Nikola Tesla’s original polyphase power system was based on simple to build 2-phase components. However, as transmission distances increased, the more transmission line efficient 3-phase system became more prominent. Both 2-φ and 3-φ components coexisted for a number of years. The Scott-T transformer connection allowed 2-φ and 3-φ components like motors and alternators to be interconnected. Yamamoto and Yamaguchi:
In 1896, General Electric built a 35.5 km (22 mi) three-phase transmission line operated at 11 kV to transmit power to Buffalo, New York, from the Niagara Falls Project. The two-phase generated power was changed to three-phase by the use of Scott-T transformations. [MYA][MYA]Mitsuyoshi Yamamoto, Mitsugi Yamaguchi, “Electric Power In Japan, Rapid Electrification a Century Ago”, EDN, (4/11/2002). http://www.ieee.org/organizations/pes/public/2005/mar/peshistory.html
The Scott-T transformer set, Figure above, consists of a center tapped transformer T1 and an 86.6% tapped transformer T2 on the 3-φ side of the circuit. The primaries of both transformers are connected to the 2-φ voltages. One end of the T2 86.6% secondary winding is a 3-φ output, the other end is connected to the T1 secondary center tap. Both ends of the T1 secondary are the other two 3-φ connections.
Application of 2-φ Niagara generator power produced a 3-φ output for the more efficient 3-φ transmission line. More common these days is the application of 3-φ power to produce a 2-φ output for driving an old 2-φ motor.
In Figure below, we use vectors in both polar and complex notation to prove that the Scott-T converts a pair of 2-φ voltages to 3-φ. First, one of the 3-φ voltages is identical to a 2-φ voltage due to the 1:1 transformer T1 ratio, VP12= V2P1. The T1 center tapped secondary produces opposite polarities of 0.5V2P1 on the secondary ends. This ∠0o is vectorially subtracted from T2 secondary voltage due to the KVL equations V31, V23. The T2 secondary voltage is 0.866V2P2 due to the 86.6% tap. Keep in mind that this 2nd phase of the 2-φ is ∠90o. This 0.866V2P2 is added at V31, subtracted at V23 in the KVL equations.
We show “DC” polarities all over this AC only circuit, to keep track of the Kirchhoff voltage loop polarities. Subtracting ∠0o is equivalent to adding ∠180o. The bottom line is when we add 86.6% of ∠90o to 50% of ∠180o we get ∠120o. Subtracting 86.6% of ∠90o from 50% of ∠180o yields ∠-120o or ∠240o.
In Figure above we graphically show the 2-φ vectors at (a). At (b) the vectors are scaled by transformers T1 and T2 to 0.5 and 0.866 respectively. At (c) 1∠120o = -0.5∠0o + 0.866∠90o, and 1∠240o = -0.5∠0o – 0.866∠90o. The three output phases are 1∠120o and 1∠240o from (c), along with input 1∠0o (a).
Article extracted from Lesson in Electric Circuits AC Volume Tony R Kuphaldt under Design Science License