CHAPTER-FIVE Synchronous Machines

1. CHAPTER-FIVESynchronous Machines

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. ECEg3202:- Introduction to Electrical Machine By Dawit Habtu

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

4. 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

5. N + Vf - If Rotor If Stator If S R DC Machines • The DC excitation current in the stator generates a Resultant flux. and induced voltage in the rotor. ECEg3202:- Introduction to Electrical Machine By Dawit Habtu

6. N If Rotor + Vf - Stator If S R Synchronous machines • 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. ECEg3202:- Introduction to Electrical Machine By Dawit Habtu

7. Synchronous Machine Structures Stator and Rotor • 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. ECEg3202:- Introduction to Electrical Machine By Dawit Habtu

8. Rotor structures: There are two types of rotor structures • 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 ECEg3202:- Introduction to Electrical Machine By Dawit Habtu

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

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

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

12. ECEg3202:- Introduction to Electrical Machine By Dawit Habtu

13. ECEg3202:- Introduction to Electrical Machine By Dawit Habtu

14. ECEg3202:- Introduction to Electrical Machine By Dawit Habtu

15. ECEg3202:- Introduction to Electrical Machine By Dawit Habtu

16. Mechanical and Electrical Angle • 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 3600 or 2pi radian. • Electrical Angle • The Generated Voltage depends on the number of poles is called Electrical Angle. 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 emfvaries 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. 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 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. ECEg3202:- Introduction to Electrical Machine By Dawit Habtu

20. Phase c 120o 120o Phase a 120o Phase b 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 120o(electrical) ahead of phase ‘b’ and 240o(electrical) ahead of phase ‘c’. ECEg3202:- Introduction to Electrical Machine By Dawit Habtu

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

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

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

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

27. M IaXs Eg IaRa Vt VtSinΦ Φ O Ia N VtCos Φ IaRa Contd. • Consider the terminal Voltage leads the current by Φ. Determine the Generated Voltage • Consider Triangle ONM ECEg3202:- Introduction to Electrical Machine By Dawit Habtu

28. Ia1 Open- and short-circuit characteristics of a synchronous machine. Open- and Short-Circuit Characteristics Ia2=In • The Short-Circuit -Ratio (SCR) is the ratio of the field current Of ’=If1 needed to generate rated open-circuit Armature Voltage to the field current Of ’’=If2 needed to produce rated armature current The SCR exactly equals the per unit synchronous reactance so that ECEg3202:- Introduction to Electrical Machine By Dawit Habtu

29. ECEg3202:- Introduction to Electrical Machine By Dawit Habtu