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Cogging torque

QuickField simulation example

Here we will review an example of poor design of the wind turbine generator. Number of rotor poles is chosen to be 2p=12. It turns out that the ratio of the number of stator slots (Z1 = 48) to the number of rotor poles is an integer.

Engineering question
How to find cogging torque in permanent magnet generators

Engineering answer
Set up a plane-parallel QuickField DC Magnetics problem for a permanent magnet wind generator and evaluate cogging torque from computed field results.

Typical applications
wind turbine generators, permanent magnet generators, slot-pole generator structures

Problem Type
Plane-parallel problem of DC magnetics.

Geometry
Axial length is 400 mm.
Cogging torque Cogging torque of wind turbine generator Stator Rotor R 200 mm R 135 mm R 140 mm 60 mm

Given
Stator slots number Z1 = 48
Rotor poles number 2p = 12
Permanent magnet coercive force Hc = 950 kA/m
The B-H curve for the stator and the rotor:

Task
Calculate the cogging torque.

Solution
Cogging torque* is a result of permanent magnet magnetic field and the stator core interaction. We do not specify current in the stator armature and measure the torque for a set of rotor positions. The calculations are automated using the LabelMover parametric tool.

Results
Airgap flux density fundamental harmonic is 1.04T. Cogging torque peak value is 324 N*m per 400 mm of the generator axial length.

Field picture of the flux density distribution and the fundamental harmonic magnitude.
Cogging torque

Reference
*Wikipedia: Cogging torque