Question.
(a) With reference/to the insulation testing
of marine electrical plant:
State the reason for insulation
testing;
State the precautions to
be observed when testing intrinsically safe equipment
Type Ex. I'
Describe the overhaul of a a.c.\
Answer.
APART from the fact that good
insulation resistance is an essential condition for maintaining service, the
regular recording of insulation resistance values is undoubtedly the best
method of detecting deterioration and of indicating when remedial action is
desirable, or perhaps essential, in order to prevent complete failure.
Insulation resistance (IR) should
accordingly be measured and recorded at regular intervals, the recording being
preferably on a separate log-sheet for each important machine or circuit, so
that each fresh reading can be compared with previous values and any downward
trend immediately observed.
It cannot be too strongly emphasised that, subject of course to reasonable minimum
values being maintained, trends are more important than actual values; a single
value is of comparatively little significance.
The intelligent interpretation of
results is therefore of great importance and a general appreciation of the
factors involved is essential.
Precaution
Exi –
Equipment where circuit condition limits to < 30 V or B 50mA most commonly
used circuit are communication and alarm
Mostly they are used in pump room and
battery room
Here in addition to intrinsically
safe circuit an electric safe barrier is also fitted in the circuit
Purpose of such barrier is to limit
voltage and current in hazardous area when fault occurs in the circuit
All electrical equipment and
apparatus associated with intrinsically safe circuit needs periodic insulation
testing
it is important that IR test are not
made in such a way that safety devices and insulation used in Exi apparatus and circuit are not damaged by excess test
volts [megger cannot be used it is 500V]
The energy output of intrinsically
safe equipment is very low
Intrinsically safe circuit has to be
tested for insulation by using insulation tester having drooping characteristic
to prevent high current
Tester should be intrinsically safe
when applied to circuit of small inductance and capacitance
Also when carrying out insulation
testing, test leads should be firmly connected to equipment through out and on
completion of testing they should not be detached
Until circuit has been discharged
through testing instrument
For this reason leave tester at least
for 1minutue after test has finished
Test carried out in hazardous zone
should not produce high voltage as it will damage equipment
If this has capacitor it has to be
discharged through equipment only
Therefore to be kept connected for
one minute after test has finished
Answer.
Overhaul of a.c.
motor [damped condition]
If a cage rotor motor has been
flooded with sea water, the insulation resistance can drop down to zero
MΩ. The main problem is to restore the insulation value of the stator
winding to a high value. This is achieved in three stages:
(i)
Cleaning
(ii) Drying
(iii) Re-varnishing
Salt contamination can be removed by
washing with clean, fresh water. Any grease or oil on the windings has to be
removed using a degreasant liquid such as Armaclean. Dry the stater
windings with low power electric heaters or lamps with plenty of ventilation to
allow the dampness to escape. Alternatively, the windings can be heated by
current injection from a welding set or from a special injection transformer.
Be sure to keep the injected current level well below the motor's full load
rating.
With the windings clean and dry, and
if the IR test remains high over a few hours, apply a couple of coats of good
quality air-drying insulating varnish.
Starter and other motor control gear
should be regularly inspected to check and maintain the following items:
a)Enclosure
- Check for accumulations of dirt and rust. Any corroded parts must be cleaned
and repainted. Examine the starter fixing bolts and its earth bonding connection
- particularly where high vibration is present, e.g. in the steering flat and
the foc'sle.
b)Contactors
and relays - Check for any signs of overheating and loose connections.
Remove any dust and grease from insulating components to prevent voltage
breakdown by surface tracking.
c)Contacts
- Examine for excessive pitting and roughness due to burning. Copper contacts
may be smoothed using a fine file and copper oxide, which acts as a high
resistance, can be removed using glass-paper. DO NOT file silver alloy contacts
or remove sliver oxide as it acts as a good conductor. A thin smear of
electrical contact lubrication helps to prolong the life of all contacts. When
contacts have to be replaced, always replace both fixed and moving contacts in
pairs.
Check contact spring pressure and
compare adjacent contact sets forequal pressure.
Check power and control fuse contacts for signs of overheating - lubricate
moving contacts on fuse-holder.
(d) Arc
chutes - Ensure that the magnet armature of contactors moves freely. Remove any
dirt or rust from magnet faces which may prevent correct closing.
(e) Connections - Examine all power
and control connections for tightness and signs of overheating. Check flexible
leads for fraying and brittleness.
(f)
Overcurrent relays - Check for proper size
(relate to motor full-load current). Inspect for dirt, grease and corrosion and
for freedom of movement. A thorough OCR per-
formance test can only be carried out by
calibrated current injection.
(g) Control operation — Watch the
sequence of operation during a normal start-up, control and shut-down of the
motor. Particularly look for excessive contact sparking. Remember to check
operation of emergency stop and auto restart functions
Overhaul of
Check insulation resistance between
coil and earth and between winding
Make sure that connection are marked
[to avoid reverse rotation]
Main switch is put off take off fuse
Men at work board to be put up
Dismantle the motor
Rotor to be packed in plastic and to
be wiped out with cloth and kept in safe place
Considering S.W. has entered wash
stator winding with duistilled water
To wash thoroughly intricate places
use gun
Therefore most of the salts gets
dissolved and will come out
Put heating lamp to heat up the
winding
Check insulation resistance again
Check for any improvement
If no improvement then repeat process
till all salt is removed
Clean winding with electro cleaner
If insulation has improved and near
to 50 mohm spray the winding with quick drying
varnish
Assemble the motor
Check I.R. again between coil and
between winding W.R.T. body