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Alternator armature reaction
Alternator armature reaction
How the load's power factor sets the angle between F_a and F_f — magnetising, demagnetising, or cross-magnetising.
Step 1 — No-load alternator: rotor produces F_f, induces E_f in stator
0.55×
cos θ —
∠(Fa, Ff) —
effect —
Reference notes
Use Next → on the narrator above to step through six load conditions, watching the armature-reaction MMF Fa swing relative to the rotor field Ff.
Two rotating MMFs — Ff and Fa
In a synchronous alternator, two MMFs coexist in the air gap:
- Ff — produced by DC current in the rotor field winding. Magnitude is constant; it rotates with the rotor at synchronous speed ns.
- Fa — the armature-reaction MMF, produced by the three-phase load currents in the stator. This is exactly the rotating field from the previous lesson, but at load-current magnitude. It also rotates at synchronous speed.
Because both rotate at the same speed, they are stationary with respect to each other. What changes with load is the angle between them, ψ — and that angle depends on the load power factor.
How load PF sets the angle between Fa and Ff
| Load PF | Angle ψ | Effect on field |
|---|---|---|
| Unity (resistive) | 90° | Pure cross-magnetising — distorts the flux density, doesn't weaken or strengthen |
| Zero, lagging (inductive) | 180° | Pure demagnetising — directly opposes Ff |
| Zero, leading (capacitive) | 0° | Pure magnetising — aligned with Ff |
| General lagging PF, angle θ behind unity | 90° + θ | Partly cross, partly demag |
| General leading PF, angle θ ahead of unity | 90° − θ | Partly cross, partly mag |
Why utilities care
- Lagging loads (most industrial loads — motors, fluorescent lights) demagnetise the alternator → for the same terminal voltage, the field current has to be increased. This is the underlying reason for voltage regulation in alternators (next lesson).
- Leading loads (heavy capacitor banks, long lightly-loaded HVDC lines) magnetise → the terminal voltage can actually rise above the no-load EMF. Utilities sometimes deliberately under-excite alternators on long lightly-loaded transmission systems.
- Unity PF distorts but doesn't shift the magnetic axis significantly → minimal regulation, this is the "design point" for most alternators.
Mental model. Two MMFs spinning together in lockstep, separated by a load-PF-dependent angle. Everything about alternator regulation, exciter sizing, and reactive-power dispatch falls out of this one picture.
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