 The two engines each rotated in the same relative direction, but were mounted at opposite ends with their drive through Geislinger couplings, and so drove the rotors contra-rotating.Each diesel engine ran at up to 1,500 rpm so the effective rotational speed of the alternator was 3,000 rpm, between the two parts.The inner rotor carried the field, supplied by two slip rings, with the outer rotor delivering its current through three more slip rings.