A control system for a cycloconverter of the circulating current type produces an ac current of a variable frequency. The cycloconverter is configured to connect a power factor compensating capacitor at its receiving-end, thus to control a lagging reactive power due to a circulating current flowing in the cycloconverter so as to cancel a leading reactive power due to the power factor compensating capacitor. Such a control is carried out under the condition of an operation below the rated load. In contrast, under the condition of an operation above the rated load, a control is effected so that the circulating current flowing in the cycloconverter is substantially kept constant.