Static breakdown torques from run-up dynamic tests in medium-large induction motors

The study deals with the difficulties in measurement of the breakdown torques through routine run-up factory acceptance tests (FAT) which do not usually yield the true breakdown potential the motor possesses. The origin of these differences is investigated. It is established that the FAT run-up tests (dynamic torques) can never yield the calculated figures (static torques). It is shown that even a more than 10 times increase of inertia (as compared to the motor inertia) results in a dynamic torque that is -5% lower than the static value. Same is found to be the case for tests at very low supply voltages. Distinct dependencies of the dynamic torques upon voltage (flux level), inertia, saturation and temperature are noted, some of which have never been reported earlier. The literature reported theoretical criteria to avoid dynamic effects presence during run-up tests are investigated. It is shown that those can at best be only served as guidelines. For real machines, the criterion varies and is found to be different from that reported in literature by a factor 5-100. It is shown that some of the uncertainties associated with the breakdown torques are introduced by the conventionally used and standards recommended scaling methods that consider the saturation effect of voltage on static torques only. The standards need to be updated so as to take into account the dynamic torques dependencies and means to translate these values to static torques.