Rotor

Rotor

There are two types of rotors used in alternators

1) Salient pole type

2) Smooth cylindrical type.

1. Salient Pole Type

This is also called projected pole type as all the poles are projected out from the surface of the rotor.

The poles are built up of thick steel laminations. The poles are bolted to the rotor as shown in the Fig. 1.6.1.

The pole face has been given a specific shape. The field winding is provided on the pole shoe. These rotors have large diameters and small axial lengths. The limiting factor for the size of the rotor is the centrifugal force acting on the rotating member of the machine. As mechanical strength of salient pole type is less, this is preferred for low speed alternators ranging from 125 r.p.m. to 500 r.p.m. The prime movers used to drive such rotor are generally water turbines and I.C. engines.

2. Smooth Cyllndrical Type

This is also called non salient type or non-projected pole type of rotor.

The rotor consists of smooth solid steel cylinder, having number of slots to accommodate the field coil. The slots are covered at the top with the help of steel or manganese wedges. The unslotted portions of the cylinder itself act as the poles. The poles are not projecting out and the surface of the rotor is smooth which maintains uniform air gap between stator and the rotor. These rotors have small diameters and large axial lengths. This is to keep peripheral speed within limits. The main advantage of this type is that these are mechanically very strong and thus preferred for high speed alternators ranging between 1500 to 3000 r.p.m. Such high speed alternators are called 'turboaltemators'. The prime movers used to drive such type of rotors are generally steam turbines, electric motors.

The Fig. 1.6.2 shows smooth cylindrical type of rotor.

Let us list down the differences between the two types in tabular form.

3. Difference between Salient and Cylindrical Type of Rotor

Salient Pole Type

1. Poles are projecting out from the surface.       

2. Air gap is non uniform.      

3. Diameter is high and axial length is small.      

4. Mechanically weak.  

5. Preferred for low speed alternators.      

6. Prime mover used are water turbines, I.C. engines.  

7. For same size, the rating is smaller than cylindrical type.  

8. Separate damper winding is provided.  

Smooth Cylindrical Type

1. portion of the cylinder acts as poles hence poles are non projecting.

2. Air gap is uniform due to smooth cylindrical periphery.

Small diameter and large axial length is the feature.

4. Mechanically robust.

5. Preferred for high speed alternators .e. for turboaltemators.

6. Prime movers used are steam turbines, electric motors.

7. For same size, rating is higher than salient pole type.

8. Separate damper winding is not necessary.

Review Questions

1. List the differences between salient type and non salient type of rotor construction. Give the constructional details of two of alternators.

2. Give the constructional details of two types of alternators. AU : May-03, 05, Dec.-06, 12, 14, Marks 8

3. Describe with neat sketches, the constructional details of a salient pole type alternator. AU : Dec -16, May-18, Marks 10