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been superseded by a more complicated code based on international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superseded later by BS 4999 :1977, Part 23):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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