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Control of Linear Compressor System Using Virtual AC Capacitor
ShinHyun Park* and JongWoo Choi¢Ó 
Abstract 
Recently, linear compressors of cooling systems such as refrigerators, which have a free
piston driven by a linear motor, have attracted much attention because of their high efficiency. For
structural reasons, linear compressors applied in refrigerators should use an AC capacitor to ensure
stable control. However, AC capacitors are expensive and bulky. In this paper, we propose a new
method to realize stable control without a real AC capacitor by implementing a virtual AC capacitor
with software. To realize a virtual AC capacitor, a pure integral is calculated. Nonetheless, if an offset
current exists, the calculation may diverge to infinity. To solve this problem, a highpass filter is
applied and the compensation for the phase angle and magnitude are realized with a new method.
Finally, a virtual AC capacitor enables variable frequency operations. Hence, in case of a lack of
voltage, we can compensate by running the linear compressor in highfrequency operations. To
improve efficiency, we may optimize the operation frequency. The validity of a virtual AC capacitor
has been verified through simulations and experimental results. 
Keyword 
AC capacitor, Linear compressor, Virtual AC capacitor

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