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Monday, January 7, 2013

CHAPTER 3 ENCLOSURES AND COOLING



3.1       ENCLOSURES


3.1.1    General

All motors are made with a stator yoke which acts as a case for enclosing the motor.  Enclosures are of many different types, depending on the service to which the motor will be put.  The minimum requirement is that it will protect the motor from direct damage caused by articles being dropped on it, and that it will protect the operator or any passer-by from danger either by contact with live conductors or with rotating parts.

Enclosures progress from this minimum through various stages of increasing protection to a form which allows the motor to be completely submersed in water and subjected to a specified pressure.  There are additional requirements for motors to be used in hazardous areas; they are discussed in the manual entitled ‘Electrical Safety’.

3.1.2    Classification of Enclosures

The types of motor enclosure are indicated by a system of coding.  There are two such coding systems: one has been in general use for many years, but it has for some time been superseded by a more complicated code based on international agreement.  It is listed in BS 4999, Part 20, and is indicated below.

It consists of the letters ‘IP’ followed by two digits whose meanings are as follows:


First Digit



Second Digit
0
Non-protected
0
No special protection
1
Protected against solid bodies not less than 50mm
1
Protected against dripping water
2
Protected against solid bodies greater than 12mm
2
Protected against water drops up to 15° from vertical
4
Protected against solid bodies greater than 1 mm
3
Protected against spraying water
5
Protected against dust
4
Protected against splashing water


5
Protected against water jets


6
Protected against conditions on ship’s deck


7
Protected against immersion


8
Submersible to specified pressure

Thus a motor classified ‘IP44’ indicates a machine protected against ingress of particles greater than 1mm and of splash-proof design

3.2       COOLING


3.2.1    Temperature Rise

The maximum temperature to which the stator windings may be allowed to rise by the cooling system depends on the type of insulation material round the conductors.  Motors are classified according to the insulating material used, and to each class is allotted a maximum ultimate temperature.  The classification is as follows (according to BS 2757 :1956 - to be superseded later by BS 4999 :1977, Part 23):


Class
Typical Insulating Material

Ultimate Temperature
Y
Cotton, silk, paper, etc., unimpregnated
90°C
A
Impregnated cotton, silk, etc.; paper; enamel
105°C
E
Paper laminates; epoxies
120°C
B
Glass fibre, asbestos (unimpregnated); mica
130°C
F
Glass fibre, asbestos, epoxy impregnated
155°C
H
Glass fibre, asbestos, silicone impregnated
180°C
C
Mica, ceramics, glass, with inorganic binders
>180°C

It should be noted that the classification letters do not follow an alphabetical sequence.  This is because there were originally only three classes - ‘A’, ‘B’ and ‘C’.  Later intermediate classes were added, and it was decided not to disturb the original three which are well understood.  Most motors on offshore installations are Class ‘B’ or ‘F’.  This classification applies not only to motors but also to generators, transformers and similar electrical equipment.

Certain of the higher-temperature materials may be hygroscopic and therefore not always suitable in any particular environment, particularly where dampness is severe.

It should be particularly noted that the classification depends on the ultimate temperature to which the insulating material may be subjected, for it is this which determines whether or not it will suffer damage when heated.  It does not depend on temperature rise alone.  If, for instance, the ambient temperature is 40°C, a Class ‘B’ material may be used if the designed temperature rise will not exceed 90°C, so making the ultimate maximum temperature 130°C.  Designed temperature rises must therefore take into account the greatest expected ambient temperature in which the machine will operate.

3.2.2    Classification of Cooling Methods

The various methods of cooling are indicated by a system of coding based on international agreement.  It is listed in detail in BS 4999, Part 21, and consists of the letters ‘IC’ followed by two digits signifying the cooling circuit arrangement and the method of supplying power to circulate the coolant.  The meanings of the digits are as follows:


First Digit


Second Digit
0
Free circulation
0
Free convection
1
Inlet duct ventilated
1
Self-circulation
2
3
Outlet duct ventilated
Inlet and outlet duct ventilated
2
Integral component mounted on separate shaft
4
Frame surface cooled
3
Dependent component mounted on the machine
5
Integral heat exchanger (using surrounding medium)
5
Integral independent component
6
Machine-mounted heat exchanger (using surrounding medium)
6
Independent component mounted on the machine
7
Integral heat exchanger (not using surrounding medium)
7
Independent and separate device or coolant system pressure
8
Machine-mounted heat exchanger (not using surrounding medium)
8
Relative displacement
9
Separately mounted heat exchanger



This classification applies not only to motors but also to all rotating machines such as generators.

Where it is desired to specify the nature of a coolant, the following letter-code is used in conjunction with the cooling code:

                                                                      -   air                               A
                                                                      -  hydrogen                     H
                                                Gases             -  nitrogen                       N
                                                                       -  carbon dioxide            C
                                                                      -   helium                         L


 
                                                Liquids            - water                           W
                                                                        - oil                                U

When nothing but air is used, the letter ‘A’ may be omitted.

A motor formerly referred to as ‘TEFC’ (totally enclosed, fan-cooled) would have an enclosure code ‘IP55’ or ‘IP56’ and a cooling code ‘IC41’.

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