Coach/carriage electrics.

All the dc dynamos that I have been involved with appear to only generate when rotated in only one direction. As coaches operate in both directions and still generate, how is it done?

 

Interesting question... Obviously in cars the engine and hence dynamo rotates in one direction so no problem. However for an alternator they could be using a bridge diode system to get DC from AC, could they use a similar thing for coaches?

 

Early diodes were aimed at radio detection, not power rectification, so a non-electronic solution was needed... Friction & cunning:—



"A Polarity Changer, of simple form, is embodied in the dynamo; it ensures the same polarity or direction of current for both directions of running. The brush-holder, O, at each side of the dynamo is mounted on an arm that runs out parallel with the shaft of the machine; and this arm, carrying a contact plate, M, at its end, is attached to a rocker, D (which forms the armature of an electro-magnetic clutch), allowing it to rock through a small angle in either direction of rotation.

"When the dynamo starts, the friction of the brushes on the commutator carries the rocker over until the contact plates rest against the two switchboard brushes, L (one above and one below the line of the shaft), mounted on the small switchboard, N, at the end of the dynamo; this completes the circuits of the field magnet and the clutch coil. As the speed of the dynamo increases, it excites itself and also energises the clutch coil, E, which then holds the brush rocker firmly over, and ensures good contact between the contact plates and the switchboard brushes. At the stopping of the dynamo, the clutch is not released until the field current has fallen to zero; so there is no sparking at the contacts of the automatic polarity-changer."

(Source: The "Liliput" Dynamo – General Instructions, J. Stone & Co, 1927)

Pete S.

 

Thank you Pete S......Yet another example of the teaching and learning from NP!

 

Thanks for that info.