Operation-When the double pole switch is closed the navigation light is illuminated. Current in the relay circuit causes the relay coil to energise so contact 'a' ,may be attracted to 'b'. A low voltage lamp only is needed for the indicating lamp, so there is a small voltage drop across that part of the circuit. If the indicating lamp fails the circuit is completed through the resistance C, so the navigation light does not fail.
If the navigation light fails, or if a fuse blows the current in the circuit ceases and the relay de-energises. Contact 'a' springs back to contact 'b' and the buzzer circuit is completed.
In case of failure of ships mains, the double pole switch may be switched over to emergency supply.
Types of fluorescent lights
Cold cathode type-only used for decorative lighting
a, High pressure fluorescent
(i)Mercury vapour types )Used mainly for street lighting
(ii) Sodium type , )but sometimes used for deck lighting
b, Low pressure Hot Cathode Fluorescent type discharge Lamp
Principle- A length of glass tubing contains a small amount of mercury vapour and argon gas, at a very low pressure ( 10-6 atmospheres ). A heater element forming an electrode is situated at each end of the tube. These electrodes may be coated with an oxide to improve thermionic emission. The interior of the tube is coated with fluorescent powder ( a Phosphor coating)
If a suitable voltage is applied between the two electrodes a DISCHARGE strikes between them and the mains voltage is then sufficient to maintain the discharge. This occurs in low pressure so that the lamp will run at a comparatively low temperature and so will not effect the fluorescent coating. The electrons from the electrode collide with the mercury atoms. This dislodges an electron from the atom making the mercury atom a positively charged ION. As the dislodged electron returns to the influence of the ION ( i.e. the electron changes from one energy level to another) a certain amount of electro-magnetic radiation (i.e. a photon) is given off in the form of Ultra-violet light. These rays activate the fluorescent coating and the luminous surface provides a glare free efficient light.
Operation -With switch start circuits to start a discharge across the tube a large Voltage Impulse is required.
This may be obtained by the following methods;
Glow type switch and choke
Thermal type switch and choke
There are also methods using starterless circuits, referred to as rapid start or instant start,. where a drop in potential between the electrode and an earth strip is sufficient to ionise the gas adjacent to the electrode and this ionisation then spreads across the whole tube.
C1- Radio suppresser
C2- Power factor correction
Glow type switch starter circuit
When the control switch is closed the contacts on the bimetal strips which are open form the electrodes of a small discharge lamp. The mains voltage is sufficient to cause a glow discharge in the starter which warms the bimetal strips. The strips bend until contacting and a large current flows through the electrodes of the main tube, forming an electrode cloud around their cathodes (thermionic emission)
Shortly the bimetal strips cool sufficiently to break contact, This sudden reduction in current flow causes a large e.m.f to be generated in the choke ( typically four to five times mains)
The voltage surge across the tube is sufficient to ionise the gas, reducing the resistance to electron flow and allowing the discharge to occur and be sustained by the mains voltage.
Operation may still occur if the mains voltage is reduced, however the tube is unlikely to start hence this type of light is not used for emergency lighting
The choke has a second purpose other than providing the start voltage. It maintains a constant correct potential difference across the tube when the mains is an alternating current.. If a d.c is used then a ballast resistor ( which may be an incandescent light) must be used
Advantages and disadvantages of Fluorescent lighting
On Tankers all cables must be either lead alloy sheathed and armoured, mineral insulated copper sheathed ( the ends must be sheathed to prevent moisture being absorbed by the hygroscopic insulation material ) or non-mineral impervious sheathed and wire braided so long as they are laid in a pipe ( the csa of the wire must be less than 30% of pipe bore.
Glands fitted to bulkheads must allow for expansion and be weather tight, water tight bulkheads should only be penetrated by a suitable gland.
Cables should be laid away from hot surfaces
All installations must be flame proof
Metal casing should be adequately rust protected and earthed. PVC conduit must not be used in fridge spaces or on deck unless specially approved as liable to breakdown in cold. Cable sheathing, unless galvanised, should have a rust preventative coating. The cable should not be laid behind insulation.
Flame test of insulation
A standard 4ft length of cable is held vertical and is burnt by a flame of known strength
If the flame travels the full length the cable is graded as flame extending,( not in common use.
If the flame is extinguished before it reaches the top end, it is classified as Flame retardant,
, If a cable is graded as flame retardant it must be able to resist the flame, and also after cooling be able to withstand an a.c voltage of twice the rated voltage for one minute.
Relays and Solenoids
The wire coil of the electromagnet, without its core of magnetic material is called the solenoid. If this solenoid is provided with a movable soft iron core and current flows through the turns of the coil , the magnetic field tends to pull the plunger in to the centre of the coil. Accordingly the coil with its moving centre is called the solenoid.
The plunger can be used to operate a great many mechanical applications. A spring is often fitted above the plunger to positively return it to its start position once the current is turned off. The plunger may also be used via an non-magnetic extension be used as a pusher, a spring again returning it to the start position.
Either a direct or alternating current may be used to energise the solenoid, since either type will produce the magnetic field around the coil. There is one precaution however. The core of the electromagnet finds itself in the magnetic field of the coil. If a steady direct current flows through the coil , no current will be induced in the core since both the core and field are stationary. But if an alternating current flows through the coil, the changing magnetic field will cause a current to be induced in the core. This is called the eddy current.
The eddy current is undesirable on two counts. The flow of current through the core represents a power loss , which must come from the source. Also the flow of current may cause the core to get quite hot. To reduce the eddy current, the core is not built solid but is made up of many thick slices, called laminations. Each lamination is insulated from its neighbour by a coat of varnish or similar material. This offers considerable resistance, and as a result, the eddy currents are cut down. In solenoids operating on alternating current, the plunger is built up of laminations.
An alternating current coil will offer a greater resistance to current flow than a direct current coil of same ohm resistance due to its inductive reactance. Hence, if a coil designed to be operated by an alternating current is connected to a source of direct current at the same voltage, the flow of current may be great enough to burn out the windings.
On very common application of an electromagnet is in the operation of an electric switch. In this form it is known as an electromagnetic relay.
The sensitivity and current draw of a relay is determined by the wire wound on the core. This is determined by size and therefore breaking capacity of the contacts.
The relay coil may be energised by either direct or alternating current. Where direct current is employed, there are no special problems. Alternating current may be employed since the polarity does not effect the attraction of the armature. However, the rapid alternations of the magnetic field cause the armature to vibrate, or 'chatter'. Since the contacts are controlled by the armature, the controlled circuit too, will be affected..
One method of remedying the fault is to rectify the alternating current before applying it to the relay. Small semi conductor diodes are employed. Another method is to connect a fairly large capacitor across the coil. Frames are laminated to prevent eddy current losses.
Overload circuit breaker is a variation on a relay.
A big advantage of A.C. transmission is the ability to easily change voltage by means of a transformer. As there are no moving parts maintenance is very low and efficiency very high ( typically 98%).
A transformer consists of two insulated coils wound separately over a closed magnetic field, usually iron, of low reluctance. An alternating supply E1 acts across a coil called the primary, a voltage is induced in the secondary coil E2.
Details of a small transformer-the core is built up of stamped laminated sheets of silicon iron about 0.35 mm thick insulated from one another by a thin layer of paper or varnish. The purpose of laminating the core is to reduce the loss due to eddy currents induced by the alternating magnetic flux. The vertical portions of the yoke are called the 'limbs' and the horizontal portions are called the 'yokes'
Auto transformers are used in;
In power factor corrections with capacitance's.
Incorporated in portable appliances such as radios.
Iron losses- As the magnetic field sweeps across the conducting material a voltage is induced which sets up a current within it. When this current, the EDDY CURRENT, flows the resistance within the material causes heat to be produced. The material does not have to be magnetic for the eddy currents to be set up but must be a conductor.
If the material is magnetic the magnetic field around the coil magnetizes that material and rearranges its molecules. Each time the magnetic field reverse the molecules are rearranged. As the result of molecular friction called HYSTERESIS LOSS this rearrangement produces heat
Eddy currents- losses reduced by laminating material
Hysteresis loss-reduced by using soft iron or annealed steel.
Copper losses-These are the I2R losses in the copper wires of the primary and secondary coils. Increase in temperature of these coils will increase their resistance.
Iron losses are constant, but copper losses are proportional to the square of the current.
Vs/Vp approx equals Ns/Np approx equals Ip/Is
Efficiency = Output power/ I nput power
= Output power/ Output power - Iron losses + Copper losses
= Vs Is Cosf/ Vs Is Cosf + Iron losses + Ip2Rp + Is2Rs
Maximum efficiency occurs when iron losses and copper losses are equal. The losses in a transformer for a given frequency are largely determined by the value of the working flux density in the windings. With a small transformer it is possible to work with fairly high densities in both the iron and the copper without exceeding the maximum temperature so air cooling is satisfactory. However in larger transformers either the flux density or the current density must be reduced or somemethod of cooling used. This is normally achieved by immersing the transformer in insulating oil in a chamber with cooling fins or tubes.
It is usual in A.C. installations to fit the ammeter to the secondary circuit of a current transformer. This avoids heavy current connections to the meter and allows an ammeter switch to be fitted to read the current in each phase. The same current transformer can be used for the wattmeter and the reverse power relay. Voltage transformers are also normally provided for use with voltmeters, wattmeters, synchroscopes and reverse power relays.
Safety-The secondary circuit of a current transformer must never be opened or left open under load. The large voltage induced (due to high flux density produced in the core with no 'back' ampere turns from the secondary coil) will cause the transformer to overheat. The secondary circuit must be such that short circuit conditions will not cause damage.
In the event of breakdown of insulation between primary and secondary windings it is a requirement that one end of the secondary winding of the current transformer and the voltage transformer and the metal cases of the instruments shall be earthed.
The principle of the current transformer is that the primary winding carries the full load current and as such is made of large diameter low resistance wire. The secondary winding steps up the small volt drop that occurs over the length of the primary wire