On (29 Jul 96) Elvis Hargrove wrote to Alec Cameron...
-> B Boveri also supply the railroad 1500v dc steel tank pumped
-> rectifiers
 EH> What the heck is THAT?  Sounds ominous!
Pretty harmless. Was a development of the earlier glass bulb rectifier as 
sed
in streetcar substations, to convert AC to DC. The steel tank version was
built in very high ratings, as an apprentice I was in a team installing one
rated 3000kW in 1948.
The generic name is Mercury Arc Rectifier. A simple three phase version is
connected thus-
From a wye connected secondary winding of a HV trannie, the three phases are
wired to three anodes ["plates"] of the single tube, which has a fourth
terminal connected to an internal pool of mercury.
That fourth terminal is cabled to the streetcar rails.
The centre ["star"] terminal of the wye trannie winding, is connected to the
trolley wire.
The trannie secondary voltage is high enough to start an arc from the three
anodes to the one mercury pool. This arc spins rapidly to each of the three
anodes, while always rooted to the mercury pole. Like a rotating switch if
you will!
But, the nature of a mercury arc encourages current flow in one direction
only, so the arc behaves like a beaut modern diode. AC in and DC out.
The DC is rather rippley, even though a choke [inductor] was often placed in
series with the negative cable to the rails. Therefore 6 phase supplies were
often used.
But your utility ain't in the six phase business, they offer three. So most
railroad and streetcar substations used a cunning interconnection of 
econdary
windings so that a second three phase supply was given, out of phase with the
other. The particular connection was called ZIG ZAG or SCOTT, so that as well
as the star point supplied [positive, to trolley wire] there were six AC
cables wired off to the six- anode rectifier.
The steel tank version, came with a vacuum pump to keep the internal chamber
de- gassed. And the steel tank, being at rail potential [not perfectly
earthed] was supported on insulators.
Are you sorry you asked?
-> 50 cycles, three or six phase. I *imagine* that the contacts were
-> carbon and that the synchronising was such that overlap, put most of
-> the sparking on the transformer primary circuit, the LV side contacts
-> moving at zero current.
 EH> Jeeze, I think I need a diagram.....  SIX phase syncronous zero
 EH> crossing makes my HEAD HURT!  What drove the contacts?
A synchronous motor I guess! With a neat, durable cam shaft and adjustable 
am
followers to drive the DC side commutating switches.
-> Here, auto radios were a novelty until about 1946. The synch and
-> asynch vibrators, were standard then.
 EH> I had a 1937 Chevy sedan that had a "Factory" radio in it.  I suspect
 EH> all of them were actually dealer installed, but it had been in it since
 EH> new.
My first car was a 1937 Morris. It had wire wheels, and solenoid dipped
headlamps. That is, the headlamp globes were single filament 6v 24 w, and the
complete reflector/lamp assembly was tilted by solenoid as you hit the dip
switch with your left foot! Lucas had some good points. Delco Remy learned a
lot from them!! And beside the rear licence plate, was the switch for the 
ail
light. It was not permitted to park a car on the street unless the tail lamp
was on.         Cheers...ALEC
... ........When I contemplate the moon, my head aches [Galileo]
--- PPoint 1.92
---------------