MEO Orals on Marine Electro Technology Function 5- Part 4

Why AC is popular on board ship ?

1. Smaller, lighter and compact machine size, for a given kW
2. High power and high voltage AC generator can be easily manufactured
3. Voltage can be raised or lowered by transformer
4. AC can be easily converted to DC

Precautions while working on electrical machinery ?

1. Switch off the main switch
2. Out mechanical lock on 
3. Take out fuse
4. Put a signboard “Man Working on Line”

What is AVR ?

• A.V.R  or Automatic Voltage Regulators are used in conjunction with generator for controlling the terminal voltage to give a steady voltage under varying load.
• It senses and controls an A.C generator’s output voltage within (+ or -) 1 to 2%.

What are the types of A.V. R ?

• Carbon pile regulator
• Vibrating contact regulator
• Static A.V. R.
• Rotating sector
• Multi contact
• Magnetic amplifier
• Electronic amplifier

Carbon Pile Regulator (AVR)

1. A resistance from a carbon pile (stack), which is varied by pressure changes, controls the current flow through exciter shunt field 
2. Pressure is applied by springs and relieved by magnetic field strength of electromagnetic coil
3. Current for electromagnetic coil is supplied from alternator output to switchboard
4. AVR is designed such that variations in alternator voltage, due to load change will effect strength of electromagnetic coil and hence alters carbon pile resistance
5. When alternator voltage is low, spring exerts greater pressure and carbon pile resistance becomes low, so more currents flow through exciter shunt field and then increase the output voltage
6. When Alternator voltage is high electromagnetic coil relieves pressure on carbon pile and resistance becomes high. Less current flows through exciter shunt field and decreases the voltage (Strength of Electromagnetic coil relieves spring pressure on carbon pile)

What is rectifier ?

• It is an electric device or circuit capable to allow an A.C current to be converted to D.C current.

What is excitation ?

• To supply and control the correct D.C current for the rotor pole winding to produce the required generator A.C output voltages.
• Production of an electromagnetic field of a generator by supplying exciting current for magnetizing the field magnet
• For excitation, DC is used because DC produces constant rate of magnetic flux
• Continually regulated to maintain the generator output voltages as the load demand fluctuates.

Exciters  

• The source, which generates the field current for excitation of field magnets.

Equalizing bar

• Equaliser is a low resistance circuit, connected across armature ends of series coils of parallel compound generators, via a special bar in switchboard
• Equaliser is fitted to stabilize parallel operation of compound Dc generators.

Rotary converter

1. A rotating diode to convert AC to DC current for alternator excitation
2. Synchronous motor and generator combined unit
3. Filed and armature coils are similar to DC generator, except that slip rings are located on the end of the shaft opposite to commutator
4. AC turns the converter (as synchronous motor) and DC is taken from commutator brush

Procedure for alternator overhaul ?

1. Shut down the generator prime mover and lock off the starting system.
2. Lock off the circuit breaker and isolated the alternator electric heater. Tag the label on the switchboard.
3. Inspect the tightness of terminal connection and insulation.
4. Check the bearing oil level and condition.
5. Clean the cooling air intake filter and exhaust opening.
6. Clean the rotor and stator windings by means of vacuum cleaner with rubber hose and nozzle.
7. If the oil is deposited on the surface of winding, open the drain plug and then removed by special degreasing liquids. (Electro cleaner)
8. Measure the air gap clearance between rotor and stator. (Carefully at lower part)
9. Baking the alternator with lamp. Keep a temp not more than 43°C
10. Disconnect the neutral point from the terminal box.
11. Measure insulation resistance between – rotor to earth, phase to earth, stator to earth, phase to phase
12. It is must at least one mega-ohm, if mega reading has reasonable value, the windings are to be covered with high quality air drying insulation varnish.

 After maintenance work

• Reassemble all necessary parts. 
• Check no load running, the synchronizing and loading.
• On load, practically check for excessive temperature rises and load sharing stability when running in parallel.

Reference:

Oral Guide by – MIN ZAR TAR

MEO Orals on Marine Electro Technology Function 5- Part 4