Lec # 01 synch generator introduction

Lec # 01 synch generator introduction

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  SYNCHRONOUS GENERATOR Introduction & Construction NOORNABISHAIKH DEPARTMENT OF ELECTRICAL ENGINEERING MUET
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  N.N.SHAIKH 8 INTRODUCTION Synchronous generators(also called Alternators or AC generators) are synchronous machines used to convert mechanical power to ac electric power.  Synchronous generators are the primary source of all the electrical energy we consume.  These machines are the largest energy converters in the world. They range in size from a fraction of a KVA to 1500MVA. [1991 Charles I. Hubert]  Commonly used to convert the mechanical power output of steam turbines, gas turbines, engines, hydro turbines into electrical power for the grid.  The bulk of electric power is generated with high-speed steam turbines driving cylindrical (Nonsalient) rotor machines, and low speed hydraulic turbines driving salient pole machines.  Synchronous generators are built with either a stationary or a rotating dc magnetic field.
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  N.N.SHAIKH 9 BASIC WORKINGPRINCIPLE  A DC current is applied to the rotor winding, which produces a rotor magnetic field.  The rotor of the generator is then turned by a prime mover, producing a rotating magnetic field within the machine.  The rotating magnetic field induces a 3 – phase set of voltages within the stator windings of the generator.  The electrical frequency of the 3-phase output depends upon the mechanical speed and the number of poles 120 Pn f m e =
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  N.N.SHAIKH 10 Synchronous GeneratorRotor It is essentially a large electromagnet. The magnetic poles on the rotor can be either:  Salient  Nonsalient Salient means “sticking out” and a salient pole is a magnetic pole that sticks out from the surface of the rotor. These type of rotors are used with 4 or more poles. A nonsalient pole is a magnetic pole constructed flush with the surface of the rotor. These types of rotors are normally used for 2 and 4 – pole rotors. NOTE Due to changing magnetic fields, rotor is constructed of thin laminations to reduce eddy current losses.
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  N.N.SHAIKH 11 A salient6 – pole rotor A nonsalient 2 – pole rotor
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  N.N.SHAIKH 12 A dccurrent must be supplied to the field circuit on the rotor. Since rotor is rotating, a special arrangement is required to get the dc power to its field windings. There are two common approaches to supply this dc power. 1. Separately Excited System 2. Self (brushless) Excited System
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  N.N.SHAIKH 13 In thissystem dc power is supplied from an external dc source to the rotor by means of slip rings and brushes. Slip rings are metal rings completely encircling the shaft of a machine but insulated from it. One end of the dc rotor winding is tied to each of the 2 – slip rings, and a stationary brush (graphite like carbon compound that conducts electricity) rides on each slip ring. 1. Separately Excited System: NOTE Slip rings and brushes increase the amount of maintenance required on the machine, since brushes must be checked for wear regularly. Brush voltage drop can be the cause of significant power losses on machines with larger field currents.
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  N.N.SHAIKH 14 Slip Rings Theyare metal rings completely encircling the shaft of a machine but insulated from it. One end of the dc rotor winding is tied to each of the two slip rings, and a stationary brush rides on each slip ring. Brush A “brush” is a block of graphite like carbon compound that conducts electricity freely but has very low friction, so that it doesn’t wear down the slip ring. If the positive end of a dc voltage source is connected to one brush and negative end is connected to the other, then the same dc voltage will be applied to the field winding at all times regardless of the angular position or speed of the rotor.
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  N.N.SHAIKH 15 Self (brushless)Excited System: a brushless exciter is a small ac generator with its field circuit mounted on the stator and armature circuit mounted on the rotor shaft. The 3 – phase output of the exciter generator is rectified by 3-phase rectifier also mounted on the shaft of the generator, and is then fed into the main dc field circuit.
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  N.N.SHAIKH 16 By controllingthe small dc field current of the exciter generator (located on the stator), it is possible to adjust the field current on the main machine without slip rings and brushes. Refer figure shown above.
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  N.N.SHAIKH 17 To makethe excitation of a generator completely independent of any external power sources, a small pilot exciter generator is often included in the system. Many generators include brushless exciters and slip rings, so that auxiliary source of dc field current is available in emergencies.
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  N.N.SHAIKH 18 Synchronous Generatorstator Stator of a synchronous generators are normally made of preformed stator coils in a double layer winding. The winding is itself is distributed and chorded in order to reduce the harmonic content of the output voltages and currents.
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