Synchronous machine

Synchronous machine construction, working principle explanation, types and operation, Characteristics, Phasor angle, application, advantages, and Disadvantages

1. SYNCHRONOUS machines UNIT 4

2. CONTENT Construction Operation Types EMF Equation Power angle curve Regulation Phasor diagram Starting methods Application

3. introduction A synchronous machine is an AC machine whose speed is proportional to the frequency of armature current A machine which rotates at a speed fixed by the supply frequency and the number of rotor poles is called synchronous machine It always runs at synchronous or constant speed and has no slip Synchronous machine includes synchronous generators and synchronous motors Used principally in large power applications because of their high operating efficiency, reliability and controllable power factor Rotated at constant speed in the steady state It is a doubly excited machine – ROTOR poles are excited by a DC current, STATOR are connected to the ac supply

4. construction STATOR: The stator is the stationary part of the synchronous generator It works as a armature in synchronous machine The armature winding is housed in it.

5. construction ROTOR: Rotor is the rotating part of the synchronous machine It works as a field of the synchronous machine Field system is rotating in the synchronous machine Field winding is housed on it Field winding is excited by DC supply through brushes and slip ring.

6. construction

7. Rotor types SALIENT POLE ROTOR: Magnetic pole stick out from the surface of the rotor Its rotor poles projecting out from the rotor core Is use for low speed hydroelectric generator Need large number of poles to accumulate in projecting on a rotor Almost universal adapt Has non uniform air-gap Range from 100 RPM to 500 RPM

8. Rotor types NON SALIENT or CYLINDRICAL POLE ROTOR: Magnetic pole constructed flush with the surface of the rotor Has its rotor in cylindrical form with dc field winding embedded in the rotor slots Has uniform air-gap Provide greater mechanical strength Per unit more accurate dynamic balancing For use in high speed turbo generator Range 1000 to 3000 RPM

9. Working principle It works on the principle of Electromagnetic induction In the synchronous generator field system is rotating and armature winding is steady Its works on principle opposite to the dc generator High voltage AC output coming from the armature terminal

10. Generated voltage The magnitude of the voltage induced in the given stator phase is given as: E A = √2 π N C φ f or E A = K φ ω Where φ = flux in the machine f = frequency of the machine ω = speed rotation of the machine K = Constant representing the construction of the machine From the equation, it can concluded that E A proportional to flux and speed Flux proportional to field current I F Thus E A also proportional to I F

11. Generated voltage The internal generated voltage E A vs Field current I F plot

12. emf When a conductor moves past a pair of poles, one cycle of sinusoidal voltage is completed if P = total number of poles in the machine, then Number of cycles per revolution = P/2 If N = RPM of the motor, then the rotor makes N/60 RPS. Hence, the frequency of the induced EMF is given by F = P/2 x N/60 = PN / 120 Hz

13. emf If P = number of poles in the machine, and φ = flux per pole, magnetic flux cut by a conductor in one revolution of the rotor = P φ . If N is the RPM then time taken by the rotor to make one revolution = 60 / N seconds. Therefore, Flux cut per second by a conductor = P φ N / 60 But average induced EMF in a conductor = flux cut per second. Therefore Average induced EMF in a conductor = P φ N / 60

14. emf If T = total number of turns connected in series per phase, and since each turn will have two conductor, we have Z = Total number of conductors in series per phase = 2 T . so, Average EMF induced per phase = |E av | = P φ N / 60 . 2 T The air gap flux in the generator will have more or less sinusoidal distribution. Then the induced EMF in each phase will also be sinusoidal. For a sinusoidal waveform we have Form Factor = | E ph | / | E av | = 1.11 Where E ph = RMS value of the induced voltage per phase.

15. emf Therefore, the RMS voltage induced per phase is | Eph | = 1.11 x P φ N / 60 x 2 T =2.22 P φ N T / 60 But the frequency of the induced EMF is given by F = PN / 120 ; 2f = PN / 60 We get | Eph | = 4.44 φ f T volts

16. Phasor Diagram The load for synchronous machine may be of three types Pure resistive load ( Unity power factor) Inductive load ( Lagging power factor) Capacitive load ( Leading power factor)

17. Phasor Diagram Synchronous generator with pure resistive load ( Unity power factor )

18. Phasor Diagram Synchronous generator with pure inductive load ( Lagging power factor )

19. Phasor Diagram Synchronous generator with pure capacitive load ( Leading power factor )

20. Efficiency and voltage regulation

21. voltage regulation Voltage Regulation is a convenient way to compare the voltage behaviour of two different generators. At a given load, power factor and at rated speed to calculate the Voltage Regulation V = The full load terminal voltage E = no load terminal voltage at rated speed

22. Phasor Diagram Synchronous motor : Unity Power Factor

23. Phasor Diagram Synchronous motor : Lagging Power Factor

24. Phasor Diagram Synchronous motor : Leading Power Factor

25. starting The three methods to start a synchronous motor: Reduce electrical frequency, f Use external prime mover Use damper windings

26. Power angle P >0: generator operation P <0: motor operation Positive Q : delivering inductive vars for a generator action or receiving inductive vars for a motor action Negative Q : delivering capacitive vars for a generator action or receiving capacitive vars for a motor action

27. Power angle The real and reactive power delivered by a synchronous generator or consumed by a synchronous motor can be expressed in terms of the terminal voltage V t , generated voltage E A , synchronous impedance Z s , and the power angle or torque angle d. I t is convenient to adopt a convention that makes positive real power P and positive reactive power Q delivered by an overexcited generator. The generator action corresponds to positive value of d, while the motor action corresponds to negative value of d.

28. Power angle The complex power output of the generator in volt-amperes per phase is given by where : V f = terminal voltage per phase I A * = complex conjugate of the armature current per phase Taking the terminal voltage as reference The excitation or the generated voltage

29. Power angle Armature Current Where X S is the synchronous reactance per phase

30. Power angle The above two equations for active and reactive powers hold good for cylindrical-rotor synchronous machines for negligible resistance To obtain the total power for a three-phase generator, the above equations should be multiplied by 3 when the voltages are line-to-neutral If the line-to-line magnitudes are used for the voltages, however, these equations give the total three-phase power

31. Power angle & Torque angle

32. application Synchronous motors are usually used in large sizes because in small sizes they are costlier as compared with induction machines. The principle advantages of using synchronous machine are as follows: Power factor of synchronous machine can be controlled very easily by controlling the field current It has very high operating efficiency and constant speed For operating speed less than about 500 rpm and for high power requirements synchronous motor is cheaper than induction motor In view of these advantages, synchronous motors are preferred for driving the loads requiring high power at low speed; e.g.; reciprocating pumps and compressor, crushers, rolling mills, pulp grinders etc.