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| Revolving Field Salient Pole Type Alternator |
Construction and Working Principle
The electrical machine, which generates ac, is known as ac generator or alternator. Similar to dc generator it operates on the fundamental principle of electro-magnetic induction. It essentially consists of two parts namely the armature and field magnet system. The alternator may be constructed with either the armature or the field structure as revolving member. Small ac generators and of low voltage rating are commonly made with rotating armature. In such generators the required magnetic field is produced by dc electro-magnets placed on the stationary member, called the stator, and the current generated is collected by means of brushes and slip-rings on the revolving member, called the rotor.
Practically all large rating generators are made with revolving field. In such generators revolving field structure or rotor has slip-rings and brushes for supply of excitation current from an outside dc source and the stationary armature, (also called the stator), which is made up of thin silicon steel laminations securely clamped and held in place in the steel frame, accommodates coils or winding in the slots. The slots provided on the stator core are mainly of two types viz open type or semi-closed type slots. Totally closed type slots are never used.
The field system is just like that of a dc generator which is excited from a separate source of 125 or 250 V dc supply. The excitation is usually provided form a small dc shunt or compound generator, called the exciter, mounted on the shaft of the alternator itself.
The stationary armature and revolving field system have some inherent advantages such as (i) light revolving system (ii) easier to insulate the high voltage winding when mounted on stationary structure (iii) connection of load circuit without passing through slip-rings for 125 V or 250 V 9v) cooling of armature winding is more readily possible. Rotors are of two types viz salient-pole type and non-salient or smooth cylindrical types.
Salient pole type rotor is used almost entirely for slow and moderate speed alternators (hydro generators), since it is least expensive and provides ample space for the field ampere-turns. Salient poles cannot be employed in high speed generators, such as turbo-generators, on account of very high peripheral speed (100 to 170 m/s) and the difficulty of obtaining sufficient mechanical strength. Such rotors have large diameter and small axial length.Smooth cylindrical or non-salient pole type rotors are used in very high speed alternators, such as turbo-generators. Such rotors have two or four poles and small diameter and long axial length. These are better in dynamic balancing and quieter in operation. Windage losses are also less.
EMF Equation. The emf equation for an alternator is given as
Erms per phase = 4.44 Kd Kp Ø f T volts
Where Kp is coil span factor or pitch factor and is equal to cos α/2 where α is angle by which coil span falls short and is equal to unity for full pitch winding, Kd is the distribution or breadths factor and is equal to (sin mβ/2 / msin β/2) ; m being the number of slots per pole per phase and β = 1800/ number of slots per pole , is the useful flux per pole in webers, f is the frequency in hertz and T is the number of turns per phase.
Rating of Alternators. The alternators are rated in KVA or MVA.
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