TASK2 Search strategy : fill in the missing words from the word bank
UNIT 2 Maintenance and trouble shooting
The proper operation of the electrical equipment depends on the maintenance and supervision of the Electrical Engineer. He must always check readiness of the electric machines, the change of their mode of operation as well as supervise starting and shutdown operations of the equipment.
After repairs or long standstill electric machines should always be tested before start. Trial start is usually performed under idling conditions. Then the load gradually increased to the rated value.
Types of maintenance:
1. Break maintenance (corrective)
2. Planned (preventive)
3. Condition monitoring (preventive)
Causes of soon breakdown of the equipment
· Poor installation as a result of work by unqualified personnel
· Heat increases the resistance of the circuit and in this way increases the current causing the material to expand, dry out, crack, and wear down
· Moisture like water and liquids causes expansion of the material, abnormal current flow, or short circuit.
· Dirt, fumes, vapors, grease, oils, etc. causing clogging and abnormal operation of electrical devices until breakdown
· Vibration or physical abuse
Repair of generators
Before performance of any repair electric machines should be tested for troubles and a repair list is made. The repair list must have the following information: a machine registration certificate, a number of faults and repairs, all necessary materials, spare parts, and wiring accessories.
A DC machine repair can be performed in two ways:
1. The machine poles are removed
2. The machine poles are not removed.
1. When the machine poles are removed:
- Disassemble the machine
- Clean it
- Blow and wash the inner parts, the bearings, the commutator and the brushgear
- Find and remove minor defects
- Replace the bearings
- Slot and grind the commutator
- Dry, enamel, and redry the windings
- Partially repair the brush gear with replacing and grinding the brushes
- Assemble the machine, adjust, and test it
- Mount the machine and test it for operation
2. When the machine poles are removed, additionally you must do the following:
- Remove the main and commutating poles with coils
- Disassemble, clean, and wash the poles
- Dry and varnish the coils
- Replace and repair the gaskets
- Restore micanite gaskets of pole cores
- Check insulation resistance of coils
- Assemble, mount, and test the pole coils for polarity
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The repair of an AC synchronous or asynchronous generator is done the same way as of DC machine, but the poles are never removed as well as the commutator is not repaired.
The overhaul of DC and AC machines is usually carried out in the workshops of ship-repairing factories.
BASIC METHODS USED IN TROUBLESHOOTING
· Voltage measurement of a circuit is usually taken by using a voltmeter, which zero reading indicates an open circuit while a low-voltage reading may indicate a shorted component. Connect the voltmeter in parallel with the circuit.
· Amperage measurement is usually taken by an ammeter which indicates and locates common circuit faults such as short, open and grounds. Connect the ammeter in series with the circuit.
· Resistance measurement. An ohmmeter is used to measure the continuity resistance of a circuit or a component. Shut off the power before measuring the resistance
· Substitution is a technique of replacing a faulty components with a good spare one.
· Other methods are signal tracing, using testers, test lamp, resoldering, adjusting and by-passing.
TASK 1 General Electrical Maintenance: matching
1. All equipment is subject to
2. The purpose of maintenance is to
3. The purpose of maintenance is also to
4. the marine environment is not good to
5. The continuous operation of equipment on board ship
A. electrical equipment due to the damp, salt, extreme temperatures
B. wear and tear and will reach the end of it's useful life.
C. help keep operational costs to a minimum.
D. extend the useful life by repair or replacement the defective parts
E. maintain the equipment in a safe and serviceable condition.
Insulation resistance (IR)
Measurement of IR for a diesel generator is made periodically with a portable megger. Before measurement the circuit must be isolated from the rest of the system, in order to avoid false measurement or damage to other electric machines because of high voltage of the megger.
If some of the consumers changed their IR and overload protection doesn’t indicate it, then these consumers should be found by consecutive disconnection of working consumers.
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Factors which influence IR dropping and potential danger:
· Splashing of consumers with water
· Mechanical break of cables insulation and dielectrics in the commutators
· Condensation
· Looseness of commutator cable connection because of vibration
Dropping of IR can cause equipment breakdown and is dangerous for the personnel working on this equipment.
Faults and Effects
Generators
Prime Mover Failure
If only one generator is running the vessel will suffer a blackout.
If running in parallel, the failed generator will try to run as a synchronous motor to drive the engine, causing the reverse power trip to operate.
The remaining generator will then take full load.
This may cause overload again, resulting in blackout.
Excitation Failure
If a single generator is running there will be a loss of voltage causing a blackout.
If the generators are running in parallel they will supply a large leading current, causing the overcurrent trip to operate.
On a brushless generator the rectifier diodes on the shaft can also fail.
If one fails in an open circuit the field current will reduce.
The AVR will try to compensate for this reduction causing a risk of the field winding overheating.
If a diode short circuits there will be an increase in excitation current which could also cause damage to the field winding.
Motors
Single Phasing
This is when an open circuit occurs on one line of a three phase winding causing the motor to act as a single phase machine.
This can be due to a blow fuse, a broken wire on one line, a loose terminal, a dirty contactor, or a burnt out winding.
Effects on a stopped motor:
- The motor will not start.
- The motor takes a large current.
- There is a loud humming noise.
- Overheating takes place rapidly.
Effects on a running motor:
- The motor continues to run with an increased current.
- The motor eventually overheats and can burn out.
Overheating
This may be due to the following factors:
- Overload.
- Dirtywindings or dirty air intake grilles.
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- Damaged cooling fan.
- High ambient air temperature or inadequate ventilation.
- Incorrect voltage or frequency.
TASK2 Search strategy : fill in the missing words from the word bank
WORD BANK: locate, analyze, collect, check, rectify, determine, remove
A six-step approach should be utilized as a search strategy:
1. ____________evidence (stop and think)
2._____________evidence (check assumptions)
3. ____________fault (inspect and test)
4. ____________and _________ cause
5. ___________ the fault
6.____________ the system
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