Electrically Driven Automotive Air Conditioner

In recent years, the fraction of air conditioning systems installed in American automobiles has increased to approximately 85%. Concurrently, efforts to improve fuel economy have resulted in smaller engines. The point has been reached in which load from a cycling fixed-displacement compressor can significantly affect vehicle drivability and performance. To remedy this problem, variable-capacity A/C systems have been developed with mechanically controlled compressor displacements.

An alternative electronic approach to cooling capacity modulation is presented in this paper. The system utilizes a variable-speed brushless dc motor to drive a fixed-displacement compressor, thus achieving continuous control of cooling output independent of the engine drive belt. Compared to variable-displacement compressors, electrically driven A/C systems provide the following additional benefits:

• Hermetic motor/compressor assembly (no shaft seal leakage)
• Smaller, less complex compressor
• Flexible packaging (no drive belt)
• Full cooling capacity at any engine speed
• Control of conditioned air temperature without reheat

This paper deals with the development of a brushless dc drive system suitable for powering an automotive air conditioner. Two 5 hp motors were designed and built with maximum operating speeds of 6000 and 7000 rpm. These motors were then tested in a typical automotive air conditioning system over a range of operating conditions in order to determine their performance envelopes. The results are discussed, and conclusions are drawn as to the type of compressor and control strategy best suited for electric automotive A/C. Issues that are critical to the further development of electrically driven A/C systems are identified. An example simulation of annualized A/C system performance is also provided to quantify some of the benefits of cooling capacity modulation.