The term "rectifiers" rings some bells actually, maybe my memory of the electrics wasn't as good as I thought, although "Inverters" doesn't. I'm not entirely sure where in the training I remember rectifiers from first though.
Transformer: Steps the voltage of AC either up or down.
Rectifier: Converts AC into DC.
Inverter: Converts DC into AC.
The alternator must be the invertor and also step down the power, but if the 317 uses two windings to get to 920v, then that doesn't explain my memory of the transformer doing it in one stage and the alternator stepping down twice for 240v and 110v.:?
The Cl317 transformer only uses one winding to to get the voltage down to 920V AC, the secondary winding.
Imagine the transformer to be a box with one input and two outputs. 25kV AC goes in to the box through the input (primary winding) and from one output comes 920V AC (secondary winding) and 240V AC (tertiary winding). Effectively what you have is two transformers in one.
Each winding inside the transformer is formed of an iron core around which copper wire is wound. As an alternating current is passed into one of the windings it generates a magnetic field that acts on each of the other windings and induces a current in it. The voltage of the current induced in the other windings depends on the number of turns in the copper wire on the iron core and the voltage being fed into the primary winding. Therefore when the primary winding is energised at 25kV AC it induces a current at 920V AC in the secondary winding and 240V AC in the tertiary winding.
The 240V AC is being supplied from it's own winding directly from the 25kV AC system the same as the 920V AC for the traction supply and not stepped down again from the 920V AC supply. They are both seperate systems that originate in the transformer, so you have:
25kV AC -> 920V AC (Traction supply from the secondary winding)
25kV AC -> 240V AC (On-train systems supply from the tertiary winding)
The only two-step process you have is the 110V DC supply for the battery charger which takes the 240V AC supply.
In order to supply the TMs 920/900v the Transformer would have to do it before it hits the smoothing choke, the DC would already be at 750v at this point. So the Transformer steps down twice and the Alternator once? That's really not how I remember it.:?
I'm not sure if the DC from the CRE has to go through the smoothing chokes at all as it is already a useable power source and not the choppy looking rectified AC that comes out the other end of the thyristors. But yes, the smoothing chokes are the last piece of kit the power from the OLE goes through before it hits the traction equipment cases. Basically the chain the traction supply follows is:
OLE (25kV AC) -> Transformer Secondary Winding (920V AC) -> Thyristor Converters (920V Rectified AC) -> Smoothing Chokes (900V DC) -> Traction Equipment -> Traction Motors (0-900V DC) -> Return Path
The inverter on the Cl319 is there to turn 750V DC from the CRE into 240V AC for the on-train systems and, by extension, power at 110V DC for the battery charger. The transformer doesn't step down twice, it steps down just the once though it can produce two outputs at different voltages.
Electronics was never my strong point at school and memories of 319s from five years ago are somewhat sketchy it seems.
No, nor me. And my traction training was about the same time ago. I'm having to make copious reference to my "Big Book of the Class 317".
O L Leigh
