2. 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.

3. Why is cooling needed?
Energy transfer and energy conversion in electrical
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.

4. COOLANTS AND HEAT
EXCHANGERS
A. Primary Coolant : It has lower temperature than machine part.
Example: Water
B. Secondary Coolant : It has Lower temperature than primary
coolant.
Example: Ethylene Glycol, Propylene Glycol, Calcium Chloride,
Sodium Chloride.
C. Heat Exchanger : Component that keeps two coolants separate
but allows transfer of heat energy between them.
Example: They are widely used in space heating, refrigeration,
air conditioning, power plants, chemical plants, petrochemical
plants.

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 Based on manner of
cooling: cooling:
• Natural Cooling • Open Circuit Ventilation
• Self Cooling • Surface Ventilation
• Separate Cooling • Closed Circuit
Ventilation
• Liquid Cooling

7. TYPES OF VENTILATION
 INDUCED  FORCED
 Fan -> Decrease in air  Fan -> Sucks air from
pressure inside machine - atmosphere-> Forces it
> Air sucked in -> Pushed into machine -> Air
out by fan pushed out
 Small, medium machines  Temperature of cooling
air rises due to heat loss
 More amount of air
required

8. RADIAL VENTILATION
Most common, Up to 20kW rating
Large machines -> Large core lengths -> Core
subdivided to provide radial ventilating ducts
 Advantages: Minimum energy loss for ventilation,
almost uniform temperature rise in axial direction
 Disadvantages: Makes machine length larger, cooling
might be unstable with amount of cooling air flowing

9. AXIAL VENTIALTION
Used in induction machines (medium output, 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
-> Including flux density in core -> Increase in iron loss

10. 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 and rotor use forced self ventilation

11. 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

12. 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 < 25kW
 For rating > 25kW,internal fan (Primary coolant) +
External fan (Secondary coolant)
 Internal fan -> Inside machine, avoids temperature
gradient across air gap

13. VENTIALTED RADIATOR
MACHINES
 Internal fan circulates air inside machine
 External fan -> Sucks hot air from inside -> Pushes it
back to radiators (Heat exchanger) on frame of
machine
 Totally enclosed machine up to 5 MW
 At higher ratings, air may be cooled by water if
convenient

14. COOLING CIRCUIT
OPEN CIRCUIT CLOSED CIRCUIT
VENTIALTION: VENTILATION:
 Cool air drawn in, forced  Same volume of air
out after passing over passes through a closed
heated machine parts circuit -> Path has fans,
 Filters required to clean coolers, drying agents ->
air, driers to remove Hot air from outlet is
moisture cooled -> Cool air enters
 Unsuitable for large
through the inlets
machines

15. COOLING OF
TURBO-ALTERNATORS
 Closed circuit ventilation
 Long core length, small diameter
Methods :
 Air cooled (One side axial, two side axial, multiple inlet
system)
 Hydrogen cooled
 Direct cooled

16. COOLING OF
TRANSFORMERS
Different Cooling
Methods
Air Cooling Cooling For Oil
For Dry Type Oil Immersed
Transformer Immersed Water
Transformer Cooling
Air Forced type Oil Natural Air Natural
Type (O.N.A.N.)
Oil Forced Water
(A.F.) Forced (O.F.W.F.)
Oil Natural Air Forced
Type (O.N.A.F.)
Air natural Type Oil Natural Water
(A.N.) Forced (O.N.W.F.)
Oil Forced Air Natural
Type (O.F.A.N.)
Oil Forced Air Forced
Type (O.F.A.F.)

18. AIR COOLED
TURBO-ALTERNATORS
 For small units used as auxiliaries in large power
stations
 1 side axial ventilation : Up to 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

19. 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

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. 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

22. DIRECT COOLING OF
TURBO-ALTERNATOR
Conventional cooling
Direct cooling : Losses dissipated to medium
circulating in windings
Called supercharged/Conductor cooled/Inner cooled
machines
Advantages : Hydrogen, Water, Oil

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

24. 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 circulations
Advantages :
1) Higher load is possible as no temperature difference
between conductors and water
2) For rating
up to 600MW

25. Conclusion:
 Cooling is very much essential for modern output
machines
 Makes the output of machine stable
 Increases efficiency, rating of machine
 Increases longevity of machines, maintains safe
operation
 Protects the inner parts of machine from thermal
damages

26. Thank You!
RAHUL DEY
Dept. ELECTRICAL
Roll. 09187016017