CONTENTs Page

1. Introduction.................................................. 02

2. Theory of solid body heating and cooling........... 04

3. Methods of cooling......................................... 05

4. Induced and forced ventilation......................... 06

5. Radial and axial ventilation.............................. 07

6. Cooling of turbo-alternators............................. 09

6.1 Air cooled turbo-alternator................................... 09

6.2 Hydrogen cooled turbo-alternator........................... 09

6.2.1 Advantages of hydrogen cooling.................... 10

6.2.2 Hydrogen cooling system............................ 11

6.3 Direct cooled turbo-alternators............................... 13

6.3.1 Advantages of direct cooling........................... 13

6.3.2 Coolants used in direct cooling........................ 15

6.3.3 Direct cooling system..................................... 16

Conclusion

References

1

INTRODUCTION

An Alternator is an electro-mechanical device that coverts

mechanical energy into electrical energy. The process of energy

transfer in case of rotating electrical machines involves currents

in conductors, and fluxes in ferromagnetic parts. Thus there are

various losses in windings. The losses appear as heat and

therefore the temperature of the system increases. These

alternators are in continuous operation and due to mechanical

reasons such as friction, insulation, etc. there is a considerable

wear and tear of system due to heating.

Heat is dissipated in form of

1. conduction given by

Qcon= (θ1-θ2)/RC

Where Qcon= heat dissipated by conduction, W;

θ1, θ2= temperature of two bounding surfaces, °C;

RC= thermal resistance of conducting medium, °C/W.

2. radiation given by

Qrad= 5.7×10-8e(T14-T0

4)

Where Qrad=heat dissipated by radiation W/m2

2

Θ1, 2= temperature of emitting surface and ambient mediumθ

e= coefficient of emissivity

3. convection given by

Qconv= KC(θ1-θ0)n

Where Qconv= heat dissipated by convection W/m2

Such overheating may cause heavy damage to the system leading

to alternator failure leading to losses in capital. Thus cooling

techniques are applied to cool the alternator and prevent excess

heating of the alternator. Various cooling techniques are applied

such as hydrogen cooling, direct cooling and air cooling which are

used according to the requirement of alternator. Various

ventilating system also aid to the cooling of the alternator such as

axial ventilation, radial ventilation, etc. which are designed

according to alternator requirement.

3

Theory of solid body heating and cooling:

The temperature of a machine rises when it is run under load conditions starting from cold conditions. Electrical machines are not homogeneous bodies. Their parts are made up of different materials like copper, iron and insulation. These materials have different thermal resistivities and due to this, it is rather difficult to calculate the temperature of a part of machine and so we take theory of heating of homogeneous bodies as basis for analyzing the process of machine heating. The results obtained are applicable to different parts of machine too.

The equation of temperature rise with time is given by

θ=θm(1−e−tTh )

Where, θ= temperature rise at any time t, °C

θm= final steady temperature rise while heating, °C

t= time, s

T h = heating time constant, s

The equation of cooling is given by

θ=θi e−t /T c

Where, θi= initial temperature rise over ambient medium

T c=¿Cooling time constant, s

4

Methods of cooling

In the near future, the greatest gains of output from a given size of machine are likely to arise from improvements in cooling techniques. Small electrical machines in the fractional horse power range may be cooled by natural means. In this method no external devices are used and the cooling is done by natural radiation and convection assisted by random air currents set up by the rotor where the frames are open. But all modern electrical machines are characterized by large losses per unit area of surfaces of machine, which dissipate heat into the ambient medium and hence artificial cooling methods are necessary in order to avoid excessive temperature rise during machine operations.

In most cases the cooling of electrical machine is done by air streams and this cooling is called “Ventilation”. In high speed machines such as turbo-alternators, hydrogen is used for cooling. There are machines in which water is used for cooling.

The cooling of machines according to the manner of cooling is of following types:

1. Open circuit ventilation- the heat is given up directly to the cooling air through the machine which is being continuously replaced.

2. Surface ventilation- the heat is given up by the cooling medium from external surface of a totally enclosed machine.

3. Closed circuit ventilation- the heat is transferred to the cooling medium through an intermediate cooling medium circulating in a closed circuit through the machine and cooler.

4. Liquid cooling- parts of machine carry water or another kind of liquid flowing through them, or they are immersed into a liquid.

5

INDUCED AND FORCED VENTILATION:

The ventilation of a machine is induced if the fan produces a decreased pressure of air inside the machine causing the air to be sucked into the machine under the external atmospheric pressure. The air is then pushed out by the fan into the atmosphere. It is most commonly used in machines of small and medium power output.

The ventilation of machine is said to be forced if the fan sucks the air from the atmosphere and forces it into the machine, from where it is then pushed out to the atmosphere. In induced ventilation cold air enters the machine whereas in forced ventilation the temperature of cooling air rises on account of losses in the fan. Thus the amount of air required to cool the machine is higher with forced ventilation.

6

RADIAL AND AXIAL VENTILATION:

RADIAL VENTILATING SYSTEM

This system is most commonly employed because the movement of rotor induces a natural centrifugal movement of air which may be augmented by provisions of fans if required. The end shields are shaped to guide air on the overhang and then on the back of the core. This method is suitable for machines up to about 20kW. For larger machines, which have large core lengths, the core is subdivided in order to provide radial ventilating ducts. The air now passes radially through these ducts, the path of air in the ducts being parallel to that over the overhang. The core is normally divided into stacks 40 to 80 mm thick, with ventilating ducts of width 6.5 mm width. The advantages of radial system are:

1) Minimum energy losses for ventilation2) Sufficiently uniform temperature rise of machine in axial direction

The disadvantages are:

1) Makes machine lengths larger.2) The ventilating system sometimes becomes unstable in respect to

quantity of cooling air flowing.

7

AXIAL VENTILATING SYSTEM

The axial ventilating system is employed in induction machines. This system of machine is suitable for machines of medium output and higher speed. This is because in high speed machines, a solid rotor construction with restricted spider is used in order to avoid centrifugal stresses and this restricts the provision of radial ventilating ducts and hence axial ventilating ducts are used. In order to increase the cooling surface holes may be punched where considerable heat dissipation occurs. This system improves cooling but has some disadvantages like non-uniform heat transfer and increased iron losses. However, in a large number of cases this loss is more than compensated by improved cooling.

COMBINED AXIAL AND RADIAL VENTILATING SYSTEM

This method is usually employed for large motors and small turbo-alternators. This is because of these machines the area of axial ducts required to carry sufficient quantity of cooling air becomes excessive giving rise to a large iron loss and therefore a mixed axial and radial system has to be used. The air is drawn from one end and is encouraged to pass through the ducts by baffling the fan end of the rotor spider. The rotor mounted fan forces out the air.

8

COOLING OF TURBO-ALTERNATORS

The problem of cooling of turbo-alternators is one of the most complex problems in electrical engineering since being high speed machines their dimensions as compared with those of hydro-electric generators for instance, are much smaller. In fact the problems associated with cooling arise because turbo-alternators are characterized by long core lengths and small diameters.

AIR COOLED TURBO-ALTERNATORS

The cooling of turbo-alternators by air is used for small units. The various methods used for air cooling of turbo-alternators are:

1. One Sided Axial Ventilation. Machines for power outputs up to 3 MW permit the use of one sided axial ventilation. In this method the machine is supplied with air by propeller fan and air enters the machine from one side and leaves from the other. The disadvantage of one sided axial ventilation is the great difference in the temperature rise of the winding along the length of machine.

2. Two Sided Axial Ventilation. In this method the air is forced through the machine from both the sides. It has the advantage that end windings have same temperature rise.

3. Multiple inlet system. In a multiple inlet system the outer stator casing is divided into a number of compartments, these being used alternatively as inlet and outlet chambers. In the inlet chambers the air is directed radially inwards while in outlet chamber the air is directed radially outwards. Air is forced under pressure into the stator casing where it enters the inlet chamber from where it flows radially inwards into the stator ducts. Part of this air passes through the axial ducts, the remainder flowing along the air gap. It then passes radially outwards in the adjacent sections of the core to the outlet chambers. The air is drawn from the outlet chambers and is sent to the coolers where it is cooled and is recirculated. This method can be used for machines up to 60 MW.

HYDROGEN COOLING OF TURBO-ALTERNATORS

Building of air cooled turbo-alternator above 50 MW rating presents serious ventilation difficulties, not only in circulating the requisite quantity

9

of air through the machine but also because the high fan power required to circulate the air. An idea of the quantities involved can be had from the data of 60 MW machine which has a total loss of 1000 kW and requires a 150 tonnes of air per hour and a fan power of about 100 kW. Thus the air circulating fan power required for this duty is high and on this score alone the air cooled generator could be ruled out when it becomes necessary to increase generator rating. Thus hydrogen cooling method was found.

ADVANTAGES OF HYDROGEN COOLING:

1. Increased efficiency. An increase in efficiency results from reduction in the ventilation losses which are a major portion of the total losses in a high speed machine. This is because the density of hydrogen is only 0.07 times the density of air and therefore the power required to circulate hydrogen should be about 1/14 of the power required for an equivalent quantity of air. Thus efficiency is raised by approx. 0.8% at full load values of 99 to 99.2%.

2. Increase in rating. Hydrogen has a heat transfer coefficient 1.5 times and its thermal conductivity is 7 times that of air. Consequently when hydrogen is used as a coolant, the heat is more readily taken up from the machine parts and given out. Therefore heat generated is more effectively removed and the active materials can be loaded more than in possible with air cooling. The higher value of thermal conductivity of hydrogen, the temperature gradients across the film barrier between cooled surface and coolant and across the coolant are reduced. On account of above two factors, an increased output can be taken from a given frame size if hydrogen cooling is used.

3. Increase in life. The life of a machine is mainly the life of winding insulation and air pockets in insulation can be sources of such high local temperature that there is always the risk of insulation breakdown and fire. As thermal conductivity of hydrogen is of same order as winding insulation, therefore when there are pockets filled with hydrogen, heat conduction through them will be as good as through winding insulation and consequently high local temperature raises are not there.

4. Elimination of fire hazards. The outbreak of fire inside the machine is impossible as hydrogen does not support burning.

5. Smaller size of coolers. The size of coolers required to cool the gas are smaller in size when hydrogen is used as coolant.

10

HYDROGEN COOLING SYSTEM

Hydrogen when mixed with air forms an explosive mixture over a wide range (4% to 76%) of hydrogen in air. Therefore the frame of hydrogen cooling machines has to be made strong enough.

All joints in cooling system are made gas tight and oil film shaft seals are used to prevent leakage of hydrogen. The seals must accommodate axial expansion of rotor shaft and stator frame. The seal bearing is a small thrust bearing held by springs to a collar machine on the rotor shaft. The seal oil is forced in a groove in the bearing face. The seal rings are two short bearing fitting closely to the rotor shaft but held apart by a greater spring. Sealing oil is forced into the space between the two bearing rings and passes in two axial directions along the shafts. A part of the oil flows towards the inner (gas) side and other part to the air side. The oil that flows towards air mixes with air, while the oil that goes towards hydrogen is collected and degassed.

Fans mounted on the rotor circulate hydrogen through the ventilating ducts and internally arranged gas coolers. The gas pressure is maintained by an automatic regulating and reducing valve controlling the supply from cylinders. When filling and emptying the casing of machine, an explosive hydrogen air mixture must be avoided, so that air is first

11

displaced by carbon dioxide gas before hydrogen gas is admitted. The process is reversed when emptying the machine. It is usual to provide a drier to take up water vapour entering through seals. The purity of hydrogen is checked by measuring its thermal conductivity.

12

DIRECT COOLED TURBO-ALTERNATORS

Conventionally cooled machines dissipate their losses to a coolant which

is entirely outside the coil insulation. The cooling methods described till

now are conventional methods.

Direct cooling is the process of dissipating the armature and field

winding losses to a cooling medium circulating within the winding

insulation wall. Machines cooled in this manner are also called

“supercharged”, “inner cooled” or “conductor cooled”. In this method the

coolant either is in direct contact with conductor copper or is separated

only by materials having negligible thermal resistance.

ADVANTAGES OF DIRECT COOLING:

1. Increase in rating. The attempt is always to increase the rating.

One way of doing it is by increasing the hydrogen pressure in

conventional cooling. Rotor copper heating is the most serious

limitation to the output from a conventionally cooled generator and

increasing the hydrogen pressure beyond 300 kN/m2. It is therefore

logical to adopt direct cooling of rotor conductors in order to

increase machine rating.

2. Decrease in temperature gradient. The stator of a large

turbo-alternator is provided with both axial as well as radial

ventilating ducts through which hydrogen(or air) under pressure is

forced through a ventilating duct provided below the bottom of the

rotor slot. This arrangement is suitable up to ratings of about 100-

125 MW but above this rating the temperature gradient across the

conductor insulation becomes high necessitating the use of direct

cooling. In conventionally hydrogen cooled turbo-alternator a

considerable temperature drop exists between the winding

13

conductor and the coolant. Since in direct cooling the coolant comes

in direct contact with the conductor, the temperature gradients

across the slot insulation, teeth and surface barrier are almost

completely eliminated with the results that winding temperatures

are considerably brought down.

14

COOLANTS USED FOR DIRECT COOLING

1. HYDROGEN. In direct cooled system using hydrogen, both stator

and rotor conductors are made hollow. Hydrogen is pumped

through these conductors from one end to the other. The rotor

conductors consist of rectangular tubes which are ventilated by a

cooling circuit separate from that of the stator. The hydrogen gas is

admitted to the tubes through flexible insulating connections at the

ends from a centrifugal impeller mounted on the out board end of

the rotor shaft. The rotor slot tubes are electrically connected at the

overhang by suitably shaped copper bars to form inlet and outlet

parts. It is possible to build machines up to ratings of about 300 MW

by employing direct hydrogen cooling.

2. Water. The transition from direct hydrogen cooled to direct water

cooled turbo-alternator stator was a logical step for two reasons.

Firstly, as the ratings of turbo-alternators increase, more space has

to be provided for the conductors for the flow of requisite quantity

of cooling gas. Secondly, mechanical limitations on rotor diameter

makes it necessary to increase the physical size of the machine by

increasing its length, and to circulate hydrogen through very long

conductor lengths requires high pressure heads. Water is superior

to hydrogen not only because of its superior heat transfer capacity

but also because the viscosity of pure water is small and is possible

to maintain flow in small tubes without building up dangerously

high pressure heads.

15

DIRECT COOLING SYSYTEM

Turbo-alternators of highest possible ratings so far contemplated

are likely to use hydrogen cooled stator cores and direct water

cooled stator and rotor windings. In the rotor slot of direct water

cooled turbo-alternator the width is reduced near the bottom in

order to increase the tooth width so that the stresses due to

centrifugal forces are kept within limits. An inlet seal is attached to

the outboard (non-coupling) end to enable the water to enter the

bore of the shaft. Water enters at the coil ends and leaves at the

midpoint of each coil side. Water can be circulated through holes in

the slot wedges which are used as damper windings in a turbo-

alternator. The high temperature rise of water flowing through the

copper is, therefore, only 30°C. High grade transformer oil is an

effective coolant and is being used in USA for direct cooling of stator

conductors. But oil has a flash point which can be reached by

machine under fault conditions and therefore can damage the

armature insulation. Water is preferred in UK because it has no fire

risk and if leaking occurs it can be restored by simple drying

process.

16

CONCLUSION

Heating is a natural process and due to presence of ferromagnetic

materials like iron, copper,etc. occurs in alternators and would

dissipate in form of conduction, convection and radiation.

Alternators are used for continuous duty which makes it necessary to

be cooled for increased efficiency and minimum damage. Thus

various cooling techniques like hydrogen cooling, direct cooling and

air cooling are applied to an alternator according to the power

ratings and requirements which have been mentioned in the

previous pages. With the continuous innovation and technology

various new techniques of cooling are being employed for increased

efficiency.

17

REFERENCES

1. A course in electrical machine design by A.K.

SAWHNEY & A. CHAKRABARTI

18

ABSTRACT

The process of energy transfer in case of rotating electrical

machines involves currents in conductors, and fluxes in

ferromagnetic parts. Thus there are various losses in windings.

The losses appear as heat and therefore the temperature of the

system increases. Heat is dissipated in form of

1. conduction

2. radiation

3. convection

Such overheating may cause heavy damage to the system leading

to alternator failure leading to losses in capital. Thus cooling

techniques are applied to cool the alternator and prevent excess

heating of the alternator. Various cooling techniques applied are

1. Hydrogen cooling

2. Air cooling

3. Direct cooling

Which are used according to the requirement of alternator.

19