Cogging

Positioning and objective

The aim of the test “Characterization - Open circuit – Motor & Generator - Cogging torque” is to get the characteristics of the cogging torque of the machine.

Thanks to this test it is possible to evaluate the impact of the machine topology (slots and magnets – number and dimensions) on the cogging torque characteristics (magnitude and period).

The following table helps to classify the test “Open circuit - Cogging torque”.

Table 1. Positioning of the test “Characterization - Open circuit – Motor & Generator - Cogging torque”
Family Characterization
Package Open circuit
Convention Motor & Generator
Test Cogging Torque

Advice for use

The cogging torque period depends on the number of slots (NS) and the number of poles (NP).

It is given by the dividing 360°, circular mechanical degree in space, by the least common multiple of number of stator slots (NS) and number of rotor poles (NP).



In general, with increasing number of slots and poles, cogging torque period decreases as shown in the following table.

Figure 1. Cogging torque period in mechanical degrees


Note: An extremely small cogging torque period implies the need of a very thin mesh to get accurate results (by using a low airgap mesh coefficient value – See advanced parameter definition). Thus, the number of elements in the corresponding model will be very high (it means a high mesh density).

In the case of a very small cogging torque period, consideration of the airgap length is also relatively important.

In these conditions it is not possible to get good results because the numerical error induced by the mesh (meshing noise) has the same magnitude order than the cogging torque.

Dedicated mesh methods are needed for such situations but are not yet implemented in FluxMotor.

In those cases, the advanced solution is to export the model to Altair Flux and to adapt the mesh manually to reach the expected accuracy.

For information, the following table gives a general idea of easy computations (green cells), difficult ones (orange cells) and in some instances impossible ones (red cells) in FluxMotor.

Figure 2. Cogging torque computation: Green= easy / Orange = difficult / Red = very difficult or even impossible


To be noted that in case of square shape lamination, the analysis of the cogging torque will be done at least over 90 mechanical degrees (see the following example).

Figure 3. Cogging torque analysis over 90 mechanical degrees


Subsections

Inputs

For more details, please refer to Inputs.

Thermal

For more details, please refer to Thermal.