AC generator (alternator) - construction and working

NOTE: This article is about alternator. The term 'AC generator' commonly refers to 'alternator'. Read about 'Induction Generators' here.

An alternator is an electrical machine which converts mechanical energy into alternating electric energy. They are also known as synchronous generators.

How does an AC generator work?

The working principle of an alternator or AC generator is similar to the basic working principle of a DC generator.
working of alternator or AC generator

Above figure helps you understanding how an alternator or AC generator works.  According to the Faraday's law of electromagnetic induction, whenever a conductor moves in a magnetic field EMF gets induced across the conductor. If the close path is provided to the conductor, induced emf causes current to flow in the circuit.
Now, see the above figure. Let the conductor coil ABCD is placed in a magnetic field. The direction of magnetic flux will be form N pole to S pole. The coil is connected to slip rings, and the load is connected through brushes resting on the slip rings.
Now, consider the case 1 from above figure. The coil is rotating clockwise, in this case the direction of induced current can be given by Fleming's right hand rule, and it will be along A-B-C-D.
As the coil is rotating clockwise, after half of the time period, the position of the coil will be as in second case of above figure. In this case, the direction of the induced current according to Fleming's right hand rule will be along D-C-B-A. It shows that, the direction of the current changes after half of the time period, that means we get an alternating current.

Construction of AC generator (alternator)

alternator (AC generator) salient pole construction
Salient pole type alternator

Main parts of the alternator, obviously, consists of stator and rotor. But, the unlike other machines, in most of the alternators, field exciters are rotating and the armature coil is stationary.

Stator: Unlike in DC machine stator of an alternator is not meant to serve path for magnetic flux. Instead, the stator is used for holding armature winding. The stator core is made up of lamination of steel alloys or magnetic iron, to minimize the eddy current losses.

Why armature winding is stationary in an alternator?

  • At high voltages, it easier to insulate stationary armature winding, which may be as high as 30 kV or more.
  • The high voltage output can be directly taken out from the stationary armature. Whereas, for a rotary armature, there will be large brush contact drop at higher voltages, also the sparking at the brush surface will occur.
  • Field exciter winding is placed in rotor, and the low dc voltage can be transferred safely.
  • The armature winding can be braced well, so as to prevent deformation caused by the high centrifugal force.
Rotor:  There are two types of rotor used in an AC generator / alternator:
(i) Salient and (ii) Cylindrical type
  1. Salient pole type: Salient pole type rotor is used in low and medium speed alternators. Construction of AC generator of salient pole type rotor is shown in the figure above. This type of rotor consists of large number of projected poles (called salient poles), bolted on a magnetic wheel. These poles are also laminated to minimize the eddy current losses. Alternators featuring this type of rotor are large in diameters and short in axial length.
  2. Cylindrical type: Cylindrical type rotors are used in high speed alternators, especially in turbo alternators. This type of rotor consists of a smooth and solid steel cylinder havingg slots along its outer periphery. Field windings are placed in these slots.
The DC suppy is given to the rotor winding through the slip rings and and brushes arrangement.

Connecting an alternator in grid is called as synchronization of alternator, read more about it at the link.