2. Nozzle Control Governing. In this method of control, the steam supplied to the different nozzle groups is controlled by uncovering as many steam passages as
5.7 ALTERNATOR .1 Introduction
The alternator is universally used in automotive applications. It converts mechanical energy into electrical energy, by electro-magnetic induction.
In a simple version, a bar magnet rotates in an iron yoke which concentrates the magnetic field. A coil of wire is wound around the stem of the yoke. As the magnet turns, voltage is induced in the coil, producing a current flow. When the North Pole is up, and South is down, voltage is induced in the coil, producing current flow in one direction.
Fig.5.7 Alternator.
As the magnet rotates, and the position of the poles reverses, the polarity of the voltage reverses too, and as a result, so does the direction of current flow. Current that
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changes direction in this way is called alternating current, or AC. The change in direction occurs once for every complete revolution of the magnet.
5.7.2 Theory of Operation
Alternators generate electricity by the same principle as DC generators. When magnetic field lines cut across a conductor, a current is induced in the conductor. In general, an alternator has a stationary part (stator) and a rotating part (rotor). The stator contains windings of conductors and the rotor contains a moving magnetic field. The field cuts across the conductors, generating an electrical current, as the mechanical input causes the rotor to turn.
Fig.5.8 Alternator Working Principle.
The rotor magnetic field may be produced by induction (in a "brushless" generator), by permanent magnets, or by a rotor winding energized with direct current through slip rings and brushes. Automotive alternators invariably use brushes and slip rings, which allows control of the alternator generated voltage by varying the current in the rotor field winding.
Permanent magnet machines avoid the loss due to magnetizing current in the rotor but are restricted in size owing to the cost of the magnet material. Since the permanent magnet field is constant, the terminal voltage varies directly with the speed of the generator. Brushless AC generators are usually larger machines than those used in automotive applications.
5.7.3 Alternator Protection
An alternator is an important aspect of a power plant's electrical system. Any kind of obstacle in its performance can mar the working of the power plant's overall electrical system. It is for this reason that it requires adequate protection systems to prevent any kind of hindrance to the power plant's functionality.
The main types of protection system are:
1. Over Current Protection 2. Reverse Power Protection
Over Current Protection. Every alternator has an over current protection. With the help of this trip, the alternator and distribution system can be protected from various faults
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but the main thing to be considered in this method is to maintain power to the distribution system till the time the alternator trips on any other protection devices.
For this reason, the protection device has been designed in such a way that in case the over current is not high enough, a time delay provided by an inverse definite minimum time ( IDMT ) relay occurs, which prevents the alternator from tripping in case the over current values reduces back to normal within the IDMT characteristics. But in case of a major fault such as short circuit, the alternator will trip instantaneously without any delay, protecting all devices on the distribution system. Overload of alternator is caused either due to increased switchboard load or serious fault causing very high current flow.
If sudden over load occurs then, the load is reduced with the help of preferential trips which removes non essential load such as of air conditioning, ventilation fans etc., from the switchboard. These preferential trips are operated by relays which are set to about 110% of the normal full load of alternator.
Reverse Power Protection. There is not much difference between an alternator and electric motors from the engineer's perspective. They are both based on similar principles. So just imagine what would happen if an alternator suddenly would act as a motor. This is only possible in systems where two or more generators are running in parallel,
Hence this type of protection system is used only if there is more than one alternator on board a ship. The system is designed in such a way that it will release the breaker and prevent motoring of alternator if a reversal of power occurs. This protection device is also used to prevent damage to the prime mover, which might be stopped due to some fault.
Though it is extremely difficult to detect reverse current with an alternating current system, reverse power can be detected and protection can be provided by reverse power relay.
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6.1 Introduction
A steam condenser is a device or an appliance in which steam condenses and heat released by steam is absorbed by water. The use of condenser in the power plant improves the efficiency of the power plant by decreasing the exhaust pressure of the steam below atmosphere. Another advantage of the condenser is that the steam condensed may be recovered to provide a source of good pure feed water to the boiler and reduces the water softening plant capacity to a considerable extent.
The maximum possible thermal efficiency of a power system is given by (T1 – T2)/T1
where T1 and T2 are the supply and exhaust temperatures. This expansion of efficiency shows that the efficiency increases with an increase in temperature T1 and with the decrease in temperature T2. The maximum value of temperature T1 of the steam supplied to a steam prime-mover is limited by the material consideration. The temperature T2 can be reduced if the exhaust of the steam prime mover takes place below the atmospheric pressure. This is because; there is definite relation between the steam temperature and pressure. Low exhaust pressure means low exhaust temperature. The steam cannot be exhausted to atmosphere if it is expanded in the turbine below atmospheric pressure. Under this condition, the steam is made to exhaust in a vessel known as condenser where the pressure inside is maintained below the atmospheric pressure by condensing the steam with the circulation of the cold water.
A closed vessel in which steam is condensed by abstracting heat from steam and the pressure is maintained below atmospheric pressure is known as condenser. The efficiency of the steam plant is considerably increased by the use of condenser.