(a) Explain why it is not goo practice to
replace only single fuse in a 3 phase motor and why fuse clips and connections
need examination during fuse renewal. (5m)
(b) State the main difficulty as to why an
overload protection for an AC motor current be set fairly low, and what is done
to overcome this difficulty. (5m)
Answer :-
(a)
It is not a good practice to replace only a single fuse
in a three phase motor because:
1) due
to aging of the fuses and cause dampness of the fuse.
2) Material
properties f the fuse will be weakened due to expose to the increased current.
3) Rating
will be drop and could operate at a much lower operating value than it was
originally designed.
Thus the loss of machinery during
normal operation could occur and it could be dangerous if it were on essential
service equipment. As we know, burnt of one fuse can cause single phasing
resulting the current in the energised winding approx. to double. Frequent
single phasing can damage the respective machinery which is unsafe while
operating.
Fuse clips and connections should
be examined when renewing fuses as a scored contact may cause, operation below
the correct value due to being oxidised, melted or weakened by the high
current.
(b)
The main difficulty as to why an overload protection
for an AC motor cannot be set fairly low because :
1) direct
on line starting for motors causes an initial rushing of starting current that
is 5 to 8 times of full load current. Thus the protective device must be inoperative during the starting periods but
operative on normal moderate overloads, on short circuit and during stalled
condition.
2) similarly
same motors with starting sequence that requires an initial current of 15% to
20% overload at short time interval, requires that the overload device will not
trip the motor when a high starting current before the starting operation has
completed.
The methods to overcome this
problems are:
1) to
leave the relay out of circuit until the starter is in the running condition.
this is done for centrifuge driver which have considerable inertia and long
starting times of the operation of 45 to 60 secs. Thus time delay or relay
units that have a variable time features are incorporated.
To feed the relays from the current transformers
having magnetic circuits designed to become saturated at about 2 to 4 times of
full load current. Although the line current may exceed these values, the
current in the relay coil will be restricted.
(a) Explain why it is important to ensure
that supply connections to a three phase motor are made in the correct order. (2m)
(b) Explain how the correct order of phase
connection from three phase supply lines to motor terminals can be deduced from
standard markings, and state a necessary precaution in guarding against
incorrectly marked terminals. (4m)
(c) Describe the effect of running a motor
with the stator windings connected in star when the motor is designed for
running with delta connected windings. (4m)
Answer :-
(a)
It is important to ensure that the supply connections
to a three phase motors are made in the correct order so that :
1) to
ensure that the direction of motor is correct before couples to a machinery,
otherwise damage may be done to the driven machinery e.g. electro hydraulic
steering pumps.
2) In
case of lifts, hoists, all electric steering gear, the limit switches may not
afford protection. It the lift travels up, instead of down, the top limit
switch may open control circuit, which is not energised and the motor will not
stop.
(b)
Standard markings with British Standard, if generator
is driven with clockwise rotation when looking at its driven end, the terminal
phase sequence will be in the order A, B, C. (For colour marking, the sequence
in Red, Yellow, Blue). If the motor is connected to a supply terminal phase
sequence in the order of A,B,C, then the motor would rotate in clockwise
direction.
A necessary precaution is guarding
against incorrectly marked terminal is by run the motor with coupling to
mechanical equipment disengaged. Ensure correct running direction. In case of
air compressor if wrong direction, compressed air still can be produced but lub
oil pump engaged to compressor will not working, no lubrication cause bearing
damage.
(c)
The effect of running a motor with stator windings
connected in star when the motor is designed for running with delta connected
windings are :
1) a
star delta motor starter gets stuck in star position, malfunction in control
timer, contactors, etc.
2) On
full load, each stator winding will be carrying an overload of 1.73 x rated
line current.
3) This
will cause overheating, because of I² R effect. The motor will run 3 times
hotter.
4) But
the line current will be normal and thermal overload protection will not
protect the motor. This will cause overheating and eventually burn out.
The vessel you have joined has AC
alternators generating a voltage of 3.3 kV to the main switchboard. Discuss
after an inspection of the main switchboard, how would you :
(a)
control access to the live busbars or circuits, for
purposes of inspection or maintenance. (2m)
(b)
Precautions to be taken before work commence on any
unprotected bus bar or circuit. (4m)
(c)
What information would you require to be included in a
‘permit to work’ authorisation form. (4m)
Answer :-
(a)
Control and access to the live bus bars of circuit for
inspection or maintenance:
1) barriers
circuit of access to enclosures containing high voltage conductors shall kept
locked. Only authorised personal shall have access to such enclosures.
2) Working
plate, entry ways kept clear and free of obstruction.
3) Portable
tools used should be earthed or double insulated.
4) Prevent
ingress of moisture, dirt, foreign matters into electrical equipment, when work
is in progress, cover should be closed as soon as access is not required.
(b)
Precautions to be taken before work commence on any
unprotected bus bars or circuit.
No person should undertake repair,
maintenance, cleaning of high voltage system unless such parts are :
1) isolated
locked off, fuses removed, circuits such as alarms, interlocks, heating and
lighting may still be alive, receiving power from other source (reverse
energising of transformer and open circuit of current transformer secondary
coil is a hazard).
2) Dead
voltage indicator must be proves before and after this test.
3) Permanently
display caution notices.
4) All
phases of dead circuit are earthed.
5) Screen
where necessary to prevent danger – on live circuit nearby.
6) Permit
to work is issued.
7) Person
is competent to do the work.
(c)
Information that C/E should give :
1) items
on which you are to work on.
2) Points
on which system is isolated.
3) Caution
notice posted.
4) Earthing
of apparatus.
5) Who
to carry out the job.
6) What
work to be done.
To give:
1) time
and date of work to carry out.
2) Time
and date of completion.
3) C/E
and E/E to sign signature.
On medium voltage, 440V marine, AC,
3 phase, distribution system, the neutral wire is invariably insulated, whereas
in shore practice it is otherwise. Briefly list and give reasons for :
(a) the advantages of insulating the
neutral, and (5m)
(b) the disadvantages of insulating the
neutral. (5m)
Answer :-
(a)
Advantages of insulating the neutral:
1) protect
the essential service in case of earth fault occurred.
2) Less
fire risk.
3) Less
prone to single phasing.
4) No
danger of triple current (3rd harmonic current), overheat will
activated earth fault relay which may trip off the generator.
(b)
Disadvantages:
1) earth
fault detection harder.
2) Phase
voltage at line voltage (if one phase earthed, insulation highly stressed due
to star connected).
3) Less
suitable in high voltage system.
4) Required
step down transformer, double pole switches/fuses for/and lower voltage sub
circuits.
5) Subjected
to over voltage due to seasoned or intermittent fault.
With
respect to silicon rubber insulated cables briefly discuss :
(a) its advantages (6m)
(b) its disadvantages. (4m)
Answer :-
(a)
Advantages of silicon rubber insulated cables:
1) silicon
rubber has outstanding resistance to high temperature.
2) It
can continuously operate at temperature up to 150ºC.
3) In
case of fire, it burns to form silicon oxide which is an electric insulators.
Hence, operation can continue even after a fire, provided conductor is not
melted.
4) It
is not mechanically robust, but remains flexible down to very low temperature.
5) However,
modern grade has adequate mechanical strength to withstand rough handling.
(b)
Disadvantages:
1) not
oil resistant. Hence sheathing of glass braiding with heat resisting varnish
needed.
2) It
is very expensive and only used for special applications.
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