The easiest way to transform three-phase voltages into two-phase voltages is with two conventional single-phase transformers. The first transformer is connected phase-to-neutral on the primary (three-phase) side and the second transformer is connected between the other two phases on the primary side. The secondary windings of the two transformers are then connected to the two-phase circuit. The phase-to-neutral primary voltage is 90out of phase with the phase-to-phase primary voltage, producing a two-phase voltage across the secondary windings. This simple connection, called the T connection, is shown in Figure below.
The main advantage of the T connection is that it uses transformers with standard primary and secondary voltages. The disadvantage of the T connection is that a balanced two-phase load still produces unbalanced three-phase currents; i.e., the phase currents in the three phase system do not have equal magnitudes, their phase angles are not 120 apart, and there is a considerable amount of neutral current that must be returned to the source.
In order to overcome the disadvantage of the T connection, the Scott connection uses two single-phase transformers of a special design to transform three-phase voltages and currents into two-phase voltages and currents. The first transformer, called the ‘‘main,’’ has a center-tapped primary winding connected to the three-phase circuit with the secondary winding connected to the two-phase circuit. It is vital that the two halves of the center-tapped primary winding are wound around the same core leg so that the ampere-turns of the two halves cancel out each other. The ends of the center-tapped main primary winding are connected to two of the phases of the three-phase circuit.
The second transformer, called the ‘‘teaser,’’ has one end of its primary winding connected to the third phase of the three-phase circuit and the other end connected to the center tap of the primary winding of the main. The Scott connection requires no primary neutral connection, so zero-sequence currents are blocked. The secondary windings of both the main and teaser transformers are connected to the two-phase circuit. The Scott connection is shown in Figure below for a two-phase, five-wire circuit, where both secondary windings are center-tapped and the center taps are connected to the neutral of the five wire circuit. Three-wire and four-wire configurations are also possible.
If the main transformer has a turns ratio of 1: 1, then the teaser transformer requires a turns ratio of 0.866:1 for balanced operation. The principle of operation of the Scott connection can be most easily seen by first applying a current to the teaser secondary windings, and then applying a current to the main secondary winding, calculating the primary currents separately and superimposing the results.
In Figure below the current distribution in a Scott group is shown for three different conditions. Figure (a) shows the current distribution when the teaser transformer only is loaded; Figure (b) shows the corresponding distribution when the secondary of the main transformer only is loaded;
Figure(c) is a phasor diagram of currents showing a combination of the conditions in the first two figures for the main transformer only.
Referring to Figure (a) it can be seen that the current in the teaser windings on the three-phase side divides into two equal parts on passing to the main transformer, these two parts being in opposite directions. If the two halves of the primary winding on the main transformer are wound in such a way that there is a minimum magnetic leakage between them, these two currents will balance one another, and the main transformer will offer very little impedance to the flow of current even though its secondary is open circuit.
If, however, the coupling between these two halves is loose, the main transformer will appear as a choke to the current of the teaser transformer. It can be seen that the Scott connection will operate as a fairly effective means of supplying a single-phase load from a three-phase supply provided the main transformer is wound with its primary halves closely coupled. This is best achieved by winding them as two concentric windings on the same limb of the core. With this arrangement the single-phase load is distributed between the three phases of the supply equally in two phases with double the current in the third phase. When used in this way no load is applied to the secondary of the main transformer.