Turbo-pumps have weak points, such as the pumping operation becomes unstable in the rising portion of the head characteristics and/or the cavitation occurs under the intolerably low suction head. These points have been individually overcome in general, since such phenomena originate in the essentially different causes. To overcome simultaneously the above weak points, however, this paper proposes the unique pumping system with counter-rotating mechanism and discusses the wonderful advantages of the system. This system consists of the two-stage impellers and the peculiar motor with the double rotors. The front and the rear impellers are driven respectively by the inner and the outer rotors of the motor, keeping the relative rotational speed constant and counterbalancing the rotational torque. Such driving conditions not only improve smartly the unstable performance in the low discharge but also suppress smartly the cavitation in the high discharge, under the optimum cooperation with the impeller works and the rotor outputs. The characteristics of the model impellers were predicted numerically and the performances of the model pump were examined experimentally. The model test proved successfully that there is not the rising portion of the head characteristics and the front impeller speed is gradually slowed down with increase of the discharge as if taking the place of the inducer.
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