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Direct Current Supply & Distribution, D.C. Generators Field Connection & Earth Lamp Circuit

A 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.

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With many coils connected to the commutator, sparking will occur as the current collecting brushes move across the insulated segments. Commutating poles or interpoles are used to reduce this sparking. They are in fact electromagnets having a polarity the same as the main pole which follows in the direction of rotation.

The magnetic field between the poles is produced by what are known as 'field coils'. These coils are excited or energised by the current produced in the machine.

The soft iron core of the field coils retains some magnetism which enables a preliminary current generation to build up eventually to the full machine output. The field windings can be connected to the output current in a number of ways—shunt, series or compound. The compound wound arrangement is usual since it provides the best voltage characteristics.

D.C. generators field connection

Fig:D.C. generators field connection

The compound wound generator has two sets of field coils . The shunt coil has many turns of fine wire and the series coil has a few turns of heavy wire. The shunt field produces full voltage on no-load which falls off as the load current increases. The series field creates an increase in voltage as the load increases. Properly combined or compounded the result is a fairly constant voltage over a range of load (Figure below).

D.C. generators characteristic curves

Fig: D.C. generators characteristic curves


Direct current distribution

The generated supply is provided to conductors known as 'bus-bars' which are located behind the main switchboard. The supply then passes through circuit breakers to auxiliaries directly or to section or distribution boards. A circuit breaker is an isolating switch. A section board is a grouping of electrical services fed from the main board. A distribution board feeds minor supplies such as lighting and may itself be fed from the main board or a section board. The distribution system is shown in diagram

A two wire system is usual to provide a supply and return to each item of equipment. An earth lead would be the only electrical connection between any item of equipment and the ship's structure. With compound wound generators a third bus-bar would be introduced as the equalising connection between machines.

A fuse is a type of switch which isolates a circuit if an excessive current flows. To reconnect the circuit, after discovering the cause of the overload, the fuse must be rewired or replaced. The fuse is in effect a weak link in the circuit designed to break and protect equipment from damaging high currents. A semi-enclosed or rewirable fuse will have provision for a wire to be replaced after it has burnt out. The correct rating of fuse wire should be replaced within the holder to reinstate the circuit. A cartridge fuse has the wire enclosed within a ceramic body and it is not rewirable. A 'blown' cartridge fuse must be replaced by a new one, The cartridge fuse is to be preferred since the fusing current value is more reliable than for a rewirable type.

A circuit breaker is an isolating switch which also functions as a fuse. It has two designed ratings: one of the normal safe working current, the other the overload current. The breaker is closed against the action of a spring to make the circuit and supply the section board or auxiliary. A trip mechanism opens the breaker, a fast opening being ensured by the spring. When desired the breaker is tripped or opened manually. It will also open if the overload current rating is exceeded for a period of time.

A delay mechanism prevents the breaker opening for short-period overload currents. The circuit breaker opens or closes both supply and return leads in the circuit. Where a circuit breaker feeds the generator supply to the bus-bars a third 'make-or-break' arm will be provided for the equaliser connection.

Preferential tripping is a means of retaining essential electrical supplies. In the event that a generator cannot supply all the load then non-essential loads are disconnected by preferential trips. The intention is to reduce the generator load while ensuring essential equipment such as steering gear, navigation lights, etc., retains its electrical supply. Various circuit faults can occur as a result of either a break in the conductor (cable) or a break in the insulation. An open-circuit fault results from a break in the conductor and no current flow will take place, A short-circuit fault is due to two breaks in the insulation on, for example, adjacent conductors. The two conductors are connected and a large current flow takes place. An earth fault occurs when a break in the insulation permits the conductor to touch an earthed metal enclosure (or the hull).

D.C. generators Earth lamp circuit

Fig:D.C. generators Earth lamp circuit

Earth faults are usually detected by the use of earth indicating lamps. Two lamps are used, each rated for the full system voltage, but connected in series across the system with the mid point earthed , If the system is correctly insulated then both lamps will glow at half brilliance. The lamps are placed close together to enable a comparison to be made. A direct earth in one pole will short circuit its lamp, causing the other to shine brighdy. A slight insulation breakdown would produce a difference in bulb brightness between the two. Where an earth fault is detected the circuit breakers for each separate circuit must be opened in turn until the fault location is discovered. The particular section or distribution box would then have to have its circuits investigated one by one to locate the fault and enable its correction.


Direct current supply

The supply to a distribution system will usually come from two or more generators operating in parallel. Each generator must be provided with certain protective devices to ensure against reverse currents, low voltage or an overcurrent. There must also be ammeters and voltmeters in the circuits to enable paralleling to take place.

D.C. generators parallel operation

Fig:Protective trips for the parallel operation of two d,c. generators



The circuit for two generators operating in parallel is shown in Figure above. A triple-pole circuit breaker connects the supply to the bus-bars and also the equaliser bus-bar. The arrangement of the various protective trips can be seen, with excess current protection being provided in each pole. The reverse current trip prevents the generator operating as a motor if, for instance, the prime mover stopped.

The voltmeters and ammeters are provided in the generator supply circuits for paralleling purposes. A voltmeter is positioned across the bus-bars to indicate their voltage. Consider the situation where one generator is supplying the bus-bar system and a second generator is to be paralleled with it. The second machine is run up to speed and its field current adjusted until the two machines are at the same voltage. The circuit breaker connecting the second machine to the bus-bar can now be closed and the Field current adjusted to enable the generator to take its share of the load. When the load is evenly shared the two machines can then be left to operate in parallel. The equalising connection will cater for any slight changes in load sharing that occur.





Related Info:

  1. A.C. motors for ships machinery
    2. 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. ...

  2. Use of A.C. generators
    3. A 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....

  3. D.C. motors for ships machinery
    4. 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.....

  4. Use of D.C. generators
    5. A 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....

  5. Emergency power supply for ships machinery operation
    6. 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 .

  6. Maintenance requirement for ships electrical equipment
    7. 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. ...

  7. Choice of batteries for ships machinery spaces - Lead acid and alkaline batteries
    8. 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.....

  8. Ships battery maintenance guidance
    9. 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.....

  9. Operating characteristics of battery for ships machinery spaces
    10. 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.....

  10. Insulation resistance measurement
    11. 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....

  11. Use of navigational light circuits
    12. The 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.....

  12. Ward—Leonard speed control system
    13. 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 .....

  13. Danger of electric shock to human body
    14. The 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.....




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