- Stator (Armature )
- Rotor (DC Field System)
The armature is an iron ring, formed of laminations of special magnetic iron or steel alloy (silicon steel) having slots on its inner periphery to accommodate armature winding and is known as the stator. The whole structure is held in a frame which may be of cast iron or welded steel plates.
The field rotates in between the stator, so that flux of the rotating field cuts the core of stator continuously and therefore, causes eddy current loss in the stator core. To minimize the eddy current loss, the stator core is laminated.
The rotor is of two types namely:
- Salient pole type
- Smooth cylindrical type
Salient Pole Rotor
It is like a flywheel which has a large number of alternate North and South bolted on it. The magnetic wheel is made of cast iron or steel of good magnetic quality. These magnetic fields are energized or excited by a DC source.
The salient pole rotors are used only by low and medium (120 – 500 rpm) speed synchronous generators such as those driven by water turbines. Because of their low speeds, they require a large number of poles.
Such rotors have large diameters and short axial lengths. The salient pole structure is simpler and cheaper to manufacture than a cylindrical rotor.
Smooth Cylindrical Rotor
It consists of a smooth solid forged-steel cylinder having a number of slots milled out along its outer surface for housing field magnetizing field coils. Two or four regions are left un-slotted for creating non-projecting poles. Such rotors are used in steam turbine driven alternators which run at very high speeds (up to 3600 rpm). Such rotors have small diameters and very long axial lengths.
Excitation System of Synchronous Generator
The rotor poles of a synchronous generator are electromagnets. They require excitation for their operation. The field winding needs a DC supply to produce the required flux. There are several excitation systems which are used to provide DC excitation current for the generator.
In one system, power is taken from the AC generator terminals, is rectified and then supplied to the rotor field system by means of slip-rings and brushes.
In some systems, excitation supply is obtained from a small DC shunt generator called as an exciter. It is mounted on the same shaft as that of the synchronous generator.
One another system, which is known as brushless excitation system of synchronous generator, a small 3-phase generator mounted on the shaft of the main generator itself is used as an exciter. The output of the exciter is rectified and fed directly to the rotating field poles of the synchronous generator. The brushless excitation system has no commutator, slip-rings or brushes which make the system simple and reliable.
There are two methods of ventilation:
- Natural Ventilation
- Closed Circuit Ventilation
In natural ventilation method, a fan is attached to one end of the machine. Air is the medium by which ventilation takes place and the heat of machine parts is carried away.
In the closed circuit ventilation method the medium used for ventilation is hydrogen. Hydrogen is circulated with the help of water cooled heat exchangers. In the modern large capacity alternators, this method is preferred.
Generation of Three Phase EMFs
The three-phase EMFs are generated by three-phase synchronous generators (or alternators). A three-phase synchronous generator has three identical windings. Every phase winding is displaced at 120o electrical apart.
When a magnet (rotor) is rotated by means of some prime-mover in these windings, the stator conductors are cut by the magnetic field of the rotor, hence an EMF is induced in each winding. These EMFs are of same magnitude and frequency but are displaced from one another by 120 electrical degrees.
1. The frequency of voltage generated by an alternator having 4-poles and rotating at 1800 r.p.m. is .......hertz.
2. A 50-Hz alternator will run at the greatest possible speed if it is wound for ....... poles.
3. The main disadvantage of using short-pitch winding in alterators is that it
4. Three-phase alternators are invariably Y-connected because
5. The winding of a 4-pole alternator having 36 slots and a coil span of 1 to 8 is short-pitched by ....... degrees.
6. If an alternator winding has a fractional pitch of 5/6, the coil span is ....... degrees.
7. The harmonic which would be totally eliminated from the alternator e.m.f. using a fractional pitch of 4/5 is
8. For eliminating 7th harmonic from the e.m.f. wave of an alternator, the fractional-pitch must be
9. If, in an alternator, chording angle for fundamental flux wave is α, its value for 5th harmonic is
10. Regarding distribution factor of an armature winding of an alternator which statement is false?
11. When speed of an alternator is changed from 3600 r.p.m. to 1800 r.p.m., the generated e.m.f./phases will become
12. The magnitude of the three voltage drops in an alternator due to armature resistance, leakage reactance and armature reaction is solely determined by
13. Armature reaction in an alternator primarily affects
14. Under no-load condition, power drawn by the prime mover of an alternator goes to
15. As load p.f. of an alternator becomes more leading, the value of generated voltage required to give rated terminal voltage
16 . With a load p.f. of unity, the effect of armature reaction on the main-field flux of an alternator is
17. At lagging loads, armature reaction in an alternator is
18. At leading p.f., the armature flux in an alternator ....... the rotor flux.
19. The voltage regulation of an alternator having 0.75 leading p.f. load, no-load induced e.m.f. of 2400 V and rated terminal voltage of 3000 V is ............... percent.
20. If, in a 3-φ alternator, a field current of 50 A produces a full-load armature current of 200 A on short-circuit and 1730 V on open circuit, then its synchronous impedance is ....... ohm.
- Parts of Synchronous Generator
- Synchronization of Alternators
- Synchronous Motor Working and Construction
- Methods of Starting of Synchronous Motor
- Damper Winding in Synchronous Motor
- Over Excited Synchronous Motor
- Advantages and Disadvantages of Synchronous Motor
- Compare Synchronous Motor and Induction Motor