COOLING OF TURBO-ALTERNATORS

(a)The various systems of cooling turbo-alternators are,
(i).Axial system(for rotor):This is the conventional method of cooling rotors of turbo-alternators. Narrow sub-slots, just below the main slots, are formed through the rotor core as shown in fig(a).Large quantities of air are forced through these sub-slots.
(ii).Radial system:Here, the stator core has radial ventilating ducts every 5 to 7cm of its length as shown in fig(b).All the air for cooling the stator is forced into the air gap from both ends. This air than flows radially outward through the radial ventilating ducts as shown.
(iii).Radial Axial System: This is also used for stator and is a combination of the first and second systems.
(iv).Multiple Inlet System: This is the modern method of cooling and applicable to any length of machine.In this system, the stator frame and core are divided into a number of compartments as shown in fig(c).In some compartments ,the direction of air is radially outwards and in the others it is radially inwards. Air under pressure is forced into the stator casing from where it flows radially inwards in to the stator ducts.The air passes down and goes into other compartments through the airgap and axial holes from where it flows radially inwards into the stator ducts. The air passes down and goes into other compartments through the airgap and axial holes from where it flows radially outwards through ducts.
The air is drawn from these latter compartments, cooled and recirculated(fig c).
(b).Cooling media:Cooling media nay be either air or hydrogen giving correspondingly air-cooled alternators and hydrogen-cooled alternators.

Air-Cooled alternators:Here air may be circulated through alternator by anyne of the above four methods. In small alternators air may be supplied by fans mounted on their shafts whereas in large machines it is a separate system with motor driven fans.Modern practice has become to spray the air with water so that, in addition to getting cooled, the air loses all dust particles that it may carry.
Hydrogen cooled Alternator: It is applied for alternator of more than 60MW .Advantages of Hydrogen as the medium when compared to air are,
(i).Noise and losses due to windage are less.(density ofH2=1/14 air).(ii).Better cooling medium.(Specific heat of H2=14 times of air-Rapid cooling).(iii).Low-temperature gradient-Thermal conductivity of H2=7 times air.Hot particles of H2distributes its heat to surrounding particles, more rapidly than a particle of air.This property has lower the temperature gradients.(iv).Higher rating can be secured.(v).Power required for circulation is less.(10% of air).