3-phase synchronous generator construction

A three-phase synchronous generator (alternator) converts mechanical energy into three-phase a.c. electrical energy. The frequency is tied to the speed: f = PN/120.

Constructional details

  Stator (armature)               Rotor (field)
  +----------------+          Salient pole      Cylindrical
  | slots with 3ph |          (hydro, low       (turbo, high
  | winding on     |           speed)            speed)
  | laminated core |          ____              ______
  +----------------+         / N  \             | N    |
  Frame / end covers        |  S   |            |__S___|

1. Stator (armature): a hollow cylindrical frame carrying a laminated silicon-steel core (to reduce eddy-current loss) with slots. A three-phase winding, placed 120° apart in the slots, delivers the output at high voltage. The armature is stationary so that high voltages (11 kV or more) can be taken out without slip rings, and it is easier to insulate and cool.
2. Rotor (field system): carries the d.c. excited field winding and has two types:
   • Salient-pole: projecting poles, many poles, large diameter and short axial length; speed 100 to 1500 rpm; used with hydro and diesel turbines; has damper winding.
   • Cylindrical (non-salient, turbo): smooth cylinder with distributed slotted winding, 2 or 4 poles, small diameter and long axial length; speed 1500 to 3000 rpm; used with steam and gas turbines.
3. Exciter: a small d.c. generator on the same shaft (or a static rectifier system) supplies the field current through two slip rings and brushes.
4. Other parts: bearings, cooling system (air or hydrogen), and a prime mover (turbine) to rotate the shaft.

Working principle

Faraday's law of electromagnetic induction is used.

1. The field winding is excited by d.c., creating alternate N and S poles on the rotor.
2. The prime mover rotates the rotor at synchronous speed N_s. The rotating flux cuts the stationary armature conductors.
3. An emf is induced in each phase winding. Because the three phase windings are 120° apart in space, the three emfs are 120° apart in time: balanced three-phase voltage.
4. Frequency is f = PN_s/120 Hz. For 50 Hz, a 4-pole machine runs at 1500 rpm, and a 2-pole machine at 3000 rpm.
5. The rms emf per phase is E = 4.44 f Φ T_ph K_w volts (Φ = flux per pole, T_ph = turns per phase, K_w = winding factor).
6. When a load is connected, armature current flows and creates its own rotating mmf at the same speed. This armature reaction and the winding impedance reduce the terminal voltage below the induced emf: V = E − I_a (R_a + jX_s).