AC generators always run at a constant speed (synchronous speed). That is why they are also known as synchronous generators. Practically in all medium and large synchronous generators, the armature is housed in the stator while the DC field system is placed on the rotor.
Synchronous Generator Parts
Basically, there are mainly two synchronous generator parts:
- 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.
Thanks for reading about synchronous generator parts and construction.
#1 The maximum power developed in the synchronous motor will depend on
rotor excitation, maximum value of coupling angle and supply voltage.
#2 A synchronous motor switched on to supply with its field winding shorted on themselves. It will
start as an induction motor
#3 The back EMF set up in the stator of a synchronous motor will depend upon
rotor excitation only
#4 With the increase in the excitation current of synchronous motor the power factor of the motor will
#5 The armature current of a synchronus motor has large values for
both low and high excitation
#6 A 3 phse 400 V, 50 Hz salient pole synchronus motor is fed from an infinite bus bars and is running at no load. Now if the field current of the motor is reduced to zero
the motor will run at synchronous speed
#7 A 3 phase, 400 V, 50 Hz 4 pole synchronous motor has a load angle of 10 degrees electrical. The equivallent mechnical degrees will be
#8 A 3 phase, 400 V, 50 Hz synchronous motor has fixed excitation. The load on the motar is doubbled. The torque angle, ɸ will become nearly
#9 A 3 phase, 400 V, 50 Hz synchronous motor has fixed excitation. The load on the motar is doubbled. The torque angle, ɸ will become nearly
#10 The break down torque of synchronous motor varies as
#11 The name plate of an induction motor reads 3 phase, 400 V, pf 0.8 lagging, 1440 RPM. On similar lines a name plate of synchronous motor should read
3 Phase, 400 V, 50 Hz, 0.8 pf leading, 1500 RPM
#12 In a 3 phase synchronous motor, the magnitude of field flux
remains constant at all loads
#13 A four pole synchronous machine has 48 slots. A coil having one coil side in slot number 1 and the other coil side in slot number 13 will be termed as
full pitch coil
#14 A three phase synchronous motor is running clockwise. In case, the direction of its field current is reversed
the motor will continue to run in the same direction
#15 In a synchronous motor out of the following losses, which one will have the highest proportion ?
#16 When a synchronous motor is connected to an infinite bus, while operating on leading power factor
the excitation voltage will be more than the supply voltage
#17 In a synchronous motor hunting can be minimised
by any of the above method
#18 While starting a synchronus motor by induction motor action, very high EMF is induced in the field. This induced EMF may damage the insulation of the field winding and of the slip rings. This insulation damage can be prevented by
either short-circuiting the field winding by field discharge resistance or splitting the field winding into several sections
#19 Synchronous motors are gererally of
salient pole type machines
#20 The fact that a synchronous motor with salient poles will operate, even if the field current is reduced to zero, can be explained by
magnetization of rotor poles by stator magnetic field
#21 Hunting of a synchronous motor may be due to
any of the above.
- 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