IBX5980432E7F390 Electrical Test part 4 - BLOG PELAUT

Electrical Test part 4



At what value of current should the OCR(= Over Current Relay) be set?
To protect a modern MCR (CONTINUOUS MAXIMUM RATING) motor the thermal OCR should be set at the full-load current of the motor. This will ensure that tripping will not occur within 2 hours at 105% FLC. At 120% FLC tripping will occur within 2 hours

A motor is protected by a thermal OCR and back-up fuses. Can the motor exceed its rated temperature without being tripped by the protection?
YES! Although overheating is usually indicated by the current drawn by the motor rising above its rated value, a number of other situations can contribute to motor oyerheating. For example - very high ambient temperature; inadequate ventilation; a star-delta starter remains in the star connection; stopping and starting too often; worn or dry shaft bearings.

A dual-wound induction motor is arranged to create either 6 poles or 10 poles. Estimate the rated speeds assuming that the rotor slips by 5% and the power supply is at a frequency of 60Hz.
From ns = f/p (ns = speed of rotating flux pattern) High speed (6-pole): ns = 60/3 = 20 rev/sec or 1200 rpm but rotor runs at 95/100 % x 1200 = 1140 rpm
Low speed (10 pole);   ns = 60/5 = 12 rev/sec or 720 rpm but motor runs at 95/100 % x 720 = 684 rpm.

What is the most common cause of induction motor failure?
Failure of starter insulation due to dampness is a major problem with marine motors. Open ventilated motors are most at risk, particularly when they are not used for long periods. Anti-condensation heaters should be regularly checked to see that they are actually working and keeping the motor dry.

How often should a motor be cleaned?
Basically this will be determined by the local conditions and the type of ventilation. Only the external surfaces of totally enclosed motors will require regular cleaning. But both the outside and inside of open ventilated motors will require routine attention. The inside of a totally enclosed motor can be cleaned if the motor has been dismantled for bearing replacement. Motors in areas where considerable amounts of air-borne dust are expected, hatch cover motors are an example, will obviously require more frequent cleaning than other motors.

How do you check the insulation resistance between phases on an induction motor?
Larger motors are usually 'six terminal', which means that all six ends of the stator windings are brought out to the terminal block. Links between the terminals are used to star qr delta-connect the motor. Disconnect the supply leads and remove the links. Test between phases with an insulation resistance tester.

Test Between: A l - B1; B1 - C1 and A1 - C1

A cage-rotor induction motor has been flooded with sea water. Insulation resistance is down to zero MQ. What is the procedure for putting the motor back into service?
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 stator 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.

Describe in short the regular inspection on housing, starter contacts and motor controls
(a) Enclosure;
(b) Contactors and relays;
(c) Contacts
(d) Connections;
(e) Over-current relays.
(f) Controlling the operation.
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. Ensure that the magnet armature of contactors moves freely. Remove any dirt or rust from magnet faces which may prevent correct closing.
(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 silver 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 for equal pressure Check power and control fuse contacts for signs of overheating - lubricate moving contacts on fuse-holder.
(d) Connections - Examine all power and control connections for tightness and signs of overheating. Check flexible leads for fraying and brittleness.
(e) Over-current 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 performance test can only be carried out by calibrated current injection.
(f) 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.

What would happen if the RIS capacitor in a glow-switch failed to a short-circuit?
The tube would not strike but would glow at its ends while the choke may overheat and eventually fail A similar result would occur if the bi-metal strips of the starter welded together.

Give 3 methods to alleviate a stroboscobic problem
(a) Use a mixture of incandescent and discharge lighting in the same area.
(b) Use twin discharge lamp fittings with each lamp wired as a 'lead-lag' circuit, i.e. the lamp currents are phase displaced so that they go through zero at different times, hence the net light output is never fully extinguished.
(c) Where a 3-phase supply is. available, wire adjacent discharge lamps to different phases (Red, Yellow, Blue) so the light in a given area is never extinguished.

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