PURPOSE: To operate an induction generator apparently as a voltage source by controlling a secondary current vector by an exciting current reference vector corresponding to the reference value of a primary crossing magnetic flux vector and a difference vector from a primary current vector.

CONSTITUTION: The output of an angular frequency detector 7 and the output of a rotating angle frequency detector 9 are input to a vector divider 12. The primary current vector i1 of a wound-rotor type induction machine 1 is detected by a detector 8, and the outputs of the detectors 8 and 9 are input to a vector divider 14. The reference value 1 of a primary crossing magnetic flux vector is multiplied by 1/M by a coefficient unit 11, and the outputs of the unit 11 and the divider 12 are input to a vector multiplier 13. The output signals of the divider 14 and the multiplier 13 are led to a coefficient unit 15 and a vector subtractor 16 to obtain the reference value i2 of a secondary current vector, and it is converted by 2-phase/3-phase converter 17 to 3-phase reference values.