It also closely resembles real life where the ideal source is the power grid. Now if the load onthe motor is increased; A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. Your Comments: 15. December 4, 2021. If the excitation is further increased (i.e., over excitation), the motor power factor becomes leading. The Effect of Load Changes on a Synchronous Generator Operating Alone If is constant, what does vary with a changing load at (the same power factor? Correct answer: 3. Normal Excitation : The armature current is minimum at a particular value of field current, which is called the normal field excitation. Synchronous Motor Starting ; For a constant load, the power factor of a synchronous motor can be varied from a leading value to a lagging value by adjusting the DC field excitation (Figure 1). Therefore, for fixed excitation, any increase in the load on a synchronous motor will cause an increase in the line current, at a lower power factor. Question is If excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to, Options are (A) increase in both armature current and power factor angle, (B) increase in back e.m.f. It is because the load torque will be more than the torque produced by the motor. The only way that an in increase in electrical load can decrease speed is if the load is greater than the generation. The Effect of the Load Change under Constant Excitation 5 Figure 5-6 (a) Phasor diagram of a motor operating at a leading power factor. V curve is a plot of the stator current versus field current for different constant loads. V R = [ (V nl V fl )/ V fl] x 100%. Characteristics of Synchronous Motor are constant speed motors. When the load is increased, the armature current drawn by it increases because; A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. 1. Figure 3: Effect of field excitation on performance of a synchronous motor XX locus of armature current at constant power; YY locus of open-circuit voltage at constant power. If the excitation of a synchronous motor is kept constant, the load current changes with the variation of load. Since the speed is constant in a synchronous motor, therefore, field flux is substantially constant within the normal limit of operation. The magnetic field in a motor are related to the voltages. If the generator is islanded and you add the load to it, the speed would obviously go down and the controller would infuse in higher quantity of fuel to bring back the turbine speed to the normal operating speed. In three phase system, induction and synchronous motors are used. When the synchronous motor operates with constant power input, the variation of armature current with field current is thus a -shaped curve, as illustrated in . Increase in but decrease in Eb. If the load on the motor further increases, then the torque angle () also increases, i.e., the rotor field poles lag behind the stator field poles by a greater angle Let us see the effect of the load change on the motor. A. Effect of Varying Field Excitation If the load applied to a synchronous motor is constant, the power input to the motor is also constant. b. When fully loaded and running at unity power factor the input power is 35 kW. What could be the reasons if a repulsion-induction motor fails to start? d. Decrease la but increase p.f. Explain the effects of the varying field excitation of a synchronous motor. but decrease in armature current C. increase in both armature current and power factor which is lagging But the phase of E R also changes. Since the speed is assumed as constant, the magnitude of the internal generated voltage is constant also. Hence, armature develops more power than the load on the motor. If excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to A. increase in both armature current and power factor angle B. increase in back e.m.f. Since the motor is running with a constant speed and since the field current is also constant, the magnitude of the excitation voltage (E f) remains constant with change in the load on the shaft of the motor. View Answer. The phase of E R changes so that I a becomes leading with respect to V ph in over excited condition. This causes a hunting of the rotor and finally leads to stability problems.
What I gather is that you would like to know the loading effects on the generator. Increase in Eb but decrease in Ia. It is interesting to study how synchronous motor reacts to changes in the load condition. The frequency of voltage applied to the motor is increased keeping V/f constant. the stator takes a magnetising current from 3 phase AC supply mains to help the DC field. With constant mechanical load on the synchronous motor, the variation of field current changes the armature current drawn by the motor and also its operating power factor. 2- For unity power factor With the same constraints as before, it can be seen that the voltage V. decreases only slightly. This magnetizing current lags by almost 90 o to the supply voltage.
101. 9 Effect of Changing Field Excitation at Constant Load In a d. c. motor, the armature current Ia is determined by dividing the difference between V and Eb by the armature resistance Ra. (b) The effect of an increase in load on the operation of a synchronous motor, with If kept constant. A synchronous generator connected to an ideal AC voltage source (constant magnitude, constant frequency) thorough a series inductance is the simplest case. If the load increases, in other words, the torque increases and the current will increase. In a d.c. motor, armature develops an e.m.f.after motoring action starts, which opposes supply voltage, called back e.m.f. (b) the load angle will decrease. Characteristics of Synchronous Motor are constant speed motors. The effect of the excitation system on the synchronous generator can be reflected from the following aspects. (d) None of the above. Synchronous motors, however, are generally more expensive and a d.c. supply is a necessary feature of the rotor excitation. Effects of load changesA increase in the load is an increase in the real and/or reactive power drawn from the generator. The advantages of the synchronous motor are the ease with which the power factor can be controlled and the constant rotational speed of the machine, irrespective of the applied load. Synchronous Motor 1501 (b) Excitation Increased The effect of increasing field excitation is shown in Fig. A 30 kW star-connected synchronous motor is supplied from a 415 volt 50 Hz source. The I a increases to keep I a cos constant as shown in Fig.1(c). (2) Phasor excitation emf is always behind the phasor terminal voltage. The advantages of the synchronous motor are the ease with which the power factor can be controlled and the constant rotational speed of the machine, irrespective of the applied load. The balance is restored when the torque supplied by the motor matches the torque required to maintain the speed of the load. which is lagging. What will be the value of input power if the load remains constant and the excitation (i) is reduced to 5 A (ii) is increased to 20 A? Armature current, I a increases. 38.4) but it still continues to run synchronously .The value of this load angle or coupling angle (as it is called) depends on the The speed of the motor is decided by the number of poles and frequency. As the excitation is increased, the power factor improves till it becomes unity at normal excitation. It is interesting to study how synchronous motor reacts to changes in the load condition. B. increase in back e.m.f. Since the field resistor is unaffected, the field current is constant and, therefore, the flux is constant too. The armature current I a drawn from the supply increases. 2. C Both its rotor and sator are excited. E b. In other words, the function of this magnetizing current or lagging VA drawn by Academia.edu uses cookies to personalize content, tailor ads and improve the user experience. The torque angle increases. C. increase in both armature current and power factor which is lagging. The effect of increasing the load on a synchronous motor running with normal excitation is to a) decrease both armature current and power factor b) decrease armature current but increase power factor c) increase armature current but decrease power factor d) increase both its armature current and power factor 1. Start studying Synchronous Motors (8hr). 3- For leading power factor An increase in the load in the As increasing the strength of the magnets will increase the magnetic attraction, and thereby cause the rotor magnets to have a closer alignment with the corresponding opposite poles of the rotating magnetic poles of the stator. Rotor winding produces a constant magnetic field. With increased field strength, the torque angle decreases and Eres increases. Please note field excitation current is remaining constant during all these different load conditions. Question is The effect of increasing the load on a synchronous motor running with normal excitation is to, Options are (A) decrease both arrnature current and power factor, (B) decrease armature current but increase power factor, (C) increase armature current but decrease power factor, (D) increase both its armature current and power factor, (E) , Leave 3 A synchronous machine is called a doubly excited machine because. but decrease in armature current. 8) 11. Your query seems to be a confusing. 26 Full PDFs related to this paper. 1) If the alternator is regulated for constant voltage, which I am assuming, & the load current increases, it is also true that the power has increased. Correct answer: 1. 16. Engineering Electrical Engineering Q&A Library e. 4- The effect of increasing load on a synchronous motor running with normal excitation is to a. If the excitation is further increased (i.e., over excitation), the motor power factor becomes leading. Note. The armature current (Ia) is minimum at unity p.f and increases as the power factor becomes poor, either leading or lagging. In power system, the over exited synchronous motor is used as a synchronous condenser. These effects are minimized by using a Self Controlled Synchronous Motor Drive equipped with two three phase windings on stator with a phase shift of 30 between their axes and feeding them from two series connected six-pulse If the field current is increased, the power factor will become: Leading. #2. Adjust the excitation can maintain voltage stability. Explain the power flow within a synchronous motor Q6. V If the mechanical load is constant, a change in excitation (i.e. The resultant voltage ER 1 causes a leading current Ia 1 whose in-phase component is larger than Ia. Effect of changes in field excitation on synchronous motor performance. The harmonics in motor current produce torque pulsations and losses in rotor and damper windings due to induced harmonic currents. How can we reverse the direction of rotation of repulsion-induction and repulsion - Question: Q3. Remain same. The excitation is then increased to the new operating condition shown by the dashed phasors. In dc motor or induction motor, an increase in load will drop the speed. Effect of varying excitation of synchronous motor with constant load Under excitation. Since electric power has increased, the mechanical power inputted must increase. (b) Will become lagging. If the load on a synchronous motor is increased the following points are considered. V Curve of a Synchronous Motor. In a d.c. motor, armature develops an e.m.f.after motoring action starts, which opposes supply voltage, called back e.m.f.
8. As the synchronous motor is a constant speed motor, the speed is independent of excitation while the power factor depends on excitation.
If the mechanical load remains constant and the excitation current is increased, Egen increases. 7 Answers. When load on an over-excited or underexcited synchronous motor is increased, rate of change of its armature current as compared with that of power factor is A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. If the mechanical load remains constant and the excitation current is increased, Egen increases. principle and starting methods of synchronous motor. Note. A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. A synchronous motor runs at constant synchronous speed, regardless of the load. If the excitation of a synchronous motor is changed, keeping the load constant, the motor power factor will get automatically adjusted to keep its active component constant under all excitation conditions. B. Now = E R ^ I a = is constant, hence I a also changes its phase. The torque angle increases. D It needs twice the normal exciting current. Feb 26, 2010. Question is The effect of increasing the load on a synchronous motor running with normal excitation is to, Options are (A) decrease both arrnature current and power factor, (B) decrease armature current but increase power factor, (C) increase armature current but decrease power factor, (D) increase both its armature current and power factor, (E) , Leave Jun 13, 2009. If the load is further increased. B It has two sets of rotor poles. 70. The effect of changing the excitation at constant load is shown by the phasor diagram of Figure 2. B. If the shaft load is reduced, (a) the load angle will increase. Synchronous motors, however, are generally more expensive and a d.c. supply is a necessary feature of the rotor excitation. Lagging. Zero. A synchronous motor is running with normal excitation. If excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to A. increase in both armature current and power factor angle B. increase in back e.m.f. An alternator transduced mechanical power to electric power. An over excited motor (more than 100% excitation) takes leading power factor current. Increase Ia but decrease p.f. If excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to. Ans.3 because for a given fixed out put load, an increse in excitation of the generator will result in increased lagging pf. The resultant voltage E R 1 causes a leading current I a 1 whose in-phase component is larger than I a. Explain the effect of increased load with constant excitation. In the last article, we have seen the behavior of synchronous on loading i.e, by varying load on the motor. For a constant field excitation, when the load on the synchronous motor increases the current drawn by the motor increases. Prior to understanding this synchronous motor excitation, it should be remembered that any electromagnetic device must draw a magnetizing current from the AC source to produce the required working flux. A synchronous motor is operating with its excitation adjusted for unity power factor at constant load. Effect of Change of Excitation of Synchronous Motor The terminal voltage V of a synchronous motor is constant. The motor continues to run at synchronous speed. The excitation system supplies the DC power supply of the excitation winding of the synchronous generator. (c) power factor will increase. As we move from normal excitation ( E = V) to overexcitation (E > V), I a increases from a minimum at normal excitation (Unity PF) 2. With increased field strength, the torque angle decreases and Eres increases. If its excitation is now increased, it will draw. The excitation current under these conditions is 10 A. Iarm increases but now leads Vphase. cos ) decreases and becomes more and more leading. #2. Compared to an induction motor, it is very sensitive to sudden changes of load. (c) Will become zero. In an electric motor, almost all types, the current is proportional to the torque. It receives 3 phase AC electrical supply to its armature winding and DC supply is provided to
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