The construction of an alternator consists of field poles placed on the rotating fixture of the machine. An alternator is made up of two main parts: a rotor and a stator. The rotor rotates in the stator, and the field poles get projected onto the rotor body of the alternator. The armature conductors are housed on the stator.
An alternator is basically a type of AC generator. The field poles are made to rotate at synchronous speed Ns = 120 f/P for effective power generation. Where, f signifies the alternating current frequency and the P represents the number of poles.
In most practical construction of alternator, it is installed with a stationary armature winding and a rotating field unlike in the case of DC generator where the arrangement is exactly opposite. This modification is made to cope with the very high power of the order of few 100 Megawatts produced in an AC generator contrary to that of a DC generator.
To accommodate such high power the conductor weighs and dimensions naturally must be increased for optimum performance. For this reason, is it beneficial to replace these high-power armature winding by low power field windings, which is also consequently of much lighter weight, thus reducing the centrifugal force required to turn the rotor and permitting higher speed limits.
A synchronous machine essentially consists of two parts:
- Armature (rotor)
- Field magnet system
Small AC generators and of low voltage rating are commonly made of rotating armature. In such generators, the required magnetic field is produced by DC electromagnet placed on the stationary member called stator and the current generated is collected by means of brushes and slip ring on the revolving member called the rotor.
Practically all large rating generators are made of 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, which is made of thin silicon sheet steel laminations securely clamped and held in place of steel frame, accommodates coils or winding in the slots. The slots are provided on the stator core and of mainly two types viz. open type or semi-closed type. Totally closed type slots are never used.
EMF EQUATION OF SYNCHRONOUS GENERATOR OR ALTERNATOR
The emf equation of Synchronous Generator or Alternator is given as
Φ = Flux per pole, in Wb
P = Number of poles
N = Synchronous speed in rpm
f = Frequency of induced emf in Hz
Z = Total number of conductors
Zph = Conductors per phase connected in series = Z/3 as number of phases = 3
Consider a single conductor placed in a slot.
The average value of emf induced in a conductor = dΦ/dt.
For one revolution of a conductor,
Eavg per conductor = (Flux cut in one revolution/Time taken for one revolution).
Total flux cut in one revolution is Φ x P.
Time taken for one revolution is 60/Ns seconds.
But in ac circuits, RMS value of an alternating quantity is used for the analysis. The form factor is 1.11 of sinusoidal emf.
Pitch Factor or Coil Span Factor (Kc)
In practice, short pitch coils are preferred. So coil is formed by connecting one coil side to another which is less than one pole pitch away. So actual coil span is less than 180°. The coil is generally shorted by one or two slots. The angle by which coils are short pitched is called angle of short pitch denoted as ‘ alpha’ where = Angle by which coils are short pitched.
It is defined as the ratio of resultant emf when the coil is short pitch to the result emf when the coil is full pitched. It is always less than one.
Distribution Factor (Kd)
Similar to full pitch coils, concentrated winding is also rare in practice. Attempt made to use all the slots available under a pole for the winding which makes the nature of the induced emf more sinusoidal. Such a winding is called distributed winding.
Consider 18 slots 2 pole alternator.
So, slots per pole i.e. n = 9.
m = Slots per pole per phase = 3
β = 180°/9 = 20°
Let E = Induced emf per coil and there are 3 coils per phase.
In concentrated type, all the coil sides will be placed in one slot under a pole. So, induce emf in all the' coils will achieve maxima and minima at the same time i.e. all of them will be in phase. Hence resultant emf after connecting coils in series will be algebraic sum of all the emf's as all are in phase.
The distribution factor is defined as the ratio of the resultant emf when coils are distributed to the resultant emf when coils are concentrated. It is always less than one.
ARMATURE REACTION IN ALTERNATOR OR SYNCHRONOUS GENERATOR
Every rotating electrical machine works based on Faraday’s law of electromagnetic Induction. Every electrical machine requires a magnetic field and a coil (Known as armature) with a relative motion between them.
In case of an alternator, we supply electricity to rotor to produce magnetic field and output power is taken from the armature. Due to relative motion between field and armature, the conductor of armatures cut the flux of magnetic field and hence there would be changing flux linkage with these armature conductors. According to Faraday’s law of electromagnetic induction there would be an emf induced in the armature. Thus, as soon as the load relates to armature terminals, there is a current flowing in the armature coil.
As soon as current starts flowing through the armature conductor there is one reverse effect of this current on the main field flux of the alternator (or synchronous generator). This reverse effect is referred as armature reaction in alternator or synchronous generator.
In other words, the effect of armature (stator) flux on the flux produced by the rotor field poles is called armature reaction.
In an alternator like all other synchronous machines, the effect of armature reaction depends on the power factor i.e. the phase relationship between the terminal voltage and armature current.
Reactive Power (lagging) is the magnetic field energy, so if the generator supplies a lagging load, this implies that it is supplying magnetic energy to the load. Since this power comes from excitation of synchronous machine, the net reactive power gets reduced in the generator.
Hence, the armature reaction is demagnetizing. Similarly, the armature reaction has magnetizing effect when the generator supplies a leading load (as leading load takes the leading VAR) and in return gives lagging VAR (magnetic energy) to the generator. In case of purely resistive load, the armature reaction is cross magnetizing only.
The armature reaction of alternator or synchronous generator depends upon the phase angle between, stator armature current and induced voltage across the armature winding of alternator.
The phase difference between these two quantities, i.e. Armature current and voltage may vary from -90o to + 90o
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