Construction wise, an alternator generally consists of field poles placed on the rotating fixture of the machine i.e. rotor as shown in the figure above. Once the rotor or the field poles are made to rotate in the presence of armature conductors housed on the stator, an alternating 3 φ voltage represented by aa’ bb’ cc’ is induced in the armature conductors thus resulting in the generation of 3φ electrical power. All modern day electrical power generating station use this technology for generation of 3φ power, and as a result the alternator or synchronous generator has become a subject of great importance and interest for power engineers of late.
An alternator is basically a type of a.c generator also known as synchronous generator, for the simple reason that the field poles are made to rotate at synchronous speed Ns = 120 f/P for effective power generation.
Where f signifies the alternating electric 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 Mega watts produced in an ac generator contrary to that of a DC generator. To accommodate such high power the conductor weigh and dimension naturally has to be increased for optimum performance. And for this reason is it beneficial to replace these high power armature windings 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.
There are mainly two types of rotor used in construction of alternator,
- Salient pole type.
- Cylindrical rotor type.
Salient Pole Type
The salient pole type of rotor is generally used for slow speed machines having large diameters and relatively small axial lengths. The pole in this case are made of thick laminated steel sections riveted together and attached to a rotor with the help of joint.
An alternator as mentioned earlier is mostly responsible for generation of very high electrical power. To enable that, the mechanical input given to the machine in terms of rotating torque must also be very high. This high torque value results in oscillation or hunting effect of the alternator or synchronous generator. To prevent these oscillations from going beyond bounds the damper winding is provided in the pole faces as shown in the figure. The damper windings are basically copper bars short circuited at both ends are placed in the holes made in the pole axises. When the alternator is driven at a steady speed, the relative velocity of the damping winding with respect to main field will be zero. But as soon as it departs from the synchronous speed there will be relative motion between the damper winding and the main field which is always rotating at synchronous speed. This relative difference will induce electric current in them which will exert a torque on the field poles in such a way as to bring the alternator back to synchronous speed operation.
The salient features of pole field structure has the following special feature-
- They have a large horizontal diameter compared to a shorter axial length.
- The pole shoes covers only about 2/3rd of pole pitch.
- Poles are laminated to reduce eddy electric current loss.
- The salient pole type motor is generally used for low speed operations of around 100 to 400 rpm, and they are used in power stations with hydraulic turbines or diesel engines.
Cylindrical Rotor Type
The cylindrical rotor is generally used for very high speed operation and are employed in steam turbine driven alternators like turbo generators.
The cylindrical rotor type machine has uniform length in all directions, giving a cylindrical shape to the rotor thus providing uniform flux cutting in all directions. The rotor in this case consists of a smooth solid steel cylinder, having a number of slots along its outer periphery for hosing the field coils.
The cylindrical rotor alternators are generally designed for 2-pole type giving very high speed of Ns = (120 × f)/P = (120 × 50) / 2 = 3000 rpm.
Or 4-pole type running at a speed of Ns = (120 × f) / P = (120 × 50) / 4 = 1500 rpm. Where f is the frequency of 50 Hz.
The a cylindrical rotor synchronous generator does not have any projections coming out from the surface of the rotor, rather central polar area are provided with slots for housing the field windings as we can see from the diagram above. The field coils are so arranged around these poles that flux density is maximum on the polar central line and gradually falls away as we move out towards the periphery. The cylindrical rotor type machine gives better balance and quieter-operation along with lesser windage losses.