Sync motor question

Hello List

I am building a RSG for a 5kVA TC.

I have found a good 50Hz sync motor (single phase) in a second-hand parts
outfit. The motor label reads "Dual speed hysteresis motor" and
"1500rpm/3000rpm". The motor has 3 leads and a small circuit board
containing a relay and motor cap etc. The quoted power is 1/14Hp and 1/7Hp
depending on motor speed. 

I know a little about the insides of induction motors, but I have never
seen or heard about dual speed designs. Could someone explain how these
motors operate? I am having trouble imagining the internal geometry.

Thanks in advance for your help,

Gavin Hubbard