Purpose -Magnetic slot wedges are usually installed in open slot high-voltage induction machines. They reduce the no load losses and the magnetization current. Additionally, the leakage inductance increases. However, machines and the slot wedges are getting frequently damaged with a decreasing maintenance interval. The usage of magnetic slot wedges leads to unknown effects. It is possible, that direct magnetic forces or indirect forces, caused by the deformation of the stator or stator teeth during operation, results in the damage of the slots wedges. The purpose of this paper is to fully understand the influence of the magnetic slot wedges and the intrinsic effects. Design/methodology/approach -A finite element model of the affected machine is verified with current and torque values from the data sheet of the affected machine. Three types of forces, which are working on the slot wedges, are considered and compared. Findings -There are direct forces working on the slot wedges. The origin of this forces and a coherence between this forces and the slot number relationship, between stator and rotor slots is shown as well as reasons for the damage to the slot wedges. Originality/value -There are investigations about the influence of the behaviour of an induction machine by magnetic slot wedges. This investigations consider the influence on the network models of such machines. The paper at hand deals with the intrinsic effects caused by the slot wedges and its consequences.(1) the decreasing magnetic air gap leads to a reduction of the no load losses and decreases the magnetization current; and(2) the increasing slot leakage causes decreasing start-up and short-circuit currents.A typical material, used for magnetic wedges in high-voltage induction machines, is MAGNOVAL ® . It has a relative permeability up to m r % 2:9: Table I shows the non-linear magnetic properties of the material, the resistivity κ and its elasticity for mechanical stress analysis.
Magnetic saturation in electrical machines is of major influence on the operating characteristics during transients. For induction machines, the assumption of linear magnetic behavior of the iron core in classical machine models may be adequate for the calculation of steady-state rated operation. In operating points or during transients following a short-circuit or out-of-phase synchronization, where currents can reach values as high as 25 pu, especially the magnetic paths of the leakage fluxes are highly saturated, leading to a significant raise in currents and machine torque. Therefore an investigation on the effect of magnetic saturation has been performed by comparing inductances calculated by analytical methods for assumed linear magnetic behavior with results of static finite-element-method (FEM) models taking magnetic saturation into account. These results are subsequently converted into simplified current-depending parameter functions that can be used in transient machine models. Therefore, based on an enhanced space phasor approach, a newly modified machine model is presented, where the parameters can be used according to the dependencies worked out in the paper in order to identify deviations with the classical approach using constant inductances. Furthermore the newly acquired set of current-depending machine inductances is validated by simulating a series of transients using dynamic FEM calculation, showing a significant improvement in accuracy compared to sets of constant parameters.
Magnetic saturation in electrical machines is of major influence on the operating characteristics during transients. For induction machines, the assumption of linear magnetic behavior of the iron core in classical machine models may be adequate for the calculation of steady-state rated operation. In operating points or during transients following a short-circuit or out-of-phase synchronization, where currents can reach values as high as 25 pu, especially the magnetic paths of the leakage fluxes are highly saturated, leading to a significant raise in currents and machine torque. Therefore an investigation on the effect of magnetic saturation has been performed by comparing inductances calculated by analytical methods for assumed linear magnetic behavior with results of static finite-element-method (FEM) models taking magnetic saturation into account. These results are subsequently converted into simplified current-depending parameter functions that can be used in transient machine models. Therefore, based on an enhanced space phasor approach, a newly modified machine model is presented, where the parameters can be used according to the dependencies worked out in the paper in order to identify deviations with the classical approach using constant inductances. Furthermore the newly acquired set of current-depending machine inductances is validated by simulating a series of transients using dynamic FEM calculation, showing a significant improvement in accuracy compared to sets of constant parameters.
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