Electrical DC Generators

D.C. Generators

Operation of a d.c. generator relies (as with alternators) on the principle that when magnetic lines of force are cut by a conductor(Fig. 6.1) a voltage is induced in the conductor. Size of induced voltage and resulting current are dependent on magnetic field strength, length of conductor and speed of cutting.
The direction of current flow is dictated by the relationship between magnetic field and direction of movement of the conductor. Ii can be found from Fleming’s Right Hand Rule, which is applied to give direction of conventional current flow during generation,A simple generator can be constructed from a loop or coil of wire mounted on a spindle and arranged for rotation between opposite magnetic poles The field-cutting action of the straight sides will cause current flow as the result of induced voltage. Direction of flow is shown by the arrows (found from the Right Hand Rule) and can be seen to be continuous around the loop. The voltages generated are in series and therefore add to give twice the voltage produced in one side.
Direct current can be collected from the wire ends through the commutator which consists of two half-rings with brushes. Each brush takes current from one half-ring in turn, so that current flow is always in the same direction f or each collecting brush. The out put is not steady but has a wave form.

Electrical Fuses

Fuses
High current flow through a thin fuse wire will raise its temperature causing it to melt and break the circuit before the current excess reaches a level sufficient to damage other, more substantial, parts of the system. Melting temperature depends on the material used (tinned copper in rewireable fuses melts at 1080ºC, the silver in cartridge fuses at 960ºC). The wire is sized so that the normal current is carried without overheating, but due to the resistance of the relatively small wire, that excess current will produce heat sufficient to melt it. Current rating gives the normal current that may be carried: minimum fusing current is the smallest current that will cause melting.

A fuse will melt much quicker with very large fault current than when the value of fault current is only just above the minimum fusing current. Time/current characteristics are found by testing six or more of the same type of fuse at different currents and plot ting the results. The bottom current for the test is not more than 1.05 X minimum fusing current, and the top current is one that will melt the wire in not more than 0.5 second. The other test currents are equally spaced between these. Fuses are rated for particular ac. and/or d.c. voltages



Cartridge Fuses
High Rupture Capacity (HRC) fuses have silver wire enclosed in a quartz powder filled ceramic tube with metal end caps (Fig. 9.1). Arcing when this type of fuse blows is buried in the powder, fusion of which in the arc path helps to extinguish it.
HRC fuses can be used for very high fault levels: deterioration is negligible; they have accurate time/current characteristics and reliability for discrimination; they are safer if accidentally inserted on a fault: there is no issue of smoke or flame; cartridges are sized to ensure that the correct value fuse is fitted.
Semi-enclosed Fuses
The rewireable fuse has an insulated carrier for safe handling and containment of the wire in an asbestos lined tube
The wire is easily replaced after operation, but the des is open to abuse as too heavy a wire can be used which could mask a fault and also cause severe arcing if it did operate. Another fault is that of premature failure if the wire is made thinner by oxidation or con tact with air, or by being stretched when fitted (a problem with wire made of lead, tin or an alloy of the two).
Fuses in Service
Fuses may be used as the only protection in a steady load circuit, such as for lighting. An ac. motor with its high starting current and varying load has fuses in each of the supply conductors, but fitted as a backup for the other forms of protection and to break the circuit in the event of a short-circuit current greater than that which the ordinary contact breakers are designed to interrupt without damage. Very accurate time/current characteristics are needed for fuses used in conjunction with other safety devices, to ensure that the overload trip is allowed time to operate for moderate over- current but that the fuse blows first if there is very high short-circuit current.
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Electrical DC Switchboards Miniature Circuit Breaker MCB

D.C. Switchboards
The switchgear of d.c switchboards, unlike that of enclosed a.c. equipment was mounted on what was literally a board or screen of panels. On the front the open main circuit breakers, distribution breakers, knife switches and instruments were easily accessible for inspection and maintenance. Safeguards against contact were only an insulated handrail and a rubber mat’ closed gate normally prevented entry to the passage behind the board where the copper bus-bars and rheostats were mounted.
Enclosed or dead-front boards are only required for d.c. installations if the voltage is greater than 250 volts. Most of the D.C. switchboards on British vessels just prior to the general change to ac. were for 220 volt supplies.


Main Circuit Breaker
The brush type moving contacts of the generator breaker are closed against a resisting spring by a lift-and-latch handle. The handle, when raised, slips the operating pin over the latch, where it is clipped into position. Downward movement of handle pushes
the cam plate against the hinged contact holder. The leverage and pressure applied forces the brush contacts to spread on the fixed studs. This brushing action is beneficial in that contacts are wiped and any film or deposit removed. Vibration with consequent burning or welding is prevented by the pressure, sustained by the resilience of copper strips, which are clamped In the manner of a leaf 3oring. Conductivity of the copper is slightly reduced by a small amount of alloying material added to prevent work-hardening and to give the necessary resilience.
The tripping of a main breaker due to overload or when load current was being carried would causes arcing and damage to the main contacts. Arcing contacts are therefore fitted and arranged to make first and break last to protect main contacts from burning. They are of metallized carbon, sintered silver-tungsten or other material which does not weld at high temperature; such compositions are not suitable for carrying high current over long periods.
There is a set of main and arcing contacts for each connection to be made from the generator to the switchboard bus bars. They are fixed together by bars for simultaneous movement by one handle. Thus, for a two-wire insulated d.c. system the triple pole circuit breaker connects the positive, negative and equalizing connections in one operation. The equalizing contact is given a lead so that it closes first Short-circuit’ between poles is prevented by insulation.
The breaker is held in the closed position by the latching device but is easily opened by the trip. Tripping is automatic in the event of short-circuit, overload, reverse current or low-voltage faults. For normal opening, the trip can be operated manually after reducing the load with the rheostat.

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Ships electrical plant and distribution system for the A.C. generators

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.
 
How alternating current is produced onboard ?

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.

The magnetic field is provided by electromagnets so arranged that adjacent poles have opposite polarity. These 'field coils', as they are called, are connected in series to an external source or the machine output.



Fig: Three-phase alternator output

If separate coils or conductors are used then several outputs can be obtained. Three outputs are usually arranged with a phase separation of 120°, to produce a three-phase supply. The supply phasing is shown in fig . The three-phase system is more efficient in that for the same mechanical power a greater total electrical output is obtained. Each of the three outputs may be used in single-phase supplies or in conjunction for a three-phase supply. The separate supplies are connected in either star or delta formation .

The star formation is most commonly used and requires four sliprings on the alternator. The three conductors are joined at a common slipring and also have their individual siipring. The central or neutral line is common to each phase. The delta arrangement has two phases joined at each of the three sliprings on the alternator. A single-phase supply can be taken from any two sliprings.



Fig: Star and delta three-phase connections

So far, alternator construction has considered the armature rotating and the field coils stationary. The same electricity generating effect is produced if the reverse occurs, that is, the field coils rotate and the armature is stationary. This is in fact the arrangement adopted for large, heavy duty alternators.

The field current supply in older machines comes from a low-voltage direct current generator or exciter on the same shaft as the alternator. Modern machines, however, are either statically excited or of the high-speed brushless type. The exciter is required to operate to counter the effects of power factor for a given load.

The power factor is a measure of the phase difference between voltage and current and is expressed as the cosine of the phase angle. With a purely resistance load the voltage and current are in phase, giving a power factor of one. The power consumed is therefore the product of voltage and current. Inductive or capacitive loads, combined with resistance loads, produce lagging or leading power factors which have a value less than one. The power consumed is the product of current, voltage and power factor.

The alternating current generator supplying a load has a voltage drop resulting from the load. When the load has a lagging power factor this voltage drop is considerable. Therefore the exciter, in maintaining the alternator voltage, must vary with the load current and also the power factor. The speed change of the prime mover must also be taken into account.

Hand control of excitation is difficult so use is made of an automatic voltage regulator (AVR). The AVR consists basically of a circuit fed from the alternator output voltage which detects small changes in voltage and feeds a signal to an amplifier which changes the excitation to correct the voltage. Stabilising features are also incorporated in the circuits to avoid 'hunting' (constant voltage fluctuations) or overcorrecting. Various designs of AVR are in use which can be broadly divided into classes such as carbon pile types, magnetic amplifiers, electronic types, etc,

The statically excited alternator has a static excitation system instead of a d.c. exciter. This type of alternator will more readily accept the sudden loading by direct on-line starting of large squirrel cage motors. The static excitation system uses transformers and rectifiers to provide series and shunt components for the alternator field, that is, it is compounded. Brushes and sliprings are used to transfer the current to the field coils which are mounted on the rotor.

The terminal voltage from the alternator thus gives the no-load voltage arid the load current provides the extra excitation to give a steady voltage under any load condition. The careful matching of components provides a system which functions as a self regulator of voltage. Certain practical electrical problems and the compensation necessary for speed variation require that a voltage regulator is also built into the system.

The brushless high speed alternator was also developed to eliminate d.c. exciters with their associated commutators and brushgear. The alternator and exciter rotors are on a common shaft, which also carries the rectifiers. The exciter output is fed to the rectifiers and then through conductors in the hollow shaft to the alternator field coils. An automatic voltage regulator is used with this type of alternator.



Fig: Alternator construction

The construction of an alternator can be seen in Figure above. The rotor houses the poles which provide the field current, and these are usually of the salient or projecting-pole type. Slip rings and a fan are also mounted on the rotor shaft, which is driven by the auxiliary engine. The stator core surrounds the rotor and supports the three separate phase windings. Heat is produced in the various windings and must be removed by cooling. The shaft fan drives air over a water-cooled heat exchanger. Electric heaters are used to prevent condensation on the windings when the alternator is not in use.

In addition to auxiliary-engine-driven alternators a ship may have a shaft-driven alternator. In this arrangement a drive is taken from the main engine or the propeller shaft and used to rotate the alternator. The various operating conditions of the engine will inevitably result in variations of the alternator driving speed. A hydraulic pump and gearbox arrangement may be used to provide a constant-speed drive, or the alternator output may be fed to a static frequency converter. In the static frequency converter the a.c. output is first rectified into a variable d.c. voltage and then inverted back into a three-phase a.c. voltage. A feedback system in the oscillator inverter produces a constant-output a.c, voltage and frequency.



Fig: A.C. distribution system


Distribution system

An a.c. distribution system is provided from the main switchboard which is itself supplied by the alternators (Figure above). The voltage at the switchboard is usually 440 volts, but on some large installations it may be as high as 3300 volts. Power is supplied through circuit breakers to larger auxiliaries at the high voltage. Smaller equipment may be supplied via fuses or miniature circuit breakers. Lower voltage supplies used, for instance, for lighting at 220 volts, are supplied by step down transformers in the distribution network.

The distribution system will be three-wire with insulated or earthed neutral. The insulated neutral has largely been favoured, but earthed neutral systems have occasionally been installed. The insulated neutral system can suffer from surges of high voltage as a result of switching or system faults which could damage machinery. Use of the earthed system could result in the loss of an essential service such as the steering gear as a result of an earth fault. An earth fault on the insulated system would not, however, break the supply and would be detected in the earth lamp display. Insulated systems have therefore been given preference since earth faults are a common occurrence on ships and a loss of supply in such situations cannot be accepted.

In the distribution system there will be circuit breakers and fuses, as mentioned previously for d.c. distribution systems. Equipment for a.c. systems is smaller and lighter because of the higher voltage and therefore lower currents. Miniature circuit breakers are used for currents up to about 100 A and act as a fuse and a circuit breaker. The device will open on overload and also in the event of a short circuit. Unlike a fuse, the circuit can be quickly remade by simply closing the switch. A large version of this device is known as the 'moulded-case circuit breaker' and can handle currents in excess of 1000 A. Preferential tripping and earth fault indication will also be a part of the a.c. distribution system. These two items have been mentioned previously for d.c. distribution systems.


Alternating current supply

Three-phase alternators arranged for parallel operation require a considerable amount of instrumentation. This will include ammeters, wattmeter, voltmeter, frequency meter and a synchronising device. Most of these instruments will use transformers to reduce the actual values taken to the instrument. This also enables switching, for instance, between phases or an incoming machine and the bus-bars, so that one instrument can display one of a number of values. The wattmeter measures the power being used in a circuit, which, because of the power factor aspect of alternating current load, will be less than the product of the volts and amps. Reverse power protection is provided to alternators since reverse current protection cannot be used. Alternatively various trips may be provided in the event of prime mover failure to ensure that the alternator does not act as a motor.

The operation of paralleling two alternators requires the voltages to be equal and also in phase. The alternating current output of any machine is always changing, so for two machines to operate together their voltages must be changing at the same rate or frequency and be reaching their maximum (or any other value) together. They are then said to be 'in phase'. Use is nowadays made of a synchroscope when paralleling two a.c. machines. The synchroscope has two windings which are connected one to each side of the paralleling switch. A pointer is free to rotate and is moved by the magnetic effect of the two windings. When the two voltage supplies are in phase the pointer is stationary in the 12 o'clock position. If the pointer is rotating then a frequency difference exists and the dial is marked for clockwise rotation FAST and anti-clockwise rotation SLOW, the reference being to the incoming machine frequency.

To parallel an incoming machine to a running machine therefore it is necessary to ensure firstly that both voltages are equal Voltmeters are provided for this purpose. Secondly the frequencies must be brought into phase. In practice the synchroscope usually moves slowly in the FAST direction and the paralleling switch is closed as the pointer reaches the 11 o'clock position. This results in the incoming machine immediately accepting a small amount of load.

A set of three lamps may also be provided to enable synchronising. The sequence method of lamp connection has a key lamp connected across one phase with the two other lamps cross connected over the other two phases. If the frequencies of the machines are different the lamps will brighten and darken in rotation, depending upon the incoming frequency being FAST or SLOW. The correct moment for synchronising is when the key lamp is dark and the other two are equally bright.

Marine electricity & ancillary equipments

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Brushless DC Motor Back EMF

Brushless DC motors (BLDC) are used where there are limitations in the use of the brush-type DC motors. In this article we discuss how it is possible operate a DC motor with no brush arrangement and also about the back EMF in a brushless DC motor (BLDC).

Introduction

A DC motor is a one which operates on supply from a DC source. The DC source may be either DC generator or from a battery. DC motors may be classified as:

·         Series wound DC motor

·         Shunt wound DC motor

·         Compound wound DC motor

·         Separately wound DC motor

In all types of DC motors, the supply is given to both stators to make it as an electromagnet. This supply is necessary because the operation of a DC motor depends on the attraction and repulsion principles of magnetism.

In the stator, the supply voltage from a DC source is given directly, and in the rotor of DC motor it is supplied by means of a brush arrangement. But in case of brushless DC motors, this supply voltage to the rotor should be supplied without any brush arrangement. Brushless DC motors are more complicated than ordinary DC motor with brush arrangements, but certain applications needs this brushless DC motor, and hence it exists.

In a brushless DC motor (BLDC), we have an exciter rotor mounted on the same shaft of the rotor of a DC motor. This exciter stator induces an EMF when a small voltage is applied to the stator of this exciter. The voltage induced in the exciter rotor is an AC voltage and this is rectified to DC by means of a rotating rectifier diode arrangements mounted on the same shaft of the motor. The rectified DC voltage is applied to the rotor of DC motor, and there is no brush required so the DC motor with this type of complicated arrangement is called a brushless DC motor (BLDC).
 

Back EMF in Brushless DC Motor (BLDC):

According to Faradays law of electromagnetic induction, when a current carrying conductor is placed in a magnetic field that is if the conductor cuts the magnetic field), an EMF is induced or produced in a conductor and if a closed path is provided current flows through it.

When the same thing happens in a brushless DC motor (BLDC) as a result of motor torque, the EMF produced is known as “back EMF.” It is so called because this EMF that is induced in the motor opposes the EMF of the generator.

This back EMF that is induced in the brushless DC motor (BLDC) is directly proportional to the speed of the armature (rotor) and field strength of the motor, which means that if the speed of the motor or field strength is increased, the back EMF will be increased and if the speed of the motor or field strength is decreased, the back EMF is decreased.

This back EMF created acts as a resistance and we all know that any resistance in a line reduces and opposes the current flow so if the speed of the DC motor or field strength is increases, the back EMF increases which it turn increases the resistance to the current flow in windings and hence only less amount of current is delivered to the armature of DC motor. Also if the speed of Dc motor armature or field strength decreases, the back EMF decreases, which in turn reduces the resistance and hence more amount of current flow to the armature of DC motor.

When the DC motor is first started, there is no back EMF induced and as discussed above there is maximum current flow from the DC generator or distribution lines to the motor armature and as a result the motor toque will be maximum. In this case there is no resistance offered by back EMF. The only resistance available is the motor winding resistance.

During normal operation (rated speed) of DC motor, the back EMF induced will be maximum which will reduces the motor armature current to its minimum level and as a result the motor torque will also be reduced.

When the load on the motor is increased, the motor speed (RPM) is decreased and this will reducs the back EMF. This decreases in back EMF will automatically increase the motor torque thereby bringing the motor to its rated speed.

Marine Generators – Starting Checks & Procedure

Starting of Generator Engine

Starting of an engine from “stop” state is something which needs to be done with care, especially if the interval of starting is sufficiently long. The following is a checklist of all the checks which ideally need to be carried out before starting the generator. In actual practice sometimes the engineers might take some of these for granted and skip, but it is advisable not to indulge in such a practice. In fact these checks are generic for any four stroke engine starting process

1.    Check the turbocharger sump oil level, governor, alternator, forward and aft lube oil levels, and diesel oil level in service tank

2.    Open the indicator cock

3.    Prime the lube oil to all parts by hand pump or by motor driven priming pump

4.    Ensure that all jacket cooler valves, lube oil cooler valves, air cooler valves should be in open position

5.    With use of the Turning bar turn the fly wheel and check for any resistance on the bottom end bearing and check any water / fuel coming out through indicator cocks

6.    While turning engine, check all visible lube oil points are lubricated

7.    Remove the turning bar from fly wheel and put in the place

8.    Drain the auxiliary air bottle

Blow through engine (i.e.: by turning engine with air). In order to ensure that no water is inside combustion chamber if it is present it may cause water hammering

9.    Close the indicator cocks and pull lever from stop to start

10.  When the needle in RPM indicator deflects to some value of (0-25 rpm) put the lever in run condition

11.  The engine will run on fuel oil once the generator picks up the rated speed

12.  Put generator on load by closing air circuit breaker

13.  For checking the alternator fore and aft bearing lube oil level by opening oil plug in the alternator and the ring bearing while rotating splash lube oil from the sump can be seen

14.  In order to synchronize the incoming generator with running generator syncroscope method/dark lamp method is used

Starting of generator

Checks to be made while running

Once the generator has actually started to run, there are several checks which must be performed before it is left on its own to continue running. These checks pertain to verifying various parameters related to lube oil levels, temperatures and so forth. Given below is a brief checklist related to the same.

Lube oil checks

1.    Sump lube oil level

2.    Governor lube oil level

3.    Rocker arm lube oil level

4.    Alternator forward and aft bearing lube oil level

5.    Lube oil in turbine & blower side of turbo charger

Temperature checks

1.    Exhaust gas temperature

2.    Turbocharger (inlet-outlet) temperature

3.    Booster air inlet temperature

Cooler temperatures

1.    Cooling sea water (inlet – out let) temperature in cooler

2.    Jacket cooling water (inlet – outlet) temperature

3.    Air cooler (inlet -outlet) temperature

Safety Devices

Once the above mentioned parameters have been checked and found within normal range, it is safe to continue running the generator. Yet a fault can develop even at a later stage, so for this very purpose various trips and alarms are situated on the generators. An alarm gives the signal of an impeding danger and requires quick action while a trip actually trips the generator immediately because of the nature of the fault.

The various trips and alarms are mentioned as follows

1.    Alternator bearing low oil level alarm & trip

2.    Alternator bearing high temperature lube oil alarm &trip

3.    Low sump oil level alarm and trip

4.    Lube low oil pressure alarm and trip

5.    Reverse current trip

6.    Over speed trip

7.    Over load trip

8.    High and low frequency trip

9.    Jacket cooling water low pressure alarm