Three windings on each side of a three-phase transformer, and two ways to connect them: a star, whose ends meet at a neutral point, or a delta, connected in series in a closed loop. Each connection has its own electrical character and proper place.
The fundamental relationships
| Connection | Voltage relationship | Current relationship | Neutral |
|---|---|---|---|
| Star Y | VL = √3 × VPh | IL = IPh | Available from the common point |
| Delta D | VL = VPh | IL = √3 × IPh | Not directly available |
A 400V low-voltage network with a star connection: the phase voltage (phase-to-neutral) = 400 ÷ √3 ≈ 230V — this is how a home gets two voltage levels from the same network.
When to use each connection?
- Star: where a neutral is needed (low-voltage distribution networks, unbalanced loads), and where it is useful that the phase voltage is lower than the line voltage (less insulation required per winding at high voltages).
- Delta: where third-harmonic trapping and waveform improvement are desired, on the medium-voltage side of distribution transformers, and for balanced loads.
- Combining both (Dyn) takes the advantages of each — see the meaning of Dyn11.
A precise point about power
The power of a three-phase transformer = √3 × VL × IL regardless of connection type; star and delta distribute the voltage and current across the windings differently, but the resulting power is the same.
Sample answer: In a star connection, the line voltage equals √3 times the phase voltage, while the line current equals the phase current. Residential distribution needs it because the common point of the windings provides a neutral that gives the consumer two voltage levels (230V phase-to-neutral for household loads and 400V phase-to-phase for larger loads), and allows grounding and handling of unbalanced loads.
Memorizing the √3 relationships in a confused way: remember that √3 appears in the voltage for star and in the current for delta. Anchoring it with the 400/230V example is more useful than rote memorization.
Want to master electrical transformers step by step?
Follow trainer Fahad Refai's Electrical Transformers course — a practical walkthrough from the fundamentals to testing and reading catalogs.
Browse Fahad Refai's Courses