Content – Energy sources
The generator converts mechanical energy into electrical energy by electromagnetic induction.
There are two main categories of electromagnetic generators:
- Dynamos or generators generating direct current (DC) (mainly for low voltage applications).
- Alternators generating alternating current (AC).
Direct current (DC): Flow of electric charge with a constant direction (constant polarity).
Alternating current (AC): Periodically reversed direction (alternating polarity) flow of electric charge. The reversed direction or alternating polarity follows a particular pattern, a sine wave. The reason why an AC generator outputs sine wave is that the voltage produced by the stationary coils due to the rotating magnet is proportional to the magnetic flux change rate perpendicular to the coils. The rate is highest when the magnet poles are close to the coils, and lower when the magnet poles are further away from the coils. The change in magnetic flux follows a sine wave pattern and therefore the voltage induces follows the same sine wave pattern.
AC generator – principle
DC generator – principle
For efficiency reasons most grid power generation is by 3 phase generators with three voltage sources separated by 120 degrees on the same shaft such that three sine shaped waves with same amplitude and frequency and a 120 degree phase shift are produced. The efficiency advantages of 3 phase systems are related to transmission, where a 3 phase system would require 25% less cable cross section than what would be reuired by a single phase system transmitting the same amount of energy. Also 3 phase electrical motors perform better than single phase AC motors.
The main components of an electrical generator are:
- The rotor
- The stator
- The shaft, to which the rotor and the commutator or the slip ring is fixed
- The commutator (DC generators)
- Slip ring (AC generators)
- The brushes, picks up or provide current to or from the commutator or the slip ring. Semiconductor switches often replace brushes.
AC Generator – salient poles
The armature windings generate the electric current. The armature can be on either the rotor or the stator.
The magnetic field of the dynamo or alternator can be provided by either electromagnets or permanent magnets mounted on either the rotor or the stator.
The commutator is the component rectifying and transferring the current generated within the magnetic field of a DC generator through the brushes to the external circuit such that direct current can be transferred to the users.
The dynamo (DC generator) converts mechanical rotation into direct current by electromagnetic induction and the use of the commutator.
The commutator is consisting of pairs of contact bars or segments that is isolated from each other, fixed to the rotating shaft and connected to the armature windings. For every half turn the armature winding makes within the magnetic field the direction of the current changes within the winding due to the position of the winding in relation to the polarity of the magnetic field. Each brush will like wise every half turn change which corresponding commutator segment of each pair the current is picked up from such that the direction of the current remain unchanged over a full rotation of the rotator and thereby provides direct current.
The slip ring is used in AC generators (alternators) with a similar function as the commutator segments in a DC generator except that the slip ring is not segmented since the purpose is to produce alternating current. In an AC generator the current often is picked up from the stator to avoid high currency levels passing through the slip ring and brushes.
Electrical power generation is mainly done by use of alternating (AC) generators.
Principle of construction:
Synchronous AC generator
The frequency of the current generated is proportional (ie. synchronous) to the rotation of the generator rotor. The vindings on the rotor are supplied DC through the brushes and slip rings from either an external DC source or from a DC generator mounted on the AC generators shaft (self magnetizing). The rotor either has alient poles or for generators with high rpms a cylindrical shaped rotor (turbo generator – turbine driven generator). The magnetic field of the rotor induce voltage within the stator armature vindings where the frequency, number of phases, phase shift and sine wave amplitude is determined by the number of and postioning of the armature windings and the rotational speed
The static field synchronous AC generator
The static field synchronous AC generator, mainly used for smaller generators ( < 4 -5 KVA) has outwards pointing poles mounted on the stator. The poles may be magnetized either by permanent magnets or by DC current.
The armature, which in most cases consists of a three-phase winding, is mounted on the shaft. The current from the armature winding is released through three sliprings (collectors) and a set of brushes sliding on them.
Asynchronous AC generator:
The main physical difference with the synchronous generator is the rotor construction. The rotor consists of a number of aluminium or copper rods connencted by metal rings (aluminium) at each end. The rotor is positioned in the center of the stator. The stator having two or more poles connected to the electricity grid or another AC power source. When the stator (poles) are connected to the power grid a current will be induced within the rotor rods from the stator rotating magnetic and the rotor will start rotating. When the speed exceed the synchoronous speed of thre magnetic field in the stator, electric current is generated and the generaror will start delivering electrical power to the grid.
The main area of utilization for the asynchromous AC generator is wind turbines or small hydro power plants.