Excitation / Excitation voltage or more precisely excitation system of a generator ( A.C ) is one of the leading sphere of doubts and questions asked in interviews. While these big machineries work on the Faraday’s law of electromagnetic induction. They need some source of energy not just to produce magnetic field but to control it avoiding voltage variations. After-all controlling magnetic field will automatically control the voltage output of the alternator.
But how do they produce such stable magnetic field? This is when excitation comes in to part; A D.C voltage is supplied to the rotor field windings from a D.C generator connected in synchronous on the same shaft. Flux is produced in the field windings of the rotor by the D.C supply; while the level of excitation depends on the load current, speed and power factor of the machine.
What is Excitation Voltage?
An excitation voltage or current is the amount of electric energy ( D.C ) feed into the field winding of an alternator rotor to produce magnetic flux / field. The output voltage of an alternator depends upon the magnetic field and so the excitation voltage. So a device called automatic voltage regulator is installed and used to control the final output by adjusting excitation voltage. Conventionally a D.C generator synchronized and coupled with the alternator on the same shaft is used for this purpose.
While most designs use conventional D.C generator, few designs also use D.C battery for this purpose. The alternator which supply its own excitation voltage are are called self excited alternators. The only problem with them for both A.C and D.C generator is too keep them isolated from the external load for the initial starting period. For D.C generators or exciter the field excitation is generated only when the poles posses some some residual magnetism. Upon rotation of generator shaft the residual magnetism generate a small voltage in the armature. This lead to generate even more field strength and so more output voltage and so on and so on.
Q. Why only D.C is used for Excitation in Alternators?
Excitation voltage or current is supplied to the field windings of a rotor to produce a static magnetic field. If we use alternating current instead of direct current; we will get a fluctuating magnetic field. This will generate alternating magnetic flux in the stator windings leading to unpredictable and unstable voltage and power supply which may cause distortion and high risk for burning armature windings. Even if we some how tap the output it won’t be pure sinusoidal three phase supply. This is why the field windings of a rotor must be exited by direct current to avoid all those disadvantages.
Types of Excitation System
Magnetic flux and speed are the two key elements for emf generation in an alternator. A rotor field winding produce a strong magnetic field when subjected to direct current. Various ways can be implemented to supply direct current to the rotor windings; but the two main types are static and rotary. While the rotary method may include A.C or D.C exciter mounted on the same shaft; a static exciter utilizes thyristor to rectify A.C current to produce D.C.
1 ) D.C Excitation
In a typical conventional alternator we have a small d.c generator called main exciter is coupled on the same shaft as of the alternator. The direct current produced by the exciter upon shaft rotation is then feed to the rotor windings through brushes and slip rings. This excitation voltage is then controlled by varying field current of main exciter through a pilot exciter.
An automatic voltage regulator is then used to control both pilot and main excitation voltage according to the demand at the output terminal. Thus by controlling excitation voltage to the rotor field windings; final output voltage of an alternator can be changed or keep constant. While for most of the design the primary and main exciter are mounted on the same shaft; but for some designs they may be separately driven by a motor.
2 ) A.C Excitation
A ) Thyristor / Static Excitation
A static excitation method used the power from the alternator itself to supply excitation voltage. It provide faster and better response with low cost of operation. Typically they utilize a thyristor rectifier to convert A.C to D.C which is then feed into the rotor using brushes and slip-rings. First the output from the alternator is step down using a excitation transformer and then rectified to feed into the rotor.
This is an unpopular but most effective mode of excitation as it helps reduce the operating costs eliminating cost of maintenance, frictional loss and loss due to commutator and windings. A separate source of energy is used during the start for the excitation process; it is done so because there is no output current at the output terminal of the alternator to rectify.
Generally, A separate battery bank is used to provide the initial excitation current to reach the desired rated rpm and rated voltage. Once it reach the rated voltage the static excitation took in and maintain the field excitation.
B ) Brush-less Excitation
It is an excitation method with armature winding of exciter on the same shaft. Basically this system consists of a rectifier, main exciter and a pilot exciter with permanent magnet fields ( I mean generator with permanent magnet producing magnetic fields ). The output of the armature is feed to the rectifier and then to the rotor field windings. This arrangement of armature and rotor on the same shaft eliminates the need for slip rings and brushes.
The absence of parts and problem such as commuter, brushes, slip rings and carbon dust ( problem ) significantly improve the reliability and performance of the alternator. It also helps reduce the cost of maintenance. In some design even additional electronic detector are fitted to give alarms and trips upon diode failure.
Automatic Voltage Regulator
A sudden change in the load current of a generator can lead to change in its output voltage. This is due to the voltage drop / dip in the winding of a generator. An unregulated excitation system would be thus unacceptable / UN-feasible for maintaining power output at variable load. On ship and land based plants there is a regular fluctuation in the load demand; if remained unregulated this would destabilize the generators.
The magnitude and structure of the voltage dip largely depends on the load and repentance characteristics of the generator; while the recovery depends upon the A.V.R, governor and the excitation system. An automatic voltage regulator acknowledge the output voltage of a generator and change the excitation voltage accordingly.
Construction & Working
Although design of the automatic voltage regulator ( A.V.R ) change with the manufacturer; there are some basic key elements like transformer ( attached to the voltage sensing unit ), rectifier, transistor and thyristor remain the same. A voltage sensing unit rectify and step down the voltage to the low D.C voltage in proportional to the alternator terminal voltage.
This D.C output voltage is then compared for the set / desired value with the help of a comparator. Any difference win between will trigger an error signal which then feed into the thyristor circuit. A thyristor acts as a rectifier and a switch controlling the amount / magnitude of the excitation voltage or current. Generally on ship there is always one or two spare A.V.R available to be replaced upon suspected failure.
Other key features of an A.V.R includes:
- Quick Response.
- High KVAR for proper current sharing during parallel operation.
- Under or Over voltage alarms and trips
- Quick excitation voltage buildup during start of the generator.
#NOTE: I will look forward for your helpful comment ( even critical ) and recommendations to improve this Article ( What is Excitation Voltage And Its System? ).
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