Rotor of generator

The rotor shaft is forged from a vacuum cast steel ingot. The high mechanical stresses resulting from the centrifugal forces & short circuit torque call for high quality heat-treated steel. Comprehensive tests are done to ensure adherence to the specified mechanical & magnetic properties as well as homogeneous forging.

The rotor consists of an electrically active portion & two shaft ends. Approximately, 60% of the rotor body circumference has longitudinal slots, which hold the field winding. Slot pitch is selected so that the two poles are displaced by 180 degree. The rotor wedges act as damper winding within the range of the winding slots.

The rotor teeth at the end of the rotor body are provided with axial & radial holes, enabling the cooling gas to be discharged into the air gap after intensive cooling of the end windings.


Cooling of Rotor :

Each turn is subdivided into four parallel cooling zones. One cooling zone includes the slot from the centre to the end of the rotor body , while other covers the half the end winding to the centre of the rotor body.
The cooling of Air for the slot portion is admitted into the slot bottom ducts below the winding . The hot gases at the end of the rotor body is then discharged into the air gap between the rotor body and stator core through radial opening in the rotor slots wedges .

The cooling air for the end winding is drawn from below the rotor-retaining ring. It rises radially along the air gap via axial and radial slots in the end portions of the rotor teeth.


Types of Rotor :

There are two types of rotors used in generators.

Salient Pole Type : It is used in low & medium speed (engine driven) generators. It has a large number of projecting (salient) poles having their cores bolted or dovetailed onto a heavy magnetic wheel of cast iron, or steel of good magnetic quality

Smooth Cylindrical Type : It is used for steam turbine or gas turbine driven generators, which run at very high speed (3000rpm). The rotor consists of a smooth solid forged steel cylinder, having a number of slots milled out at intervals along the periphery & along the shaft for accommodating field coils. To avoid excessive peripheral velocity, such rotors have very small diameters. Hence, turbo generators are characterised by small diameters & long axial (or rotor) lengths. The cylindrical construction of the rotor gives better balance, quieter operation & also less windage loss.