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1. A Presentation on SYNCHRONOUS GENERATOR PRESENTED BY: Dipesh Kumar Yadav II-EEE-”B” 22L31A0246 DEPT. OF ELECTRICAL & ELECTRONICS ENGG.. VIGNAN’S INSTITITE OF INFORMATION TECHNOLOGY DUVVADA,VISAKHAPATNAM

2. SYNCHRONOUS GENERATORS

3. SYNCHRONOUS GENERATOR Introduction Synchronous generator operates on the principle that when the magnetic flux linking a conductor changes, an e.m.f. is induced in the conductor. It has an armature winding and a field winding, It is more convenient and advantageous to place the field winding on the rotor and armature winding on the stator.

4. SYNCHRONOUS GENERATOR Advantages of Stationary Armature It is easier to insulate stationary winding for higher voltages because they are not subjected to centrifugal forces and also extra space is available due to the stationary arrangement of the armature. The stationary 3-phase armature can be directly connected to load without going through large, unreliable slip rings and brush-gear. Since the excitation current is much smaller as compared to load current, the slip rings and brush gear required are of light construction. Due to simple and robust construction of the rotor, higher speed of rotating d.c. field is possible.

5. 4 CONSTRUCTION OF SYNCHRONOUS GENERATOR Stator It is the stationary part of the machine and is built up of sheet-steel laminations having slots on its inner periphery. A 3-phase winding is placed in these slots and serves as the armature winding of the alternator. The armature winding is connected in star with neutral grounded. Rotor The rotor carries a field winding supplied with direct current through the slip rings by a separate d.c. source. This d.c. source (exciter) is generally a small d.c. generator mounted on the shaft of the alternator. Rotor construction is of salient (projected) pole type and non- salient pole type rotor.

6. CONSTRUCTION OF SYNCHRONOUS GENERATOR (Contd.) Salient Pole Type In this type, salient or projected poles are mounted on a large circular steel frame which is fixed to the shaft of the alternator. The individual field pole windings are connected in series such that when the field winding is energized by the exciter, adjacent poles have opposite polarities.

7. 6 CONSTRUCTION OF SYNCHRONOUS GENERATOR (Contd.) Rotor (Contd.) Salient Pole Type (Contd.) Low and medium-speed alternators (120-400 r.p.m.), those driven by diesel engines or water turbines, have salient pole type rotors due to the following reasons: The salient field poles would cause an excessive windage loss if driven at high speed and would tend to produce noise. Salient-pole construction cannot be made strong enough to withstand the mechanical stresses to which they may be subjected at higher speeds. For a frequency of 50 Hz, we must use a large number of poles on the rotor of slow-speed alternators. Low-speed rotors possess a large diameter to provide necessary space for the poles.

8. CONSTRUCTION OF SYNCHRONOUS GENERATOR (Contd.) Rotor (Contd.) Non-Salient Pole Type Non-salient pole type rotor is made of smooth solid forged-steel cylinder having a number of slots along the outer surface. Field windings are embedded in the slots and are connected in series to the slip rings through which they are energized by the d.c. exciter. The regions forming the poles are left unslotted. 8

9. CONSTRUCTION OF SYNCHRONOUS GENERATOR (Contd.) 9 Rotor (Contd.) Non-Salient Pole Type (Contd.) High-speed generators (1500 or 3000 r.p.m.), driven by steam turbines, use non-salient type rotors due to the reasons: It gives noiseless operation at high speeds. The flux distribution around the periphery is nearly a sine wave and hence a better e.m.f. waveform is obtained than in the case of salient-pole type. Since steam turbines run at high speed and a frequency of 50 Hz is required, we need a small number of poles on the rotor. We can not use less than 2 poles, hence, the highest possible speed will be 3000 r.p.m.

10. 9 OPERATION The rotor winding is energized from the d.c. exciter and alternate N and S poles are developed on the rotor. When the rotor is rotated in anti-clockwise direction by a prime mover, the stator or armature conductors are cut by the magnetic flux of rotor poles. Consequently, e.m.f. is induced in the armature conductors due to electromagnetic induction. The induced e.m.f. is alternating since N and S poles of rotor alternately pass the armature conductors. Direction of the induced e.m.f. can be determined by Fleming’s right hand rule and the frequency is given by; f = NP/120 where N = speed of rotor in r.p.m. P= no. of poles

11. OPERATION (Contd.) Magnitude of the voltage induced in each phase depends upon the rotor magnetic flux, the number and position of the conductors in the phase and the speed of the rotor. Magnitude of induced e.m.f. depends upon the speed of rotation and the d.c. exciting current. Magnitude of e.m.f. in each phase of stator winding is same, however, they differ in phase by 120° electrical. 11

12. F R EQ U E N CY 12 Frequency of induced e.m.f. in the stator depends on speed and the number of poles. Let N = rotor speed in r.p.m. P = number of rotor poles f = frequency of e.m.f. in Hz Consider a stator conductor that is successively swept by the N and S poles of the rotor. If a positive voltage is induced when a N-pole sweeps across the conductor, a similar negative voltage is induced when a S-pole sweeps.

13. FREQUENCY (Contd.) 13 Thus one complete cycle of e.m.f. is generated in the conductor as a pair of poles passes it.  No. of cycles/revolution = No. of pairs of poles = P/2 No. of revolutions/second = N/60  No. of cycles/second = (P/2)(N/60) = N P/120 But number of cycles of e.m.f. per second is its frequency.  f = NP/120 N is the synchronous speed generally represented by N s For a given alternator, the number of rotor poles is fixed, hence, the alternator must run at synchronous speed to give the desired frequency. For this reason, an alternator is also called synchronous generator.

14. A.C. ARMATURE WINDINGS 14 A.C. armature windings are generally open-circuit type i.e., both ends are brought out. An open-circuit winding is one that does not close on itself i.e., a closed circuit will not be formed until some external connection is made to a source or load. The following are the general features of a.c. armature windings: A.C. armature windings are symmetrically distributed in slots around the complete circumference of the armature. Distributed winding has two principal advantages: Distributed winding generates a voltage in the form of sin wave. Copper is evenly distributed on the armature surface resulting in uniform heating of winding which can be easily cooled.

15. A.C. ARMATURE WINDINGS (Contd.) 15 A.C. armature windings may use full-pitch coils or fractional-pitch coils A coil with a span of 180° electrical is called a full-pitch coil with two sides of the coil occupyng identical positions under adjacent opposite poles and the e.m.f. generated in the coil is maximum. A coil with a span of less than 180° electrical is called a fractional- pitch coil (For example, a coil with a span of 150° electrical would be called a 5/6 pitch coil) and the e.m.f. induced in the coil will be less than that of a full-pitch coil. Most of a.c. machines use double layer armature windings i.e. one coil side lies in the upper half of one slot while the other coil side lies in the lower half of another slot spaced about one-pole pitch from the first one.

16. E.M.F. EQUATION Let Z = No. of conductors or coil sides in series per phase  = Flux per pole in webers P = Number of rotor poles N = Rotor speed in r.p.m. In one revolution (60/N second), each stator conductor is cut by P  webers i.e., d  = P  ; and dt = 60/N  Average e.m.f. induced in one stator conductor = d ϕ Pϕ PϕN d t 6 Τ N 60 = = volts Since there are Z conductors in series per phase, A v erage e. m . f . /phase = PϕN x Z = 60 x 60 PϕZ 120 f P = 2 f  Z V olts N = 120 f 16 P

17. E.M.F. EQUATION 17 R.M.S. value of e.m.f./phase = Average value of e.m.f. per phase x form factor = 2 f  Z x 1.11 = 2.22 f  Z Volts  E r.m.s. per phase = 2.22 f  Z volts (i) If K p and K d are the pitch factor and distribution factor of the armature winding, then, E r.m.s. per phase = 2.22 K p K d f  Z Volts (ii) Sometimes the turns (T) per phase rather than conductors per phase are specified, in that case, eq. (ii) becomes: E r.m.s. per phase = 4.44 K p K d f  T Volts (iii) The line voltage will depend upon whether the winding is star or delta connected.

18. ARMATURE REACTION 18 When an alternator is running at no-load, there will be no current flowing through the armature winding and magnetic flux produced in the air-gap will be only due to rotor field. When the alternator is loaded, the three-phase currents will produce an additional magnetic field in the air-gap. The effect of armature flux on the flux produced by field ampere-turns is called armature reaction. The armature flux and the flux produced by rotor ampere-turns rotate at a synchronous speed in the same direction, hence, the two fluxes are fixed in space relative to each other. Modification of flux in the air-gap due to armature flux depends on the magnitude of stator current and on the power factor of the load. Load power factor determines whether the armature flux distorts, opposes or helps the main flux.

19. ARMATURE REACTION Load at Unity Power Factor When armature is on open-circuit, there is no stator current and the flux due to rotor current is distributed symmetrically in the air-gap Since the direction of the rotor is assumed clockwise, the generated e.m.f. in phase R 1 R 2 is at its maximum and is towards the paper in the conductor R 1 and outwards in conductor R 2 . No armature flux is produced since no current flows in the armature winding. 19

20. ARMATURE REACTION (Contd.) Load at Unity Power Factor (Contd.) In case a resistive load (unity p.f.) is connected across the terminals of the alternator, according to right- hand rule, the current is “in” in the conductors under N-pole and “out” in the conductors under S-pole. Therefore, the armature flux is clockwise due to currents in the top conductors and anti-clockwise due to currents in the bottom conductors. The armature flux is at 90° to the main flux (due to rotor current) and is behind the main flux. 20

21. ARMATURE REACTION (Contd.) 21 Load at Unity Power Factor (Contd.) In this case, the flux in the air-gap is distorted but not weakened. Therefore, at unity p.f., the effect of armature reaction is merely to distort the main field; there is no weakening of the main field and the average flux practically remains the same. Since the magnetic flux due to stator currents (i.e., armature flux) rotate; synchronously with the rotor, the flux distortion remains the same for all positions of the rotor.

22. ARMATURE REACTION (Contd.) Load at Zero Power Factor Lagging When a pure inductive load (zero p.f. lagging) is connected across the terminals of the alternator, current lags behind the voltage by 90°. This means that current will be maximum at zero e.m.f. and vice-versa. Figure shows the condition when the alternator is supplying resistive load. Note that e.m.f. as well as current in phase R 1 R 2 is maximum in this position. 22

23. ARMATURE REACTION (Contd.) Load at Zero Power Factor Lagging (Contd.) When the generator is supplying a pure inductive load, the current in phase R 1 R 2 will not reach its maximum value until N-pole advanced 90° electrical Now the armature flux is from right to left and field flux is from left to right. All the flux produced by armature current (i.e., armature flux) opposes the field flux and, therefore, weakens it. In other words, armature reaction is demagnetizing. 23

24. ARMATURE REACTION (Contd.) Load at Zero Power Factor Leading When pure capacitive load (zero p.f. leading) is connected to the alternator, the current in armature windings will lead the induced e.m.f. by 90°. Effect of armature reaction will be the reverse that for pure inductive load. Armature flux aids the main flux and generated e.m.f. is increased. Figure shows the condition when alternator is supplying resistive load The e.m.f. as well as current in phase R 1 R 2 is max in this position When alternator is supplying pure capacitive load, the max current in R 1 R 2 will occur 90° before occurrence of max induced e.m.f. 24

25. ARMATURE REACTION (Contd.) Load at Zero Power Factor Leading (Contd.) When the generator is supplying a pure capacitive load, the maximum current in R 1 R 2 will occur 90° electrical before the occurrence of maximum induced e.m.f. Therefore, maximum current in phase R 1 R 2 will occur if the position of the rotor remains 90° behind as compared to its position under resistive load It is clear that armature flux is now in the same direction as the field flux and, therefore, strengthens it. 25

26. ARMATURE REACTION (Contd.) 26 Load at Zero Power Factor Leading (Contd.) This causes an increase in the generated voltage. Hence at zero p.f. leading, the armature reaction strengthens the main flux. For intermediate values of p.f, the effect of armature reaction is partly distorting and partly weakening for inductive loads. For capacitive loads, the effect of armature reaction is partly distorting and partly strengthening.

27. VOLTAGE REGULATION The voltage regulation of an alternator is defined as the change in terminal voltage from no-load to full-load (the speed and field excitation being constant) divided by full-load voltage. % V oltage regulation = No load voltage − Full load voltage Full load voltage x 100 E o − V = x 100 27 V E o = Terminal voltage of generator at no load V = Terminal voltage of generator at full load whe r e E - V is the arithmetic difference and not the phasor difference. The factors affecting the voltage regulation of an alternator are: i) I a R a drop in armature winding ii) I a X L drop in armature winding iii) Voltage change due to armature reaction

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