Variable Speed Compressor in Refrigerator

1. Differences Between Variable-Speed Compressor And Fixed Speed Compressor

The differences between variable speed compressor and fixed speed compressor are their working principle and user experience.

For a fixed speed compressor, it is controlled by power switch of the indoor unit. For variable speed compressor, its frequency conversion module can change the current frequency of it to control the power and temperature.

In terms of the user experience, the temperature in the normal refrigerator is of fluctuation, while that in the inverter refrigerator is stable.

Figure 1: An opened inverter fridge.

1.1 Working Principle

The fixed speed compressor realizes the compression to control the temperature by the control board of indoor unit. Because of the constant power of the fixed speed compressor, the capacity of refrigeration is also constant.

If you want to control the temperature, you need to continue starting and stopping the compressor. When it reaches the set temperature, the compressor stops working, and the temperature rises, the compressor starts again.

The variable-speed compressor controls the temperature by changing the current frequency of the compressor and controlling the speed (power) of the compressor through the frequency conversion module.

The power of the compressor of the inverter air conditioner is adjustable, the output refrigeration capacity is also variable. When the refrigeration temperature reaches the set value, the frequency conversion module reduces the speed of the compressor and the refrigeration capacity to control the temperature.

Figure 2: Refrigerator with fixed speed compressor.

1.2 Temperature Control Performance

The internal temperature fluctuation of the fixed speed compressor is much server than that of the variable-speed compressor. The improved technology of fixed speed compressors brings low prices and stable operation. So fixed speed compressors require low cost for maintenance.

The price of the variable-speed compressor is higher. Its complex structure requires better quality components. Plus it is easier to break down, and the maintenance cost is high.

2. Applying Variable-Speed Technology to Refrigerators

The variable-speed compressor is used in the inverter refrigerator, and the inverter controller is added to the refrigerator control system. The refrigeration capacity of the refrigerator can be changed by changing the speed of the compressor. Due to the high cost of variable-speed compressor and controller, the price of the inverter refrigerator is also high. Now it is generally used in high-end refrigerator products.

2.1 Pros of Inverter Refrigerator

Because the refrigeration capacity can be adjusted to match the load of the refrigerator, thus avoiding unnecessary energy consumption. When a small refrigeration capacity is required, the compressor can run at a low speed to reduce the number of start-ups of the refrigerator.

Compared with the conventional refrigerator, the inverter refrigerator has the advantage of energy-saving from two aspects. One is to reduce the loss caused by frequent shutdown/start-up of the refrigerator. Besides, the variable-speed compressor helps reduce the energy loss caused by the re-establishment of the balance between high and low pressure when the refrigerator starts again.

When the refrigerator load is large, the compressor can run at a high speed to realize rapid cooling. The temperature control in the refrigerator can be more accurate and stable by continuously adjusting the compressor speed.

The inverter compressor is driven by a brushless DC (BLDCM) motor, reducing the excitation loss caused by a conventional AC induction motor. However, it will bring some problems of electric-magnetic compatibility and interference.

And compressors are applied in the refrigerator. The working principle and structure of the reciprocator compressor determine a narrow frequency conversion range, leading to the advantages of the inverter refrigerator not as obvious as that of the inverter air conditioner.

2.2 Refrigerator Suitable for Variable-Speed Compressor

The variable-speed compressor is more beneficial to the refrigerator with a capacity of more than 250 liters. There are three methods to improve the efficiency of the refrigeration system.

The first is the adoption of energy-saving components and technologies, such as high-efficiency compressor, high-efficiency heat exchange technology, and foam insulation technology. The second is to improve the adaptability of products and determine the structural parameters and system parameters employing optimization design.

The third method is to realize energy saving in the process of operation and use. It includes two aspects. On the one hand, it requires high efficiency of the refrigerator in a wide range of working conditions. On the other hand, it makes the output of the refrigerator adapt to its load. This is the common sense of energy-saving, and it is suitable for any equipment and energy-consuming products.

In principle, the variable-speed technology improves the energy efficiency of the refrigerator. But inverter technology requires using a variable-speed compressor and controller.

The energy efficiency of mainstream refrigerators with a capacity of about 200 liters has reached a very high level, and the power consumption has been very low, so the use of variable-speed technology may not be worthwhile. But for large refrigerators with a capacity of 250 liters (especially more than 300 liters), the variable-speed technology would be a good choice.

Figure 3: A 300 liters refrigerator.

At present, these large refrigerators are used with low-efficiency compressors. Apart from this, this kind of refrigerators have large power themselves, so the energy consumption of the inverter controller accounts for a small proportion. But it causes losses of the appliance starting and stopping. The variable-speed technology helps to save energy.

In a word, variable-speed technology is of vital importance to the energy saving of refrigerators. But we should objectively treat and comprehensively consider various factors, give full play to their role in energy-saving and realize the comprehensive benefit between income and payment.

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