Considerations for improving the non-intrusive efficiency estimation of induction machines using the air gap torque method

The air gap torque (AGT) method is a well established technique for determining the efficiency of an induction motor, yet there are still discrepancies in the literature when applying the method. Additionally, the AGT method is known to be highly accurate, however, it is considered to be too intrusive for industry applications. To resolve this, the efficiency of an induction machine can be estimated non-intrusively by implementing the non-intrusive air gap torque (NAGT) method. The NAGT method combines various estimation techniques to determine the stator resistance, rotor speed and losses non-intrusively. However, due to the various estimation techniques associated with this method, the accuracy of the method is compromised. This paper assesses the various methods for estimating these parameters. It proposes a technique for improving the estimation of SLLs and addresses the discrepancy, as seen in literature, in interpreting the AGT method. Efficiency results from the NAGT method are compared to the IEC Std 34-2-1 and direct methods. The effects of voltage supply unbalances on efficiency estimation using the NAGT method are also examined. Lastly, an error analysis is conducted on experimental data to investigate the effects of instrumental errors on efficiency determination using the IEC Std 34-2-1, direct and NAGT methods.