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 shaft load is reduced, (a) the load angle will increase. The torque angle increases. but decrease in armature current (c) increase in both armature current and power factor which is lagging #2. The I a increases to keep I a cos constant as shown in Fig.1(c). Explain the effect of increased load with constant excitation.

Remain same. Effect of Varying Field Excitation If the load applied to a synchronous motor is constant, the power input to the motor is also constant. Explain the power flow within a synchronous motor Q6. If the excitation of a synchronous motor is kept constant, the load current changes with the variation of load. If the load is kept constant and field excitation is varied, the synchronous motor operates by changing its power factor. So c. Decrease both Ia and p.f. Note. It is interesting to study how synchronous motor reacts to changes in the load condition. Synchronous Motor 1501 (b) Excitation Increased The effect of increasing field excitation is shown in Fig. With increased field strength, the torque angle decreases and Eres increases. Single machine to infinite bus is the way we teach power engineers. E b. If the load on a synchronous motor is increased the following points are considered which are given below. 2. 3. 26 Full PDFs related to this paper. 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. The The formula of the voltage regulation is given here. The effect of the excitation system on the synchronous generator can be reflected from the following aspects. 8. but decrease in armature current. So, if the load is 20MW and the frequency is stable, if the load increases to 21 MW (actually any value above 20MW) and fuel is not increased then the speed (and the frequency will decrease). The resultant voltage ER 1 causes a leading current Ia 1 whose in-phase component is larger than Ia. 101. Since the speed is constant in a synchronous motor, therefore, field flux is substantially constant within the normal limit of operation. but decrease in armature current, (C) increase in both armature current and power factor which is The harmonics in motor current produce torque pulsations and losses in rotor and damper windings due to induced harmonic currents. Effects of load changesA increase in the load is an increase in the real and/or reactive power drawn from the generator. It is interesting to study how synchronous motor reacts to changes in the load condition. V R = [ (V nl V fl )/ V fl] x 100%. V Curve of a Synchronous Motor. D It needs twice the normal exciting current. The excitation is then increased to the new operating condition shown by the dashed phasors. The maximum motor torque will. When the excitation is weak. View Answer. 3 A synchronous machine is called a doubly excited machine because. If its excitation is now increased, it will draw. 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. Armature current, I a increases. Since electric power has increased, the mechanical power inputted must increase. 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? Increase both its Ia and p.f. If the load is kept constant and field excitation is varied, the synchronous motor operates by changing its power factor. Let us see the effect of the load change on the motor. 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 B. increase in back e.m.f. In power system, the over exited synchronous motor is used as a synchronous condenser. (b) The effect of an increase in load on the operation of a synchronous motor, with If kept constant. Effect of Change of Excitation of Synchronous Motor The terminal voltage V of a synchronous motor is constant. A It can be overexcited. Compared to an induction motor, it is very sensitive to sudden changes of load. but decrease in armature current C. increase in both armature current and power factor which is lagging The synchronous motor runs at a synchronous speed, which depends on the supply frequency. #2. The excitation current under these conditions is 10 A. A synchronous generator connected to an ideal AC voltage source (constant magnitude, constant frequency) thorough a series inductance is the simplest case. 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 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. cos ) decreases and becomes more and more leading. So changes. Your Comments: 15. 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. 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. Increase in but decrease in Eb. (b) Will become lagging. B. Academia.edu uses cookies to personalize content, tailor ads and improve the user experience. Answer (1 of 7): Synchronous motor has special reaponse to increase in load than other motors .It does not eat more current to meet the load demand. It is because the load torque will be more than the torque produced by the motor. A synchronous motor running at no load with P.F = 1.

Voltage Regulation of the Synchronous Generator.

Instead it supplies increased mechanical load by shift in relative position of rotor and Characteristics of Synchronous Motor are constant speed motors. 2- For unity power factor With the same constraints as before, it can be seen that the voltage V. decreases only slightly. If the load increases, in other words, the torque increases and the current will increase. Effect of Changing Field Excitation on Synchronous Motor at In an electric motor, almost all types, the current is proportional to the torque. 1. Read Paper. Start studying Synchronous Motors (8hr). 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. In three phase system, induction and synchronous motors are used. The Graph plotted between the armature current Ia and field current If at no load the curve is obtained known as V Curve. If the load is further increased. The excitation system supplies the DC power supply of the excitation winding of the synchronous generator. A synchronous motor installed at the receiving end substation operates with such an excitation that it takes power at lagging power factor. 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. So more fuel must be burned to keep steady voltage. B. The motor continues to run at synchronous speed. Under such conditions, the current drawn from the supply is minimum. 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). V curve is a plot of the stator current versus field current for different constant loads. V 38 ( c) where increased Eb 1 is shown at the original load angle 1. Feb 26, 2010. 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. 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. Synchronous Motor 1501 (b) Excitation Increased The effect of increasing field excitation is shown in Fig. Example: Effect of changing field excitation for constant load: For a synchronous motor, as excitation (E) is increased : PF (i.e. 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. Since the field resistor is unaffected, the field current is constant and, therefore, the flux is constant too. (Full-text PDF) 0 ) 2 6 -4 Learning Objectives Rotary synchronous motor for lift applications. Now = E R ^ I a = is constant, hence I a also changes its phase.

By using our site, you agree to our collection of information through If the mechanical load is constant, a change in excitation (i.e. 70. 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. 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. A short summary of this paper. but decrease in armature current C. increase in both armature current and power factor which is lagging At some rotations, the poles of two magnetic fields attract each other while at some instant, they repel each other.

Effect of Excitation on Synchronous Motor. Due to increased magnitude of E b, E R also increases in magnitude. (c) Will become zero. A. increase in both armature current and power factor angle. 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. (2) Phasor excitation emf is always behind the phasor terminal voltage. 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 which is lagging. Increase Ia but decrease p.f. A 30 kW star-connected synchronous motor is supplied from a 415 volt 50 Hz source. A synchronous motor is operating with its excitation adjusted for unity power factor at constant load. Field excitation can be adjusted so that PF = 1 (Figure 1a). The resultant voltage E R 1 causes a leading current I a 1 whose in-phase component is larger than I a. The torque angle increases. A synchronous motor is running with normal excitation. That causes an electromagnetic imbalance that results in increase current. 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. The rotor produces a field that turns at the same speed as the supply. E b. The only way that an in increase in electrical load can decrease speed is if the load is greater than the generation. A synchronous motor runs at constant synchronous speed, regardless of the load. The balance is restored when the torque supplied by the motor matches the torque required to maintain the speed of the load. An over excited motor (more than 100% excitation) takes leading power factor current. 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 . This magnetizing current lags by almost 90 o to the supply voltage. In other words, the function of this magnetizing current or lagging VA drawn by (b) the load angle will decrease. 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 The solid phasors show the initial operating condition, in which the motor is under-excited and operating at a lagging power factor. As the synchronous motor is a constant speed motor, the speed is independent of excitation while the power factor depends on excitation. D. Increase in both Ia and SYMBOL. 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. 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. What could be the reasons if a repulsion-induction motor fails to start? If the excitation of a synchronous motor is kept constant, the load current changes with the variation of load. It also closely resembles real life where the ideal source is the power grid. 16. C. Increase in both Ia and p.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.

So we discuss the various similarities and dissimilarities of these motors. View Answer.

The motor is _____ _____ when the amount of excitation results in a leading PF B. Iarm increases but now leads Vphase. Notes for assessment contents contents learning objectives synchronous principle of operation method of starting motor on load with constant excitation power 3. A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. 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. Synchronous motors, however, are generally more expensive and a d.c. supply is a necessary feature of the rotor excitation. If the excitation is increased, the power factor (a) Will become leading. (d) power factor will decrease. Normal Excitation : The armature current is minimum at a particular value of field current, which is called the normal field excitation. Rotor winding produces a constant magnetic field. When the load is increased, the speed drops or tends to drop. Hence, armature develops more power than the load on the motor. 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. 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. Q4. 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. Increase in Eb but decrease in Ia. the speed will be constant but the angle of coupling will change to new value (h, the current 12 and power required to drive the load per phase. C Both its rotor and sator are excited. The excitation voltage E f remains constant. As the excitation is increased, the power factor improves till it becomes unity at normal excitation. An alternator transduced mechanical power to electric power. 38.20 (c) where increased E b 1 is shown at the original load angle 1. Zero. With increased field strength, the torque angle decreases and Eres increases. The operation of a synchronous motor is unique when compared to that of a DC motor and induction motor. Learn vocabulary, terms, and more with flashcards, games, and other study tools. C. increase in both armature current and power factor which is lagging. In case of synchronous motor speed always remains constant equal to the synchronous speed, irrespective of load condition. 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. Effect of changes in field excitation on synchronous motor performance. Explain the effects of the varying field excitation of a synchronous motor. 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. How can we reverse the direction of rotation of repulsion-induction and repulsion - Question: Q3. It receives 3 phase AC electrical supply to its armature winding and DC supply is provided to The frequency of voltage applied to the motor is increased keeping V/f constant. Compared to an induction motor, it is very sensitive to sudden changes of load. Hence, armature develops more power than the load on the motor. If the mechanical load remains constant and the excitation current is increased, Egen increases. 3- For leading power factor An increase in the load in the 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. This causes a hunting of the rotor and finally leads to stability problems. B It has two sets of rotor poles. b. So, we discuss the effect of excitation and V curves before 1. In a d.c. motor, armature develops an e.m.f.after motoring action starts, which opposes supply voltage, called back e.m.f. As the load on the motor is increased, the rotor progressively tends to fall back in phase (but not in speed as in d.c. motors) by some angle (Fig.

But the phase of E R also changes. When fully loaded and running at unity power factor the input power is 35 kW. Ans.3 because for a given fixed out put load, an increse in excitation of the generator will result in increased lagging pf. Correct answer: 3. the stator takes a magnetising current from 3 phase AC supply mains to help the DC field. The effect of changing the excitation at constant load is shown by the phasor diagram of Figure 2.

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. Synchronous generator will operate at lagging power factor. Correct answer: 1. Effect of changing field excitation for constant load: For a synchronous motor, as excitation (E) is increased : PF (i.e. 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. a) increase and will occur at smaller values of slip 135.A synchronous motor is floating on infinite mains at on load. 2. Both of these. principle and starting methods of synchronous motor. A. Lagging. On changing the excitation only, the armature current will have_____________? The speed of the motor is decided by the number of poles and frequency. 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? This causes a hunting of the rotor and finally leads to stability problems. Synchronous motors, however, are generally more expensive and a d.c. supply is a necessary feature of the rotor excitation. d. Decrease la but increase p.f. 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. We can very easily compare the voltage performance of the 2 generators by their voltage regulation. 4 Mostly, synchronous motors are Jun 13, 2009. 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. In dc motor or induction motor, an increase in load will drop the speed. The emf takes into account the flux produced by field excitation.

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.

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. Since the speed is assumed as constant, the magnitude of the internal generated voltage is constant also. The motor continues to run at synchronous speed. 7 Answers. Please note field excitation current is remaining constant during all these different load conditions. Characteristics of Synchronous Motor are constant speed motors.

If the excitation is further increased (i.e., over excitation), the motor power factor becomes leading. The motor is _____ _____ when the amount of excitation results in a leading PF In case of synchronous motor speed always remains constant equal to the synchronous speed, irrespective of load condition. (d) None of the above. 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 The magnetic field in a motor are related to the voltages. If the load on a synchronous motor is increased the following points are considered. If the field current is increased, the power factor will become: Leading. 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 As we move from normal excitation ( E = V) to overexcitation (E > V), I a increases from a minimum at normal excitation (Unity PF) When the field excitation is reduced, the excitation voltage E decreases (E

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