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Operational guidance for ships electrical plant- Insulation resistance measurement
Electrical insulation tester
Good insulation resistance is essential to the correct operation of
electrical equipment. A means must be available therefore to measure
insulation resistance. Readings taken regularly will give an indication as
to when and where corrective action, maintenance, servicing, etc., is
Insulation resistance may be measured between a conductor and earth
or between conductors. Dirt or other deposits on surfaces can reduce
insulation resistance and cause a leakage current or 'tracking' to occur.
Equipment must therefore be kept clean in order to ensure high values,
in megohms, of insulation resistance.
Insulation is classified in relation to the maximum temperature at
which it is safe for the equipment or cables to operate. Classes A (55 degC),
E (70°C) and B (80°C) are used for marine equipment.
One instrument used for insulation testing is shown in Figure .
Its trade name is 'Megger Tester'. A permanent magnet provides a
magnetic field for a pivoted core which is wound with two coils. A needle
or pointer is pivoted at the centre of rotation of the coils and moves
when they do. The two coils are wound at right angles to each other and
connected in such a way that one measures voltage and the other
Fig: Electrical insulation tester
The needle deflection is a result of the opposing
effects of the two coils which gives a reading of insulation resistance. A
hand driven generator provides a test voltage to operate the instrument.
Test probes are used to measure the resistance at the desired points.
- A.C. motors for ships machinery
Supplying alternating current to a coil which is free to rotate in a magnetic field will not produce a motor effect since the current is constantly changing direction. Use is therefore made in an induction or squirrel cage motor of a rotating magnetic field produced by three separately phased windings in the stator. ...
Use of A.C. generatorsA coil of wire rotating in a magnetic field produces a current. The current can be brought out to two slip rings which are insulated from the shaft. Carbon bushes rest on these rings as they rotate and collect the current for use in an external circuit. Current collected in this way will be alternating, that is, changing in direction and rising and falling in value. To increase the current produced, additional sets of poles may be introduced....
D.C. motors for ships machinery When a current is supplied to a single coil of wire in a magnetic field a force is created which rotates the coil. This is a similar situation to the generation of current by a coil moving in a magnetic field. In fact generators and motors are almost interchangeable, depending upon which two of magnetic field, current and motion are provided.....
Use of D.C. generatorsA current is produced when a single coil of wire is rotated in a magnetic field. When the current is collected using a ring which is split into two halves (a commutator), a direct or single direction current is produced. The current produced may be increased by the use of many turns of wire and additional magnetic fields....
Emergency power supply for ships machinery operation
In the event of a main generating system failure an emergency supply of electricity is required for essential services. This can be supplied by batteries, but most merchant ships have an emergency generator. The unit is diesel driven and located outside of the machinery space .
Maintenance requirement for ships electrical equipment
With all types of electrical equipment cleanliness is essential for good operation. Electrical connections must be sound and any signs of sparking should be investigated. Parts subject to wear must be examined and replaced when necessary. ...
Choice of batteries for ships machinery spaces - Lead acid and alkaline batteries
The battery is a convenient means of storing electricity. It is used on many ships as an instantly available emergency supply. It may also be used on a regular basis to provide a low-voltage d.c. supply to certain equipment.....
Ships battery maintenance guidance The electrolyte level should be maintained just above the top of the plates. Any liquid loss due to evaporation or chemical action should be replaced with distilled water. Only in an emergency should other water be used. It is not usual to add electrolyte to batteries.....
Operating characteristics of battery for ships machinery spaces
Having been 'discharged' by delivering electrical power a battery must then be 'charged' by receiving electrical power. To charge the battery an amount of electrical power must be provided in the order of the capacity.....
Insulation resistance measurement
Good insulation resistance is essential to the correct operation of electrical equipment. A means must be available therefore to measure insulation resistance. Readings taken regularly will give an indication as to when and where corrective action, maintenance, servicing, etc., is required....
Use of navigational light circuitsThe supply to the navigation lights circuit must be maintained under all circumstances and special provisions are therefore made. To avoid any possibility of accidental open circuits the distribution board for the navigation lights supplies no other circuit.....
Ward—Leonard speed control system
As a very flexible, reliable means of motor speed control the Ward-Leonard system is unmatched.The system is made up of a driving motor which runs at almost constant speed and powers a d.c. generator .....
- Danger of electric shock to human bodyThe resistance of the human body is quite high only when the skin is dry. The danger of electric shock is therefore much greater for persons working in a hot, humid atmosphere since this leads to wetness from body perspiration.....
Marine machineries - Useful tags
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