11.2 DC Generators — Construction & Operation

11.2 DC Generators — Construction & Operation

Quick Answer: A DC generator is driven mechanically; armature conductors cut field flux → induced EMF → brushes deliver DC via the commutator. Output rises with Φ, speed, and armature conductors. Reverse field or rotation → reverse polarity. Armature reaction distorts the main field under load; interpoles help keep commutation clean.

Topic 3.12 expects you to name generator parts, say what each does, and reason about what changes voltage magnitude and polarity. Aircraft DC generators (and the generate mode of starter-generators) feed buses and charge batteries — wrong polarity or poor commutation is a maintenance and safety issue.

Construction and Purpose of Components

Worked walk-through. Engine accessory gearbox turns the armature. Field coils (excited from residual magnetism then built up, or from a separate excitation source / regulated self-excitation scheme depending on design) establish N and S poles. Conductors under a north pole move one way relative to flux; Fleming’s right-hand rule gives induced current sense. Commutator segments pass under brushes so the brush that should stay positive remains positive as coils rotate.

Self-excitation note (Level 2)

Many DC generators use self-excitation: a fraction of generated output feeds the field. Build-up needs a little residual magnetism in the pole iron. If residual magnetism is lost (shock, reverse magnetisation, long storage), the generator may not build voltage until flashed or remagnetised per maintenance data — a classic practical failure mode linked to this construction.

Factors Affecting Output (Magnitude)

Induced EMF in the armature depends on how much flux is cut how fast by how many conductors. Teaching form used in Module 3:

E ∝ Φ × N × Z (qualitative / proportional form)

More precisely, generated EMF relates to flux per pole, speed, and the armature winding constants (conductors, paths). For exam reasoning:

Terminal voltage of a loaded generator is less than generated EMF by armature resistance drop (and brush drop):

V = E − Ia Ra (simplified)

Worked example 1. Generated EMF E = 30 V, armature current Ia = 50 A, Ra = 0.04 Ω. IaRa = 2.0 V → V ≈ 28 V at the terminals (idealised). That is why a generator must generate slightly above bus voltage to feed a 28 V system under load.

Worked example 2 — speed. If flux is constant and speed falls to 80% of rated, generated EMF falls to about 80% of rated (before regulator action). A voltage regulator typically adjusts field current to hold V nearly constant over a speed/load range — Module 3 wants the open-loop proportional idea first.

Factors Affecting Direction of Current / Polarity

From Fleming’s right-hand rule, induced current direction depends on field direction and motion direction. Therefore:

Worked scenario. An aircraft DC generator was reassembled with incorrect field lead polarity or was flash-magnetised the wrong way. It may build voltage with reversed polarity relative to the bus — paralleling or battery connection then risks heavy reverse current. Maintenance manuals specify polarity checks and flashing procedures for a reason.

Residual magnetism sets the initial field sense for self-excited build-up; flashing with the correct battery polarity restores the intended residual direction.

Armature Reaction (Brief, Module 3 Standard)

When the generator delivers load current, the armature windings carry current and produce their own magnetic field. That armature field crosses the main field and:

1. Distorts the flux distribution under the pole faces (crowding flux toward one pole tip).
2. Demagnetises slightly in places (weakening useful flux).
3. Shifts the Magnetic Neutral Axis (MNA) from the Geometric Neutral Axis (GNA).

Brushes are normally set near the neutral axis for sparkless commutation. If the MNA shifts with load and brushes stay fixed, coils shorted by the brush may still have EMF → sparking.

Worked qualitative example. Light load → little armature MMF → small MNA shift → good commutation. Heavy load → large armature current → larger distortion → more sparking unless design counters it.

Interpoles (Commutating Poles)

Interpoles (also called commutating poles) are small poles placed on the geometric neutral axis, in series with the armature, wound so their flux opposes armature reaction in the commutating zone and helps reverse coil current during commutation.

Brush lead (forward/backward shift) was an older remedy; interpoles are the Module 3 “modern machine” answer for load-dependent correction.

Generator Operation Summary Table

Exam Focus Checklist

1. Name yoke, field, armature, commutator, brushes and state each purpose.
2. State that output EMF depends on flux and speed (and winding constants).
3. Predict polarity change when field or rotation reverses.
4. Define armature reaction and why interpoles help.

With generator construction and behaviour clear, §11.3 turns to DC motor types — series, shunt, compound — power/torque/speed/direction factors, and aircraft starter-generators.