Synchronous Machines

Like other electrical machines, synchronous machines can be operated as either generators or motors. We will concentrate on synchronous generators, then adapt the theory for synchronous machines.

The prinicples of operation of synchronous generators are relatively simple. They operate like the simple 3-phase generator discussed in the fundamentals section of the course.

Principles of operation

  1. Create a magnetic field on the rotor of an electric machine
  2. Apply an external driving force
  3. As the rotor rotates, a voltage is induced in windings on the stator
  4. The frequency of the induced voltage will be synchronised with the speed of rotation
    \[ n_s=\frac{120 f_e}{p} \]

Synchronous machine construction

A synchronous machine has two mechanical parts: a rotor and a stator. There are also two electrical parts to the machine: a field source and an armature winding. These basic fundamentals of an electric machine are like those for a DC machine, with one significant difference: The field source of a synchronous machine is on the rotor, the armature winding of a synchronous machine is on the stator. Like DC machines, the field source creates a magnetic field, the armature winding has a voltage induced in it by the field. Also like DC machines, the field can be produced using either a field winding or by using permanent magnets. PM (permanent magnet) machines are common in small sizes, whilst large machines are usually made with field windings. (There are some exceptions to this rule, e.g. multi-MW PM motors are built for ship propulsion and direct drive wind turbine generators)

In this part of the course, we will concentrate on wound-field synchronous machines.


  • The field winding is on the rotor
  • The armature winding is on the stator
  • There is sometimes a damper or armortisseur winding on the rotor
  • The external driving force (e.g. steam or hydro turbine, diesel generator, jet engine) is called the prime mover


The main parts of a synchronous generator are identified: rotor (field); stator (armature); prime mover ; exciter