14.3 Three-Phase Transformers & Autotransformers

14.3 Three-Phase Transformers & Autotransformers

Quick Answer: Three-phase banks use star or delta connections — remember √3 factors for line vs phase. Balanced power P = √3 VL IL (unity pf). An autotransformer shares one winding (weight save, no isolation).

Sections 14.1–14.2 treated the single-phase two-winding unit. Syllabus 3.15 also requires three-phase transformer relationships and the special case of the autotransformer — both appear in aircraft and ground support equipment.

Three-Phase Transformer Arrangements

A three-phase transformer may be:

1. Three separate single-phase transformers connected in a three-phase bank, or
2. A three-limb (or five-limb) three-phase core with three primary and three secondary windings on a common magnetic structure.

Primary and secondary sides are each connected in star (wye, Y) or delta (Δ). Combinations include Y–Y, Δ–Δ, Y–Δ, Δ–Y. Each combination sets the relationship between line quantities (measured between lines) and phase quantities (across one winding).

Star (wye) connection — per side

Each winding sees phase voltage; line-to-line is higher by √3. Neutral may be available at the star point for line-to-neutral loads (e.g., 115 V from 200 V line-to-line class systems — exact aircraft figures follow the specific system).

Delta connection — per side

Each winding sees full line voltage; line current is √3 times winding (phase) current because line current is the phasor combination of two winding currents.

Memory aid

• Star: voltage gets the √3 (line > phase); currents equal.
• Delta: current gets the √3 (line > phase); voltages equal.

Worked example 1 — star secondary. Balanced star secondary with Vph = 115 V. Line voltage VL = 115 × √3 ≈ 199 V (often called ~200 V class). If each phase winding current Iph = 10 A, then IL = 10 A.

Worked example 2 — delta secondary. Same Vph = 115 V in delta → VL = 115 V. If Iph = 10 A, IL = 10 × √3 ≈ 17.3 A.

Three-Phase Power

For a balanced three-phase load at unity power factor:

P = √3 × VL × IL

Also:

P = 3 × Vph × Iph

(with Vph, Iph the phase values of that side). At power factor cos φ:

P = √3 VL IL cos φ

Module 3 often uses unity pf unless stated.

Worked example 3 — power. VL = 200 V, IL = 10 A, unity pf.

P = √3 × 200 × 10 ≈ 3464 W ≈ 3.46 kW.

Worked example 4 — from phase. Three windings each 115 V, 10 A, unity pf (star or delta internal accounting).

P = 3 × 115 × 10 = 3450 W (matches √3 VL IL within rounding when VL/IL are consistent with the connection).

Ideal three-phase transformer banks still obey per-phase turns-ratio ideas: each phase pair transforms voltage by Np/Ns; total power balances primary↔secondary aside from losses.

Why Three-Phase on Aircraft / GSE

Large aircraft electrical systems commonly use three-phase 400 Hz generation and distribution for smoother power delivery and efficient high-power machines. Ground power units and hangar supplies may present three-phase as well. Transformers (or autotransformers) match voltage levels between ground and aircraft or between buses. Single-phase transformers remain ubiquitous for avionics and lighting secondaries derived from one phase or from dedicated windings.

Autotransformers — Construction

An autotransformer has one continuous winding on a laminated core, with one or more taps. Part of the winding is common to both “primary” and “secondary” circuits.

Voltage relationship

If the full winding has Nfull turns across the supply Vp, and the tap uses Ntap turns for Vs (or vice versa depending on step-up/step-down tap):

Vs / Vp = Ntap / Nfull (for the usual proportional tap from a common end)

Part of the power is transformed electromagnetically; part is conducted directly through the shared winding segment. That conductive path is why less copper is needed for a given throughput when the voltage ratio is close to 1:1 — a major weight win.

Worked example 5 — autotransformer tap. Full winding 115 V across 460 turns. Tap at 112 turns from common end.

Vs = 115 × (112/460) ≈ 28 V.

Worked example 6 — current idea (ideal, step-down). Vs = 28 V, Is = 20 A, Vp = 115 V. Ideal Pin ≈ Pout → Ip ≈ (28×20)/115 ≈ 4.9 A. The common winding carries the difference of the larger and smaller currents (Is − Ip in the usual step-down auto case), which is why conductor rating of the common section differs from a fully isolated design — recognise the idea even if a stem only asks isolation or weight.

Advantages of Autotransformers

1. Weight and size — less copper and often smaller core for the same throughput when ratio is not extreme.
2. Cost — fewer materials.
3. Efficiency — lower winding resistance losses for comparable duty.
4. Smooth variable ratio — continuously variable autotransformers (Variacs) useful in shops and test benches.

Disadvantages — Critical for Safety

1. No electrical isolation — primary and secondary share conductors. A fault or open in the common portion can place full primary voltage on the secondary load.
2. Not for isolation-required loads — sensitive avionics, personnel-safety isolation, and many certification contexts need a true two-winding isolating transformer.
3. Fault propagation — primary-side disturbances couple more directly to the secondary.
4. Limited use on aircraft for critical low-voltage supplies — Module 3 / training texts emphasise restriction to applications where isolation is unnecessary and weight saving matters.

Exam contrast: Laminations still apply (eddy currents). Turns-ratio thinking still applies on the tapped turns. The unique autotransformer penalty is loss of isolation, not “higher eddy loss by definition.”

Three-Phase Autotransformer Banks

Three autotransformers can form a three-phase bank (e.g., for modest voltage buck/boost between buses). Line/phase √3 rules still describe the system voltages and currents; each phase unit remains a single-winding tapped device without isolation. Ground-power and industrial buck-boost applications are common; treat aircraft use with the same isolation caution.

Practical Selection Snapshot

Closing Syllabus Map for 3.15

Know √3 line/phase rules cold, compute balanced three-phase power, and state without hesitation that autotransformers do not isolate. That trio, plus the turns-ratio chain from 14.2, covers the Module 3 transformer exam surface.