What is the difference between a dual winding motor and a Dahlander motor?

Both are 2-speed motors.

In dual winding, there are two winding that are electrially separate and can each be connected to a starter , whereas for the dahlander motor, referred also as a pole change motor, is physically a single winding, but wired in a way to alter the number of poles.

The starter of the dual winding requires two contactors, whereas the starter of the Dahlander motor requires three contactors.

The Dahlander motor is a two speed motor that offers a 1 : 2 speed switching. It is a specially wound pole switching motor requirng a special dual speed starter