Thanks Mike. A web search on ‘Wound Rotor Induction Motor” yielded the the following site http://www.ele.auckland.ac.nz/~kacprzak/PE2.html which is coincidently from your part of the world! (Do they call these motors something different outside of New Zealand? [Linked Image] ).

The resultant information extinguished my falsely illuminated light (see above) but I think that I am finally starting to get the picture. A floating (w.r.t. earth) resistor potential and a change in the speed/torque characteristics produced by a changing rotor resistance – it seems. I have to admit though that the formulae going down the page explain why I am not an electrical engineer. [Linked Image]

Still, it’s amazing where - a passing online comment on a board dealing with railway restoration, numerous web searches plus of coarse helpful insight from you guys - can lead one.