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CHAPTER-FIVE Synchronous Machines

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 2 Introduction A synchronous machine is an ac rotating machine whose speed under steady state condition is proportional to the frequency of the current in its armature. The magnetic field created by the armature currents rotates at the same speed as that created by the field current on the rotor, which is rotating at the synchronous speed, and a steady torque results. Synchronous machines are commonly used as Generators especially for large power systems, such as turbine generators and hydroelectric generators in the grid power supply.

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 3 Synchronous Generator

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Facts Used principally in large power applications because of their High operating efficiency Reliability Controllable power factor Rotates at constant speed in the steady state i.e. The rotating air gap field and the rotor rotate at the same speed. It is a doubly excited machine i.e. Rotor poles are excited by a DC current Stator are connected to the ac supply It can draw leading or lagging reactive current from the ac supply system. ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 4

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5 Stator Rotor +VfVf-+VfVf- IfIf IfIf IfIfN RRRR S DC Machines The DC excitation current in the stator generates a Resultant flux. and induced voltage in the rotor.

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 6 Synchronous machines Stator Rotor +VfVf-+VfVf- IfIf IfIfN RRRR S The DC excitation current in the rotor generates a flux. The turbine drives the rotor and produces a rotating flux The rotating flux induce AC three phase voltage in the stator winding. The generated power of a synchronous machine can be adjusted by controlling the magnitude of the rotor field current.

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 7 Synchronous Machine Structures Stator and Rotor 1. The stator is termed as stationary armature where the generated power can be easily taken out. The armature winding of a conventional synchronous machine is almost invariably on the stator and is usually a three phase winding. 2. The rotor is the rotating member of the machine The field winding is usually on the rotor and excited by dc current, or permanent magnets. The dc power supply required for excitation usually is supplied through a dc generator This dc generator known as exciter, which is often mounted on the same shaft as the synchronous machine.

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 8 Rotor structures: There are two types of rotor structures 1. Salient pole rotor For low speed applications, such as hydroelectric generators, Diesel Generator etc. 2. Non-Salient pole (cylindrical) rotor For high speed synchronous machines, such as steam turbine generators, Gas turbine, etc

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Salient Pole rotor ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 9

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Cont’d ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 10

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Non-Salient(Cylindrical) Rotor ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 11

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 16 Mechanical and Electrical Angle 1. Mechanical angle Any angle Measured on the surface of a synchronous Machine is a Mechanical Angle. Always the total Mechanical angle over the complete surface being or 2pi radian. 2. Electrical Angle The Generated Voltage depends on the number of poles is called Electrical Angle.

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 17 Contd. Example, we consider a four pole machine. As the rotor rotates for one revolution the induced emf varies for two cycles and hence For a general case, if a machine has P poles, the relationship between the electrical and mechanical units of an angle can be readily deduced as,similarly, Where is the angular frequency of electrical radians per second and the angular speed of the rotor in mechanical radians per second.

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 18 Synchronous Speed The Synchronous speed defined as It can be seen that the frequency of the induced emf is proportional to the rotor speed and this speed is called Synchronous speed

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 19 Distributed Three Phase Windings The stator of a synchronous machine consists of a laminated electrical steel core and a three phase winding. The stator lamination of a synchronous machine that has a number of uniformly distributed slots. Coils are to be laid in these slots and connected in such a way that the current in each phase winding would produce a magnetic field in the air gap around the stator.

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 20 Contd. Stator coils are connected to form a three phase winding. Each phase is able to produce a specified number of magnetic poles. The windings of the three phase are arranged uniformly around the stator periphery and are labeled in the sequence that phase ‘a’ is 120 o (electrical) ahead of phase ‘b’ and 240 o (electrical) ahead of phase ‘c’. Phase a Phase b Phase c 120 o

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 21 Contd. Stator windings are sinusoidally distributed with N s equivalent turns and their magnetic axes are displaced by 120 o. Where N s : the number of turns of the equivalent sinusoidally distributed stator windings. s : the angular displacement about stator as' as bs bs' as bs cs cs' v as v bs v cs cs

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 22 Rotating Magnetic Fields Magnetic Field of a Distributed Phase Winding The magnetic field distribution of a distributed phase winding can be obtained by adding the fields generated by all the coils of the winding. The mmf distribution along the air gap is a square wave. Because of the uniform air gap, the spatial distribution of magnetic field strength is the same as that of mmf.

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 23 Magnetic Field of Three Phase Windings The resultant mmf generated by a three phase winding becomes

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 24 Graphical Analysis

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 25 Contd. The Resultant mmf wave has a constant amplitude of 3/2 F M and rotates at constant Synchronous speed Using Per-Phase Analysis the equivalent circuit of Synchronous Generator

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 26 Equivalent Circuit of Synchronous Machine

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 27 Contd. Consider the terminal Voltage leads the current by Φ. Determine the Generated Voltage Consider Triangle ONM O VtVt IaRa IaXs Eg Ia N M Φ V t Cos Φ IaRa V t SinΦ

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ECEg3202:- Introduction to Electrical Machine By Dawit Habtu 28 Open- and Short-Circuit Characteristics The Short-Circuit -Ratio (SCR) is the ratio of the field current Of ’=I f1 needed to generate rated open- circuit Armature Voltage to the field current Of ’’=I f2 needed to produce rated armature current Open- and short-circuit characteristics of a synchronous machine. I a1 I a2 =I n The SCR exactly equals the per unit synchronous reactance so that

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