Wednesday, June 22, 2011

Marine electricity & ancillary equipments

In the name of Allah who is the most beneficient the most merciful
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