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A post written by Dr. Farid ZIDAT – Senior Application Engineer – EM Solutions at Altair
In the present energy efficiency context of electrical machines, diagnosis of rotating machines is increasingly studied. Designers seek to include the on-line, non invasive diagnosis and typical signatures of the rotating machines faults in the stator winding currents, torque, leakage magnetic field…etc. Among the rotating machine’s faults, 7 to 10% are located in the rotor and some of these faults are eccentricities. These faults generate electromagnetic torque oscillations: electromagnetic forces acting on the stator, particularly the stator winding, which can accelerate wear of its insulation. Friction between the stator and the rotor is not excluded; this can also have an adverse effect on the bearing.
In the literature we often find three types of machine eccentricities: static, dynamic and mixed. Our Flux 2D/3D/Skew finite element solution can be of considerable help to predict the typical signatures of eccentricities faults and the influence of these defects on the electromagnetic and vibro-acoustic performances of these machines, a very differentiating feature of the software.
The purpose of this article is to show the feasibility of the different eccentricities with Flux 2D/3D/Skew thanks to the possibilities offered by mechanical sets.
This is the state where the rotor axis is shifted from the stator axis (Figure 1), and the rotor turns around the rotor axis. This fault is most easily dealt with with Flux. One has to just define two
• Mobile 1: this mechanical set turns around the rotor axis with velocity ω1. The rotor axis is shifted from the stator one. However, make sure you center the sliding cylinder on the rotor axis.
Remember, if the machine is defined with Overlays, the static eccentricity can be directly taken into account in this environment, by modifying parameters DX_STATOR or DX_ROTOR or DY_STATOR or DY_ROTOR.
This is the state where the rotor axis is shifted from the stator axis (Figure 2), and the rotor turns around the stator axis. This fault is also easy to model with Flux. One has just to define two
• Mobile 1: this mechanical set turns around the stator axis with velocity ω1. The rotor axis is shifted from the stator one. However, make sure to center the sliding cylinder on the stator axis.
This is the state where the rotor axis is shifted from the stator axis (Figure 3), and the rotor turns around both its axis with velocity ω1 and around the stator axis with velocity ω2. Often, if we know
ω1, ω2 is difficult to define. This is the most difficult point in the modeling of mixed eccentricity.
If we know the two speeds ω1 and ω2, the modeling of the mixed eccentricity is feasible with Flux. Indeed, with Flux one is allowed to use more than 3 mechanical sets, however, one has to take some precautions as explained in the FAQ on the Connect user portal.
To model the mixed eccentricity with Flux we have to define 4 mechanical sets located as shown in Figure 3:
• Mobile 1: this mechanical set turns around the rotor axis with velocity ω1. The rotor axis is shifted from the stator one.
• Mobile 2: this mechanical set turns around the stator axis with velocity ω2, but in the opposite direction of Mobile 1.
To model these eccentricities we have used the example of the induction machine from the tutorial provided with Flux, and we deliberately set the air gap to 2 mm. We put an eccentricity
of 0.8 mm. We launched 4 Flux projects in transient application: Healthy state, Static eccentricity, Dynamic eccentricity and mixed eccentricity.
In the video below we can see the mixed and dynamic eccentricities. Markers were placed on the stator and the rotor to identify the position of the rotor and the stator.
Thanks to the possibilities offered by mechanical sets with Flux, we can model all kinds of movements: translation, rotation and free movement. These capabilities allow us further studies on electromagnetic devices, in particular, rotating electrical machines in 2D, 3D and Skew. In this article we have seen that it is quite possible to model the eccentricities, even the most difficult one: the mixed eccentricity that combines two movements. The Flux efficient application team remains at your disposal for any advice or for demonstrations of this feature.
Learn more in detail reading our detailed article: Eccentricities Faults Magnetic Signature of an Induction Machine Determined with Flux®