--- title: "Scott connection for three-phase to two-phase" subject: "Electrical Machines I" chapter: 2 key: "semester_4/electrical_machines_i:scott-connection" source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/scott-connection site: BCT Engineering --- # Scott connection for three-phase to two-phase Electrical Machines I · Chapter 2: Transformer - Asked in: 2079 Chaitra, 6 marks - Video: [Scott Connection of Transformers (Full Concept) | Converting 3-Phase to 2-Phase Supply |](https://www.youtube.com/watch?v=mjByPB7cm4k) — Easy Electrical - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/scott-connection - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-2 ## Question Explain with the help of connection and phasor diagrams, how Scott connections are used to obtain two-phase supply from three-phase mains. ## Short answer Scott connection uses a main transformer with centre-tapped primary across two lines and a teaser transformer with 0.866 N1 winding from the third line to the tap. Secondary voltages are equal and 90 degrees apart, giving a balanced two-phase supply from three-phase mains. ## Answer The **Scott (T) connection** uses two single-phase transformers to convert a balanced three-phase supply into a balanced two-phase supply (two voltages equal in magnitude and 90° apart), or the reverse. It is used for two-phase loads such as electric furnaces and for traction supply. #### Connections - **Main transformer:** primary of $N_1$ turns connected across two lines B and C, with a **centre tap D**. - **Teaser transformer:** primary of $\frac{\sqrt3}{2}N_1$ turns (86.6%), connected between line A and the centre tap D. - Both secondaries have the same number of turns $N_2$ and feed the two phases separately. ```text A | | Teaser primary | (0.866 N1 turns) | B -------D------- C N1/2 N1/2 Main primary (N1 turns, centre tapped at D) Secondaries (N2 turns each): Teaser sec -> V_T (phase 1) Main sec -> V_M (phase 2) ``` #### Voltage relations (phasor diagram) Let the line voltage be $V_L$. ```text A /|\ / | \ / | \ VAD = 0.866 VL / | \ (perpendicular / | \ to BC) B-----D-----C VL/2 VL/2 ``` - Voltage across the main primary: $V_{BC} = V_L$. - D is the midpoint of BC, so $V_{AD}$ is the height of the equilateral triangle: $$ V_{AD} = \sqrt{V_L^2 - \left(\frac{V_L}{2}\right)^2} = \frac{\sqrt3}{2}V_L = 0.866\,V_L $$ and $V_{AD}$ is **at 90°** to $V_{BC}$. - Volts per turn on the teaser: $\frac{0.866V_L}{0.866N_1} = \frac{V_L}{N_1}$, the same as on the main. Therefore the secondary voltages are $$ V_{M} = \frac{N_2}{N_1}V_L, \qquad V_{T} = \frac{N_2}{0.866N_1}(0.866V_L) = \frac{N_2}{N_1}V_L $$ They are **equal in magnitude and 90° apart** — a balanced two-phase supply. #### Current relations (balanced two-phase load) - Teaser primary current $I_A = \frac{2}{\sqrt3}\frac{N_2}{N_1}I_2 = 1.155\,\frac{N_2}{N_1} I_2$, flowing in line A. - It divides equally at D into the two halves of the main primary, where it combines with the main load current; the resulting line currents $I_B$ and $I_C$ are equal in size to $I_A$ and 120° apart, so the three-phase side sees a **balanced load**. #### Points to note - If the neutral of the three-phase side is wanted, it is at the point on the teaser one-third of its length from D (i.e. at $\frac{N_1}{2\sqrt3}$ turns from D). - The two transformers can be identical units if each has 50% and 86.6% tappings, so they are interchangeable. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does a Scott connection achieve? A. Gives a three-phase to six-phase output B. Raises voltage by sqrt(3) C. Converts three-phase supply into balanced two-phase supply using two transformers D. Converts single-phase to three-phase using three transformers Answer: C. Two single-phase transformers convert balanced three-phase to balanced two-phase (equal magnitude, 90 deg apart). (Key point: Purpose of Scott connection) 2. How are the primary turns of the teaser related to the main transformer? A. Teaser has 0.866 N1 turns, connected from line A to the centre tap D of the main B. Teaser has N1 turns across AB C. Teaser has N1/2 turns D. Teaser has 1.155 N1 turns, connected across BC Answer: A. Teaser primary has (sqrt3/2)N1 turns between A and the main's centre tap, because VAD = 0.866 VL. (Key point: Turns of teaser) 3. Why are the two secondary voltages 90 deg apart? A. V_AD is perpendicular to V_BC as D is the midpoint of BC in an equilateral triangle B. Because of the sqrt(3) turns ratio on the secondary C. The teaser has more turns D. Both secondaries are wound on the same core Answer: A. The altitude AD of the equilateral triangle is perpendicular to BC, so teaser and main voltages are 90 deg apart. (Key point: Why 90 degrees) 4. Why are the two secondary voltages equal in magnitude? A. Both secondaries have different turns B. Volts per turn is the same on teaser and main, and both secondaries have N2 turns C. The secondaries are connected in series D. The supply is single phase Answer: B. Teaser volts per turn = 0.866VL/0.866N1 = VL/N1, same as main, so with equal N2 both give (N2/N1)VL. (Key point: Equal secondary magnitudes) ## Related in Electrical Machines I - [Three-phase transformer connections](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Athree-phase-transformer-connections) — Electrical Machines I, chapter 2 - [Fluxes in three-limb core with two air gaps](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Athree-limb-core-two-airgaps-300turns) — Electrical Machines I, chapter 1 ## Related in other subjects - [Three-phase connections and series RC phasor](https://www.bctengineer.com/question.md?key=semester_1%2Ffundamental_of_electrical_and_electronics_engineering%3Athree-phase-connections-series-rc) — Fundamental of Electrical and Electronics Engineering, chapter 3 - [Three phase transformer connections and applications](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Athree-phase-transformer-connections) — Electrical Machines, chapter 2 - [Methods of three-phase connection](https://www.bctengineer.com/question.md?key=semester_1%2Ffundamental_of_electrical_and_electronics_engineering%3Athree-phase-connection-methods) — Fundamental of Electrical and Electronics Engineering, chapter 3 - [Transformer on load, phasor diagrams](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Atransformer-phasor-resistive-capacitive) — Electrical Machine, chapter 2