An induction motor drive system, in which the drive speed is in response to a reference speed by applying a vector control method, with maintaining the exciting current command value Io for the induction motor (14). In this system, when the speed of the induction motor (14) exceeds the reference speed an exciting current amplitude control circuit (51) increases the exciting current command value in response to the speed of rotation and a supply voltage output circuit (52) detects the level of the drive suppply voltage for the induction motor and controls the exciting current amplitude control circuit (51) so as to increase the reference speed which is the threshold for changing the exciting current command value in response to the detected level.
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