Ø QUE-ANS: 10-30 words
[1] In alternators above 5000 Kva what is required in the machine windings?
In alternators above 5000KVA, it is required to have temperature detectors in the machine windings to comply with certain statutory requirements.
[2] With diesel sets what extra load occurs when one set motors?
In diesel generator sets, the extra load that occurs when one set motors, is load on the remaining set or sets.
[3] How does a current flow in the shaft of a generator?
Machines with a very strong magnetic field often generate voltage in the shaft that causes a current flow through bearings.
[4] What are the effects of shaft current in a generator?
Flow of electric current through the bearings causes electrolytic action, which will cause Bearing failure in due course and possible sludging of lubricating oil.
[5] Why both bearings are not insulated?
Insulating both bearings would permit the build up of electrical charges (static electricity). The resulting high voltage will be dangerous to operating personnel.
[6] Why the air gap of the alternator to be checked periodically.
To determine the condition of the bearings.
[7] How the air gap of an alternator is measured?
By using tapered, long blade feeler gauge.
Ø 30-60 words
[1] How the shaft current is prevented?
To eliminate shaft currents either (a) outboard bearing shell is insulated in the housing, or (b) outboard bearing pedestal is insulated from the foundation. All lub oil pipe connections that make connection with the shaft or the particular bearing must also be insulated.
[2] What is critical resistance?
It is the value of resistance in the field circuit where the generator will just excite. Above this resistance value, there will be no voltage generated. This is the tangent drawn to the linear portion of the open circuit characteristic.
[3] What is the difference between direct-connected and direct coupled units?
In the former, alternator and driving engine are directly and permanently connected. In the latter case, engine and alternator are each complete in itself and are connected by some device such as friction clutch, jaw clutch or shaft coupling.
[4] How is a direct connected exciter arranged in an alternator?
The armature of the exciter is mounted on the shaft of the alternator close to the spider hub. In some cases, it is mounted at a distance sufficient to permit pedestal bearing to be placed between the exciter and the hub.
[5] What are the advantages and disadvantages of direct connected exciter’?
The exciter has to run at the same speed as the alternator which is slower than desirable. Hence it must be larger for a given output than the gear driven type, because it can be run at high speed and so made proportionally smaller.
Ø 60-100 words
[1] Describe how a Self Excited A C Generator would work
The Self Excited A C Generator basically consists of a Synchronizes Generator, where the 3-phase supply is generated, an A C Exciter and a Rotary Rectifier and a Static Excitation Device.
The Rotating Diodes change A C into D C for the D C Excitation Current, which is supplied through Brushes. The Exciter in conjunction with the A V R is used to monitor the correct Voltage under Load Changes.
Ø 100-150 words
[1] In large DC generators where the current is in excess of 1000A, it is necessary to take steps to avoid magnetizing the shaft :
(a) Explain why this is so, and
(b) Briefly describe how this is accomplished.
In large generators where the current exceed 1000A, it is necessary to take steps to avoid magnetizing the shaft as this could lead to the penetration of stray current accumulated at the bearing, sufficient to puncture the oil film with consequent pitting on the bearing and journal faces. This can lead to bearing failure in due course and possibility of sludging of lubricating oil.
In DC generator, the compounding coils are connected in series and in effect constitute one complete turn around the shaft.
The commutating poles are similarly connected in series and act as if it is one complete turn around the shaft. By arranging for the current in the two sets of connection to in opposite directions to each other around the shaft, the current neutralize each of them, avoiding magnetizing of the shaft.
[2] What are the considerations under which minimum number of generators may be designed for? Discuss if the generators should be design for equal load sharing, or if not the design considerations you would deem essential. List the main loads you would include in your design appraisal.
Number of rating of generator sets:
Power rating of generators determined by power demand of the electrical load.
The designer estimates the number and power rating of the required generator by assessing power demand at sea, port and all other situations.
With one generator out of services:
The remaining set or sets must ensure power to essential services.
The remaining set or sets must have reserve power to permit starting of largest motor without excessive voltage dip.
4) For UMS ship:
Where one generator supplies power, stand by generator should start automatically in case of power failure.
Where 2 generators operate in parallel, any loss of one generator, and the remaining set should be able to supply power.