15.3 Star/Delta Connections & Permanent Magnet Generators
Key Takeaways
- In star (wye) connection, line voltage = √3 × phase voltage and line current = phase current; a neutral point is available for line-to-neutral loads
- In delta connection, line voltage = phase voltage and line current = √3 × phase current; no internal neutral from the winding mesh itself
- Star suits higher line voltage from a given winding insulation and single-phase line-to-neutral loads; delta suits certain motor and transformer applications and circulating-current considerations
- Permanent Magnet Generators (PMGs) use rotor permanent magnets instead of wound-field excitation — compact, brushless field source, output depends strongly on speed
- Aircraft PMGs commonly provide GCU/exciter control power or backup electrical sources; they do not replace understanding of wound-field main generators
15.3 Star/Delta Connections & Permanent Magnet Generators
Quick Answer: Star (Y): V_L = √3 V_ph, I_L = I_ph, neutral available. Delta (Δ): V_L = V_ph, I_L = √3 I_ph, no winding neutral. PMG: rotor permanent magnets induce stator AC without field windings — used on aircraft for GCU/excitation power and some backup roles; output tracks speed.
Section 15.2 established three-phase generation with 120° displacement. Topic 3.17 also requires how those three windings are interconnected and what a permanent magnet generator is. Wrong star/delta voltage factors are classic Module 3 traps; PMG questions test whether you know excitation comes from magnets, not a DC field winding.
Three-Phase Winding Ends
Each phase winding has two ends. How you join the six ends decides star vs delta.
| Connection | How windings join | External terminals |
|---|---|---|
| Star (wye, Y) | One end of each phase joined at a common neutral point N; free ends are A, B, C | Three lines + optional neutral |
| Delta (mesh, Δ) | Windings joined A-to-B, B-to-C, C-to-A in a closed triangle; corners are the lines | Three lines (no internal N) |
Phase quantity = voltage across or current in one winding. Line quantity = voltage between two line conductors or current in a line conductor.
Star (Wye) Connection — Relationships
For a balanced star system:
V_L = √3 × V_ph
I_L = I_ph
| Quantity | Relation | Memory |
|---|---|---|
| Line voltage (A to B, etc.) | √3 times phase (A to N) | Lines “see” two phases with 120° → √3 |
| Line current | Equals phase current | Each line feeds one winding |
| Neutral current (balanced) | ≈ 0 | Three phase currents cancel |
Worked example 1 — aircraft-style numbers. Phase (line-to-neutral) voltage V_ph = 115 V. Star connected.
V_L = 115 × √3 ≈ 115 × 1.732 ≈ 199 V ≈ 200 V line-to-line.
That is why aircraft AC is often described as 115/200 V three-phase: 115 V to neutral, ~200 V between lines on a star system.
Worked example 2 — current. Each phase winding carries 20 A. In star, each line also carries 20 A (I_L = I_ph).
Advantages and uses of star
| Advantage / use | Why it matters |
|---|---|
| Higher line voltage from a given phase winding insulation rating | V_L = √3 V_ph — good for transmission/distribution efficiency |
| Neutral available | Supplies single-phase line-to-neutral loads (lighting, many LRUs) at V_ph while three-phase loads use V_L |
| Phase voltage lower than line | Each winding needs insulation only for V_ph, not V_L |
| Generator stator default on many aircraft | Matches 115 V equipment from neutral and ~200 V three-phase loads |
| Unbalanced single-phase loads | Neutral carries the imbalance current — must be sized and connected correctly |
Exam phrasing. “Line voltage is greater than phase voltage by √3” → think star. “Line current equals phase current” → star.
Delta Connection — Relationships
For a balanced delta system:
V_L = V_ph
I_L = √3 × I_ph
| Quantity | Relation | Memory |
|---|---|---|
| Line voltage | Equals phase winding voltage | Each line-to-line pair is directly across one winding |
| Line current | √3 times phase (winding) current | Each line is fed by two windings with 120° current difference |
| Neutral | Not inherent in the delta mesh | Need a separate transformer or artificial neutral if required |
Worked example 3. A delta winding has V_ph = 200 V and I_ph = 10 A.
V_L = 200 V; I_L = 10 × √3 ≈ 17.3 A.
Worked example 4 — contrast. Same 115 V phase windings in star gave V_L ≈ 200 V. If those same windings were reconnectable in delta (special machines), V_L would equal the winding voltage (115 V) while currents would redistribute—always check which connection the stem states before applying √3.
Advantages and uses of delta
| Advantage / use | Why it matters |
|---|---|
| No internal floating neutral required for pure three-phase loads | Simple three-wire feed |
| Line current > phase current by √3 | Windings can be designed for lower conductor current than the lines in some ratings |
| Motor stator windings often delta or star-delta started | Industrial and some aircraft motor contexts (topic 3.18) |
| Transformer banks | Delta primary/secondary combinations manage harmonics and grounding schemes |
| Circulating third-harmonic currents | Can remain inside a delta — a power-engineering detail sometimes alluded to in advanced notes |
Limitations. No ready neutral for line-to-neutral single-phase loads unless an external source of neutral is provided. Ground-fault behaviour differs from a grounded-star system—awareness level for Module 3, detailed protection schemes later in type training.
Star vs Delta Comparison Table
| Feature | Star (Y) | Delta (Δ) |
|---|---|---|
| V_L vs V_ph | V_L = √3 V_ph | V_L = V_ph |
| I_L vs I_ph | I_L = I_ph | I_L = √3 I_ph |
| Neutral | Yes (common point) | No inherent neutral |
| Typical aircraft generator label | 115/200 V | Less common as the named bus pair |
| Good for mixed 1φ + 3φ loads | Yes (with neutral) | Poor without extra hardware |
| √3 multiplies… | Voltage (line) | Current (line) |
Memory rhyme: Star stretches voltage; delta stretches current.
Permanent Magnet Generators (PMGs)
A permanent magnet generator is an alternator whose rotor field is produced by permanent magnets rather than by a wound field supplied with DC.
| Aspect | Wound-field generator | PMG |
|---|---|---|
| Rotor field source | DC through field windings (or brushless exciter feeding a wound rotor) | Permanent magnets |
| Field control | Vary field current → regulate voltage | Flux largely fixed by magnets; little/no direct field-current trim |
| Brushes for field | May use slip rings or brushless exciter | Typically brushless (magnets on rotor) |
| Output vs speed | Voltage also depends on speed; GCU trims field | Voltage strongly tied to speed (and load); regulation architecture differs |
| Complexity | Exciter, brushes or rotating rectifiers | Simpler rotor magnetically; magnets must survive environment |
Operating principle. Same Faraday induction: rotating magnetic field from the magnets sweeps the stator windings → AC EMF at frequency set by speed and pole count. No commutator; output is AC (often rectified if DC is needed downstream).
Worked concept. If PMG speed doubles and flux is constant, induced EMF roughly doubles and frequency doubles (e ∝ Φω; f ∝ n). Wound-field main generators can reduce field current to hold voltage when speed tries to rise (within system design). A pure PMG cannot “turn down” magnet flux the same way—designers size magnets and use regulators/loads accordingly.
Aircraft relevance of PMGs
| Role | Why PMGs appear |
|---|---|
| Excitation / GCU power | Many integrated drive generators and brushless generators include a small PMG on the shaft to power the generator control unit and exciter field — self-contained control power once rotating |
| Backup / emergency generation | Some aircraft use PMG-based or PM alternator emergency sources (design-specific) for essential buses |
| Reliability / simplicity | No field winding open-circuit on the PM rotor; fewer brushes in the PMG section |
| Permanent magnet starter-generators / more-electric architectures | Emerging and type-specific — Module 3 needs the principle, not every OEM diagram |
Maintenance awareness (Level 2 flavour). Permanent magnets are strong; mishandling rotors can damage magnets or attract tools violently. High temperature can weaken certain magnet materials (demagnetisation risk)—respect OEM limits. Foreign object debris near an open PM rotor is hazardous.
Do not confuse:
- Main AC generator (usually wound revolving field with GCU) vs shaft PMG used to feed that GCU/exciter.
- PMG AC output vs battery DC — PMG output is generated AC (often conditioned/rectified before use).
- PM DC motors elsewhere in the syllabus vs PM generators here — same magnet idea, opposite energy flow.
Putting Star/Delta and PMG Together on Aircraft
A typical large-aircraft picture:
- Engine drives IDG/generator — revolving wound field on the main machine.
- Stator windings are star-connected to provide 115 V phase and ~200 V line for the AC distribution system.
- A coaxial or integral PMG supplies control power so the GCU can excite and regulate the main field once the shaft turns.
- Frequency ~400 Hz when speed is controlled (CSD/IDG) or follows engine speed on variable-frequency systems.
Module 3 may ask any piece of that chain in isolation: √3 factors, neutral purpose, or “what provides the magnetic field in a PMG?”
Common Exam Traps
| Trap | Correction |
|---|---|
| Applying V_L = √3 V_ph to delta | That √3 voltage rule is star |
| Applying I_L = √3 I_ph to star | That √3 current rule is delta |
| Saying PMG voltage is adjusted by field rheostat on the PM rotor | PM rotor has no field winding to rheostat |
| Claiming star has no neutral | Star’s defining feature is the neutral point |
| Mixing 90° (two-phase) with star/delta maths | Star/delta are connection topologies for three-phase windings |
Section Synthesis
| Topic | Lock this |
|---|---|
| Star | V_L = √3 V_ph, I_L = I_ph, neutral, 115/200 V story |
| Delta | V_L = V_ph, I_L = √3 I_ph, three-wire, motors/transformers |
| PMG | Magnets on rotor, brushless field, speed-linked output, aircraft GCU/backup roles |
Master the √3 table cold, then explain why a PMG is valuable beside a wound-field main alternator. Topic 3.18 AC Motors reuses three-phase rotating-field ideas in the motor direction.
For a balanced star-connected three-phase generator, which relationship is correct?
An aircraft AC system is described as 115/200 V three-phase. This most strongly indicates:
What is the primary source of magnetic field in a permanent magnet generator (PMG)?
On many large aircraft integrated generators, a small shaft-driven PMG is commonly used to: