VENTILATING SYSTEMS FOR ELECTRICAL MACHINES

Presented by,

C.Balaji – 15E105 ,

S.Deepak arumugam -15E108,

J.Karthickraja – 15E117,

S.Praveen kumar – 15E137

Y.Elangovan – 16E501

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What is cooling ?

Process by which heat resulting from losses

occurring in a machine is given upto 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

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Cooling and Ventilation

In most cases cooling of electrical machines is done by air streams and this cooling is called as Ventilation .

Depending upon the origin of the cooling,

cooling can be classified into

Cooling system

Natural cooling

Separate cooling

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COOLING SYSTEM

CLASSIFICATION

Based on manner of cooling:

Open circuit ventilation

Surface ventilation

Closed circuit ventilation

Liquid cooling

Inner cooling of windings

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ENCLOSURES FOR ROTATING

ELECTRICAL MACHINES

Open machine

Protected machine

Pipe ventilated / duct

ventilated

Totally enclosed

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Induced and forced

ventilation

Both self ventilation and separate ventilation

can be subdivided into two categories

1) Induced ventilation (with internal fan /

external fan)

2) Forced ventilation (with internal fan / external

fan)

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INDUCED VENTILATION WITH

INTERNAL & EXTERNAL FAN

Internal fan’s rotation creates an decreased pressure inside the machines causing the air to be sucked inside the machine as air flows from high pressure to low pressure

Then the air is pushed out by the fan into atmosphere

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FORCED VENTILATION WITH

INTERNAL AND EXTERNAL

FAN

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Radial and Axial Ventilation

The ventilating systems can be classified

into three types depending upon how the air

passes over the heated machine parts are

Radial

Axial

Combined radial and axial

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Radial ventilating systemThis 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

This method is suitable for machines

about 20kW

For, larger machines , the core is

normally divided to provide radial

ventilating ducts

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WorkingAir flowing up the radial ducts only a layer

3mm wide in contact with the core walls is

effective in cooling

The middle 4mm wide band contributes

very little to cooling of the core

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Advantages

Minimum energy

losses for ventilation

Sufficiently uniform

temperature rise of

machines in the axial

direction

Disadvantages

It makes the

machine lengths

larger as space for

ventilating ducts has

to be provided along

the core length

It may become

unstable in respect

to quantity of cooling

air flowing

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Axial ventilating system

It is suitable for induction machines of

medium output and high speed machines.

Axial ducts are provided and two types of

axial system

1. Simple axial system

2. Double axial system

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WHY WE USED IN HIGH

SPEED MACHINES ?

By use of a solid rotor and reduced core

length and in order to avoid centrifugal stresses,

axial ventilation is preferred.

To increase the cooling surface holes are

punched where heat dissipation is more

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Advantages & Disadvantages

It improves cooling but requires a large core

diameter for the increased core depth.

Non uniform heat transfer.

Increased Iron loss.

In large number of cases this loss is more

than compensated by improved cooling.

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Combined Axial and Radial

ventilating system

This method is usually employed for large motors

and small turbo alternators.

Since the area of ducts to carry cooling air

becomes large.

So to minimise losses this type is preferred

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The air is drawn into the machines from one

end is encouraged to pass through the ducts

by and finally the rotor mounted fans forces

out the air

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Cooling of totally enclosed

machines

They are used in applications where the use of open and protected machines is inadmissible

In a totally enclosed machine, there is no air connection with outside and all heat developed inside should be dissipated into the surroundings from the external surface of the frame

It is quite uneconomical

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Types of totally

enclosed machines

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1.Ventilated Frame machines

A fan is mounted on the shaft outside the working parts of the machine and it blows the air over the carcass through the space between main housing and thin cover plate

Suitable for power outputs upto 25kw

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For larger rating , it is

necessary to provide in

addition , circulation of air

inside the machine to

improve heat dissipation.

This is done by providing a

fan inside the machine which

may circulate air in the inner

portion

Air inside the machine acts

as primary coolant

Air outside the machine

acts as Secondary

coolant

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2.Ventilated radiator machines

In such machines , internal

fan circulates air inside the

machine.

At higher ratings the

outside air may be cooled

by water if there is a

convenient source of water

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References

“A Course in electrical machine design” by

A.K.SAWNEY

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