Construction of rotor in alternator is of two types. The two main types of rotor in synchronous generator↗ are salient pole and non-salient pole rotor. Non-salient pole rotor is also known as cylindrical pole rotor. The main difference between salient pole rotor vs non-salient pole rotor are as follows:
Salient Pole Rotor:
The term salient means projected outward. Hence poles of salient type rotor are projected out from the surface of core and houses the rotor field winding.
Construction of salient type rotor is not strong enough to withstand mechanical stresses due to which this type of rotor cannot run at higher speed.
Salient pole rotors are generally used in low and medium speed alternators having a speed (100-1500 rpm). These types of rotor cause excessive windage losses if driven at high speed.
Since alternators are driven at constant speed to produce AC power of the desired frequency, hence large number of poles are required to get the desired frequency (f=Ns*P/120).
Salient pole rotor has normally 4 to 60 numbers of poles .
Let’s understand the above discussion by the following example.
Assuming the desire fixed frequency is 50 Hz at synchronous speed of 100 rpm. Lets calculate the required number of poles. Since
P= Number of poles.
F = Frequency.
N= rotating speed.
Putting the values, we have
P= 60 Poles
Salient pole rotor has large diameter in order to have space for the large number of poles and short axial length.
Salient pole rotor has non-uniform air gap between rotor and stator.
Flux distribution is poor due to non-uniform air gap. Hence pole faces of the rotor are shaped in radial manner (can be seen in the above figure) so that the generated emf is of sinusoidal shape.
Because of non-uniform air gap, salient pole rotor produce oscillations during the operation.
Damper windings↗ are needed in salient pole rotor to damp out the rotor oscillations.
These types of rotors are generally used in hydel power plants since hydro turbines has low speed of operation.
Non- Salient Pole Rotor:
Non-salient pole rotor is also known as cylindrical pole rotor or drum rotor.
Non salient pole type rotors are smooth cylindrical in shape and have a number of parallel slots on its outer periphery.
Each of the slot has different number of turns and produce sinusoidal flux when excited.
Flux distribution is uniform due to which sinusoidal emf is obtained in the case of cylindrical pole rotor.
Operation is smooth and less noisy because of uniform air gap.
Non- salient rotors are used in high speed alternators having a speed (1500-3000 rpm.)
Since non-salient pole rotor are used in high speed alternators, hence small numbers of poles are required to get the fixed frequency ( f=Ns*p/120).
Non salient pole rotor has normally two to four number of poles.
Cylindrical rotor machine has small diameter and very long axial length.
These type of rotors are smooth so windage losses are less.
Damper windings are not needed in non-salient type rotors.
Non salient pole rotors are used in high speed operations such as gas turbine and steam turbine.
These type of rotors are used in machines having ratings of 10kva to 1500kva.