related information of Excitation System for Alternator & AVR

Alternator excitation system
An excitation system for a brushless synchronous alternator contains an exciter having a stator which has first and second sets of field coils ,respectively coupled to a source of DC power and in series with polyphase AC output current generated by the main generator. Here, The base level of exciter field current is provided by the DC current supplied to the first set of field coils. a second variable amount of field current is provided by the AC output current supplied to the second set of field coils. a rotor structure having first and second sets of armature windings is also present in the exciter. This helps to develop first and second exciter currents in response to movement of the rotor structure within the magnetic fields set up by the first and second sets of exciter field coils. Transient response is improved and overall length and weight of the generator package are decreased By dividing the responsibility for providing exciter field current in this fashion.
The synchronous alternator includes a dual winding exciter structure for developing first and second exciter signals which are combined, rectified and coupled to thefield windings of a main generator. armature windings which develop polyphase AC output signals to drive one or more loads are present in the main generator. a voltage regulator and rectifier coupled to the armature of a permanent magnet generator supplies the base current to the first set of exciter field windings. A second set of exciter field windings is connected in series withthe armature windings of the main generator so that the second set of exciter windings receive the load current. first and second exciter magnetic fields are established by the first and second exciter field windings through which the first andsecond sets of exciter armature windings move. a polyphase rectifier converts the exciter armature currents into a DC current for energizing the main generator field winding. A transient regulation is provided by the AC exciter, the demands placed upon the voltageregulator are diminished. the PMG length and weight may be decreased, since only a base level of exciter current is required to be supplied.

For more details visit:
http://patentstorm.us/patents/4467267/fulltext.html
http://criepi.denken.or.jp/en/e_publicat...eika32.pdf
http://198.170.104.138/jas/2007/1186-1190.pdf

PRESENTED BY:
AMIT HAZRA

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What is Excitation System ?
The Magnetic field created in the Alternator is known as Excitation System.
It is created using direct current (DC) field.
The DC field obtained from external source is called an Exciter.
Principle
The Alternator is working on the principle of Fleming’s right Hand rule.
As the rule says, “When a motional conductor is placed in a magnetic field (direction of motion is perpendicular to the magnetic field), then an induced emf is produced, which circulate a current in the close loop of the motional conductor.”
Illustration
Its Importance
As far as importance is concerned it is very much essential for alternator.
Without DC excitation field an alternator can’t produce electricity.
HOW AN ALTERNATOR WORK ?
HOW THIS MAGNETIC FIELD PRODUCED ?
It is produced by wound a coil on rotor slots.
Rotor is either cylindrical type or projected pole type.
In cylindrical rotor the windings are placed on rotor slots.
In projected poles the coil is placed on the rotor projectors.
Types of Exciter
WORKING
Normally the Alternators required the excitation field of 200 V DC.
In conventional exciters a Pilot Exciter & a Main Exciter is required.
The pilot exciter is a self-excited DC generator & main exciter is a separately excited DC generator.
Battery backup is also use for excitation purpose.
A thyristor bank is used as the associate of the excitation circuit.
EXCITATION FACTOR
Excitation factor means on which factor excitation depend.
Excitation = f ( Load Pf. )
If the Pf. Load is reactive lagging, then the excitation requirement is more than unity Pf. Load.
If the Pf. Load is leading, then the excitation requirement is less than unity Pf. Load.
ADVANTAGES & DISADVANTAGES
As far as advantages & disadvantages is concerned, there is no advantages or disadvantages of excitation system for an alternator.
It is very much essential for alternator.
Without this the alternator is dead.
So it is just as the “HEART” of alternator.
Maintenance
Regular
Eye site visit.
Fault check.
Vibration & hauling of rotating parts.
Working, controlling & protection circuit continuity.
Periodical
Lubrication of rotating parts.
Cleaning.
Sparking parts should be clean.
Working, controlling & protection circuit healthiness.
Circuit elements maintenance, like relay, breaker, contactor, etc.
Circuit controlling PCB.
CONCLUSION
By this time we know how an Excitation System & an Exciter work for Alternator & how much essential. If it is looking so simple yet it is very much essential for production of electricity by an Alternator.