Alternator is an electrical generator that is used to convert mechanical energy to electrical energy in the form of alternating current to keep electronic components up and running.
Some facts about Alternator:
A rotating magnetic field with a stationary armature is used in most of the alternator for low cost and simplicity. However, linear alternator or a rotating armature with a stationary magnetic field is also used sometimes.
When a permanent magnet is used for its magnetic field, an alternator is called a magneto.
Alternators that are used in power stations and are driven by steam turbines are called turbo alternators.
Most of the electric power in the world is generated by large 50 or 60 Hz three-phase alternators in power plants, which is distributed by electric power grids.
Alternating current is produced naturally from Rotating generators which is normally converted into direct current via the addition of a commutator in the generator because of its limited use.
Principle of operation of Alternator:
An electromotive force (EMF) is developed in a conductor moving relative to a magnetic field. The polarity of EMF will be reversed when it moves under magnetic poles of opposite polarity. A rotor which is a rotating magnet turns within a stationary set of conductors that is wound in coils on an iron core, called the stator.
An induced EMF will be generated when the field cuts across the conductors, as the rotor is turn by the mechanical input. An AC voltage will be induced in the stator windings by the rotating magnetic field.
An alternator is a synchronous generator as the currents in the stator windings varies in step with regards to the position of the rotor.
The magnetic field of the rotor may be produced by permanent magnets, or by a field coil electromagnet.
Ccontrol of the generated voltage of alternator is allowed by varying the current in the rotor field winding when a rotor winding is used by an automotive alternators.
The loss due to magnetizing current in the rotor can be avoided by the permanent magnet machines. The terminal voltage varies directly with the speed of the generator as the permanent magnet field is constant.
Usually brushless AC generators are larger than the genertaors used in automotive applications.
The field current is controlled by an automatic voltage control device to keep output voltage constant.
More current is fed into the rotating field coils through the voltage regulator (VR) if the output voltage from the stationary armature coils drops due to an increase in demand. So, greater voltage in the armature coils will be induced because of increase in the magnetic field around the field coils. Hence, the output voltage is brought back to its original value.
The field current is controlled by the alternators to regulate reactive power and to help stabilize the power system against any effect of momentary faults that are used in central power stations.
A three phase current will be produced by the rotating magnetic field, displaced by one-third of a period with respect to each other by the three sets of stator windings, physically offset.
Classifications of Alternator:
Alternators may be classified based on the method of excitation, number of phases, the type of rotation, cooling method and their application. These include:
The magnetic field used in the alternators can be produced in two ways. This can be done either using permanent magnets which create their own persistent magnetic field or by using field coils. The alternators that use permanent magnets are called as magnetos.
Wound field coils form an electromagnet to produce the rotating magnetic field in other alternator.
When permanent magnets are used in an alternator to produce alternating current, that is called a permanent magnet alternator (PMA). Either alternating current, or direct current will be produced by a permanent magnet generator (PMG) if it has a commutator.
There will be a smaller direct current (DC) generator in a direct connected direct current (DC) generator method of excitation which is fixed on the same shaft with the alternator. A small amount of electricity will be generated by the DC generator which is sufficient enough to excite the field coils of the connected alternator to generate electricity.
The residual magnetism retained in the iron core is used in the transformation and rectification method to generate weak magnetic field which would allow a weak voltage to be generated. Stronger voltage as part of its build up process can be generated as the field coils will be excited by the voltage generated for the alternator. The field is supplied with rectified voltage from the alternator after the initial AC voltage buildup.
There will be two alternators built end-to-end on one shaft in a brushless alternator. Smaller brushless alternators may look like one unit but the two parts are identifiable on the large versions. The main alternator is the larger of the two sections and the smaller one is the exciter.
There will be stationary field coils and a rotating armature in the exciter. The opposite configuration with a rotating field and stationary armature will be used by the main alternator. A bridge rectifier, also called as the rotating rectifier assembly is mounted on the rotor.
The number of wearing parts can be reduced as neither brushes nor slip rings are used.
The 3-phase output from the exciter varies by varying the amount of current through the stationary exciter field coils which is rectified by a rotating rectifier assembly, mounted on the rotor. The resultant DC supplies the rotating field of the main alternator and hence alternator output. hence the output of the main alternator will be indirectly controlled by a small DC exciter current.
Alternators can also be classified by the number of phases of their output voltage. The output can be either single phase or polyphase. The most common type is a three phase alternators. However, polyphase alternators can be two phase, six phase or more.
High frequency alternators of the variable reluctance type can be applied commercially to radio transmission in the low-frequency radio bands.
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