Zero. Effect of changing field excitation for constant load: For a synchronous motor, as excitation (E) is increased : PF (i.e. 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. In dc motor or induction motor, an increase in load will drop the speed. Increase Ia but decrease p.f. Explain the power flow within a synchronous motor Q6. In order to draw the phasor diagram one should know these two important points which are written below: (1) We know that if a machine is made to work as a asynchronous motor then direction of armature current will in phase opposition to that of the excitation emf. Increase both its Ia and p.f. Your Comments: 15. Single machine to infinite bus is the way we teach power engineers. 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 A 3-phase synchronous motor connected to an infinite bus-bar, with constant excitation, is driving a certain load and operating at leading power factor. 70. Engineering Electrical Engineering Q&A Library e. 4- The effect of increasing load on a synchronous motor running with normal excitation is to a. E b. December 4, 2021. 70. As the excitation is increased, the power factor improves till it becomes unity at normal excitation. Effect of changes in field excitation on synchronous motor performance. V Please note field excitation current is remaining constant during all these different load conditions. 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. So we discuss the various similarities and dissimilarities of these motors. Salient points regarding the working principle of Synchronous Motor are: Due to 3 phase AC, a 3 phase rotating magnetic field is produced by stator winding. In case of synchronous motor speed always remains constant equal to the synchronous speed, irrespective of load condition. An over excited motor (more than 100% excitation) takes leading power factor current. Explain the effects of the varying field excitation of a synchronous motor. #2. 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 Iarm increases but now leads Vphase. Synchronous Motor 1501 (b) Excitation Increased The effect of increasing field excitation is shown in Fig. Let us see the effect of the load change on the motor. Characteristics of Synchronous Motor are constant speed motors. 16. Effect of Varying Field Excitation If the load applied to a synchronous motor is constant, the power input to the motor is also constant. The excitation current under these conditions is 10 A. 101. It is interesting to study how synchronous motor reacts to changes in the load condition. but decrease in armature current C. increase in both armature current and power factor which is lagging If the field current is increased, the power factor will become: Leading. With increased field strength, the torque angle decreases and Eres increases. The synchronous motor runs at a synchronous speed, which depends on the supply frequency. 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. The motor is _____ _____ when the amount of excitation results in a leading PF B It has two sets of rotor poles. Feb 26, 2010. 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). 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 (d) None of the above. This causes a hunting of the rotor and finally leads to stability problems. A synchronous generator connected to an ideal AC voltage source (constant magnitude, constant frequency) thorough a series inductance is the simplest case. What I gather is that you would like to know the loading effects on the generator. Compared to an induction motor, it is very sensitive to sudden changes of load. Ignoring the effects of armature reaction, if excitation of a synchronous motor running with constant load is decreased from its normal value, it leads to. which is lagging. 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. 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 V curve is a plot of the stator current versus field current for different constant loads. Effect of change of Excitation in synchronous motor: The real power is P = VI cos .The value of excitation for which back emf, E b is equal to applied voltage V, is known as 100% excitation. 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. D It needs twice the normal exciting current. A short summary of this paper. 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? Hence, armature develops more power than the load on the motor. The resultant voltage ER 1 causes a leading current Ia 1 whose in-phase component is larger than Ia. The emf takes into account the flux produced by field excitation. C. Increase in both Ia and p.f. In case of synchronous motor speed always remains constant equal to the synchronous speed, irrespective of load condition. 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. 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. On changing the excitation only, the armature current will have_____________? When the excitation is weak. The motor is _____ _____ when the amount of excitation results in a leading PF Since the field resistor is unaffected, the field current is constant and, therefore, the flux is constant too. 1. 8. The excitation system supplies the DC power supply of the excitation winding of the synchronous generator. V R = [ (V nl V fl )/ V fl] x 100%. C Both its rotor and sator are excited. If the load on a synchronous motor is increased the following points are considered. Field excitation can be adjusted so that PF = 1 (Figure 1a). The magnetic field in a motor are related to the voltages. At some rotations, the poles of two magnetic fields attract each other while at some instant, they repel each other. Due to increased magnitude of E b, E R also increases in magnitude. Hence, in order to operate the motor with fairly constant flux in the air gap, it is necessary to vary the magnitude of the applied voltage in the same ratio as the frequency of the supply (i.e V/f should be kept constant) and to keep the excitation current constant. 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. Since the speed is assumed as constant, the magnitude of the internal generated voltage is constant also. 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. Remain same. The harmonics in motor current produce torque pulsations and losses in rotor and damper windings due to induced harmonic currents. Ans.3 because for a given fixed out put load, an increse in excitation of the generator will result in increased lagging pf. If the mechanical load is constant, a change in excitation (i.e. B. Synchronous generator will operate at lagging power factor. If the excitation is increased, the power factor (a) Will become leading. Characteristics of Synchronous Motor are constant speed motors. Lagging. When the field excitation is reduced, the excitation voltage E decreases (E
In other words, the function of this magnetizing current or lagging VA drawn by So more fuel must be burned to keep steady voltage. but decrease in armature current, (C) increase in both armature current and power factor which is 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. By using our site, you agree to our collection of information through Hence, armature develops more power than the load on the motor. So, we discuss the effect of excitation and V curves before 1. A synchronous motor installed at the receiving end substation operates with such an excitation that it takes power at lagging power factor. 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. A. increase in both armature current and power factor angle. That causes an electromagnetic imbalance that results in increase current. Since the speed is constant in a synchronous motor, therefore, field flux is substantially constant within the normal limit of operation. With increased field strength, the torque angle decreases and Eres increases. If the load is kept constant and field excitation is varied, the synchronous motor operates by changing its power factor. 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. Synchronous Motor 1501 (b) Excitation Increased The effect of increasing field excitation is shown in Fig. But the phase of E R also changes. The speed of the motor is decided by the number of poles and frequency. (2) Phasor excitation emf is always behind the phasor terminal voltage. The balance is restored when the torque supplied by the motor matches the torque required to maintain the speed of the load. (b) The effect of an increase in load on the operation of a synchronous motor, with If kept constant. cos ) decreases and becomes more and more leading. (b) Will become lagging. The armature current I a drawn from the supply increases. The Graph plotted between the armature current Ia and field current If at no load the curve is obtained known as V Curve. A synchronous motor is operating with its excitation adjusted for unity power factor at constant load. Example: Effect of changing field excitation for constant load: For a synchronous motor, as excitation (E) is increased : PF (i.e. but decrease in armature current C. increase in both armature current and power factor which is lagging the stator takes a magnetising current from 3 phase AC supply mains to help the DC field. 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. 26 Full PDFs related to this paper. The solid phasors show the initial operating condition, in which the motor is under-excited and operating at a lagging power factor. 7 Answers. This magnetizing current lags by almost 90 o to the supply voltage. The increased current causes the motor to develop more torque. Synchronous motors, however, are generally more expensive and a d.c. supply is a necessary feature of the rotor excitation. If the mechanical load remains constant and the excitation current is increased, Egen increases. If the shaft load is reduced, (a) the load angle will increase.
If the load is further increased. D. Increase in both Ia and SYMBOL. 38.20 (c) where increased E b 1 is shown at the original load angle 1. The excitation voltage E f remains constant. When the load is increased, the speed drops or tends to drop. Correct answer: 1. Effect of Changing Field Excitation on Synchronous Motor at A. 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. 38 ( c) where increased Eb 1 is shown at the original load angle 1. 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. E b. 3- For leading power factor An increase in the load in the