Cooling is the very important aspect in the construction and operation of the alternators. The cooling in the alternators or synchronous generators are classified into two types −
In open-circuit cooling, the air is drawn into the alternator by the means of fans and blowers and circulated inside. This air is later released back into the atmosphere. The open-circuit cooling method is suitable for small sized alternators.
The closed circuit cooling method is a totally enclosed system and clean hot air from the alternator is cooled by oil or water-cooled heat exchanger and then, this cooled air is forced through the alternator by fans.
In order to increase the surface area in contact with the cooling air, ducts are provided in the stator and rotor cores and sometimes in the field coils of the alternator. Depending upon the direction of the flow of air through the ducts, they can either be radial or axial.
In the radial flow ventilation system, the cooling air enters the ducts through stator by the way of air-gap and passes radially to the back of the stator from where it is exhausted.
The radial ventilation system is suitable for both small and large alternators.
Minimum energy loss for cooling.
Sufficient uniform temperature rise of the generator in axial direction.
It increases the size of the generator because the ventilation ducts occupy about 20 % of the armature length.
The heat dissipation in less as compared to the other ventilation systems.
The radial ventilation system in certain cases is unstable to quantity of cooling air flowing through the alternator.
In the axial flow ventilation system, the cooling air is forced in the axial direction through the ducts formed in the stator and rotor cores. This is a highly effective method for cooling of the alternators except for the generators with considerable axial length.
The chief disadvantage of the axial ventilation method is non-uniform heat transfer. In this method, the air outlet part of the alternator is cooled less since the air is passing through the axial ducts has become heated by the time of exhaust.
In the circumferential ventilation system, the cooling air is supplied at one or more points on the outer periphery of the stator core and forced circumferentially through the ducts between the laminations to suitable outlets. In this method the duct area can be increased.
In some cases, the circumferential and radial flow systems are combined but the resultant interference in the two streams of cooling air have to be avoided. This is done by closing the alternating radial ducts at the outer surface.
The cooling air should be free from dust and soot especially in industrial surroundings. These will clog the ventilation ducts and reduce the duct area which results in reducing heat transfer by conduction. In order to remove the dust particles from the air, the air filters are used. Generally, cheese filters are used which can be renewed frequently. Sometimes, air may have to be washed in a spray chamber. In practice, the air is cooled by water coolers and used again.
For large-sized alternators, the air cooling becomes uneconomical since the sizes of fans required for circulation of cooling air increase and hence require greater power and corresponding expensive auxiliary equipment.
There is ultimate rating of the alternator beyond which the air cooling will not be able to keep the temperature of the alternator within the safe limit.