The familiar state space averaging technique for DC-DC convertors is extended for modelling a three phase AC-DC convertor without current sensor. The space vector representation technique is used to simplify greatly the modelling processes without sacrificing the accuracy and valid frequency range. Both steady-state and small-signal analyses are made. The results show that for the proposed duty ratio control law the convertor can achieve an ideal property, namely, sinusoidal input current with unity power factor, zero output voltage ripple resulting from line frequency and fixed switching frequency. Moreover, the convertor also possesses the regenerative capability which is useful for many applications. Finally, some experimental results are presented for verification.