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AC synchronous generator (alternator) - construction and working

NOTE: This article is about alternators. The term 'AC generator' commonly refers to an AC Synchronous generator or alternator. Although, the term "AC generators" includes any type of electricity generator that generates Alternating Current. Read about 'Induction Generators' here.

Basically, an AC generator is an electrical machine that converts mechanical energy into electrical energy in the form of Alternating Current (AC). Basic principle behind the working of an AC synchronous generator is also Faraday's law of electrical induction, somewhat similar to working of a DC generator.

How does an AC synchronous generator or alternator work?

working of alternator or AC synchronous generator

The above figure illustrates how an alternator or AC synchronous generator work. According to Faraday's law of electromagnetic induction, whenever a conductor moves in a magnetic field,  EMF gets induced across the conductor. If a closed path is provided to the conductor, induced emf causes current to flow in the circuit.

Now, in the above figure, let the conductor coil A-B-C-D is placed in a magnetic field. Direction of the magnetic flux will be from N pole to S pole. The coil is connected to slip rings, and the load is connected through brushes that are resting on the slip rings.

Now, consider the case 1 from the 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, the position of the coil will be changed after half of the rotational period, as shown in the second case of the 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 every half of the rotational time period, that means we get an alternating current.

Construction of AC synchronous generator (alternator)

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

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

Stator: Unlike in a DC machine, the stator of an alternator is not meant to serve a 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 is easier to insulate the stationary armature winding, which may be as high as 11 kV or even more in some cases.
  • The generated 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 be a problem to look after.
  • If the field exciter winding is placed in the rotor, low voltage DC can be transferred safely to the exciter winding via slip-rings.
  • The armature winding can be braced well, to prevent deformation caused by high centrifugal force if it was in the rotor.
Rotor: There are two types of rotor used in an AC synchronous generator/alternator: (i) Salient and (ii) Cylindrical type
  1. Salient pole type: Salient pole type rotor is used in low and medium speed alternators. The construction of an AC synchronous generator of a salient pole type rotor is shown in the figure above. This type of rotor consists of a 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 having slots along its outer periphery. Field windings are placed in these slots.

A DC supply is given to the rotor winding through the 'slip-rings and brushes' arrangement.

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