diagram of an embodiment in which a system for driving an ac motor is driven using a pwm inverter is applied to the present invention in fig1 ac motor 1 is the ac unit whose type is to be identified and whose rotor has a position detector 10 ac motor 1 also has a connected pwm inverter 2 which supplies a variable frequency ac voltage to ac motor 1 in accordance with a pwm signal three phase voltage command signals vu vv and vw are created by a dq uvw coordinate converter 3 this dq uvw coordinate converter is constructed so as to receive as an input the phase signal u03b8 that has been created from a d axis voltage command vd a q axis voltage command vq and a first order angular frequency command u03c9 through an integrator 61 and perform arithmetic operations on three phase voltage command signals vu vv and vw a current detector 4 a for detecting a u phase current and a current detector 4 b for detecting a w phase current are provided in the line that connects pwm inverter 2 and ac motor 1 and the currents that have been detected as the outputs from the detectors are supplied to a uvw dq coordinate converter 5 this uvw dq coordinate converter is constructed as a current component creating means for creating from the detection currents of current detectors 4 a and 4 b and from the above mentioned phase signal u03b8 a d axis current component id and a q axis current component iq which correspond to the two axes denoted as the d axis and the q axis respectively signals relating to the current components are supplied to a vector control unit 6 provided with a motor type identification.