MEO Examination Study Materials Archives - Page 31 of 106

MEO Orals on Marine Electro Technology Function 5- Part 4

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%.

A resistance from a carbon pile (stack), which is varied by pressure changes, controls the current flow through exciter shunt field
Pressure is applied by springs and relieved by magnetic field strength of electromagnetic coil
Current for electromagnetic coil is supplied from alternator output to switchboard
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
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
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)

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

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.

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.

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

Shut down the generator prime mover and lock off the starting system.
Lock off the circuit breaker and isolated the alternator electric heater. Tag the label on the switchboard.
Inspect the tightness of terminal connection and insulation.
Check the bearing oil level and condition.
Clean the cooling air intake filter and exhaust opening.
Clean the rotor and stator windings by means of vacuum cleaner with rubber hose and nozzle.
If the oil is deposited on the surface of winding, open the drain plug and then removed by special degreasing liquids. (Electro cleaner)
Measure the air gap clearance between rotor and stator. (Carefully at lower part)
Baking the alternator with lamp. Keep a temp not more than 43°C
Disconnect the neutral point from the terminal box.
Measure insulation resistance between – rotor to earth, phase to earth, stator to earth, phase to phase
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