T3b = 3 Ph. Delta pri. to 6 Phase secondary [Delta / Wye] - Rectifier
X1, X2, X3 = Delta sec. - Y1, Y2, Y3 = Wye sec.
Here's another Rectifier Transformer, but this one has a 6 phase output, instead of a 9 phase output. Primary is 3 phase Delta.
X1, X2, X3 = Delta 3 phase 3 wire,
Y1, Y2, Y3 = Wye 3 phase 3 wire.
These coils have epoxy resin potting, which is why they have the black coating on them.
FYI: Note the "Common" conductor at the bottom of the coils, which connects the "X2" ends of the coils for the Wye Secondaries in series additive connection.
That wire at the bottom of the coils is the one which is connected to the Grounded Conductor [AKA "Neutral"] on 4 wire Wye systems]. That's where the common grounded conductor makes a complete circuit to the 3 phase coils.
As you can see, it is nothing at all like the Grounded Neutral of a 1 phase 3 wire system - it's not a center tap on a winding, but is a common connection to all coils.
[This message has been edited by Scott35 (edited 04-18-2001).]