Construction and operation of synchronous generator

A three-phase synchronous generator (alternator) converts mechanical energy into three-phase AC electrical energy at a frequency fixed by its speed, $f = \dfrac{PN_s}{120}$.

Construction

           stator frame
     +------------------------+
     |  laminated stator core |
     |  with 3-ph armature    |
     |  winding in slots      |
     |      +----------+      |
     |      |  rotor   |      |  DC field via
     |      |  N    S  |------+--- slip rings
     |      +----------+      |  and brushes
     +------------------------+
          shaft <- prime mover

Stator (armature) – stationary part

• Frame (cast iron/welded steel) for support and ventilation.
• Core of 0.35–0.5 mm silicon-steel laminations with slots, to reduce iron losses.
• Three-phase, double-layer, distributed, short-pitched winding, displaced 120° electrical, usually star connected.

Rotor (field) – rotating part, excited by DC from an exciter through slip rings (or brushless exciter):

1. Salient-pole rotor: projecting poles on a spider, large diameter, short length, many poles; used for low-speed hydro turbines. Damper bars in pole faces.
2. Cylindrical (non-salient) rotor: smooth forged-steel cylinder, small diameter, long axial length, 2 or 4 poles; used for high-speed steam/gas turbines.

Why stationary armature, rotating field? High voltage (11 kV and above) armature is easier to insulate when stationary; output is taken directly without slip rings; only low-voltage DC (100–400 V) is fed through slip rings; the rotor is lighter and better for high speed; the stator can be cooled better.

Principle of operation

1. The field winding is excited with DC, producing alternate N and S poles on the rotor.
2. The prime mover drives the rotor at synchronous speed $N_s$.
3. The rotating magnetic field cuts the stationary armature conductors. By Faraday's law of electromagnetic induction, an emf is induced in them: $e = Blv$.
4. As N and S poles pass a conductor alternately, the emf alternates; one cycle is produced per pair of poles passing. Hence $f = PN_s/120$.
5. The three phase windings are 120° electrical apart in space, so three emfs equal in magnitude and 120° apart in time are produced.
6. The rms emf per phase is $E = 4.44\,k_p k_d f\Phi T$.
7. When a load is connected, current flows; the armature mmf (armature reaction) and the impedance drop change the terminal voltage, which is corrected by adjusting the field current.