In the previous Tip of the Week, we discussed how inductive imbalance can vary between motors. This week we will discuss how to use the data from the Rotor Influence Check (RIC) to determine the correct inductive imbalance and alarm setpoints for each asset.
Residual magnetic fields from the rotor iron influence the inductance values of each phase in the stator windings. Rotating the rotor to different positions like in the RIC test offer a comprehensive range of the influence that the varying rotor magnetic field has on the stator inductance. For MCEGold® users after performing and saving the RIC, go to the Test History and double click the last test to view the three phase inductance values recorded in millihenries (mH). Open the Graph Option menu and select Show Inductive Imbalance. The three phase inductance values recorded in millihenries (mH) will change to a calculated % Inductive Imbalance at each position of rotation throughout the RIC. The variation between the high and low levels of % Inductive Imbalance may be as high as 2 to 3% on a typical motor and represent the varying levels of residual magnetic field. This graph should be used to determine the range of normal Inductive Imbalance on any given asset and also used to re-establish the caution and severe setpoints in the event that the Inductive Imbalance values go outside the normal inductive imbalance range. Additionally, while the Inductive Imbalance recorded in the AC Standard test may vary between tests based on the position of the rotor, the values are expected to stay within the range shown in the Inductive Imbalance graph associated with the RIC.
To learn more about the rotor residual flux and the impact on stator inductance go to our website at https://pdma.com/wp-content/uploads/2021/01/Influence_of_Residual_Flux_on_the_Measurement_of_Inductance.pdf