UNIT1. ALTERNATOR OBJECTIVE
The objective of this chapter is to learn about the following 1. construction of synchronous machines and its types 2. emf equation, synchronous reactance and voltage regulation 3. predetermination of regulation of alternators by EMF, MMF and ZPF methods 4. synchronizing of alternator and effect of change of excitation to alternator. 5. determining the direct-axis and quadrature-axis reactance for salient pole type alternators. INTRODUCTION
With the development of the technology and the way in which human labour is getting minimized and the comforts increasing tremendously the use of electrical energy is ever increasing. Basically electric power is the main source of energy for carrying out many functions, as it is a clean and efficient energy source, which can be easily transmitted over long distances. With the availability of Transformer for changing the voltage levels to a very high value (of say 132kV to 400kV) the use of AC power has increased rapidly and the DC power is used only at remote places where AC power cannot be supplied through power lines or cables or for a few social purposes. A synchronous generator is an electrical machine producing alternating emf (Electromotive force or voltage) of constant frequency. In our country the standard commercial frequency of AC supply is 50 Hz. In U.S.A. and a few other countries the frequency is 60 Hz. The AC voltages generated may be single phase or 3-phase depending on the power supplied. For low power applications single phase generators are preferable. The basic principles involved in the production of emf and the constructional details of the generators are discussed below.
CLASSIFICATION OF AC ROTATING MACHINES
Synchronous Machines: •
Synchronous Generators: A primary source of electrical energy.
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•
Synchronous Motors: Used as motors as well as power factor compensators
(synchronous condensers) Asynchronous (Induction) Machines: •
Induction Motors: Most widely used electrical motors in both domestic and
industrial applications. •
Induction Generators: Due to lack of a separate field excitation, these machines
are rarely used as generators. BASIC AC GENERATORS Prime Movers
All generators, large and small, ac and dc, require a source of mechanical power to turn their rotors. This source of mechanical energy is called a prime mover. Prime movers are divided into two classes for generators-high-speed and low-speed. Steam and gas turbines are high-speed prime movers, while internal-combustion engines, water, and electric motors are considered low-speed prime movers. The type of prime mover plays an important part in the design of alternators since the speed at which the rotor is turned determines certain characteristics of alternator construction and operation. Regardless of size, all electrical generators, whether dc or ac, depend upon the principle of magnetic induction. An emf is induced in a coil as a result of (1) a coil cutting through a magnetic field, or (2) a magnetic field cutting through a coil. As long as there is relative motion between a conductor and a magnetic field, a voltage will be induced in the conductor. That part of a generator that produces the magnetic field is called the field. That part in which the voltage is induced is called the armature. For relative motion to take place between the conductor and the magnetic field, all generators must have two mechanical parts - a rotor and a stator. The ROTor is the part that ROTates; the STATor is the part that remains STATionary. In a dc generator, the armature is always the rotor. In alternators, the armature may be either the rotor or stator.
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There are two types of rotor construction for alternators. They round rotor and salient pole rotors. Round Rotor Machine Concept (two poles)
The stator is a ring shaped laminated iron-core with slots.
Three phase windings are placed in the slots.
Round solid iron rotor with slots.
A single winding is placed in the slots. Dc current is supplied through slip rings.
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Salient Pole Rotor Machine Concept (two poles)
The stator has a laminated iron-core with slots and three phase windings placed in the slots.
The rotor has salient poles excited by dc current.
1. DC current is supplied to the rotor through slip-rings and brushes.
Six poles
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RELATIVE POSITION OF FIELD SYSTEM AND ARMATURE
For lower power rating, the armature of the alternator can be made as a rotating member and the field system stationary just like a dc machine. However, this is unsuitable for medium and high power alternators. For these alternators, the most convenient and economical construction is to have field system rotating and armature stationary. Stationary-Field Alternators (Rotating-Armature)
The rotating-armature alternator is similar in construction to the dc generator in that the armature rotates in a stationary magnetic field. In the dc generator, the emf generated in the armature windings is converted from ac to dc by means of the commutator. In the alternator, the generated ac is brought to the load unchanged by means of slip rings. The rotating armature is found only in alternators of low power rating and generally is not used to supply electric power in large quantities. Rotating-Field Alternators (stationary armature)
The rotating-field alternator has a stationary armature winding and a rotating-field winding. The advantage of having a stationary armature winding is that the generated voltage can be connected directly to the load. A rotating armature requires slip rings and brushes to conduct the current from the armature to the load. The armature, brushes, and slip rings are difficult to insulate, and arcovers and short circuits can result at high voltages. For this reason, high-voltage alternators are usually of the rotating-field type. Since the voltage applied to the rotating field is low voltage dc, the problem of high voltage arc-over at the slip rings does not exist. The stationary armature, or stator, of this type of alternator holds the windings that are cut by the rotating magnetic field. The voltage generated in the armature as a result of this cutting action is the ac power that will be applied to the load.
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The stators of all rotating-field alternators are about the same. The stator consists of a laminated iron core with the armature windings embedded in this core The core is secured to the stator frame. BRUSSLESS ALTERNATORS
A number of arrangements for supplying direct current to the fields of synchronous machines have come into use. Adjustments in the field current may be automatic or manual depending upon the complexity and the requirements of the power system to which the generator is connected.
Figure. Conventional excitation systems for synchronous machines Excitation systems are usually 125 V up to ratings of 50kW with higher voltages for the larger ratings. The usual source of power is a direct-connected exciter, motor- generator set, rectifier, or battery. A common excitation system in which a conventional dc shunt generator mounted on the shaft of the synchronous machine furnishes the field excitation is shown in figure below. The output of the exciter (i.e., the field current of the synchronous machine) is varied by adjusting the exciter field rheostat.
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Brushless Excitation System
The brushless excitation system eliminates the usual commutator, collector rings, and brushes. One arrangement in which a permanent magnet pilot exciter, an ac main exciter, and a rotating rectifier are mounted on the same shaft as the field of the ac turbogenerator is shown in figure. The permanent magnet pilot excitor has a stationary armature and a rotating permanent magnetic field. It feeds 400 Hz, three-phase power to a regulator, which in turn supplies regulated dc power to the stationary field of a rotating-armature ac exciter, The output of the ac exciter is rectified by diodes and delivered to the field of the turbo generator.
Figure. Brushless excitation system
Brush less excitation systems have been also used extensively in the much smaller generators employed in aircraft applications where reduced atmospheric pressure intensifies problems of brush deterioration. Because of their mechanical simplicity, such systems lend themselves to military and other applications that involve moderate amounts of power. PRINCIPLE OF OPERATION
The three-phase alternator, as the name implies, has three single-phase windings spaced such that the voltage induced in any one phase is displaced by 120° from the other two. A schematic diagram of a three-phase stator showing all the coils becomes complex, and
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it is difficult to see what is actually happening. The simplified schematic of figure below, view A, shows all the windings of each phase lumped together as one winding. The rotor is omitted for simplicity. The voltage waveforms generated across each phase are drawn on a graph, phase-displaced 120° from each other. The three-phase alternator as shown in this schematic is made up of three single-phase alternators whose generated voltages are out of phase by 120°. The three phases are independent of each other.
Figure. Three-phase alternator connections. Rather than having six leads coming out of the three-phase alternator, the same leads from each phase may be connected together to form a wye (Y) connection, as shown in figure, view B. It is called a wye connection because, without the neutral, the windings appear as the letter Y, in this case sideways or upside down. The neutral connection is brought out to a terminal when a single-phase load must be supplied. Single-phase voltage is available from neutral to A, neutral to B, and neutral to C.
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In a three-phase, Y-connected alternator, the total voltage, or line voltage, across any two of the three line leads is the vector sum of the individual phase voltages. Each line voltage is 1.73 times one of the phase voltages. Because the windings form only one path for current flow between phases, the line and phase currents are the same (equal). A three-phase stator can also be connected so that the phases are connected end-toend; it is now delta connected (figure, view C). (Delta because it looks like the Greek letter delta, Δ.) In the delta connection, line voltages are equal to phase voltages, but each line current is equal to 1.73 times the phase current. Both the wye and the delta connections are used in alternators. GENERATED EMF IN A SYNCHRONOUS GENERATOR
It is now possible to derive the computed or expected EMF per phase generated in a synchronous generator. Let us assume that this generator has an armature winding consisting of a total number of full pitched concentrated coils C, each coil having a given number of turns Nc. Then the total number of turns in any given phase of an m-phase generator armature is
But Faraday’s law states that the average voltage induced in a single turn of two coil sides is
The voltage induced in one conductor is 2Ø/(1/s) = 2Øs, where s=speed of rotation in r.p.s, for a 2 pole generator. Furthermore, when a coil consisting of Nc turns rotates in a uniform magnetic field, at a uniform speed, the average voltage induced in an armature coil is
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where Ø is the number of lines of flux (in Webers) per pole, Nc is number of turns per coil, s is the relative speed in revolutions/second (rps) between the coil of Nc turns and the magnetic field Ø. A speed s of 1 rps will produce a frequency f of 1 Hz. Since f is directly proportional and equivalent to s, (for a 2-pole generator) replacing the latter in above equation, for all the series turns in any phase,
However, in the preceding section we discovered that the voltage per phase is made more completely sinusoidal by intentional distribution of the armature winding. The effective rms value of a sinusoidal ac voltage is 1.11 times the average value. The effective ac voltage per phase is
But above equation is still not representative of the effective value of the phase voltage generated in an armature in which fractional-pitch coils and a distributed winding are employed. Taking the pitch factor kp and the distribution factor kd into account, we may now write the equation for the effective value of the voltage generated in each phase of an AC synchronous generator as
FREQUENCY OF AN A.C. SYNCHRONOUS GENERATOR
Commercial ac synchronous generators have many poles and may rotate at various speeds, either as alternators or as synchronous or induction motors. The emf equation was derived for a two-pole device in which the generated EMF in the stationary armature winding changes direction every half-revolution of the two-pole rotor. One complete revolution will produce one complete positive and negative pulse each cycle. The frequency in cycles per
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second (Hz) will, as stated previously, depend directly on the speed or number of revolutions per second (rpm/60) of the rotating field. If the ac synchronous generator has multiple poles (having, say, two, four, six, or eight poles...), then for a speed of one revolution per second (1 rpm/60), the frequency per revolution will be one, two, three, or four ..., cycles per revolution, respectively. The frequency per revolution, is therefore, equal to the number of pairs of poles. Since the frequency depends directly on the speed (rpm/60) and also on the number of pairs of poles (P/2), we may combine these into a single equation in which
where P is the number of poles N is the speed in rpm (rev/min) f is. the frequency in hertz ωm is the speed in radians per second (rad/s) ωe is the speed electrical radians per second.
SYNCHRONOUS REACTANCE
The operation of the synchronous machine can be reduced to comparatively simple expression by the convenient concept of synchronous reactance. The resultant linkage of flux with any phase of the armature of a synchronous machine is due, as has been seen, to the combined action of the field and armature currents. For a simple treatment it is convenient to separate the resultant flux into components: (a) the field flux due to the field current alone; and (b) the armature flux due to the armature current alone. This separation does not affect qualitative matters, but its quantitative validity rests on the assumption that the magnetic circuit has a constant permeability. In brief the simplifying assumptions are: 1. The permeability of all parts of the magnetic circuit of the synchronous machine is constant - in other words the field and armature fluxes can be treated separately as proportional to their respective currents so that their effects can be superposed. 2. The air gap is uniform, so that the armature flux is not affected by its position relative to the poles - in other words we assume the rotor to be cylindrical 11
3. The distribution of the field flux in the air gap is sinusoidal. 4. The armature winding is uniformly distributed and carries balanced sinusoidal currents. In other words, the harmonics are neglected so that the armature flux is directly proportional to the fundamental component of the armature reaction mmf implying that the armature reaction mmf is distributed sinusoidally and rotates at synchronous speed with constant magnitude.
Assumption (1) is roughly fulfilled when the machine works at low saturation; (2) and (3) are obviously inaccurate with salient-pole machines and assumption (4) is commonly made and introduces negligible error in most cases. The behaviour of an “ideal” synchronous machine can be indicated qualitatively when the above assumptions (1) to (4)are made. The phasor diagrams below for the several conditions contain the phasors of two emfs viz. Eo and E . The latter is the e.m.f actually existing, while the former is that which would be induced under no-load conditions, i.e. with no armature current (or armature reaction).
Figure. Phasor diagrams for different operating conditions 12
Thus Eo is the e.m.f. corresponding to the flux produced by the field winding only, while E is that actually produced by the resultant flux due to the combined effect of stator and rotor ampere-turns. The actual e.m.f. E can be considered as Eo plus a fictitious e.m.f. proportional to the armature current. The figure is drawn in this manner with Er such that the following phasor relationship is satisfied: E = Eo + Er It can be seen from figure, that Er, is always in phase-quadrature with armature current and proportional to it (as per the four assumptions (1) to (4) above). The emf Er is thus similar to an emf induced in an inductive reactance, so that the effect of armature reaction is exactly the same as if the armature windings had a reactance Xa = Er/Ia . This fictitious reactance Xa can added to the armature leakage reactance X l and the combined reactance (Xa+Xl ) is known as the synchronous reactance Xs. The armature winding apart from these reactance effects, presents a resistive behavior also. Synchronous impedance is a tern used to denote the net impedance presented by each phase of the alternator winding, consisting of both resistive and reactive components. The behavior of a synchronous machine can be easily predicted from the equivalent circuit developed using this synchronous reactance Xs. LOSSES AND EFFICIENCY
To calculate the efficiency of a synchronous generator, a procedure is to be followed for establishing the total losses when operating under load. For generators these losses are, 1. Rotational losses such as friction and windage. 2. Eddy current and hysteresis losses in the magnetic circuit 3. Copper losses in the armature winding and in the field coils 4. Load loss due to armature leakage flux causing eddy current and hysteresis losses in the armature-surrounding iron.
With regard to the losses, the following comments may be made,
1. The rotational losses, which include friction and windage losses, are constant, since the speed of a synchronous generator is constant. It may be determined from a no-load test.
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2. The core loss includes eddy current and hysteresis losses as a result of normal flux density changes. It can be determined by measuring the power input to an auxiliary motor used to drive the generator at no load, with and without the field excited. The difference in power measured constitutes this loss. of Technology Madras 3. The armature and field copper losses are obtained as I a2Ra and Vf If Since per phase quantities are dealt with, the armature copper loss for the generator must be multiplied by the number of phases. The field winding loss is as a result of the excitation current flowing through the resistance of the field winding.
4. Load loss or stray losses result from eddy currents in the armature conductors and increased core losses due to distorted magnetic fields. Although it is possible to separate this loss by tests, in calculating the efficiency, it may be accounted for by taking the effective armature resistance rather than the dc resistance.
After all the foregoing losses have been determined, the efficiency η is calculated as,
where η = efficiency, kVA = load on the generator (output) PF = power factor of the load
The quantity (kVA*PF) is, of course, the real power delivered to the load (in kW) by the synchronous generator. Thus, it could in general be stated as
The input power Pin = Pout + Plosses is the power required from the prime mover to drive the loaded generator.
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Practice Questions 1. In a synchronous machine, the induced emf Phasor: o. a) leads the flux phasor by 90 b) is in phase with the flux phasor. o. c) Lags behind the flux phasor by 90 d) Is in phase opposition to the flux phasor. 2. In a synchronous machine, the voltage induced by armature reaction flux acts like: a) In Voltage drop inductive reactance. b) In Voltage drop resistance. c) In Voltage drop capacitive reactance. d) In Voltage drop inductive impedancde. 3. Synchronous reactance is: a) The difference of armature leakage reactance and reactance equivalent of armature reaction. b) The same of armature leakage reactance. c) The reactance equitant of armature reaction. d) The sum of armature leakage reactance and reactance equivalent of armature reaction. 4. Voltage regulation of a synchronous generator calculated by synchronous impedance method is: a) Higher than actual because of saturation of magnetic circuit. b) Lower than actual because of saturation of magnetic circuit. c) Nearly accurate because it takes account of magnetic saturatioon. d) Nearly accurate because the generator is normally operated with an unsaturated magnetic circuit. 5. Armature reaction AT of a synchronous generator at rated voltage zero power factor lagging is: a) Magnetizing. b) Demagnetizing c) Cross-magnetizing d) Both magnetizing and cross- magnetizing 6. Armature reaction AT of a synchronous motor at rated voltage zero power factor lagging is: a) Magnetizing. b) Demagnetizing c) Cross-magnetizing d) Both magnetizing and cross- magnetizing
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Answers for Practice Questions 1. o. c) Lags behind the flux phasor by 90 2. a) In voltage drop inductive reactance. 3. d) The sum of armature leakage reactance and reactance equivalent of armature reaction 4. a) Higher than actual because of saturation of magnetic circuit. 5. b) Demagnetizing 6. a) magnetizing REGULATION OF ALTERNATORS
Voltage regulation of an alternator is defined as the change in terminal voltage of the machine from no load to load expressed as a fraction of load voltage, the speed and excitation remaining unchanged. The experiment involves the determination of the following characteristics and parameters: 1. The open -circuit characteristic(the O.C.C) 2. The short-circuit characteristic(the S.C.C) 3. The effective resistance of he armature winding. The procedure to conduct the open circuit test and short circuit test given below: 1. Open circuit characteristic
Connect the alternator as shown in figure. The prime move in this experiment is a D.C. shunt motor, connected with resistances in its armature and field circuits so as to enable the speed of the set to be controlled. Run the set at the rated speed of the alternator, and for each setting of the field current, record the alternator terminal voltage and the field current. Note that there is no load on the alternator. Record readings till then open circuit voltage reaches 120% of the rated voltage of the machine. The O.C.C is drawn between open circuit voltage and the field current. 2. Short circuit characteristic
Connect as in figure but short-circuit the armature terminals through an ammeter. The current range of the instrument should be about 25-50 % more than the full load current of the alternator. Starting with zero field current, increase the field current gradually and cautiously till rated current flows in the armature. The speed of the set in this test also is tom be maintained at the rated speed of the alternator. The S.C.C is drawn between the short circuit current and the field current.
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3. Measure the D.C. resistance of he armature circuit of the alternator. The effective a.c resistance may be taken to be 1.6 times the D.C. resistance.
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EMF method (Synchronous impedance method)
Conduct tests to find OCC (upto 125% of rated voltage), SCC (for rated current) and Armature resistance (per phase)
Synchronous impedance per phase,
for same value of field current.
For any load current I and phase angle Φ, find E as the vector sum of V, IRa and IX s E = V + IZs
(Here bold letters indicate complex numbers.)
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For lagging power factor
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After calculating Zs and Xs, no load emf E o is calculated using the phasor diagrams and formula as shown above for different type of load (power factors).
Regulation is found by the following expression
where V is the terminal voltage and E 0 is the induced voltage on no load.
At higher values of field current, saturation increases and the synchronous impedance decreases. The value of Zs calculated for the unsaturated region of the O.C.C is called the unsaturated value of the synchronous impedance. Due to this, this method gives a higher value of regulation and so called as pessimistic method.
MMF METHOD (AMPERE TURNS METHOD)
Conduct tests to find OCC (upto 125% of rated voltage) SCC (for rated current)
Steps:
1. By suitable tests plot OCC and SCC 2. From the OCC find the field current I f1 to produce rated voltage, V . 3. From SCC find the magnitude of field current I f2 to produce the required armature
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current. 4. Draw I f2 at angle (90+ Φ) [for lagging power factor] from I f1, where Φ is the phase angle of current from voltage. If leading power factor, take the angle of I f2 as (90-Φ). 5. Find the resultant field current, I f and mark its magnitude on the field current axis. 6. From OCC. find the voltage corresponding to I f , which will be E 0. 7. Calculate regulation by using the formula
Practice Questions 7. In a generating synchronous machine carrying load (usual symbols are used): a) E leads V by angle δ. b) E lags V by angle δ c) E leads V are in phase d) E leads V are in phase opposition 8. Potier’s method uses OCC and ZPFC to yield information about: a) Synchronous reactance. b) Leakage reactance only c) Field current equivalent of armature reaction only d) Leakage reactance and field current equivalent of armature reaction 9. Synchronous generator voltage obtained by the synchronous impedance method is: a) Higher than actual as it does not account for magnetic saturation. b) Lower than actual as it does not account for magnetic saturation. c) Nearly accurate as it does not accounts for magnetic saturation. d) Nearly accurate as the generator is normally operated in the unsaturated region of magnetization. 10. Prime mover for salient pole synchronous machine is -------------------11. Prime mover for cylindrical rotor synchronous machine is -------------------12. A synchronous generator is synchronized to bus-bars. If the governor setting of its prime mover is raised, its power factor will-----------------Answers for practice questions 7. a) E leads V by angle δ 8. d) Leakage reactance and field current equivalent of armature reaction 9. a) Higher than actual as it does not account for magnetic saturation.
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10. hydro turbine 11. steam turbine 12. improve Two mark questions
1. Write down the equation for frequency of emf induced in an Alternator? 2. Why do cylindrical Alternators operate with steam turbines? 3. What are the advantages of salient pole type construction used for Synchronous machines? 4. How does electrical degree differ from mechanical degree? 5. What is the relation between electrical degree differ from mechanical degree? 6. Why is short pitch winding preferred over full-pitch winding? 7. Why are Alternators rated in KVA and not in KW? 8. What do you mean by Synchronous reactance? 9. What is meant by load angle of an Alternator? 10. Define the team voltage regulation of an Alternator? 11. What is the necessity for predetermination of voltage regulation? 12. How synchronous impedance is calculated from OCC and SCC? 13. In What way does the ampere-turn method differ from synchronous impedance method? 14. Why is the MMF method of estimating the voltage regulation considered as the optimistic method? 15. How do the synchronizing lamps indicate the correctness of phase sequence between existing and incoming Alternators? 16. How does change in excitation affects the load sharing? 17. What is meant by infinite bus-bars? 18. Compare salient pole rotor and cylindrical pole rotor. 19. What is synchronous reactance? 20. What are the reasons for the variation in the terminal voltage of a loaded alternator? 21. Define voltage regulation. What are the methods available for determine of voltage regulation of alternator?
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22. Why is the synchronous impedance used to determine voltage regulation of synchronous machine called pessimistic? 23. What is the need of parallel operation of alternator and what are the condition for parallel operation?
Detail Answer Questions
1. Explain the operating principle of three phase alternator. 2. Explain the construction details of a three phase alternator which is used for slow speed operation. 3. Explain the terms distribution factor and pitch factor of an alternator armature winding and derive the emf equation of an alternator. 4. Derive the emf equation of an alternator. Explain pitch factor and distribution factor. 5. Explain and drive the expression for distribution factor and coil span factor. 6. Discuss the synchronous impedance method of calculating regulation of an alternator. 7. What is the effect of armature reaction at different power factors on synchronous machine? 8. Explain the procedure for POTIER method to calculate voltage regulation. 9. Explain with neat diagrams the armature reaction and it’s effects on alternator. 10. State requirements for paralleling alternators. 11. Bring out the characteristics of two alternators working in parallel. What is the effect of change in excitation on load sharing? 12. For a salient pole synchronous machine, prove the d-axis synchronous reactance X
d
can be obtained from its OCC and SCC. Neglect armature resistance. 13. Explain the two reaction theory of synchronous machines. 14. How can Xd and Xq be determined? 15. Explain the procedures that are followed to connect a synchronous machine to infinite busbars.
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