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?
).
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.
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.