QUES-ANS:10-30 words
[1] Armature coils are tested with a low voltage source. What will be the indications for short circuited coils?
For short circuited coils, the voltmeter reading will show low or zero, other coils will have higher readings.
[2] What do you understand by the term ‘varnished cambric’?
This is an insulating material made of cotton cloth dipped (or coated) with insulating varnish.
[3] How a console circuit is trouble-shooted?
1.) Check operating voltages 2) clean the card
3) Make sure wired connection and push on connectors are tight.
[4] When a component is to be removed from a PCB, what precaution will be taken?
Before desoldering the component a tightly knit metal braid wire will be used.
[5] What is meant by breakdown maintenance?
Break down maintenance is where equipment is untouched until a breakdown occurs.
[6] What is meant by planned maintenance?
Planned maintenance is where equipment is regularly inspected and maintained according to a set timetable.
[7] What is meant by condition monitoring?
Condition monitoring is where equipment is regularly monitored and tested.
[8] Give an example of condition monitoring.
An example of condition monitoring is where a motor may be checked regularly and a record kept of this, and any deterioration can be seen and be dealt with before a breakdown occurs.
[9] What is required for condition monitoring?
Requirements for condition monitoring are; insulation tests, temperature readings and vibration tests and an ongoing record of the results kept.
[10] What should alternator windings be checked for?
Alternator windings should be checked for signs of oil or water contamination and any damage.
[11] What should alternator terminal boxes be checked for?
Alternator terminal boxes should be checked for tightness of terminal connections also damaged or frayed insulation.
[12] What should alternator air-ducting be checked for?
Alternator air ducting should be checked for blockages and that they are free from dirt and dust.
[13] What must be guarded against when cleaning windings with low pressure air?
When cleaning windings with low-pressure air, care should be taken that dirt is not driven deeper into the windings.
[14] How is the alignment of contacts checked?
Alignment of contacts is checked by removing the arc chutes on ACB’s to expose the contact assembly. On some high voltage designs, the chute tilts forward to provide access
[15] What should be used to dress copper contacts?
To dress copper contacts, a fine file or fine glass paper can be used
[16] What maintenance is usually carried out on silver plated contacts?
The maintenance carried out on silver plated contacts is very little but if cleaning is necessary, metal polish may be used
[17] What can excessive application of petroleum jelly cause?
Excessive applications of petroleum jelly can cause burning and pitting of contacts
[18] What is dash points checked for?
Dash pots are checked for correct oil levels. Particular care should be taken to use the correct grade of oil when topping up or replenishing, as proper operation of switch gear is entirely dependent on this.
[19] How are trips and relays checked?
Trips and relays are checked by using a current injection tester.
[20] What do all moving contacts have?
All moving contacts used in control gear have what is known as a wipe or follow through (a cleaning action).
[21] What could happen it ‘wipe’ is lost from contacts?
If the wipe action is lost from moving contacts, a buildup of oxide could occur on the contact surfaces and thus cause overheating.
[22] What condition do contacts need to be in before requiring filing?
The conditions contacts have to be in before filing are badly burnt or pitted, and then a fine file should only be used sparingly.
[23] What reduces mechanical wear?
Mechanical wear can be reduced by applying a thin layer of oil on the contacts.
[24] What causes overheating with copper contacts?
Overheating in copper contacts can be caused by an oxide film forming on them (due to being closed for long periods).
[25] How is rust removed from a magnet?
Rust is removed from a magnet by using fine emery clothe.
[26] What should starter enclosures be checked for?
Starter enclosures should be checked for accumulation of dirt or dust, corroded parts, fixing bolts, earth bonding connection.
[27] What should be checked in high vibration areas?
In high vibration areas, fixing bolts, earth bonding connections, all terminal connections should be checked.
[28] Should fixed and moving contacts be replaced separately?
Fixed and moving contacts should be replaced in pairs.
[29] Before starting work what tests should be carried out?
Before starting work, the actual equipment should be checked with a live line detector, not only between phases but also between phases and earth.
[30] What should be done if an alternator is laid up for a long period
If an alternator is to be laid up for a considerable period, it should be made sure that the windings are suitably heated to prevent internal condensation forming on its insulation.
[31] What precautions will you take when replacing a blown fuse?
Replace the blown fuse with the one of equal voltage and ampere capacity’ in 3 phase circuits. Even if one fuse is blown, replace all the three.
30-60 words
[1] What precautions must be taken when doing work on switchgear?
Precautions to be taken when working on switch gear are; that it is isolated and to ensure that it cannot be made live. Where interlock circuits, pilot lights and control circuits are involved, they may be supplied from a different source, so this must be checked as well.
[2] State with reason the immediate action to be taken upon failure of a fuse in such a system.
Upon failure of a fuse in such a system, it must be replaced by a fuse of same type and same rating. The reason is because loss of one fuse out of three can cause single phasing, resulting the current in the energized winding approximately to double.
[3] What should alternator slip rings be checked for?
Alternator slip rings should be checked for even wear and that carbon brushes have free movement in their boxes. Correct brush pressure can be checked a pull type spring balance (normal pull around 1 – 1.5 kg is usual or according to manufacturer’s instructions). Minimum brush lengths should be no less than 2cm or spring pressure will be too little, which could cause sparking.
[4] What must be done before replacing a fuse?
Before replacing a fuse, the supply must be switched off. The cause of the fault must be located and repaired. The replacement fuse link must be of the correct current rating, grade and type. All three fuses in a three phase supply should be replaced even if one has found to have blown after a fault.
60-100 words
[1] State why the air gap needs to be checked occasionally.
Air gap checked with feeler gauge. It is need to be checked periodically so as to keep a record of the air gap taken approximately 90° apart. This to ensure that a worn out bearing (especially on the power take off side) does not permit rotor to rub against the stator.
Grinding the rotor lamination, filing the stator teeth, increasing in magnetizing current can definitely lower the power factor and air gap measurement is to keep a check so that it is not excessive.
100-150 WORDS
[1] Suggest with reasons the problem that may arise if air gap change is not corrected.
If air gap measurement takes at 90° apart indicates that it is due to worn out bearing, then there is a possibility that the rotor will rub against a stator. A very slight rub will generate sufficient heat causing overheating which can destroy the coil insulation.
If the air gap is uneven, it will reduce the life of bearing due to unbalanced torque and vibration occurs as a result of uneven magnetic flux. Excessive heat generate to the bearing may cause seizure, resulting noise sound in the motor.
Directly, it will result in damaging the moving part of the pump which can reduce the efficiency of the pump performance.