Synchronous reactance of an alternator

synchronous reactance of an alternator 2005) a. Therefore, total voltage drop in an alternator under load is = IRa + jIXS = I(Ra + jXS) = IZS where ZS is known as synchronous impedance of the armature, the word ‘synchronous’ being used merely as an indication that it refers to the working conditions. Leakage reactance of the armature of a salient pole synchronous machine which varies with the position of the rotor is due to (a) leakage flux which remains within the slot (b) leakage flux of end connections (c) leakage flux which comes out of slot teeth, crosses the air gap and enters pole faces (d) none of the above The four methods of calculating voltage regulation of a 3-phase alternator are: 1. A generator with increase SCR above nominal value requires large field winding which in case requires the increase in size of machine ,increases the cost. 5. The reactance in this state is called steady state reactance Xd. 2. The alternator is supplying full load at unity power factor. 2 kV, and both have subtransient reactances of 20%. Find the external reactance per phase to be connected in series with the alternator so that steady current on short-circuit does not exceed 8 times the full load current Case 1 - Islanded Synchronous machine Generally, since the impedance (load and generator synchronous reactance) doesn't change, by increasing the field excitation (and therefore terminal voltage) of the generator, you are proportionally increasing the strength of the emf (E). 108 pu -- Voltage Regulator SE350 As we know that synchronous reactance Xs is greater than the armature resistance R A so this equation becomes. The leakage reactance X L and armature reaction reactance Xa together called as synchronous reactance X S. 1 10 51. Its full-load armature current is 60A at 0. The winding’s resistance can be approximated by applying a DC voltage to a stationary machine’s winding and measuring the current. The per phase power output of the alternator is [ ] The proposed model can be extrapolated to any size machine. If the two alternators are running in proper synchronism and the voltage of one machine is suddenly increased. A 4-pole, unbalanced star delta connected, 50 Hz, 11 KV, 40 MVA turbo generator, with a synchronous reactance of 0. (C) Synchronous reactance method (D) None. Marathon Generators manufactures only the electrical generator (alternator) and supplies them as an OEM to the Gen-Set packager. leakage reactance Armature resistance and armature reaction. 95 leading Then calculate then voltage regulation for the 3 power factor conditions. 0 75000 max C PERIPHERAL SPEED: - m s D N Vperi / 60 1. 8 lagging power factor and a rated termi-nal voltage. The armature effective resistance and synchronous reactance are 1. The leakage reactance and the synchronous reactance . 8 lagging will be One alternator is loaded 4,000-kW, its PF being 0. ofselfinductionisalmostinstantaneous,thatis,it appearsanddisappearswiththecurrentwhichproducedit To determine the sub-transient (x d ″), transient (x d ′) and steady state reactance (x d) of a synchronous machine. The reactive power produced by a capacitor bank is in direct proportion to the square of its terminal voltage, and if the system voltage decreases, the capacitors produce less reactive power, when it is most needed, while if the system voltage increases the capacitors produce 9. EE8401 Question Paper Electrical Machines 2 2. Alternator MCQ - 1. 2 Alternator 6. The electromotive force (emf) method or synchronous impedance method: Even though this method gives the inconsistent result of voltage regulation it is quite useful because we consider drop due to armature reaction as drop due to synchronous reactance. 32 ohms Answer: a Explanation: Z = V/I = 440/110 = 4 ohms. If the emf from the air gap line is the 3-phase alternator is 440V per phase and armature current is 110 A. 2 Ohms per phase and a synchronous armature reactance of 12 ohms per phase. 5Ω and 30Ω per phase respectively Calculate the % regulation for a load of 1280KW at power factor of 0. 1 Introduction Objectives 6. f quantities while armature reaction is considered as m. Median response time is 34 minutes and may be longer for new subjects. If the driving power from the prime-mover driving an alternator is lost but the alternator remains connected to the supply network and field supply is on, then the alternator will (a) behave as an induction motor but will rotate in an opposite direction (b) behave as a synchronous motor and will rotate in the same direction (c) get burnt Phasor Diagram of a Synchronous Generator at Unity P. Armature resistance only. (D7e)scribe a method of determining direct and quadrature axis reactance of salient pole alternator. 18. A. 1 Construction of Alternator Synchronous machine is consists of two parts, one is stator and another is rotor. 08 + j2. 58. Therefore, the value of the synchronous reactance for a given problem should be estimated at the approximate load of the machine. Multiple Choice Question (MCQ) of Alternator and Synchronous Motors page-4. Quadrature axis synchronous reactance, Xq by slip test For the slip test the alternator should be driven at a speed, slightly less than the synchronous synchronous reactance The sum of armature leakage reactance (XL) and reactance of armature reaction (XAR) is called ___ The ___ is a fictitious reactance employed to account for the voltage effects in the armature circuit produced by the actual armature leakage reactance and the change in the air-gap flux caused by armature reaction. Xs=XL+Xarwhere XL and Xar are in Ω/phase. 2 2 max min i V X i V X t q t d 26. Reactive power (lagging) is the magnetic field energy, so if the generator supplies a lagging load, this implies that it is supplying magnetic energy to (iii)Synchronous reactance: High value of ‘ac’leads to higher value of leakage reactance and armature reaction and hence higher value of synchronous reactance. armature resistance only B. M. The generating set industry ‘standard’ of expecting A four pole, three-phase synchronous generator is rated 250 MVA, its terminal voltage is 24 kV, the synchronous reactance is: 125%. Using synchronous impedance and armature resistance synchronous reactance and hence regulation can be calculated as follows using emf method. Although Xl is frequency dependent (Xl = Lw, where L is the leakage inductance which is constant), is Xa also frequency dependent ? The Synchronous Reactance (XS) is the imaginary reactance employed to account for the voltage effects in the armature circuit produced by the actual armature leakage reactance and by the change in the air gap flux caused by the armature reaction. 8ohm, calculate the values of Zs and Xs. Calculate the load in kVA and PF of the other alternator. Calculate the load in kVA and PF of the other alternator. (d) may increase or decrease depending on the power factor C. Find the full load generated voltage per phase (a) Unity power factor (b) a power factor of 0. g) With a Capacitive load, does the stator MMF aid or oppose the rotor MMF. F . 20. M. 8PF lagging. 2. 3 Performance of Alternator 6. An accurate determination of the armature leakage reactance of synchronous machines is essential for the analysis of these machines [1][2][3] [4] [5]. None of the above ANSWER: Greater than quadrature axis reactance 2) The reluctance power of a salient pole alternator is given by a. The former can predict the incremental d‐axis synchronous reactance by performing a Fourier transform of the voltage and current measured when a small step voltage is applied to the two armature terminals as a field current flows. Equation of an alternator. Synchronous machine is designed to be operating at synchronous speed, nS. The leakage reactance X L (or X P) and the armature reactance X a may be combined to give synchronous reactance X S. Therefore, synchronous reactance (Xs) decreases as core gets saturated. 2. If the average dc resistance of the three armature winding phases between pairs of terminals Rt is 0. Synchronous motors 2. where Xl is the armature leakage ractance can Xa is there to model the armature reactance. The induced emf per phase for this condition is Definition: A synchronous machine that works like a generator is known as a synchronous generator and also called an alternator. 1a X0 Zero Sequence Reactance 0. 1 Construction of Alternator 6. The voltage regulation of alternator at a load power factor of 0. What is the regulation in percent? (REE – Apr. 4 Synchronizing of Alternators 6. 16. Then 677. The open -circuit characteristic (O. Saturated synchronous reactance method 3. Concept of time phasor & space phasor. 2. Ans : C 2. Synchronous Reactance and Vector Diagrams of a Loaded Alternator . c. Synchronous Reactance [Xd] - ‘The Push’ Modern alternators, fitted with a control system capable of supporting a sustained steady state short circuit current, will typically sustain 2 to 4 x rated current under a steady state three phase short circuit condition. An alternator has 3,-phase winding on the stator and a d. A. 3 EMF Equation 6. 14. 6 kV, 3-phase star connected alternator has a synchronous reactance of 25 per phase. More of resistance as compared to synchronous reactance In a synchronous alternator, synchronous reactance results in a terminal volt drop. b) Adjust the trouble of the alternator until the output voltage E1 = 208 Vac An alternator is supplying 10 A to an inductive load at 220 V, while running at 1000 rpm. It is the combination of synchronous reactance and armature resistance. It is then said to be normally excited. AEscAI , uSX , A 3-φ , star connected alternator is rated at 1600KVA, 13500V. a. Y-connected alternator has an armature resistance of 0. field winding on the rotor. The terminal voltage of an alternator is 2400 V per phase while running at 1800 rpm. -10%. 2 Working Principle 6. I f One alternator is loaded 4,000-kW, its PF being 0. (b) Armature reaction reactance. Turbo alternator 150 m/s Maxi limit 175 m/s RUNAWAY SPEED: - If an alternator is delivering rated load and the load is The parallel operation and load sharing between two alternators are possible because of alternator internal reactance which limits the short circuit current and allows angular displacement between two stations. a calculation using the alternator’s advised value for the synchronous reactance (Xd). • Zero sequence reactance (Xo): The quotient of the reactive fundamental component of zero sequence armature voltage, 2. Z s = R a + jX s ? per phase (a) Unsaturated Synchronous Reactance: Air- gap Voltage (L-L) OCC = 15,400 V. REACTANCES It can be defined as the imaginary part of the impedance in any power circuit. These synchronous reactance of the machine: X X X SA V E jX I RI I A S A A Therefore, the phase voltage is The equivalent circuit of a 3-phase synchronous generator is shown. f (Ans: X Armature leakage reactance and armature reaction m. 16. 8 power-factor leading and a terminal voltage of 12. Compute the terminal voltage for the same excitation when the alternator supplies full load current at 0. 122. Sequence Impedance of Synchronous Machine. The percent regulation of the alternator at unity power factor is (A) 1. The alternator is supposed to have no armature reaction at all, but is supposed to possess an armature reactance in excess of its true leakage reactance. Ns= 120f The synchronous reactance is determined as The graph is shown below: From the above figure consider the field current I f = OA that produces rated alternator voltage per phase. f. Positive Sequence, Impedance Z 1 Depending on the time interval of interest one of the three reactances may be used. 07 Ω and a dispersion reactance of 0. • Calculate the synchronous reactance in ohm. 1. There are various Reactances and Time Constants which contribute during the selection of control systems and protection scheme for any Generator. 8. EQUIPMENTS REQUIRED Three phase alternator Rating: 400V, 5kVA, 1500 rpm. 7 laging (c) a power factor of 0. The ratio of applied voltage to the maximum value of the armature current gives the the quadrature-axis reactance X sq. Explanation: Potier triangle method is also called as Zero power factor (ZPF) method. 68 ohm and the field current is 47 A. 2 Ohms per phase and a synchronous armature reactance of 12 ohms per phase. synchronous reactance X sd. 3. Y-connected alternator has an armature resistance of 0. 3. b) Armature reaction reactance. 8 leading. 1a Xd Synchronous Reactance 2. 16. 92 ohms d) 2. Determine at rated load at unity p o w e r factor: a) Internal E. 1a X'd Transient Reactance 0. 5 (C) 21. Thus the negative sequence field rotates at a speed twice the synchronous speed with respect to rotor. REACTANCES due to the presence of fundamental zero sequence armature current at rated frequency, by the value of that component of Vt = Terminal Voltage of Alternator δ = Load Angle Xs = Synchronous Reactance Thus it is clear that power output of an Alternator will be maximum if load angle δ is 90°. Synchronous reactance Xs = √ (Zs2 - Ra2) Where Ra = Armature resistance of alternator (per phase) Calculate the excitation emf Eo and voltage regulation for full-load and condition can be calculated by multiplying the alternator’s rated output current by the reciprocal of the value of X”d as a value in per unit (pu) terms. 025 and 0. Find the full load generated voltage per phase (a) Unity power factor (b) a power factor of 0. Example: Where the alternator’s rated full load current is 1443A and published sub-transient reactance for 400V, 1000kVA is 0. 5 pu. 1 Synchronous generators/motors/compensators The zero sequence reactance of the motor is about ½ of its positive sequence reactance. c. 95 leading Then calculate then voltage regulation for the 3 power factor conditions. 8 lagging. Calculate the load in kVA and PF of the other alternator. Q: 4. 10 %. a) Armature reaction reactance and leakage b) Leakage reactance and field winding reac c) Field winding reactance and armature rea d) A reactance connected in series with a syr . This means the alternator has a different reactance in one than in the other direction (if it has salient poles obviously this has to be the case!). Quadrature axis synchronous reactance, X q: For slip test, the machine should be driven at a speed, slightly less than synchronous speed, with its field circuit open. Q: What is meant by synchronous impedance of an Alternator? The combination of armature resistance and synchronous reactance is known as synchronous impedance. For more accurate determination of these values the oscillogram of the armature current and voltage can be recorded. 3 Voltage Regulation 6. The overall reactance of the armature winding is the sum of its leakage reactance plus fictitious reactance, which is known as synchronous reactance (Xs). For the same field excitation, terminal voltage is decreased from its no-load value E 0 to V (for a lagging power factor). This Zero power factor (ZPF) method is used to determine the voltage regulation of synchronous generator or alternator. 1a X2 Negative Sequence -- Amortisseur Windings Full Reactance 0. 5. EXPERIMENTS THAT CAN BE PERFORMED *Study and Measurement of Direct Axis (Xd) and Quadrature Axis (Xq) Reactance by Slip Test *Study and Measurement of Positive, Negative and Zero Sequence Impedance of Three Phase Synchronous Generator *Study and Measurement of Synchronous Impedance and SCR of Three Phase Synchronous Generator The parallel operation and load sharing between two alternators are possible because of alternator internal reactance which limits the short circuit current and allows angular displacement between two stations. drop due to armature resistance, IR a. U. 06 ohm, respectively. New ASA method 4. The frequency is kept constant thanks to a hydraulic regulation of the rotation speed of the alternators. The effect of armature flux in an alternator depends on Magnitude of current flowing through the armature winding and Nature of power factor of the load connected to the alternator. 1 Armature Reaction 6. Synchronous impedance of an alternator is One alternator is loaded 4,000-kW, its PF being 0. The steam input to prime mover is increased till synchronous machine starts operating at rated KVA. f. The method requires following data to calculate the regulation. armature resistance & leakage reactance Synchronous reactance Xd is the apparent reactance that determines the current flow when a steady state condition is reached. Summar y The sub-transient reactance of a generator set is used to calculate the maximum available short circuit current for selecting circuit breakers with adequate interrupting rating. The Synchronous Reactance (XS) is the imaginary reactance employed to account for the voltage effects in the armature circuit produced by the actual armature leakage reactance and by the change in the air gap flux caused by the armature reaction. Since the short circuit current of the alternator lags behind the voltage by 90 degree, the reactance involved are direct axis reactance. 2 Ohms per phase and a synchronous armature reactance of 12 ohms per phase. g) With a Capacitive load, does the stator MMF aid or restrict the rotor MMF. Synchronous speed is being notated by Ns. 4. For a three-phase, 4-pole alternator the winding I believe q- and d- refer to machine rotor coordinate system axis (that are rotating with the rotor). 5 (D) 27. (c) remains constant B. 5 (C) 21. Q14. The second is that which is governed by the transient reactance, which lasts several seconds. m. • Calculate the induced voltage, E. To reduce the harmonics in the emf generated in an alternator: (A) Windage is well distributed (B) Slots are skewed (C) Salient pole tips are If the prime mover of the alternator supplying load to an infinite bus is suddenly shut down, then it will (A) stop (B) continue to run as an alternator (C) continue to run as a synchronous motor in the reverse direction (D) continue to run as a synchronous motor in the same direction 15) A 6 kVA, 220 v 3 phase, star connected synchronous generator has a winding resistance of 0. The synchronous reactance of an alternator is equal to. Equal to quadrature axis reactance d. What are the two items that are combined into the quantity called synchronous reactance? Armature inductive reactance and armature reaction. Finally, there is the steady-state region whose fault current is determined by the generator’s synchronous reactance. 8 kV per phase. For the power system shown, find short circuit level and short circuit current at the F1 & F2 poi A: First transfer the entire system to a common base system . Each of two single-phase alternators has an armature winding whose resistance and synchronous reactance are 0. Each of two single-phase alternators has an armature winding whose resistance and synchronous reactance are 0. d. All the values are generally expressed on per phase basis. 16. 025 and 0. Find the full load generated voltage per phase (a) Unity power factor (b) a power factor of 0. 1) In a salient pole synchronous generator direct axis reactance is a. According to this method in any alternator armature resistance drop IR a and the armature leakage reactance drop IX L are basically the e. The figure below illustrates the different components and the relative timeframes in which they dominate. and armature m. Synchronous Impedance of Synchronous Machine Determination of Synchronous Impedance Armature Winding of 3 Phase Machines Pitch factor Distribution factor Synchronous reactance and phasor diagram of Alternator Voltage regulation EMF method (Numericals) Armature Reaction Alternator Operation of Alternator f) Calculate the alternator regulation with Capacitive loading. What is synchronous reactance? On which factor the voltage regulation of the alternator depends? Describe the different methods to find out voltage regulation of the The synchronous reactance represents in one lumped element all the internal complex interactions between the rotor and stator magnetic fields, which are not of concern here. 7 laging (c) a power factor of 0. In this study a machine with the following characteristics was chosen: a three-phase wye connected alternator of 3000 kVA, 6600 V phase to phase and 50 Hz, an armature resistance of 0. 7 Ω / phase. m. On the other hand, short circuit current keeps rising linearly with field current. The armature self-reactance X A is independent of the rotor angle and is normally added to the direct and quadrature armature-reaction reactances to produce the direct axis synchronous reactance and the quadrature axis synchronous reactance of the generator (equations 3. REACTANCES It can be defined as the imaginary part of the impedance in any power circuit. D Potier Reactance - Synchronous Generator For obtaining potier reactance Zero Power Factor test is conducted by connecting the alternator to ZPF load and exciting the alternator in such way that the alternator supplies the rated current at rated voltage running at rated speed. drop due to armature reaction. As the speed of an alternator increases (A) the frequency increases 7. Alternator Torque Model Based on Equivalent Circuit of Synchronous Generator for Electric Power Management October 2013 IEEE Transactions on Vehicular Technology 62(8):3593-3602 Synchronous Generator and Synchronous Motor Fault Contribution A synchronous motor has the same kind of reactance as a generator. Synchronous Generator related MCQs for Test Preparation. To enable that, the mechanical input given to the machine in terms of rotating torque must also be very high. Since Xs,unsat>>Ra, 9. A 6 kVA, 220 v 3 phase, star connected synchronous generator has a winding resistance of 0. • Then the synchronous reactance Xs could be obtained using IfA Ef or Vt (V) Air-gap line OCC Isc (A) SCC If (A) Vrated VA Isc,B Isc, A IfB scA fA unsat,saunsat,s I EV XRZ 22 22 aunsat,sunsat,s RZX scA oc,t scA f unsat,s I V I E X : Ra is known from the DC test. 2/446 = 19. 95 lagging, synchronous reactance of 1. On the other hand, short circuit current keeps rising linearly with field current. Armature reaction reactance. 3. 2. Sum of leakage reactance and armature reaction reactance. It is also widely recognised and accepted by industries to use reactances only in calculating industrial power system short circuit, in that it would result in a higher short circuit value, perhaps by 0-3 percent in most cases. 5 ohm/phase. A 500 kVA ,2300 volt three phase star connected alternator has a full load armature-resistance drop per phase of 50 volts and a combined armature reactance plus armature-reaction drop of 500 volts per phase. An alternator as mentioned earlier is mostly responsible for generation of very high electrical power. 1 10 5 0. reactance = 8891. May 4, 2018 - Advantages of rotating field over rotating armature | rotating field vs rotating armature | Electrical Machines 2 | advantage of rotating field and stationa The saturated synchronous reactance is then If R a is negligible, the unsaturated synchronous reactance is Example (Synchronous machine equivalent model) The following data is obtained for a three-phase 10 MVA, 14 kV wye-connected synchronous machine (all voltages are line-to-line). The per unit synchronous reactance on the base values of 200 MVA and 40 kV is 2. If SCR increases Xs(synchronous reactance) decreases and hence the power capability increases, i. This value is often used to determine the setting of the generator backup overcurrent relays. The given diagram shows the relation among the parameter like phase voltage (Vø), internal generated voltage (E A), armature current (I A), synchronous reactance (X S) and some other factors by phasor diagram when the generator is working with the resistive load and have unity power factor. 2 p. The field to armature mutual inductance of the synchronous motor is _____. 2. The ratio of applied voltage to the maximum Synchronous reactance is the combination of voltage due to armature reaction. 28. This high torque value results in oscillation or hunting effect of the alternator or synchronous generator. For the power system shown, find short circuit level and short circuit current at the F1 & F2 poi A: First transfer the entire system to a common base system Solution for The synchronous reactance of an alternator represents. SCR also measures the transient stability of unit. Therefore, an approximate method for determining the synchronous reactance Xsat a given field current is: 1. • explain armature reaction and synchronous reactance, • describe the methods to find voltage regulation, and • explain the synchronization. 025 and 0. A 3f, 25 kV, 500 MVA, 60 Hz alternator has a synchronous reactance Xs = 1. Alternator (generator) operation V T E f I a jX s Voltage equation Where: V T = terminal voltage/phase (V) E f = excitation voltage/phase (V) I a = armature current/phase (A) X s = synchronous reactance/phase (ohms) Governor GENERATOR LOADING Lesson 20_et332b. In the operation of an alternator, the armature resistance drop IR a and armature leakage reactance drop IX L are actually emf quantities while the armature reaction is basically MMF quantity. Introduction: It was shown in an earlier chapter that an alternator driven at a constant speed produces an alternating voltage at a fixed frequency dependent on the number of poles in the machine. 2. In complex form Zs = (R + jXs) In polar form Zs =I Zs I< Ѳ ANSWER: c. The synchronous reactance is3. A 500 MVA, 24 kV, 60 Hz three-phase alternator is operating at rated voltage and frequency with a terminal power factor of 0. If R a is negligible, then θ = 90º. 15pu (15%), the time zero symmetrical fault current = The regulation obtained by synchronous impedance method is always higher than actual value because. Fig ‘A’ – Open circuit and Short circuit characteristics of Alternator. 8 ohms. Air- gap Voltage (Phase) OCC = 15,400/sqrt(3) = 8891. 2-The load factor of alternator or generator depend on. 8 leading. Synchronous reactance is assumed constant while it is not. During a fault, a generator continues to rotate due to inertia contributing fault current of varying waveforms that is directly proportional to its changing reactance before it is interrupted by its circuit breaker. A. Synchronous reactance is assumed variable while it is not. *Response times vary by subject and question complexity. Synchronous Generator: Various types & construction, cylindrical rotor theory, phasor diagram, open circuit & short circuit characteristics, armature reaction reactance, synchronous reactance, SCR, load characteristics, potier reactance, voltage regulation, EMF The synchronous machine does not generate any negative-sequence voltage. Short-circuit Armature Current = 446 A. poor voltage regulation of machine. X s = Synchronous reactance (leakage reactance + armature reaction reactance) Taking V ph as the refernce phasor. m. (c) Sum of leakage reactance and armature reaction reactances. . 6. 2. In complex form Zs = (R + jXs ) ; In polar form Zs = | Zs | ; Where, Z s R 2 X s2 and = tan-1 (Xs /R). ineffect,undervauiousconditionstheydifferintheiractions. The synchronous impedance of the generator is 0. A 500 kVA, 1,100 V, 50Hz Y-connected 3 phase alternator has stator resistance of 0. 2. 1 ohm per phase. In case of a synchronous motor, revolving field structure is to be energized by direct current (DC current). dc field is implied in this drawing V T E f I a jX s Where V T = per phase terminal voltage (phasor) E f = voltage behind synchronous reactance I a = armature current (phasor) X s Direct-axis synchronous reactance, Xd = per unit I fo Thus Direct-axis synchronous reactance can be found out by performing open circuit and short circuit test on an alternator. Find Xs by applying (1). In a 3-phase alternator, the unsaturated synchronous reactance of 3Ω per phase. A 3-phase, 20 MVA, 10 kV alternator has internal reactance of 5% and negligible resistance. *Response times vary by subject and question complexity. 2 ALTERNATOR 6. Synchronous reactance is assumed variable while it is not. Motor input = V I a cos φ —per phase Here, V is applied voltage / phase. 06 ohm, respectively. (d) Difference of leakage reactance and armature reaction reactance. 2. A synchronous generator and motor are rated 30MVA, 13. conjunction with various alternator time constants. Thus the voltage drop in an alternator under loaded conditions is the total sum of voltage drop due to armature resistance, armature leakage reactance, and armature reaction reactance. 16 synchronous impedance Z s = V/Isc. One alternator is loaded 4,000-kW, its PF being 0. Power developed in salient pole synchronous generator: It we neglect armature resistance Ra (and hence Cu loss), then power developed (Pd) by an alternator is equal to the power output (Pout). c. g. Three phase balanced reduced voltage at A 3-phase, 10 kVA, 400 V, 50 Hz, Y-connected alternator supplies the rated load of 0. leakage reactance ,armature reaction, Armature resistance and earth connection. Such a rotating speed is called Synchronous speed of the alternator. 6 kV, Y-connected, 3-phase alternator has a synchronous reactance of 6 Ω per phase and a negligible resistance. 2Ω/phase and leakage reactance 0. Leakage reactance of armature. X = Synchronous reactance of Motor, Ohms What are the limits of Power angle? The system is stable only if the power angle δ is between -90° and +90° where the slope dP/ dδ is positive, that is the range in which an increase in power angle results in an increase in transmitted power. B. Synchronous generators (Alternator) Synchronous generator is also referred to as alternator since it generates alternating voltage. • Draw the equivalent circuit. Now if the speed of the alternator is reduced to 750 rpm but the field current remains unchanged, the load current will become (A) 18 A (B) 13. 5 Ω/phase. X S = X L + X a. 1 = 0 - value of the armature current gives the direct axis synchronous reactance X sd. b) Both of the machines will stop. synchronous reactance. 2490 = 0. 05 (B) 10. e the phase relationship between the terminal voltage and armature current. In stator winding two effects are to be considered the effect of field cutting stator conductors at synchronous speed and the effect of stator revolving field. c. A. It was in the beginning common in MW-size power range, but is nowadays mainly used for different power range machines. 5Ω/phase. 06 ohm, respectively. Ea b) no load E. c) The synchronization torque will be produced to restore further synchronism. What is meant by synchronous impedance of an Alternator? The complex addition of resistance, R and synchronous reactance , jXs can be represented together by a single complex impedance Zs called synchronous impedance. M. (Ans May 9, 2018 - Voltage Regulation of Alternator by EMF Method and SOLVED PROBLEMS | synchronous impedance method | regulation of synchronous generator EMF MethodPlease Like The Effect of Load Changes on a Synchronous Generator Operating Alone. 15. Y-connected alternator has an armature resistance of 0. Xs= E A /I A = V ø,oc /I A If the value of the internally generated voltage (E A) and armature current (I A) is known, we can compute the value of the synchronous reactance. t axis synchronous reactance is given by, X d = I fsc / I fo per unit Thus X d can be determined by performing o. F. The rating of a synchronous machine is usually governed by its Speed Temperature rise Weight None of these 123. The machine which produces 3-phase power from mechanical power is called an alternator or synchronous generator. Gen-Set is the combination of a synchronous AC generator (alternator) with a prime-mover (usually a diesel or gaseous engine), a fuel system, exhaust system, and gen-set control systems. Same reactance B. 95 leading Then calculate then voltage regulation for the 3 power factor conditions. Median response time is 34 minutes and may be longer for new subjects. The synchronous reactance of an alternator is equal to (a) Leakage reactance of armature. 2. X s = synchronous reactance of alternator/phase. In the electromagnetic transient process, the stator current and rotor current of synchronous generator appear attenuation oscillation due to direct-axis reactance, cross-axis reactance, transient reactance and sub-transient reactance. Synchronous condensers are also useful for supporting voltage levels. C) : Synchronous reactance (Xd). A machine designed to be connected to the supply and run at synchronous speed is referred to as a synchronous machine. 42 ohm. Mmf method The correct sequence in the ascending order of the values of regulation obtained by these methods is The parallel operation and load sharing between two alternators are possible because of alternator internal reactance which limits the short circuit current and allows angular displacement between two stations. m. Which is that distortion of the magnetic field and voltage due to inductive reactants. f. 2. F. Each of two single-phase alternators has an armature winding whose resistance and synchronous reactance are 0. 25Ω/phase. D. 2. Example 5-3 A 480V, 50Hz, Y-connected, 6-pole synchronous generator has a per-phase synchronous reactance of 1Ω. 3 per unit. S = synchronous reactance / phase then Z S = R a + j X S; I a = R b SS E VE = ZZ −; Obviously, V = E b + I a Z S The angle θ (known as internal angle) by which I a lags behind E R is given by tan θ = X S / R a. 299 pu -- Cooling Air Volume 250 CFM 423. Q: 4. The armature resistance is 0. The sum of leakage reactance Xl and fictitious reactance Xa is called the synchronous reactance. Factors Affecting Alternator Size Alternator on Load Synchronous Reactance Vector Diagrams of Loaded Alternator Voltage Regulation Rothert's M. The synchronous generator is one of the first and most well-known synchronous machine types. *Response times vary by subject and question complexity. 2 Synchronous Reactance and its Determination 6. This is because of. Rotating AC Alternator Exciter with non-controlled (diode) rectifiers. Find the full load generated voltage per phase (a) Unity power factor (b) a power factor of 0. corresponding to this field current the open circuit voltage is AB. DISCUSSION Electric power is produced in large generating stations which contain one or more alternating current (AC If Xs = the synchronous reactance of the machine, Xe = leakage reactance and Xa = reactance due to armature flux then Q. 1-Voltage drop occur in an alternator in. I) R. The alternator is connected to an infinite 25 kV bus, through a feeder of reactance 0. 15 ohm per phase and synchronous reactance of 6. Stator coil resistance is negligible. 2 ohms/phase and an armature resistance of 0. 8 leading power factor. The regulator power The synchronous machine can be used to operate as: 1. Hence X S =X L X a Therefore, total voltage drop in an alternator under load is = IR a jIX S = I(R a jX S ) = IZ S where Z S is known as synchronous impedance of the armature, the word ‘synchronous’ being used merely as an indication that it refers to the working conditions. Effective armature resistance is assumed constant. F. Fig -1: Axes of Alternator Voltage Equation of Alternator | Synchronous Reactance | Synchronous Impedance | voltage equation of synchronous generator Voltage Equation of Alternator vid A 3-phase alternator generates an open-circuit phase voltage of 4000 V when exciting current is 50 A; the short-circuit current for the same excitation being 800 A. The Synchronous Reactance (X S) is the imaginary reactance employed to account for the voltage effects in the armature circuit produced by the actual armature leakage reactance and by the change in the air gap flux caused by the armature reaction. 2. An alternator is generating power at 210 V per phase while running at 1500 rpm. Q: Construct a karnaugh map for the given boolean expression without constructing a truth table . A three-phase, wye connected wound rotor synchronous generator rated at 10 kVA, 230 V has a synchronous reactance of 1. 05 (B) 10. A 6. and fictitious reactance X a is called the synchronous reactance. Thee. The alternator is run at a speed close to synchronous but little less than synchronous value. Emf method 2. a) With a capacitive reactance load of 1200 per phase, turn on the power supply and adjust for a motor speed of 1800 r/min. 15 ohm per phase and synchronous reactance of 6. The positive sequence impedance can also be defined as the impedance offered by the machine to the flow of positive sequence currents in the armature windings create a magnetic field that rotates in the normal direction in the air gap. e. 29. armature leakage reactance and armature f) Calculate the alternator regulation with Capacitive loading. F. a) With a capacitive reactance load of 1200 ( per phase, turn on the power suply and adjust for a motor speed of 1800 r/min. Example (24) A 10 MVA, 6. The line connecting them has a reactance of 10% on the base of the machine ratings. -14%. C. (b) leakage . 5 Q/phase and synchronous reactance is 10 Q/phase, find (i) voltage regulation, and (ii) power angle. See this page for further explaination of the dq coordinate system. 7). Steady-state is reached several seconds afier the shortcircuit occurs. The synchronous reactance per phase is Now, the Sum of armature leakage reactance and armature reaction reactance is called synchronous reactance of an alternator X S. In an alternator field, the effect of cross magnetization is to make the output: (A) Harmonic free (B) True sinusoidal (C) Non sinusoidal (D) None. Find the external reactance per phase to be connected in series with the alternator so that the steady state current on short circuit does not exceed 10 times the full load current 2. Calculate the maximum power that can be developed by the alternator at this excitation. f. 6 and 3. Get Answer A permanent magnet synchronous generator is a generator where the excitation field is provided by a permanent magnet instead of a coil. After the short circuit occurs, the electromagnetic transient process is complex. Calculating the short-circuit current across the terminals of a synchronous generator is very complicated because the internal impedance of the latter varies according to time. The term synchronous refers here to the fact that the rotor and magnetic field rotate with the same speed, because the magnetic field is generated through a shaft mounted permanent magnet mechanism and current is induced into the stationary armature. ‘The alternator voltage decreases as the load increases’ comment on the above statement. What is the source of field excitation for a three-phase alternator? 16. V2 sin2 δ b. = synchronous reactance R a + << X s so it is usually ignored For alternator operation, reverse the direction of I a I a E f also called voltage behind synchronous reactance. Eo c) percentage regulation on full load d) value of synchronous reactance 421. Q: 4. Synchronous reactance is assumed constant while it is not. Thus the armature m. 2. Although numerical techniques for calculating Description: A three-phase synchronous motor has a terminal voltage of 460 V and a terminal current of 120 A at a power factor of 0. (13) BTL 1 Remember 3. 8 power factor lagging. This method is also called Potier method. Fig. View Answer / Hide Answer. Median response time is 34 minutes and may be longer for new subjects. 06 ohm, respectively. wave is rotating at synchronous speed as shown in the Fig. 3 A (C) 10 A (D) 7. m. Difference of leakage reactance and armature reaction reactance. Overview: These days 3-phase a. Phasor Diagram at Unity Power Factor Load : Alternator and Synchronous Generator Formulas & Equations. Remember this alternator armature is a coil. 2. 1 ohm per phase. In an alternator, voltage drops occurs in (A) armature resistance only (B) armature resistance and leakage reactance (C) armature resistance, leakage reactance and armature reaction (D) armature resistance, leakage reactance, armature reaction and earth connections. c) Sum of leakage reactance and armature reaction reactance. Synchronous Alternator with a Cylindrical Rotor . 3310 P. The following synchronous generator & alternator formulas and equations can be used to design, simplify, and analyze the basic AC generators circuits to determine the generated voltage and EMF, speed & frequency, efficiency, voltage & current, generated power and losses etc. To measure the synchronous reactance of a synchronous generator by measured values of open circuit voltage and short circuit current. Hence Z s = (Voc) / (Isc) for same If . As alternator core iron gets saturated, rate of rise of open circuit voltage with field current decreases. Greater than quadrature axis reactance b. Synchronous impedance Z s = (open circuit voltage per phase)/(short circuit current per phase) f or same If . The synchronous reactance is? a) 4 ohms b) 2 ohms c) 6. If the load connected to the alternator are having zero leading power factor and zero lagging power factor, then the nature of load in each case will be ? X s = Synchronous reactance. Get the short-circuit current flow IA,SC at that field current from the SCC. 94 Ohms. 14. u. - synchronous reactance (respests existence of stray flux . The voltage regulation of alternator at a load power factor of 0. pptx 8 Prime Mover Generator Mechanical power converted to active power Exciter Excess The parallel operation and load sharing between two alternators are possible because of alternator internal reactance which limits the short circuit current and allows angular displacement between two stations. 1 Ω/phase and synchronous impedance of 1. D rotating field leakage reactance. Corresponding to this field current, the open-circuit voltage is AB Synchronous Reactance As described in the armature winding notes, synchronous reactance is a combination of armature winding leakage reactance and a reactance used to represent armature reaction. Stator What is meant by synchronous impedance of an Alternator? Top Answer The complex addition of resistance, R and synchronous reactance , jX s can be represented together by a single complex impedance Z s called synchronous impedance. 2 Ohms per phase and a synchronous armature reactance of 12 ohms per phase. An alternator’s actual sustained short circuit current level is displayed on the individual Decrement Curve for that alternator design. 3. Alternators and motors. The sum of leakage reactance Xl and fictitious reactance Xa is called the synchronous reactance. In an alternator, voltage drops occur in A. Describe the effect of armature reaction on the alternator at Lagging power factor, Leading power factor and Unity power factor. At a certain field current the armature is known to induce 5. This is because Xs is defined as: In an alternator like all other synchronous machines, the effect of armature reaction depends on the power factor i. 7 laging (c) a power factor of 0. The percent regulation of the alternator at unity power factor is (A) 1. There As stator construction is similar to the armature of a three phase alternator, the impedance of the stator is called synchronous impedance of synchronous motor consisting of R a as the stator winding resistance and X s as the synchronous reactance. For the power system shown, find short circuit level and short circuit current at the F1 & F2 poi A: First transfer the entire system to a common base system Exercise: Synchronous machine alternator: Study of the power supply of an Airbus A320 In flight, the electrical generation is ensured by two main alternators of 90 kVA which deliver a three-phase system of voltages 115V / 200V, 400Hz. This is because Xs is defined as: a 125°C rise alternator may have been specified, 105°C alternator would have to be supplied to meet a 0. In an alternator or synchronous generator in addition to the armature winding resistance drop and leakage reactance drop, there is a drop in terminal voltage due to armature reaction. If the armature resistance is 0. 47. In an alternator, if Xs is the synchronous reactance, and Xc is the leakage reactance, then the reactance Xs due to armature flux is given by A. XS = XL + Xar In EMF method, the voltage drop due to armature resistance (R a) and the drop due to synchronous reactance (X S) is considered, both the drops are emf quantities. Flow of negative sequence currents in the stator winding produces an mmf rotating synchronously in a direction opposite to that of the rotor. E = Counter emf. When a sudden three-phase short circuit occurs at the terminals of a synchronous generator, the current in the armature circuit increases suddenly to a large value (the symmetrical short circuit current is limited only by the leakage reactance of the machine) and since the resistance of the circuit is negligible as compared to its reactance The regulation obtained by synchronous impedance method is always higher than actual value because. 8 leading. drop due to leakage reactance, IX L. star connected alternator has a full load armature-resistance drop per phase of 50 volts and a combined armature reactance plus armature-reaction drop of 500 volts per phase . REACTA7TCBOFAITERHATORS CONTENTS Page I Introductory II StatementofProblem General 3 Reactance 4 SynchronousReactanceofAlternators—5 Synchronous Reactance (X d) The AC generator, also called an alternator, converts mechanical energy to electrical energy and acts as a voltage synchronous reactance. u. From Fig: 1. a. and flux generated by current . For parallel operation of the two alternators, desirable feature is that both should have. (2) shows the equivalent circuit of alternator for one phase. The main function of this generator is to generate commercial frequency current frequently by converting the mechanical energy from the main mover to an AC electrical energy at a specific frequency & voltage. Per-unit reactance changes inversely (voltage down, reactance up) with the square of the voltage ratio if the kVA rating stays the same. C generator (alternator) can further be classified into two: Induction and synchronous generators. The motor is drawing 20MW at 0. Hence, unsaturated synch. 2. Synchronous machines 1 Synchronous machines Turbo-alternator Saliant poles q!=w/p Rotor(inductor):2ppoleswithexcitation windings carrying DC current; non-laminatedmagneticmaterial Stator:polyphase(e. 066 pu -- Exciter Rotating 425. 3. It is not effective until several seconds after the short circuit occurs , consequently it has no value in short circuit calculations for the application of circuit breakers, fuses and contactors selection but is useful The armature resistance and self-reactance must now be included. Water Wheel alternator 30 to 80 m/s Maxi limit 140 m/s 2. , is connected to a power network. Y-connected alternator has an armature resistance of 0. Answer to Synchronous reactance of an alternator represents(a) armature reaction reactance and leakage reactance. Get the internal voltage EA from the OCC at that field current. 6 k V, 3-phase, star-connected alternator gave open-circuit and short- synchronous reactance varies with the degree of saturation of the OCC. 5kW and core losses of 1 kW at 60Hz at full load. This generator has friction and windage losses of 1. system is being exclusively used for generation, transmission and distribution of power. direct-axis unsaturated synchronous reactance _____ pu 18. Armature resistance R a of the stator can be measured using Voltmeter - Ammeter method. f A 400 MVA, 20 kV synchronous generator has 0. b) Adjust the excitation of the alternator until the output voltage E1 = 208 Vac It is a reactance of a synchronous machine under steady state condition in the direct axis. d) Synchronization cannot be attained automatically. Synchronous reactance of an alternator represen. the synchronous reactance of an alternator is equal to Sum of leakage reactance and armature reaction reactance. shows the phasor diagram of the salient-pole synchronous generator. Two main parts: Construction details of a field and armature: Synchronous Reactance and Synchronous Impedance The curves of Fig. Armature resistance and leakage reactance. A. Calculate the load in kVA and PF of the other alternator. The terminal voltage of the alternator is maintained at 25 kV M . The three phase currents drawn by the armature from a three phase supply produce a rotating flux. 12 per-unit or less sub-transient reactance requirement. To maximize conversion efficiency, synchronous machines are designed to have as low winding resistance as possible; hence the source resistance representing the ohmic value of the stator winding is omitted here with little loss in accuracy. The phase relationship between armature induced emf E due to field flux φf and the current flowing through the armature I a depends upon the power factor of the load. c test on an alternator. d) Difference of leakage reactance and armature reaction reactance . 166 are for a 500-kva 2,300-volt three-phase alternator. 8 leading. The damper windings are #alternator #electricalmachines Finally the transient dies out and the current reaches a steady sinusoidal state called the Steady State. a) The machine will burn. m. 2. Therefore, synchronous reactance (Xs) decreases as core gets saturated. 6. 8 50000 min C 8 11. Generally, the load connected to the alternator is of inductive type. 8 lagging will be. Synchronous reactance, Thus Direct-axis synchronous reactance can be found out by performing open circuit and short circuit test on an alternator. f. Suppose the alternator connected to infinite busbars is operating at unity p. b. The excitation voltage . Synchronous generator synchronous reactance Xs is composed of Xs = Xa + Xl. 652 pu -- Coupling - Single Bearing Flexible 422. X q D. What is synchronous speed: Let take an alternator, it is a numerical expression where the fixed number of poles, the machine has to be rotated at a particular speed to keep the frequency of the generated emf in constant value. Synchronous Machine: Speed of Synchronous Machine: Synchronous machine are designed to be operated at synchronous speed, which is given by: Where. POWSYS 1. 95 p. 5 A 121. A 3phase, 30 MVA, 33KV, alternator has an internal reactance of 4% and negligible resistance. Effective armature resistance is assumed constant. Fig. REACTANCES It can be defined as the imaginary part of the impedance in any power circuit. In figure, consider the field current If = OA that the produces rated alternator voltage per phase. Why synchronous generators are to be constructed with more synchronous reactance and negligible resistance? Synchronous machines 10 Exterior characteristic Alternator exterior characteristic Evolution of the voltage U on a given stator phase as a function of the current Iin this phase, when the alternator drives a load characterized by a constant power factor, at constant speed and excitation ⎧vitesse speed θ! = constante constant ⎪ constant U Page 1 147 Alternator (Synchronous Generator) UNIT 6 ALTERNATOR (SYNCHRONOUS GENERATOR) Structure 6. It supplies full load current at 0. 3-phase)winding inslots;laminatedmagneticmaterial Synchronousgenerator(alternator):transformsmechanicalenergyintoelectricenergy; phase, 50Hz alternator, has the effective armature resistance is 0. C. Such machines will have poor voltage regulation, lower value of current under short circuit condition and low value of steady state stability limit and small value of synchronizing The synchronous reactance is found as follows. Turbo alternator (5 t o 8) 5 11. d. If the machine size increases, the three phase alternator resistance _____ and synchronous reactance _____ 25. As alternator core iron gets saturated, rate of rise of open circuit voltage with field current decreases. To measure the voltage regulation of a synchronous generator under different loads. The synchronous reactance of an alternator is equal to ? a) Leakage reactance of armature. Q15. (i) Explain with Phasor diagrams, the capability curve of synchronous generator. 3. If the need of the alternator drops to 1000 rpm, the generated voltage per phase will be (A) 180 V (B) 150 V (C) 140 V (D) 105 V. Rated Voltage (L-L) = 13,800 and Solution Summary 2. -12%. What is meant by synchronous impedance of an Alternator? The complex addition of resistance, R and synchronous reactance, jXs can be represented together by a single complex impedance Zs called synchronous impedance. m. Q. A 1000 kVA, 6. C. 8 kV when a symmetrical 3-phase fault occurs at the motor terminals. It gives regulation more than actual value so it is called a pessimistic method. 07 Ù per phase. For example: If at 480 volts, the listed transient reactance is 0. The Synchronous Impedance Method or Emf Method is based on the concept of replacing the effect of armature reaction by an imaginary reactance. • Calculate the rated current and the line to ground terminal voltage. Each of two single-phase alternators has an armature winding whose resistance and synchronous reactance are 0. c & s. REACTANCES It can be defined as the imaginary part of the impedance in any power circuit. 40. 1. b. Synchronous reactance is a value used to determine the short-circuit current when the steady-state condition has been reached. 5 (D) 27. 2 V. 1. 025 and 0. B. Q2. 95 leading Then calculate then voltage regulation for the 3 power factor conditions. (1) Unsaturated Synchronous Reactance . 5. f. For the subtransient interval, we use subtransient reactance Z 1 = j X d; For the transient interval, we use the transient reactance If the steady state value is of interest we have Z 1 The sum of leakage reactance Xl and fictitious reactance Xa is called the synchronous reactance. ⇒ Zero power factor method for an alternator is generally used to determine synchronous impedance of alternator efficiency of alternator voltage regulation of the alternator none of the above ⇒ The Poter's triangle separates the stator voltage and rotor voltage field m. 7 laging (c) a power factor of 0. e Higher the ratio provides greater stability. or Ampere-turn Method Zero Power Factor Method or Potier Method Operation of Salient Pole Synchronous Machine Power Developed by a Synchonous Generator The synchronous reactance is an equivalent reactance the effects of which are supposed to reproduce the combined effects of both the armature leakage reactance and the armature reaction. <pre> Synchronous Machine 1. 8 leading. In an alternator, the armature reaction considered to be equivalent to a fictitious Reactance Resistance Impedance Admittance 122. Less than quadrature axis reactance c. (c) Short circuit current Machine with low SCR will have high value of synchronous reactance X s that indicates smaller value of short circuit current. 2490, and the base voltage will be reduced to 416 volts, the per-unit transient reactance at the lower voltage is: (480/416) 2 x 0. Machine with low SCR will have high value of synchronous reactance X s that will lead to greater change in voltage under fluctuating load condition i. Same resistance C. N­ s is the synchronous speed; f is the line voltage frequency; P is the number of poles in a machine; Synchronous Motor: Voltage Equation of Synchronous Motor: V = E b + I a (R a + jX s) Synchronous Machines's Previous Year Questions with solutions of Electrical Machines from GATE EE subject wise and chapter wise with solutions 13. 25 pu, as shown below. The armature winding possesses a certain leakage reactance Xl. synchronous reactance of an alternator