1. Overview

A Scott-T transformer is a special type of transformer arrangement used to convert three-phase electric power into two-phase power, and vice versa. It is named after Charles F. Scott, who introduced this method for phase conversion.

2. Applications

• Power supply to two-phase systems (e.g., older industrial machinery)
• Traction systems (railways with two-phase motors)
• Balancing unbalanced loads
• Interfacing two-phase legacy systems with modern three-phase grids

3. Basic Configuration

A Scott-T transformer uses two single-phase transformers:

• Main transformer
• Teaser transformer

Connection Details:

• The main transformer is connected between two phases (e.g., A and C) of a three-phase system.
• The teaser transformer is connected from the third phase (B) to a tap on the main transformer winding (typically at 86.6% or √3/2 of the winding).

Voltage Ratios:

• The teaser transformer's primary is 86.6% of the line voltage.
• The teaser’s secondary must be equal in voltage to the main transformer’s secondary to ensure balanced two-phase output.

4. Technical Specifications

5. Vector Representation

• The two-phase outputs are electrically 90° out of phase, forming a balanced two-phase system.
• The currents in the three-phase input remain balanced, if the two-phase loads are equal.

6. Design Considerations

• Magnetic coupling between windings is critical for maintaining phase relationships.
• Load balancing is important to avoid unbalanced current on the three-phase side.
• Proper tapping on the main transformer ensures correct voltage and phase angle.

7. Advantages

• Simple and cost-effective method for phase conversion.
• Allows integration of two-phase systems with standard three-phase grids.
• Provides balanced loading on the three-phase supply when used correctly.

8. Limitations

• Limited to specific use cases where two-phase supply is needed.
• Requires careful design to maintain voltage balance and phase angle.
• Less commonly used today due to the decline of two-phase systems.

9. Applications in Modern Systems

• Traction systems (where legacy infrastructure still exists)
• Test benches or laboratory setups needing two-phase power
• HVDC converter stations (in some configurations)
• Restoration of vintage industrial machines that use two-phase motors

10. Standards

Scott-T transformers generally follow the same core transformer standards:

• IEC 60076 (Power Transformers)
• IEEE C57 series
• IS 2026 (Indian Standard for Power Transformers)