The document outlines the methods of cooling for rotating electrical machines, emphasizing the necessity of cooling due to heat losses that occur during energy transfer and conversion. Various cooling methods are categorized based on their origin and mechanisms, including natural and forced ventilation, liquid cooling, and specialized systems for specific machine types like turbo-alternators. It also discusses the advantages and disadvantages of different cooling strategies, such as air, hydrogen, and water cooling, and their impact on machine efficiency and longevity.

1. METHODS OF COOLING OF ROTATING ELECTRICAL MACHINES

2. Why is cooling needed ? Energy transfer and energy conversion in rotating machines manifest losses. These losses appear as heat and increase temperature of the machine beyond its optimum level. Heat is dissipated to surroundings by conduction and convection assisted by radiation from outer surfaces.

3. What is cooling ? Process by which heat resulting from losses occurring in a machine is given up to a primary coolant by increasing its temperature. Heated primary coolant may be replaced by a new coolant at lower temperature or may be cooled by a secondary coolant in some form of heat exchanger.

4. COOLANTS AND HEAT EXCHANGERS Primary Coolant : lower temperature than machine part Secondary Coolant : lower temperature than primary coolant Heat Exchanger : Component that keeps two coolants separate but allows transfer of heat energy between them

5. Methods of cooling Size of a machine of a given duty depends on heat losses in its various parts. Small machines ( fractional H.P.) cooled by natural means. Modern machines require cooling. Cooling by air stream  ventilation

6. COOLING SYSTEM CLASSIFICATION Based on origin of cooling : Natural cooling Self cooling Separate cooling Based on manner of cooling : Open circuit ventilation Surface ventilation Closed circuit ventilation Liquid cooling

7. Enclosures for machines Open machine Protected machine Drip proof machine Pipe/duct ventilated machine Totally enclosed machine Watertight machine Weather proof machine Submersible machine Flame proof machine Totally enclosed gas circuit machine Open pedestal machine Open end bracket machine Screen protected machine Splash proof machine Hose proof machine Totally enclosed fan cooled Totally enclosed separately air cooled machine Totally enclosed liquid cooled machine Totally enclosed closed air circuit machine

8. TYPES OF VENTILATION INDUCED Fan  decrease in air pressure inside machine  air sucked in  pushed out by fan Small, medium machines FORCED Fan  sucks air from atmosphere  forces it into machine  air pushed out Temperature of cooling air rises due to heat loss More amount of air required

9. RADIAL VENTILATION Most common, up to 20 kW rating Large machines  large core lengths  core subdivided to provide radial ventilating ducts Advantages : min. energy loss for ventilation, almost uniform temp. rise in axial direction Disadvantages : makes machine length larger, cooling might be unstable with amount of cooling air flowing

10. AXIAL VENTILATION Used in induction machines (medium o/p, high speed machines) Solid rotor  restricts radial ventilation Holes punched where heat loss is more Disadvantages : non-uniform heat transfer, increased iron loss (ducts in slots of rotor reduce amount of iron  inc flux density in core  increase in iron loss)

11. AXIAL-RADIAL VENTILATION For Large motors, small turbo-alternators axial system  large iron loss  so mixed system is used Rotor mounted fan forces out the air. As a rule, induction motors having radial ducts in stator & rotor use forced self ventilation.

12. COOLING OF TOTALLY ENCLOSED MACHINE Totally enclosed machines  heavy, expensive, hence uneconomical Air  impurities  destroy insulation Natural cooling  ineffective  rating reduces Cooled by : Self ventilated frame, Ventilated radiator machines

13. VENTILATED FRAME MACHINES Self ventilated frame, fan enclosed on shaft outside working part of machine Fan enclosed by cover to secure direction of air flow for machine rating < 25 kW For rating > 25 kW, internal fan( primary coolant)+ external fan( secondary coolant) Internal fan  inside machine ,avoids temp. gradient across air gap

14. VENTILATED RADIATOR MACHINES Internal fan circulates air inside machine External fan  sucks hot air from inside  pushes it back to radiators (heat exchangers) on frame of machine Totally enclosed machine upto 5 MW At higher ratings, air may be cooled by water if convenient

15. COOLING CIRCUIT OPEN CIRCUIT VENTILATION : Cold air drawn in, forced out after passing over heated machine parts filters required to clean air, driers to remove moisture Unsuitable for large machines CLOSED CIRCUIT VENTILATION : Same volume of air passes through a closed ckt  path has fans,coolers, drying agents  hot air from outlets is cooled  cool air enters through the inlets

16. COOLING OF TURBO-ALTERNATOR Closed circuit ventilation Long core length, small diameter Methods : air cooled (one side axial, two side axial , multiple inlet system) hydrogen cooled Direct cooled

17. AIR COOLED TURBO-ALTERNATORS For small units used as auxilliaries in large power stations 1 side axial ventilation : upto 3MW Air supply by propeller fan  air enters at an end, leaves by the other In long machines, temperature rise is high along the length

18. AIR COOLED TURBO-ALTERNATORS 2 sided axial ventilation : Air forced from both sides, both windings have same temperature rise Used for machine rating up to 12MW Multiple inlet system : larger machines Outer stator  many chambers  alternate inlets and outlets  up to rating 60MW

19. HYDROGEN COOLING OF TURBO-ALTERNATORS For machines > 50MW, air cooling unsuitable No requisite amount of air, higher fan power Advantages of hydrogen cooling : Increased efficiency Increase in rating Increase in life span Elimination of fire hazard Smaller size of cooler Less noise

20. HYDROGEN COOLING SYSTEM Hydrogen(4-76 %)+air  explosive mixture Frame strong enough, all joints gas tight Hydrogen above atmospheric pressure, so leakage is from machine to atmosphere Gas pressure maintained Explosive mix avoided Purity of hydrogen checked by measuring its thermal conductivity

21. DIRECT COOLING OF TURBO-ALTERNATOR Conventional cooling Direct cooling : Losses dissipated to medium circulating in windings Called supercharged/conductor cooled/ inner cooled machines Advantages : increase in rating,winding temperature goes down and higher output Coolants used : hydrogen, water, oil

22. Coolants in direct cooling Hydrogen : Stator, rotor made hollow Hydrogen pumped from one end to other Used for machines with rating up to 300MW Oil : High grade transformer oil Used in US-direct cooling of stator conductors Flash point, can be reached in fault conditions, damages insulation

23. Water as coolant in direct cooling Higher rating  mechanical limitations for hydrogen cooling Water : superior heat transfer property, low viscosity, no high pressure heads required for circulation Advantages : smaller pumping power Higher load is possible as no temperature difference between conductors and water For rating up to 600 MW

24. Thank You ! Anwesa Nanda Reg. No. 0711014018 Branch EEE

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