Flux Skew

Flux Skew is a module dedicated to the analysis of rotating electric machines with skewing, allowing a straightforward geometric and physical description in 2D and the consideration of continuous or step skewing effects.

Introduction : skewing in rotating electric machinery

Skewing is a constructive technique that corresponds to continuously “twisting” or slanting the geometry of the slots of the magnetic circuit of a rotating electric machine away from its axial direction. Skewing is usually performed in the rotor, but the stator slots of a machine may also be skewed in certain designs. Figure 1 shows the example of an induction machine rotor with skewed slots.

Figure 1. The rotor of an induction machine with skewed slots and cage bars (a) and its planar, schematic representation highlighting the skew angle α (b).

Skewing corresponds to spreading the current-carrying conductors housed in the machine slots along a certain angle through the machine’s axial length. Consequently, it may be regarded as a special form of winding distribution that may be applied to either squirrel cage bars in induction machines or to regular stranded windings in other types of machines.

In the more specific context of the permanent magnet machine design, a related constructive technique known as step-skewing is also commonly employed. It corresponds to modifying the angular position of the permanent magnets along the axial direction of the machine in discrete steps, as shown in Figure 2.

Figure 2. The rotor of a permanent magnet machine with step-skewing (a) and its planar, schematic representation highlighting the discrete step-skew angle α (b).
Step-skewing of permanent magnets and continuous skewing of conductor slots are both design procedures conceived to modify the spatial distribution of magnetomotive force in the air gap along the machine's axial length. The objective of skewing is to improve certain performance aspects of the machine by reducing or even eliminating undesired spatial harmonics of flux. In practice, a skewed machine design will generally exhibit
  • a reduced cogging torque,
  • diminished vibration and noise and
  • reduced harmonic distortion in the induced electromotive forces of its windings
when compared to the corresponding unskewed design.

What is Flux Skew?

Accordingly with the discussion above, the cross-section of a skewed electric machine changes slightly along its axial length. Consequently, this class of machine cannot be properly represented in a Flux 2D project. On the other hand, representing a machine in Flux 3D may be laborious or time consuming, and the resulting project may require additional computer resources to be solved.

The Flux Skew module is conceived to circumvent these modeling issues. Its main features and utilization principles are provided below:
  • Flux Skew looks and feels like Flux 2D during the pre-processing stages, allowing a straightforward description of the geometry and physics of the electric machine in two dimensions, based only on one of its cross-sections (Figure 3a).
  • After solving, and while in post-processing, Flux Skew looks and feels like Flux 3D, allowing the exploitation of physical quantities in a three-dimensional representation of the skewed machine (Figure 3b)
  • The transition from a 2D description of the machine to the 3D analysis of the results is possible due to the specialized physical applications provided by Flux Skew, which consider the effects arising from the skewing.
  • Flux Skew provides users with a special command, which allows visualizing the skewed 3D representation of the model and creating sensors before resolution. Further details on this feature are available in this topic: Create preview 3D.
Figure 3. Pre-processing environment similar to Flux 2D (a). 3D graphical representation of the solution while in post-processing (b). This example shows the evaluation of the current density in the squirrel cage bars of an induction machine with a skewed rotor.

More specifically, during the creation of a new application, Flux Skew asks for additional geometric information related to the skewing. This complementary data allows Flux Skew to solve a series of 2D finite element problems, each one associated to a "slice" of the skewed machine along its axial length. Flux Skew builds a 3D representation of the machine and then these linked 2D problems are solved.

Flux Skew is a powerful tool for the design and analysis of machines with improved performance due to skewing, and may be efficiently employed together with its native data export tools and other Altair solvers in the context of NVH (noise, vibration and harshness) applications.

Flux Skew magnetic applications

As already mentioned, the project description in Flux Skew is performed in an environment similar to Flux 2D. One remarkable difference between Flux Skew and Flux 2D lies in the availability of the physical applications. In Flux Skew, only the following magnetic applications exist:

  • Rotating Machine (skewed model) in Magneto Static;
  • Rotating Induction Machine (skewed model) in Steady State AC magnetic;
  • Rotating Machine (skewed model) in Transient Magnetic.

These applications are equivalent to their counterparts of Flux 2D, but have their names slightly modified to highlight the fact that they are tailored for skewed electric machines.

Continuous skewing and step-skewing

Both types of skewing discussed in the Introduction are available for the magnetic applications of Flux Skew: continuous skewing (Figure 1) and step-skewing (Figure 2).

Further information on the skewing definition (including examples) is provided in the following documentation chapters:

Specificities of Flux Skew

Users familiar with Flux 2D and Flux 3D should not experiment any difficulties with the general usage of Flux Skew. Nevertheless, Flux Skew has its own specificities when compared to the other Flux modules. These are covered in the following documentation chapters:

Further reading