An AC/AC frequency changer operating under a one to one frequency ratio has its input coupled to a power system of same frequency and voltage, with an inductor connected between its output and input terminals. The output voltage is controlled so as to deviate by a predetermined amount from the input voltage so that reactive power flows from the output to the input which, owing to the inherent phase inversion in the frequency changer, causes reactive input current and reactive output current to be summed up directly at the input terminals. As a result, the frequency changer requires only a power rating which is half the reactive power which it can supply to the power system.