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In this paper, a novel ac automatic voltage regulator (AVR) is proposed. This AVR is configured by an input inductor, an output filter, a small-capacitance dc capacitor, and a three-arm power converter. This three-arm power converter acts as an ac boost converter when the utility voltage is lower than the specified voltage. On the contrary, this three-arm power converter acts as an ac buck converter when the utility voltage is higher than the specified voltage. Hence, the output voltage of the AVR can be maintained at the specified voltage. The salient feature of this AVR is that the power electronic switches of only one arm are switched in high frequency, while those of the other arms are switched in low frequency. Hence, the switching loss of this AVR can be reduced. Moreover, the capacitance of the dc capacitor in the three-arm power converter is very small. Hence, the size and cost can be reduced, while the reliability and power efficiency can be increased. A prototype is developed and tested to verify the performance of the proposed AVR. The experimental results verify that the proposed AVR has the desired performance.