Low-Power Motor Drive Design for Fridge Compressors

| Autor / Redakteur: Alberto Guerra * / Gerd Kucera

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Dedicated, customisable motor control ICs and single-chip drivers can help appliance designers meet new, tougher ecodesign legislation as the ErP rules of the European Union.

Given the global need to reduce energy consumption and carbon emissions, mandatory ecodesign regulations are becoming tougher and encompassing a wider range of product types and design criteria. Europe’s Energy-related Products (ErP) directive (2009/125/EC) imposes tough power consumption and efficiency targets for items such as complete appliances as well as sub-assemblies such as fans and circulator pumps. These targets must be met in order to gain the CE mark necessary to permit sale anywhere in the EU.

In addition, energy labelling of domestic appliances forces manufacturers to strive for better efficiency in order to maintain market share. The European targets have been increased in the latest Energy Labelling Directive (2010/30/EU), as more and more appliances have achieved the top "A-grade" and the EU expects new technologies to yield a further 50% reduction in the energy consumption of an A-class product. In practice, the energy efficiency targets of the latest ecodesign regulations will require OEMs to replace existing designs with more energy efficient systems.

Traditional approach to compressor design

Taking the refrigerator as an example of a typical domestic appliance covered by the ErP and energy labelling directives, the compressor is a key subsystem responsible for a significant proportion of the total energy consumed. By upgrading traditional on-off compressor control using more sophisticated techniques, designers can not only save a significant proportion of energy consumed but can also achieve improvements such as quieter operation and improved temperature control.

The established design practice is to size the compressor system to provide the maximum required cooling capacity under worst-case operating conditions, such as during the summer and taking into account opening and closing of the door. Although the maximum demand is rarely imposed, a system with simple on/off control draws the rated power whenever it is activated. Hence the system consumes excessive energy simply to maintain the desired temperature under most operating conditions.

Using a fixed-speed compressor with simple on-off control can also result in relatively unstable cabinet temperature. This is because the temperature will fall quickly to below the desired set point when the over-sized compressor is turned on, and then rise above the set point before rapid cooling begins again. Such abrupt switching between the turned-off state and full-speed operation also produces noticeable changes in acoustic noise, which can create a poor user experience.

Energy Savings and much more

Electronic compressor speed control using an inverter drive can significantly reduce total energy consumption by allowing the compressor speed to be adjusted according to the usage mode of the appliance. For example, the motor can be operated at full speed over a short period for rapid cooling, and then run at a much lower speed to maintain the cabinet in a steady state at the set-point temperature.

This approach also allows designers to specify a compressor of lower maximum capacity, thereby saving cost and size as well as power. With a more sophisticated choice of operating speeds available, appliance designers can create new products that use less energy, generate less acoustic noise by transitioning more gently between on and off modes, and maintain a more constant temperature with reduced fluctuation around the set point.

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