A Nonintrusive Efficiency Estimation Method for In-Service Motor Testing using a Modified Induction Motor Equivalent Circuit

On average, industrial motors operate at no more than 60% of their rated load, and thus at reduced efficiency which results in wasted energy. Motor efficiency evaluation enables the energy savings in industry. However, because of the uninterrupted characteristic of industrial processes, traditional methods defined in IEEE Std-112 cannot be used for these in-service motors. This paper proposes a novel nonintrusive method for in-service motor efficiency estimation based on a modified induction motor equivalent circuit using only motor terminal quantities and nameplate information. Rotor speed and stator resistance are extracted from stator voltages and currents. No load test is eliminated by using empirical values instead. Rotor stray-load loss is considered in the equivalent circuit by adding an equivalent resistor in series with the rotor circuit, and its value is derived from the assumed stray-load loss suggested in IEEE Std-112. The parameters of the equivalent circuit are solved using only stator voltage and current phasors and motor nameplate data. Three numerical root finding methods are adopted to guarantee the solution of the parameters under any situations. Finally, the proposed method is verified by testing a 7.5 hp TEFC induction motor. Experimental results are presented and analyzed.