Hello Zhan,
 The approach used by you is correct. The model can be modelled in two ways.
1) As a single hollow cylinder representing the four layers in a single entity(total turn to be 250*4). This representation can be used if the gap between the layers is not suffeciently large.
2) Model four hollow cylinder representing each of the layer. If the gap between the layer is big in comaprison to the turns in the layer. A winding can be defined and four terminals(250 per turn) from each of the layer can be grouped inside this winding.
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