SG lecture 1&2.pdf
- 1. Electrical Machine System SYNCHRONOUS GENERATOR/ ALTERNATOR Dr. M. Julkarnain EE 340 Spring 2011 Synchronous Generators
- 2. Synchronous Generators Outlines of lecture # 1 v Working principles v Construction details v How to supply field current? v Armature winding: Single layer and double layers winding, full pitched and short-pitched winding, v Different factors v Problems solution
- 3. Working Principles The working principle of an alternator or AC generator is similar to the basic working principle of DC generator. According to the Faraday's law of electromagnetic induction, whenever a conductor moves in a magnetic field EMF gets induced across the conductor. If close path is provided to the conductor, induced emf causes current to flow in the circuit. Direction of induced current can be given by Fleming's right hand rule.
- 4. Construction Main parts of the alternator, obviously, consists of stator and rotor. But, the unlike other machines, in most of the alternators, field exciters are rotating and the armature coil is stationary. Stator: The stator consist of cast-iron frame, which supports the armature core, having slots on its inner periphery for housing the armature conductors. The stator core is made up of lamination of steel alloys or magnetic iron, to minimize the eddy current losses. Rotor: The rotor is like a flywheel having alternate N and S poles fixed to its outer rim.
- 5. Construction Advantages of Stationary armature: • The high voltage output can be directly taken out from the stationary armature. Whereas, for a rotary armature, there will be large brush contact drop at higher voltages, also the sparking at the brush surface will occur. • At high voltages, it easier to insulate stationary armature winding for high ac voltages, which may be as high as 30 kV or more. • The sliding contacts i. e. slip-rings are transferred to the low-voltage, low-power dc field circuit which can, therefore, be easily insulated. • The armature winding can be braced well, so as to prevent deformation caused by the high centrifugal force.
- 6. Construction Rotor: There are two types of rotor used in an AC generator / alternator: (i) Salient and (ii) Cylindrical type (i) Salient pole type: A salient pole is a magnetic pole that sticks out radially from the shaft of the rotor. This type of rotor consists of large number of projected poles (called salient poles), bolted on a magnetic wheel. These poles are also laminated to minimize the eddy current losses. Alternators featuring this type of rotor are large in diameters and short in axial length. Salient pole type rotor is used in low and medium speed (1200 RPM or less) alternators.
- 7. Construction Rotor: There are two types of rotor used in an AC generator / alternator: (i) Salient and (ii) Cylindrical type Cylindrical type: a non-salient pole or cylindrical pole is a magnetic pole with windings embedded flush with the surface o f the rotor. This type of rotor consists of a smooth and solid steel cylinder having slots along its outer periphery. Field windings are placed in these slots. Cylindrical type rotors are used in high speed alternators, especially in turbo alternators. windings and armature windings. In general, the term field windings applies to the windings that produce the main magnetic field in a machine, and the term arma- ture windings applies to the windings where the main voltage is induced. For syn- chronous machines, the field windings are on the rotor, so the terms rotor wind- ( ings and field windings are used interchangeably. Sintilarly, the terms stator windings and armature windings are used interchangeably. The rotor of a synchronous generator is essentially a large electromagnet. The magnetic poles on the rotor can be of either salient or nonsalient construction. The term salient means "protruding" or "sticking out," and a salient pole is a magnetic pole that sticks out radially from the shaft of the rotor. On the other hand. a nOll- salientpole is a magnetic pole with windings embedded flush with the surface of the rotor. A nonsalient-pole rotor is shown in Figure 4-1. Note that the windings of the electromagnet are embedded in notches on the surface of the rotor. A salient-pole rotor is shown in Figure 4-2. Note that here the windings of the electromagnet are wrapped around the pole itself, instead of being embedded in notches on the surface of the rotor. Nonsalient-pole rotors are normally used for two- and four-pole rotors, while salient-pole rotors are normally used for rotors with four or more poles. Because the rotor is subjected to changing magnetic fields, it is constructed of thin laminations to reduce eddy current losses. A dc CUlTent must be supplied to the field circuit on the rotor if it is an elec- tromagnet. Since the rotor is rotating, a special arrangement is required to get the End View Side View FIGURE 4-1 A nonsalient two-pole rotor for a synchronous machine.
- 8. Construction Rotor: There are two types of rotor used in an AC generator / alternator: (i) Salient and (ii) Cylindrical type
- 9. Construction
- 10. Construction • Dumper winding are useful in preventing the hunting in generators. • The dumper winding also tends to maintain balanced 3-phase voltage under unbalanced load conditions.
- 11. How to supply field current? There are two common approaches to supplying this dc power: 1. Supply the dc power from an external dc source to the rotor by means of slip rings and brushes. 2. Supply the dc power from a special dc power source mounted directly on the shaft of the synchronous generator. . Slip rings and brushes Need regular maintenance: brush wearied Significant power loss due to voltage drop in brush
- 12. How to supply field current? Exciter A brushless exciter is a small ac generator with its field circuit mounted on the stator and its armature circuit mounted on the rotor shaft. The three-phase output of the exciter generator is rectified to direct current by a three-phase rectifier circuit also mounted on the shaft of the ( generator, and is then fed into the main dc field circuit.
- 13. How to supply field current? Self-Exciter To make the excitation of a generator completely independent of any external power sources, a small pilot exciter is often included in the system. A pilot exciter is a small ac generator with permanent magnets mounted on the rotor shaft and a three-phase winding on the stator. It produces the power for the field circuit of the exciter, which in turn controls the field circuit of the main machine. s o " 196 ELECTRIC MACHINERY FUNDAMENTALS 1 1 1 Pilot exciter Pilot exciter field Permanent magnets Exciter Exciter armature : ! Three- T phase rectifier : 1 + 1 1 1 Synchronous generator Main field I I I : output I I I 1 1 1 I 1 1 1 1 1 1 Three- phase rectifier Lrvv-v-,. Pilot exciter armature FIGURE 4-5 RF 1 1 Exciter field 1 1 I 1 1 1 Mum armature A brushless excitation scheme that includes a pilot exciter. The permanent magnets of the pilot exciter produce the field current of the exciter, which in turn produces the field current of the main machine.
- 14. Armature Windings Winding are two types: (i) Single Layer (ii) Double Layer Single Layer
- 18. Armature Windings COIL PITCH : The distance between the two sides of a coil is called the coil span or coil pitch. POLE PITCH: The angular distance between the central line of one pole to the central line of the next pole is called Pole Pitch. A pole pitch always 180 electrical degrees regardless of the number of poles on the machine.
- 19. Armature Windings Full Pitch Coil: A coil having a span equal to 180 electrical degree is called a full pitch coil as show in fig. Short Pitch Coil: A coil having a span less than 180 electrical degree is called Short pitch coil or frictional pitch coil. It is also called chorded coil.
- 20. Armature Windings Advantages of short pitch winding
- 21. Armature Windings For full pitch coil, α = 0 cos( α/2 ) = 1 and Kc =1 . for a short pitch coil Kc < 1. Coil Span Factor or Pitch factor: Kc is the defined as the ratio of the voltage generated in short pitch coil to the voltage generated in full pitch coil. the coil span factor is also called chording Factor.
- 23. Armature Windings Distribution/Breadth/ Spread/ Winding Factor: 3-phase, 4-pole single layer winding Total 36 slots: 9 slots/pole; 3 slots/pole/phase Angular displacement between any two adjacent slots=180/9= 20 deg
- 24. Armature Windings Distribution/Breadth/ Spread/ Winding Factor: For distributed coil the vector sum can be found from Fig. b If three coils on one slot, total emf = 3Es (Fig. a)
- 25. Armature Windings Distribution/Breadth/ Spread/ Winding Factor:
- 26. Armature Windings Distribution/Breadth/ Spread/ Winding Factor:
- 27. Armature Windings Distribution/Breadth/ Spread/ Winding Factor:
- 28. Synchronous Generators Outlines of lecture # 2 v Relation between Speed, Frequency, and Pole v Equation of induced EMF v Equivalent circuits and phasor diagram of synchronous machine v Efficiency and losses v Power and torque equations
- 29. Let P= total number of magnetic poles N= rotative speed of the rotor in rpm f= frequency of generated emf in Hz Since one cycle of emf is produced when a pair of poles passes past a conductor, the number of cycles of emf produced in one revolution of the rotor is equal to the number of pair of poles. No of cycles/revolution = P/2 No of revolutions/second =N/60 Thus, frequency, f = P/2 X N/60= PN/120 Hz Relation between Speed, Frequency, and Pole
- 30. Equation of induced EMF Let Z= No. of conductors or coil sides in series/phase = 2T where T is the no of coils or turns per phase P= No of poles f= frequency of induced emf in Hz Φ= Flux/pole in webers 𝑘! = 𝑑𝑖𝑠𝑡𝑟𝑖𝑏𝑢𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟 = "#$ %&/( % "#$ &/( 𝑘)𝑜𝑟 𝑘* = 𝑝𝑖𝑡𝑐ℎ 𝑜𝑟 𝑐𝑜𝑖𝑙 𝑠𝑝𝑎𝑛 𝑓𝑎𝑐𝑡𝑜𝑟 = 𝑐𝑜𝑠𝛼/2 𝑘+ = 𝑓𝑜𝑟𝑚 𝑓𝑎𝑐𝑡𝑜𝑟 = 1.11 N= rotor rpm
- 31. Equation of induced EMF
- 39. Equivalent Circuit of a Synchronous Machines ame equation as the one describing the armature reaction volt- armature reaction voltage can be modeled as an inductor in nal generated voltage. the effects of armature reaction, the stator coils have a self- sistance. If the stator self-inductance is called LA(and its cor- e is called XA) while the stator resistance is called RA , then the ween EAand V. is given by (4-9) ion effects and the self-inductance in the machine are both tances, and it is customary to combine them into a single reac- nchronous reactance of the machine: Xs = X + XA (4-10) equation describing V¢ is IV¢ = EA - jXsIA - RAIA I (4-11) ible to sketch the equivalent circuit of a three-phase synchro- e full equivalent circuit of such a generator is shown in gure shows a de power source supplying the rotor field circuit, y the coil's inductance and resistance in series. In series with 204 ELECTRIC MACHINERY FUNDAMENTALS + V, FIGURE 4-12 The per-phase equivalent circuit of a synchronous generator. The internal field circuit resistance and the external variable resistance have been combined into a single resistor RF. ( Power conversion reversed in motor Synchronous Generator Synchronous Motor or This is exactly the same as the equation for a generator, except that t current term has been reversed.
- 40. Equivalent Circuit of a Synchronous Machines SYNCHRONOUS MOTORS 273 202 ELECTRIC MACHINERY FUNDAMENTALS lAI + jXs RA EAJ V'I IF + Radj lA2 RF jXs VF + EII2 rv V,2 (de) LF lA3 + jXs RA + EA3 rv V,3 FIGURE 4-10 The full equivalent circuit of a three-phase synchronous genemtor. The fact that the three phases of a synchronous generator are identical in all respects except for phase angle normally leads to the use of a per-phase equiv- ( Power conversion reversed in motor Synchronous Generator Synchronous Motor
- 42. Efficiency and losses Efficiency Efficiency of a machine can be defined by 𝜼 = 𝑷𝒐𝒖𝒕 𝑷𝒊𝒏 ×𝟏𝟎𝟎% 𝜼 = 𝑷𝒊𝒏 − 𝑷𝒍𝒐𝒔𝒔 𝑷𝒊𝒏 ×𝟏𝟎𝟎% 𝜼 = 𝑷𝒐𝒖𝒕 𝑷𝒐𝒖𝒕 + 𝑷𝒍𝒐𝒔𝒔 ×𝟏𝟎𝟎% Losses in AC machines
- 43. Efficiency and losses Losses in AC machines The losses that occur in ac machines can be divided into four basic categories: • Electrical or copper losses (l2R losses) • Core losses • Mechanical losses • Stray losses
- 44. Efficiency and losses Losses in AC machines ELECTRICAL OR COPPER LOSSES: Copper losses are the resistive heating losses that occur in the stator (armature) and rotor (field) windings of the machine. The stator copper losses (SCL) in a three-phase ac machine are given by the equation 𝑃,-. = 3𝐼/ ( 𝑅/ where IA is the current flowing in each armature phase and RA is the resistance of each armature phase. The rotor copper losses (RCL) of a synchronous ac machine are given by 𝑃0-. = 𝐼1 ( 𝑅1 where IF is the current flowing in the field winding on the rotor and RF is the resistance of the field winding. The resistance used in these calculations is usually the winding resistance at normal operating temperature.
- 45. Efficiency and losses Losses in AC machines CORE LOSSES: The core losses are the hysteresis losses and eddy current losses occurring in the metal of the motor. MECHANICAL LOSSES: The mechanical losses in an ac machine are the losses associated with mechanical effects. There are two basic types of mechanical losses: friction and windage. Friction losses are losses caused by the friction of the bearings in the machine, while windage losses are caused by the friction between the moving parts of the machine and the air inside the motor's casing. These losses vary as the cube of the speed of rotation of the machine. The mechanical and core losses of a machine are often lumped together and called the no-load rotational loss of the machine.
- 46. Efficiency and losses Losses in AC machines STRAY LOSSES (OR MISCELLANEOUS LOSSES): Stray losses are losses that cannot be placed in one of the previous categories. No matter how carefully losses are accounted for, some always escape inclusion in one of the above categories. All such losses are lumped into stray losses. For most machines, stray losses are taken by convention to be 1 percent of full load.