According to the equivalent circuit of three-phase induction motor, the supplying condition for energy saving has been deduced. And the relationship between output of speed and torque and input of supply voltage and frequency has been revealed. Then a scale-controlled scheme for the motor running at energy saving has been presented. Finally the energy-saving approach has compared with that of constant ratio of voltage to frequency on efficiency by a simulation case. And the transitional processes of speed adjustment and torque change have provided to verify the approach. The simulating results reveal the feasibility of the scale control scheme to realize energy saving for induction motor.