditioning compressors with variable rotating speed are popular in China. They brought a good benefit of energy. But they all belong to rotary compressors and the input power is limited to 3 HP below. It is obvious that the higher the capacity is, the more the energy saving is. The main reason that the input power is limited is the lack of suitable compressors with high input powers. This paper introduces the development of the variable-speed scroll compressors DEVELOPMENT OF VARIABLE SPEED SCROLL COMPRESSOR Theoretically, there are two major ways to change the refrigeration capacity of such a compressor: to change its rotating speed or to change its displacement volume. That means variable speed compressor or variable volume compressor. The latter does not need to change the electrical control system of a traditional air-conditioner. But it will make the structure of the compressor become much more complex; and the manufacturing cost will increase greatly. A great amount of investment is also needed to re-construction the compressor production lines. This is very difficult for Chinese compressor manufacturers. For the variable speed compressor, though a new frequency conversion control system is needed, few structure changes and a little investment are needed. The most important thing is that compressor manufacturers and control system manufacturers could produce such compressors and control systems with existed production lines. This is the major reason that a variable speed compressor is selected. Compared with traditional one, an air-conditioners with variable speed compressor has many advantages: it has low energy consumption and low start current; there is no strike to electrical source due to its low frequency start mode; it can operate with highest speed after start to reach the expected room temperature; it is able to change its speed in accordance with the variety of the thermal load to control the room temperature within a very small range. It can also avoid the frequent on/off operation (Fig. 1). There are two kinds of compressors that can be used in the variable frequency air-conditioners with large refrigeration capacity: reciprocating compressors and scroll compressors. For a reciprocating compressor, variable speed operation will cause a series of problems, such as noise and vibration, great pulsation of performance at different speed, oil lubrication and life of valve piece etc. After a detail feasibility analysis, it is obvious that reciprocating compressors are not suitable for the variable speed control. Considering that scroll compressors are gradually replacing reciprocating compressors in such air-conditioner market, a hermetic scroll compressor was finally selected. The variable speed control of a hermetic compressor can be divided into two ways: AC frequency conversion and DC adjustment. The cost of latter is much more than that of former one in spite of the high performance of latter one, especially in large refrigeration capacity case. We just discuss the control mode of AC frequency conversion in this paper. Quick Cooling Stable Operation Traditional Air-conditioner Variable Speed Air-conditioner Shorter time Small Pulsation R S