An effective technique for estimating parameters, losses, shaft power and efficiency of three-phase induction motors in field conditions, is proposed in this paper. It is focused on the determination of the operating efficiency of motors under unbalanced voltages or deviated voltage and frequency without caring out special tests, removing the motors, or measuring the output power or torque. Only few sets of data: voltage, current, power and speed or slip, measured in the motor in-situ, are utilized. With these data, genetic algorithms techniques, applying the method of symmetrical components of voltage and considering parameters variations (especially in the rotor) related to the effect of negative sequence voltage system, are developed. Once the parameters are known, it is possible to obtain the estimated efficiency. The experimental results indicate that this technique has a high accuracy.