Scott circuit

The Scott circuit is an electrical circuit which, with the help of two different transformers, allows three-phase voltage systems to be converted into two-phase voltage systems and vice versa. The circuit was developed in the 1890s by Charles F. Scott , an engineer at Westinghouse Electric .

General

Scott circuit

Vector image of the voltages to explain the Scott circuit

The effect of the circuit can be explained with the illustrations opposite. The voltages L ₁₂ , L ₃₁ and L ₂₃ form a three-phase voltage system. With the voltages U ₁₂ and V ₁₂ a two-phase voltage system is obtained, which can be expanded to a four-phase system using −U ₁₂ and −V ₁₂ .

Two different single-phase transformers T1 and T2 are required to set up the circuit. A winding with N turns on the primary side of T1 must be designed for the triangular voltage and have a center tap. It is connected between the first two outer conductors L ₁ and L ₂ . The primary winding of the second transformer T2 must be connected according to the voltage V ₁₂ between the center tap of the first transformer T1 and the third remaining outer conductor L ₃ and designed for a voltage of  ≈ 0.866 times the value of V ₁₂ . The primary winding of T2 thus has a number of turns that is lower by this factor than the primary winding of T1. This fact is indicated by the open end of the primary winding of T2 on the far right in the circuit diagram. ${\displaystyle {\tfrac {\sqrt {3}}{2}}}$

If the two secondary voltages, which are theoretically phase-shifted by 90 °, are to be the same, the secondary number of turns of the two transformers must also correspond to and be the same as those on the primary side with N turns.

The energy flow can also be reversed, which means that a three-phase voltage system can also be generated from two 90 ° phase-shifted sinusoidal voltages (generated e.g. by a converter ). This possibility was discussed intensively in the 1960s, when power semiconductors were still very expensive. Such solutions were rejected because of falling semiconductor prices and the same prices for copper and iron .

In practice, when there is a load, there are asymmetries in the amounts of the stresses and their angles to one another.

See also

Edward Dean Adams Power Plant , where the Scott circuit was used in the opposite direction (2 → 3 phases) shortly after its invention in the 1890s.

Individual evidence

1. ↑ Harold C. Passer: The Electrical Manufacturers, 1875-1900 , Harvard, 1953, p. 315