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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Direct current motors ,Shunt wound d.c. motor,Series wound d.c. motor & d.c. motor starter

Direct current motors

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.

Additional coils of wire and more magnetic fields produce a more efficient motor. Interpoles are fitted to reduce sparking but now have opposite polarity to the next main pole in the direction of rotation, When rotating the armature acts as a generator and produces current in the reverse direction to the supply. This is known as back e.m.f. (electromotive force) and causes a voltage drop across the motor. This back e.m.f. controls the power used by the motor but is not present as the motor is started. As a result, to avoid high starting currents special control circuits or starters are used.

The behaviour of the d.c. motor on load is influenced by the voltage drop across the armature, the magnetic field produced between the poles and the load or torque on the motor. Some of these factors are interdependent. For example, the voltage drop across the armature depends upon the back e.m.f. which depends upon the speed of the motor and the strength of the magnetic field. Shunt, series and compound windings are used to obtain different motor characteristics by varying the above factors.



Fig: Shunt wound d.c. motor

The shunt wound motor has field windings connected in parallel with the armature windings. Thus when the motor is operating with a fixed load at constant speed all other factors are constant. An increase in load will cause a drop in speed and therefore a reduction in back e.m.f. A greater current will then flow in the armature windings and the motor power consumption will rise: the magnetic field will be unaffected since it is connected in parallel. Speed reduction is, in practice, very small, which makes the shunt motor an ideal choke for constant-speed variable-load duties.




Fig: Series wound d.c. motor

The series motor has field windings connected in series with the armature windings . With this arrangement an increase in load will cause a reduction in speed and a fall in back e.m.f. The increased load current will, however, now increase the magnetic field and therefore the back e.m.f. The motor will finally stabilise at some reduced value of speed. The series motor speed therefore changes considerably with load.

Control of d.c. motors is quite straightforward. The shunt wound motor has a variable resistance in the field circuit, as shown in Figure . This permits variation of the current in the field coils and also the back e.m.f., giving a range of constant speeds. To reverse the motor the field current supply is reversed, as shown in Figure .

One method of speed control for a series wound motor has a variable resistance in parallel with the field coils. Reverse operation is again achieved by reversing the field current supply as shown in Figure .

In operation the shunt wound motor runs at constant speed regardless of load. The series motor runs at a speed determined by the load, the greater the load the slower the speed. Compounding—the use of shunt and series field windings—provides a combination of these characteristics. Starting torque is also important. For a series wound motor the starting torque is high and it reduces as the load increases. This makes the series motor useful for winch and crane applications. It should be noted that a series motor if started on no-load has an infinite speed. Some small amount of compounding is usual to avoid this dangerous occurrence. The shunt wound motor is used where constant speed is required regardless of load; for instance, with fans or pumps. The starting of a d.c. motor requires a circuit arrangement to limit armature current. This is achieved by the use of a starter .



Fig: d.c. motor starter


A number of resistances are provided in the armature and progressively removed as the motor speeds up and back e.m.f. is developed. An arm, as part of the armature circuit, moves over resistance contacts such that a number of resistances are first put into the armature circuit and then progressively removed. The arm must be moved slowly to enable the motor speed and thus the back e.m.f. to build up. At the final contact no resistance is in the armature circuit. A 'hold on' or 'no volts' coil holds the starter arm in place while there is current in the armature circuit.

If a loss of supply occurs the arm will be released and returned to the 'off position by a spring. The motor must then be started again in the normal way. An overload trip is also provided which prevents excess current by shorting out the 'hold on* coil and releasing the starter arm. The overload coil has a soft iron core which, when magnetised sufficiently by an excess current, attracts the trip bar which shorts out the hold on coil. This type of starter is known as a 'face plate'; other types make use of contacts without the starting handle but introduce resistance into the armature circuit in much the same way.

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