Comparison of Different Evaluation Methods to Determine Stray Load Losses in Induction Machines With Eh-Star Method

The eh-star method is a simple and economical alternative test for stray-load-loss assessment in induction machines, also for higher efficiency above 96%. It is an asymmetrical circuit where two machine phases are connected in parallel through an auxiliary ohmic resistance R_eh without coupling of the machine and without needing any dynamometer. This resistance shall be adjusted, so that the positive sequence current I₁ stays below 30% of negative sequence current I₂. If this pure ldquoohmicrdquo resistance is not available, the eh-star measurement can be done with an impedance Z_-eh, preferably ohmic inductive. The determination of stray load losses must be done from the measured losses by decomposition into positive and negative sequence losses. Negative sequence losses at slip 2 - s ap 2 correspond to stray load losses similar to the standardized reverse-rotation test (RRT). For this evaluation, the phase angles of measured currents and voltages must be known. This can be done by calculation with three different methods A, B, and C, which yield identical results for ideal ohmic resistance R_eh. Method A is only useful for an ohmic resistance R_eh for determination of loss separation. Method B is based on the measured two line-to-line motor-input-power values P_e, _{in_UV} and P_e, _{in_WV}. Instead of R_eh, also an impedance Z_-eh (inductor, capacitor, or resistor) may be used. Method C is similar to method B, but only the measured total motor input power P_e, _in is needed. A comparison of evaluated stray load losses with the three evaluation methods is given with theoretical examples and measurements. This is compared to stray load losses and measured with RRT and input-output method according to IEEE-112.