IAM - 22523 Unit - 4 Mcq On Industrial Ac Machines

 

IAM - 22523 Unit - 4 Mcq On Industrial Ac Machines

DOWNLOAD LINK - Click Here

                       

 Unit 3 MCQ Bank Important MCQ OF Synchronous Generator or Alternator Ques.1. The salient-pole construction for field structure of an alternator is generally used for ……………. machine. 1. 4 pole 2. 2 Pole 3. 6 Pole 4. 8 Pole Hide Explanation Answer.4. 8 Pole Explanation The salient pole rotors are driven by low-speed water turbines or diesel engines. It is because the salient-pole type construction is difficult to build to withstand the stresses at high speeds. Because a salient pole rotor turns at low speed (50 to 300 r.p.m.) and because a frequency of 50 Hz is required, we must place a large number of poles on the rotor. The speed of an alternator is given as N = 120f/P Hence the speed of an alternator is inversely proportional to the number of poles. As discussed above the salient pole alternator turns at low speed, therefore, the number of poles required to be more. Ques.2. The a.c. the armature winding of an alternator is 1. Delta Star connected 2. Generally Delta Connected 3. Star Delta Connected 4. Always Star connected Hide Explanation Answer.4. Always Star connected Explanation Alternators are connected primarily in the star to achieve the following motives:  More economical:- The phase voltages in star connection are 57.7 % of the line voltages, i.e. the armature winding in star connection is less exposed to voltage as compared to the delta connection which in turn prove more economic if we consider insulation, breakdown strength, the requirement of conductor material  Availability of neutral: It is very important for an alternator to have a neutral point. This neutral point is to be grounded through a resistor, for stability purposes. The neutral allows a path for circulating currents under unbalanced loaded conditions, and also during faults. If there was no path for the flow of fault current, and a line to ground fault occurs in one of the three phases, there would be a rise in voltages in the other two healthy phases and eventually, it would cause insulation failure in the other two phases and the line to ground fault would lead to a 3 phase fault. A neutral point would avoid all this and limit the fault condition to one phase only. Insulation would be protected, and the lines can be operational after fault isolation. For all this to happen, the star connection in the stator is absolutely necessary.  Zero sequences current Path:- In star connection, if the neutral is grounded then it also provides a path for the Zero-Sequence currents during faults, whereas in the delta connection the zero sequence currents flow within the delta circuit and hence increasing the load on the winding.  Easy protection: Neutral grounding is necessary to allow zero sequence currents to flow to the ground in case of a fault.  Elimination of harmonics: Star connection facilitates a neutral connection which is instrumental in eliminating triple harmonics.  No circulating currents: In star connection, we don’t have circulating parasitic currents like in Delta which lead to heating losses. Ques.3. Drop-in alternator frequency is corrected by: 1. Automatic Voltage Regulator 2. Damper Winding 3. Increase Prime Mover Output 4. Decrease Prime Mover Output Hide Explanation Answer.3. Increase Prime Mover Output Explanation Alternator frequency depends on two things:  The NUMBER OF POLES in the rotating field  The SPEED at which the poles rotate. You can’t change the number of poles. That was determined when the alternator was built. However, you can change the speed. The rotating field poles are mounted on the shaft of the prime mover. Changing the speed of the prime mover will directly affect the frequency output of the alternator. The frequency of an alternator can be adjusted by changing the speed of the Prime Mover as we know that Ns = 120f/P Ns ∝ f Hence the speed of the prime mover is directly proportional to the alternator frequency therefore by changing the speed of Prime-mover we can correct the Drop-in the alternator frequency. Ques.4. The pitch factor for a full pitch winding of a synchronous machine is 1. 0 2. 0.5 3. 0.9 4. 1 Hide Explanation Answer.4. 1 Explanation Pitch factor is the ratio between the winding pitch and pole pitch Pitch factor = Winding pitch/Pole pitch Pole pitch may be defined as the distance between the two adjacent poles, which is nothing but the periphery of the armature divided by the number of poles. In other words, it is the number of armature conductors or the number of armature slots per pole. Pole Pitch between two adjacent poles in case of any machine will always be 180 degrees electrical. In the case of full pitch winding, the winding pitch is equal to the pole pitch. For full pitch winding the pitch factor is equal to unity and the winding is known as “full pitch winding”. Ques.5. In the case of Zero Power factor leading load on the alternator, the effect of armature reaction is 1. To demagnetize 2. To increase induced EMF 3. To cross -Magnetize 4. To decrease the Induced EMF Hide Explanation Answer.4. To increase induced EMF Explanation Armature Reaction When the load is connected to the alternator, the armature winding of the alternator carries a current. Every current-carrying conductor produces its own flux so the armature of the alternator also produces its own flux when carrying a current. So there are two fluxes present in the air gap, one due to armature current while the second is produced by the field winding called main flux. The flux produced by the armature is called armature flux. So the effect of the armature flux on the main flux affecting its value and the distribution called armature reaction. The effect of the armature flux not only depends on the magnitude of the current flowing through the armature winding but also depends on the nature of the power factor of the load connected to the alternator Zero Leading Power Factor Load (Magnetizing) Under-excited Consider a purely capacitive load connected to the alternator having zero leading power factor. This means that armature current Iaph driven by Eph leads Eph by 90° which is the power factor angle φ. Induced e.m.f. Eph lags φf by 90° while Iaph and φa, are always in the same direction. The armature flux and the main field flux are in the same direction i.e. they are helping each other. This results in the addition of the main flux. Such an effect each armature reaction due to which armature flux assists field flux is called the magnetizing effect of the armature reaction This effect adds the flux to the main flux, greater e.m.f. gets induced in the armature Hence there is an increase in the terminal voltage for leading power factor loads. For intermediate power factor loads i.e. between zero lagging and zero leading the armature reaction is partly cross magnetizing and partly demagnetizing for lagging power factor loads or partly magnetizing for leading power factor loads. Ques.6. In the synchronous motor rotor, copper losses are met by 1. Armature Input 2. AC Input 3. DC source 4. Supply mains Hide Explanation Answer.3. DC Source Explanation The synchronous motor consist of two parts: Stator: Stator is the armature winding. It consists of three-phase star or delta connected winding and excited by 3 phase A.C supply. Rotor: Rotor is a field winding. The field winding is excited by the separate D.C supply through the slip ring. The 3 phase Ac source feeds electrical power to the armature for the following component of the power  The net mechanical output from the shaft.  Copper losses in the armature winding.  Friction and the armature core losses. The power received by the DC source is used to utilized only to meet copper losses of the field winding. Ques.7. The phasor diagram of the synchronous machine connected to an infinite bus is shown in Figure. The machine is acting as a 1. Generator and operating at lagging power factor 2. Generator and operating at leading power factor 3. Motor and operating at lagging power factor 4. Motor and operating at leading power factor Hide Explanation Answer.1. Generator and operating at lagging power factor Explanation The voltage regulation of an alternator is defined as the change in its terminal voltage when the full load is removed, keeping field excitation and speed constant, divided by the rated terminal voltage. So if Vph = Rated terminal voltage Eph = No load-induced e.m.f Regulation = (Eph -Vph)/Vph The value of the regulation not only depends on the load current but also on the power factor of the load. For lagging and unity p.f. conditions there is always a drop in the terminal voltage hence regulation values are always positive. Ques.8. In the case of the leading power factor, the terminal voltage of the alternator will 1. Rise on adding the full Load 2. Rise on removing the full load 3. Fall on adding the full load 4. Fall on removing the full load Hide Explanation Answer.2. Rise on removing the full load Explanation Under the load condition, the terminal voltage of the alternator is less than the induced e.m.f(Eph). So if the load is disconnected, Vph(per phase rated terminal voltage) will change from Vph to Eph, if flux and speed are maintained constant. This is because when the load is disconnected, Ia is zero hence there are no voltage drops and no armature flux to cause armature reaction. This change in the terminal voltage is significant in defining the voltage regulation. The voltage regulation of an alternator is defined as the change in its terminal voltage when the full load is removed, keeping field excitation and speed constant, divided by the rated terminal voltage. So if Vph = Rated terminal voltage Eph = No load-induced e.m.f Regulation = (Eph -Vph)/Vph The value of the regulation not only depends on the load current but also on the power factor of the load. For lagging and unity p.f. conditions there is always a drop in the terminal voltage hence regulation values are always positive. While for leading capacitive load conditions, the terminal voltage increases as load current increases. Hence the regulation is negative in such cases. Hence the terminal voltage will fall on removing the full-load. The relationship between the load current and the terminal voltage is called the load characteristics of an alternator. Such load characteristics for various load power factor is shown in fig. Ques.9. When any one-phase of a 3-phase synchronous motor is short-circuited, the motor 1. Will overheat in the spot 2. Will fail to pull into step 3. Will refuse to start 4. Will not come upto speed Hide Explanation Answer.3. Will refuse to start Explanation Failure of a synchronous motor to start is often due to faulty connections in the auxiliary apparatus. This should be carefully inspected for open circuits or poor connections. An open circuit in one phase of the motor itself or a short circuit will prevent the motor from starting. Most synchronous motors are provided with an ammeter in each phase so that the last two causes can be determined from their indications: no current in one phase in case of an open circuit and excessive current in case of a short circuit. Either condition will usually be accompanied by a decided buzzing noise, and a short-circuited coil will often be quickly burned out. The effect of a short circuit is sometimes caused by two grounds on the machine. Difficulties in starting synchronous motors:- A synchronous motor starts as an induction motor. The starting torque, as in an induction motor, is proportional to the square of the applied voltage. For example, if the voltage is halved, the starting effort is quartered. When a synchronous motor will not start, the cause may be that the voltage on the line has been pulled below the value necessary for starting. In general, at least half voltage is required to start a synchronous motor. Difficulty in starting may also be caused by an open circuit in one of the lines to the motor. Assume the motor to be three-phase. If one of the lines is open, the motor becomes single-phase, and no single-phase synchronous motor, as such, is self-starting. The motor, therefore, will not start and will soon get hot. The same condition is true of a two-phase motor if one of the phases is open-circuited. Difficulty in starting may be due to a rather slight increase in static friction. It may be that the bearings are too tight, perhaps from cutting during the previous run. Excessive belt tension, if the synchronous motor is belted to its load or any cause which increases starting friction will probably give trouble. Difficulty in starting may be due to field excitation on the motor. After excitation exceeds one-quarter of normal value, the starting torque is influenced. With full field on, most synchronous motors will not start at all. The field should be short-circuited through a proper resistance during the starting period. Ques.10. The angular displacement between two interconnected stations is mainly due to 1. Armature reactance of both alternators 2. Reactance of the interconnector 3. Synchronous reactance of both the alternators 4. All of the above Hide Explanation Answer.1. Armature reactance of both alternators Explanation  In an interconnected station to transfer large load between the stations, it is required that the load on the line must be shared equally. Ques.11. A 10 pole AC generator rotates at 1200 rpm. The frequency of AC voltage in cycles per second will be 1. 120 2. 60 3. 160 4. 1oo Hide Explanation Answer.4. 100 Hz Explanation Ns = 120 × f/P 1200= 120 × f/10 f = 100 Hz Ques.12. The output frequency of an alternator depends on 1. Type of winding 2. Number of poles and Rotational speed 3. Numbers of poles only 4. Rotational speed only Hide Explanation Answer.2. Number of poles and Rotational speed Explanation The relation between speed and frequency is N = 120f/P  From the above equation, it is clear that the output frequency of an alternator depends on the number of poles and the rotational speed.  The output frequency of the alternator depends on the rotational speed and number of Magnetic poles of the rotor. Ques.13. The number of electrical degrees passed through in one revolution of a six-pole synchronous alternator is 1. 3600 2. 720 3. 1800 4. 1000 Hide Explanation Answer.2. Number of poles and Rotational speed Explanation Electrical degree = p/2 mechanical degree Electrical deg. = 6/2 × 360 =3 × 360 = 1800° Ques.14. If the input to the prime mover of an alternator is kept constant but the excitation is changed, then the 1. The active component of the output is changed 2. The power factor remains constant 3. Reactive components of the output are changed 4. The power factor is reduced Hide Explanation Answer.3. Reactive components of the output are changed Explanation  The prime-mover of an alternator is rotating at a constant power basis, i.e. its power and speed remain constant, now if you make variations in the exciting coils, then the output power from the alternator will vary in accordance.  The output power of an alternator is proportional to the speed, and excitation voltage.  Now if the excitation varies then the output currents vary along with voltage, but the speed is constant and power demand or load on the alternator is also constant.  Since Input to prime mover is kept constant, the real power generated remains unchanged (as per the load demand).  When the excitation current is changed, the magnetizing current in armature changes, in order to maintain the air gap flux constant.  Thus the reactive power consumption/ generation changes for a machine(ie KVAr component). Ques.15. When an alternator is running on no-load the power supplied by the prime mover of an alternator goes to 1. Meets copper loss and winding loss in an armature 2. Meet no-load looses 3. Meet iron losses 4. Produce E.M.F in the armature winding Hide Explanation Answer.2. Meet no-load losses Explanation  If the phase angle θ is exactly 90° the synchronous machine must be driven by the prime mover whose function is to supply power losses.  In this case, a synchronous machine should be considered as a generator with zero power output.  Since its power losses are covered by the electric power system its power factor can only be close to zero.  That is why the synchronous machine at this mode is usually referred to as a synchronous reactor. Conclusion: Under no-load condition synchronous machine will only draw a small current (active power) to mainly compensate friction and windage losses and it can be used to supply reactive power and control the power factor of an external system, by controlling the field current. Ques.16. The power factor of an alternator depends on 1. Speed of Motor 2. Core losses 3. Armature losses 4. Type of load Hide Explanation Answer.4. Type of load Explanation The power factor depends on the types of load, according to load the alternator provides both reactive (K VAR) and real power(KW). Consider three types of loads 1. 1. Pure Resistive Load 2. Resistive Inductive Load (RL load) 3. Resistive Capacitive Load (RC load) Resistive Inductive load:   When the load is RL type it requires lagging VAR now that power has to be provided by the alternator  Whenever the alternator provides lagging VAR it produces a demagnetization armature reaction hence excitation has to be increased to maintain flux inside the machine(constant flux machine).  At this condition, the alternator power factor is lagging. The more VAR alternator produces, the power factor becomes more lagging. Resistive Capacitive Load:   When the load is of RC type, it requires leading VAR.  Whenever Alternator provides leading VAR it’s power factor is leading. At this condition, the leading VAR current causes a magnetization effect  So excitation can be decreased to main the flux. Ques.17. During the failure of prime mover in an alternator which relay is used to restore the working condition 1. Reverse Power Relay 2. Differential Relay 3. Over-Speed Relay 4. Buchholz Relay Hide Explanation Answer.1. Reverse Power Relay Explanation  If two synchronous generator is running parallel and the total load is divided among two then one generator takes half of the total load of it.  Due to the unbalancing of load one generator shifts his total load on the second generator and runs as a motor instead of a generator.  This situation can cause damage to the winding of the generator.  In such a condition reverse current relay is usually employed to trip the generator on reverse power flow. Ques.18. Pitch factor is the ratio of emf of 1. Full pitch winding to the distributed winding 2. Full pitch coil to the short-pitched coil 3. Short pitch coil to the full pitched coil 4. Concentrated winding to the distributed winding Hide Explanation Answer.3. Short pitch coil to the full pitched coil Explanation Pitch factor Kp is given as Ques.19. Synchronizing torque comes into operation under all of the following cases except 1. Phase difference between the two current 2. Frequency difference between two voltages 3. Changing the excitation current 4. Phase difference between two voltages Hide Explanation Answer.3. Changing the excitation current Explanation  By changing excitation current power factor changes i.e either leading or lagging.  Suppose the excitation of the alternator is decreased below normal excitation then reactive power will change and the active power output (W or KW) of the alternator will remain unchanged.  The under-excited alternator delivers leading current to the infinite bus bar.  It is because the leading current produces an adding m.m.f to increase the under excitation.  Similarly, an overexcited alternator operates at lagging power factor and supplies lagging reactive power to an infinite bus bar. Ques.20. When a generator is operating by itself supplying the system load then real and reactive power supplied by the generator depends upon 1. Types of insulation 2. Inter-coil Inductance 3. Prime mover RPM 4. Amount demanded by the load Hide Explanation Answer.4. Amount demanded by the load Explanation When a generator is operating by itself supplying the system load than 1. The real and reactive power supplied by the generator will be the amount demanded by the attachment of the load. 2. The governor point on the generator will control the operating frequency of the power system. 3. The field current ( field regulator set point) controls the terminal voltage of the power system. These 3 situations found in an isolated generator in the remote field environment. Note: When there is an inductive load on the grid then the synchronous generator will supply reactive power as it will act as capacitance which absorbs leading power and gives lagging power which is required by the inductive load, but when there is a capacitive load then it will act as a reactor and it will absorb reactive power from the grid and it will give leading power and absorb lagging power from the grid. But when there is a capacitive load then it will act as a reactor and it will absorb reactive power from the grid and it will give leading power and absorb lagging power from the grid. Ques.21. Which type of rotor is best for a turbo-alternator? 1. Salient pole type 2. Cylindrical rotor type 3. Salient pole & Cylindrical rotor 4. Non-salient pole type Hide Explanation Answer.2. Cylindrical rotor type Explanation Cylindrical rotor type  In the cylindrical Rotor type, the air gap between the stator and rotor is uniform.  Cylindrical Rotor type machine is generally used for the high-speed operation.  Commercially cylindrical rotor type synchronous generator is also called as turbo-alternator or turbogenerator. Salient-Pole type  In the salient pole type synchronous generator the air gap between the stator and rotor is not uniform.  They are used for low and medium-speed operation and for a small synchronous machine.  Commercially Salient pole type synchronous generator is also called as the hydro alternator and hydro generator. Ques.22. An alternator is generating power at 210 V per phase while running at 1500 RPM. If the speed of an alternator drops to 1000 rpm the generated voltage per phase will be 1. 140 V 2. 150 V 3. 110 V 4. 230 V Hide Explanation Answer.1. 140 Explanation The voltage of an alternator is almost directly proportional to speed. Since the speed has reduced to two-thirds of what it was, so the voltage will also reduce to 2/3. i.e. = (1000/1500) x 210V = (2/3) x 210 = 140V Ques.23. An alternator is said to be over-excited when it is operating at 1. Leading Power Factor 2. Lagging Power Factor 3. Unity Power Factor 4. Leading & Lagging PF Hide Explanation Answer.2. Lagging Power Factor Explanation Reactive power of the synchronous generator is given as  When the excitation is increased i.e for the high value of If E CosδR > V. Therefore the Reactive power Q is positive.  This means that the synchronous generator is delivering the reactive power to the infinite bus and its power factor is lagging.  In that condition, the alternator is said to be over-excited. Ques.24. Armature reaction in alternator primarily affect 1. Rotor Speed 2. Terminal voltage per phase 3. Frequency of armature current 4. No-load Losses Hide Explanation Answer.2. Terminal voltage per phase Explanation  As the load on the alternator vary it’s terminal voltage also varied as in the D.C generator.  When there is no load across the generator stator (Armature), there is no current in the armature winding hence field flux around the winding will be zero.  Thus, as soon as the load is connected with armature terminals, its current will also increase proportionally causing its flux to increase.  As we know that every current-carrying conductor its own flux so armature of the alternator also produces its own flux.  The field flux ( main flux) produce by the field winding of an alternator.  The flux produced by the armature is called armature flux.  So the armature flux will distort the value of main field flux. This distortion is known as armature reaction.  The effect of armature flux depends upon the magnitude of the current flowing through the armature winding and nature of the power factor of the load connected to an alternator.  In case if the load is at unity power factor, the effect of armature reaction is of cross magnetizing type.  If the load is at a lagging power factor, the effect of armature reaction is of demagnetizing type.  In case if the load is at the leading power factor, the effect of armature reaction is of magnetizing type. Note:- In magnetizing effect armature flux assist field flux hence greater EMF is induced in the armature. Hence there is an increase in the terminal voltage for leading power factor loads. Ques.25. The function of damper winding in an alternator is 1. Damp oscillation 2. Provide stability 3. Eliminate Haunting 4. All option are correct Hide Explanation Answer.4. All option are correct Explanation  Damper windings are wound to the rotor poles of the Synchronous machine (winding is similar to that of an induction machine) which helps in two ways.  We all know that a synchronous machine is not self-starting. Thus providing damper windings helps synchronous machines to act like an induction motor ( only at starting). Which helps the machine to selfstart.  We all know that hunting is a persistent phenomenon when it comes to synchronous machines.  We can reduce hunting to a great extent by damping it. They don’t let the motor to oscillate abruptly, they damp the oscillations thus increasing the stability of the machine. Ques.26. If the armature reaction of an alternator produces a magnetization effect on the main field then the power factor should be 1. Unity 2. Zero, Lagging 3. Zero Leading 4. Leading Only Hide Explanation Answer.3. Zero Leading Explanation Zero Power factor means that either the circuit is purely resistive or purely capacitive. If the load is purely capacitive or at the leading power factor, the effect of armature reaction is of magnetizing type. Note:- In the magnetizing effect, armature flux assists main field flux hence greater EMF is induced in the armature. Hence there is an increase in the terminal voltage for leading power factor loads. Ques.27. Two alternators are running in parallel if the excitation of the one of the alternator is changed then 1. The active component will change 2. The vibration will occur in an alternator 3. The alternator will run as the motor 4. The Wattless component will change Hide Explanation Answer.4. The Wattless component will change Explanation In an a.c. (alternating current) circuit, there are two components to impedance. There is a resistive component and a reactive component. The reactive component is the algebraic sum of the total inductive and capacitive reactances.   The reactive component stores power in a magnetic field (for inductors) and an electric field (for capacitors).  When the power source is turned off, the magnetic and electric fields collapse and return the stored energy back to the circuit in the form of induced current.  Therefore the current flow in the circuit which does not dissipate any power in the circuit is called “wattless current or component“.  Now coming to the point when the prime-mover of an alternator is rotating at a constant power basis, i.e. its power and speed remain constant, now if you make variations in the exciting coils, then the output power from the alternator will vary in accordance.  The output power of an alternator is proportional to the speed, and excitation voltage.  Now if the excitation varies then the power factor at which load is delivered is also changes, but the speed is constant and the power demand or load on the alternator is also constant.  Since Input to prime mover is kept constant, the real power generated remains unchanged (as per the load demand).  When the excitation current is changed, the magnetizing current in armature changes, in order to maintain the air gap flux constant.  Thus the reactive power consumption/ generation changes for a machine (i.e KVAr component). Ques.28. When the power factor of an alternator is unity then the armature flux will be 1. Demagnetization 2. Cross-magnetization 3. Zero 4. Operating at high RPM Hide Explanation Answer.2. Cross-magnetization Explanation In case if the load is at unity power factor, the effect of armature reaction is of cross magnetizing type.   In the case of cross-magnetization, the flux at the leading pole tip is reduced while the flux at the trailing pole tip tends to increase.  Hence average flux in the air gap remains constant but its distribution gets distorted.  Such distorting effect of armature reaction at unity p.f of the load is called the crossmagnetization effect of armature reaction. Ques.29. For 50 Hz and 2 poles, the maximum speed of an alternator is 1. 6000 2. 2500 3. 4000 4. 3000 Hide Explanation Answer.4. 3000 Explanation Synchronous speed is given as Ns = 120f/p = 120 x 50/2 = 3000 Ques.30. Consider a System consisting of working at lagging power factor, a Synchronous motor working at an over-excited condition, and a direct grid-connected to the connected to induction generator. Consider capacitive VAr to be a sink of reactive power. Which of the following statement is true? 1. Synchronous motor and Synchronous generators are sources and the induction generator is a sink of reactive power. 2. Synchronous motor and Induction generators are the source and the synchr onous generator is a sink of reactive power. 3. The synchronous motor is a source and a synchronous generator and induction generator is a sink of reactive power. 4. All sources are of reactive power. Hide Explanation Answer.1. Synchronous motor and Synchronous generators are sources and the induction generator is a sink of reactive power. Explanation Given synchronous generator operating at lagging power factor i.e, an inductive load should be connected so that the machine will generate reactive power to lagging load.  Synchronous Motor is over-excited therefore it is acting as a synchronous condenser.  An induction motor will absorb the reactive power for the development of a magnetic field.  Consider the 2 generating station S-1 and S-2 supplying a Receiving station RS through line 1 and line 2.  To deliver equal power through line 1 and line 2 the phase and active component of line current I1 and I2 must be equal.  To compensate Impedance drop the regulating components (RE) are installed at the sending end of each transmission line.  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. Ques.31. If the frequency of an alternator increase then 1. Its speed increases 2. Its speed decreases 3. Speed will remain constant 4. None of the options are correct Hide Explanation Answer.2. Its speed decreases Explanation Synchronous speed is given as Ns =120f/p So the speed of an alternator is directly proportional to its frequency therefore by increasing the frequency of an alternator its speed will also increase. Ques.32. The driving power from the prime mover driving the alternator is lost but the alternator remains connected to the supply network and the field supply also remains on. The alternator will 1. Behave as a synchronous motor with the opposite rotation direction 2. Behave as an induction motor with the opposite rotation direction 3. Behave as a synchronous motor with the same rotation direction 4. Will get burn Hide Explanation Answer.3. Behave as a synchronous motor with the same rotation direction Explanation Here are the various possibilities for a synchronous generator connected to the grid when the Prime mover is accidentally decoupled. The mode depends on whether the field winding is excited. If Field Excitation is failed ( Field OFF)  Cylindrical rotor machine: It will work as Induction Motor running in the same direction with speed less than synchronous speed.  Salient pole machine: It will work as Reluctance Motor running in the same direction at synchronous speed. If field Excitation is present  When the alternator is connected to the bus bar and that point of time when the prime mover fails then instead of supplying power to the bus bar it will receive power from the grid. Therefore synchronous generator will behave as the synchronous motor.  Loss of driving force in prime mover will result in an increase in system voltage far above the safe limits. Ques.33. Two alternators A and B are sharing an inductive load equally. If the excitation of alternator A is increased then 1. Alternator A will deliver more current 2. Alternator B will Deliver more current 3. Both Alternator will deliver the same current 4. Both alternators will share the load equally Hide Explanation Answer.1. Alternator A will deliver more current Explanation  Suppose two alternators are operating in parallel and they are supplying half of the active and reactive load.  If the excitation of Alternator 1 is increased such that Ef1 > Ef2.  It will cause a circulating current “Ic” which will flow through the armature and around the bus bar.  Therefore alternator 1 will deliver load current at power factor cosΦ1 and alternator 2 will deliver load current at power factor cosΦ2 where cosΦ1 > cosΦ2 since circulating current “Ic” is added to the load current of alternator 1 and subtracted from the load current of alternator 2.  It will result in an increase of KVAR supply of alternator 1 whereas the KVAR supply of alternator 2 will decrease. Note:  If the excitation of one of the alternators operating in parallel is increased above its normal value of excitation i.e. overexcited its power factor will change in the lagging direction and its current output will increase without a change in its kW load.  When the alternator is under excited its power factor will become more leading and its current o/p will increase without changing in kW output. Thus whether the alternator is overexcited or under excited its circulating current tends to increase thereby increases the losses and reducing their useful capacity. Therefore it is desirable to keep the circulating current to the minimum value. Ques.34. The frequency of an alternator in an aircraft system is 1. 200 Hz 2. 300 Hz 3. 60 Hz 4. 400 Hz Hide Explanation Answer.4. 400 Hz Explanation  400 Hz is used in aircraft because this is the optimum frequency.  Since speed is directly proportional to the frequency and inversely proportional to the flux, therefore, flux will be inversely proportional to the frequency.  Higher the frequency lesser the flux which means the lesser the core size and weight of the machine  If a frequency of more than 400 Hz is used it will increase the losses especially skin effect will increase and this will affect the radio system performance of the aircraft.  Therefore 400 Hz is the optimum frequency which provides the balance i.e. for the given size of component or equipment losses is not that much high. Ques.35. Three-phase alternator are invariably star connected because 1. Magnetic Loss is minimized 2. Less number of wire turns are used 3. A higher terminal voltage is obtained 4. The small conductor can be used Hide Explanation Answer.3. A higher terminal voltage is obtained Explanation For the development of a rotating magnetic field in a three-phase machine, it is essential that the current in these three-phase windings must be 120 degrees placed wrt each other both in space and time.  In order to achieve it, both star or delta configurations can be used, but the star connection has certain advantages over delta connection which are-  The phase voltages in star connection are 57.7 % of the line voltages, i.e. the armature winding in star connection is less exposed to voltage as compared to the delta connection which in turn reducing the cost of insulation and conductor material.  In a star connected system, V phase = √3 V line. and Emf induced ∝ Turns,  Therefore a star connected alternator will require a lesser number of turns than a delta connected alternator for the same line voltage.  In star connection, if the neutral is grounded then it also provides a path for the Zero-Sequence currents during faults,  But in the delta connection, the zero sequence currents flow within the delta circuit and hence increasing the load on the winding.  Star connection offers high voltage and low current and hence it is suitable for transmitting high voltages over the transmission lines than that of high current.  Star Connected system is used where the low starting current is required.  Delta connection is used where high starting torque is required. Ques.36. In an alternator chording angle for a flux, the wave is α its value for the 7th harmonic is 1. 5α 2. 8α 3. 7α 4. 2α Hide Explanation Answer.3. 7α Explanation  The chording factor or pitch factor is defined as the ratio of generated emf in the short-pitched coil to the emf that would be generated in the full pitched coil. Kp = cosα/2  The chording angle for rth harmonic is always r times the chording angle.  Therefore the value of the 7th harmonic will be 7α. Ques.37. For eliminating the 7th harmonic of the emf wave of an alternator the fractional pitch must be 1. 7/8 2. 2/3 3. 3/2 4. 6/7 Hide Explanation Answer.4. 6/7 Explanation  One harmonic can be completely eliminated by selecting a coilspan (fractional pitch) that results in the pitch factor of zero for that particular harmonic.  For example, 5th harmonic is completely eliminated by selecting the coilspan of 4/5.  Similarly 3rd harmonic can be eliminated by selecting a coilspan of 2/3.  Therefore for eliminating the 7th harmonic coilspan of 6/7 is to be selected. Ques.38. When the speed of an alternator changed from 3600 RPM to 1800 rpm the generated emf/phase become 1. One-half 2. One-fourth 3. Twice 4. One-fifth Hide Explanation Answer.1. One-half Explanation The average E.m.f generated per phase is given as E = 4 f Φ T volts Where f = PN/120 Hz Φ = flux per pole T = Total number of turns per phase Since the speed of the alternator is directly proportional to the emf of an alternator. So if the speed is reduced to half then the emf will also become one-half. Ques.39. One of the following condition which is not necessary when the alternator is operating in parallel 1. The terminal voltage of both alternators must be the same 2. Same phase Rotation 3. Same frequency 4. Same rating Hide Explanation Answer.4. Same rating Explanation There are five conditions that must be met before when two alternators running in parallel. 1. Equal line voltage 2. Same frequency 3. Same phase sequence 4. Same phase angle 5. Same waveform We can use 2 alternators of 6 MVA and 4 MVA each instead of using a single 10 MVA alternator because it is economical than using a single alternator of the same rating. Ques.40. It is never advisable to connect a stationary alternator to a live bus bar because of it 1. Will behave as a synchronous motor 2. Will Behave as an induction motor 3. Will Get burn 4. Will decrease the bus bar voltage Hide Explanation Answer.3. Will Get burn Explanation  In a Synchronous machine, the stator carries the armature winding which is having a small resistance.  Under stationary conditions, the emf induced in stator winding is zero.  So if such an alternator is connected to a live bus bar, there is always a danger of short circuit (Ra is negligible and frequency is zero so there is no reactance). Ques.41. Keeping the excitation constant, if the steam supply of an alternator running in parallel with another identical alternator is increased then 1. It would over run the another motor 2. It will supply a greater portion of the load 3. Its power factor will decrease 4. Its power factor will increase Hide Explanation Answer.2. It will supply a greater portion of the load Explanation  If the steam supply of one of the alternators is increased then its power input to the prime mover will also increase.  As we know that for parallel operation the speed must be the same for both the alternators, therefore the alternator with greater steam cannot overrun the alternator having the lesser steam supply.  The alternator with more steam supply utilizes its increased power to carry more load by advancing its angular position. Ques.42. The zero power factor method is used to find 1. Voltage Regulation 2. Efficiency 3. Armature resistance 4. Synchronous Impedance Hide Explanation Answer.1. Voltage Regulation Explanation  Zero power factor means that the load connected is either purely capacitive (zero leading) or purely reactive (zero lagging), therefore, there’s no KWatt (real power) consumed by the load.  As we know, the armature MMF depends on the armature current (Ia). when the load is purely resistive, the armature MMF is 90° Electrical behind the field MMF.  When the load is purely capacitive (zero leading pf), (Ia) advances by 90°, thus armature MMF is in phase with the field MMF and in this case, it helps in increasing the resultant MMF.  When the load is purely reactive ( zero lagging pf), Ia lags behind by 90° thus the voltage leads the current by 90°. NOTE: The power factor being zero means the angle between the voltage and current phasors in 90 degrees. The term lagging implies that voltage leads current. Cosine 90 is Zero and hence called Zero Power Factor load. Ques.43. The load sharing between two steam-driven alternators operating in parallel may be adjusted by varying the 1. Power factor 2. Speed of the alternator 3. Steam supply to the prime mover 4. Frequency Hide Explanation Answer.3. Steam supply to the prime mover Explanation  Zero power factor means that the load connected is either purely capacitive (zero leading) or purely reactive (zero lagging), therefore, there’s no KWatt (real power) consumed by the load.  As we know, the armature MMF depends on the armature current (Ia). when the load is purely resistive, the armature MMF is 90° Electrical behind the field MMF.  When the load is purely capacitive (zero leading pf), (Ia) advances by 90°, thus armature MMF is in phase with the field MMF and in this case, it helps in increasing the resultant MMF.  When the load is purely reactive ( zero lagging pf), Ia lags behind by 90° thus the voltage leads the current by 90°. NOTE: The power factor being zero means the angle between the voltage and current phasors in 90 degrees. The term lagging implies that voltage leads current. Cosine 90 is Zero and hence called Zero Power Factor load. Ques.44. The active power of an alternator can be varied by 1. Increasing field excitation 2. Changing prime mover speed 3. Decreasing field excitation 4. Decreasing Frequency Hide Explanation Answer.2. Changing prime mover speed Explanation  The alternator rotor has to maintain a constant speed as it is designed to produce a certain voltage at the desired frequency.  However, the real power demand is variable and at any instant, it is the duty of the generator to match (supply) this real power demand.  When an alternator is connected to the grid if electrical load increases that mean real power increases. This increase in real power takes place by rotor deceleration for a short duration of time.  The loading on the generator should not exceed the generator rating as it may lead to the heating of the stator. Ques.45. Negative voltage regulation can be expected in case of 1. High-speed alternator 2. Slow speed alternator 3. Leading power factor load 4. Lagging power factor load Hide Explanation Answer.3. Leading power factor load Explanation The voltage regulation of an alternator is defined as the change in its terminal voltage when the full load is removed, keeping field excitation and speed constant, to the rated terminal voltage. Where Vph = Rated terminal voltage Eph =No load-induced e.m.f  The value of regulation depends upon the load current and power factor of the load.  For lagging and unity power factor there is always a drop in the terminal voltage hence regulation values are always positive.  For leading or capacitive load the terminal voltage increase as the load current increase because armature flux is added up with the main field flux hence regulation is negative. Ques.46. The main function of the pilot exciter in the larger alternator is 1. Driving dc auxiliaries 2. Battery charging 3. The exciting field winding of the main alternator 4. The exciting field winding of the main exciter Hide Explanation Answer.4. The exciting field winding of the main exciter Explanation An exciter is a device or combination of devices which supply the magnetizing current to generate the working flux.  In industrial applications basically, 3 types of exciters are used for an alternator. 1. Static exciter 2. Permanent magnet/Brushless Exciter 3. DC/Pilot exciter  To maintain the stability of the overallsystem the synchronous generator should respond quickly for sudden changes in the load and in order to attend these two dc generators are used, one as the main exciter and the other as a pilot exciter.  A pilot exciter is used to make the excitation of the generator independent of an external power source.  A pilot exciter is mounted on the rotor shaft and it excites the field coil of a separately excited dc generator called “ the main exciter. Ques.47. The advantages of the parallel operation of an alternator are 1. Continuity of the supply 2. Proper load Sharing 3. Increase in efficiency 4. All option are correct Hide Explanation Answer.4. All option are correct Explanation The advantages of the parallel operation of an alternator are  Proper load sharing  Increase in reliability  Increase in efficiency  Reduce losses  Easy to operate  Cheaper (reduce capital cost)  Continuity of supply Continuity of the supply  If the single large unit is disabled for maintenance purposes then the station will be no longer functional; Whereas if one of the several smaller units is in need of repair, the other smaller units are still available to maintain continuity of the supply. Efficiency  During light load, one or more alternators may be shut off while a few operate at full load. This provides more efficiency. Future Expansion & reduction in cost  An additional set of an alternator can be connected in parallel to meet the increasing demand thereby reducing the initial cost of setting up another unit. Ques.48. In the synchronous machine, the generated V curve is drawn between 1. Field current on X-axis and armature current on Y-axis 2. Field current on Y-axis and armature current on X-axis 3. Field current on Y-axis and Power factor on X-axis 4. Field current on X-axis and Power factor on Y-axis Hide Explanation Answer.1. Field current on X-axis and armature current on Y-axis Explanation  V curve is the graph showing the relation of armature current as a function of field current in synchronous machines.  In the case of a synchronous generator, the V curve is drawn between field current on the X-axis and armature current on the Y-axis.  The purpose of the curve is to show the variation in the magnitude of the armature current as the excitation voltage of the machine is varied. Ques.49. In the synchronous generator, the Inverted V curve is drawn between 1. Field current on X-axis and armature current on Y-axis 2. Field current on Y-axis and armature current on X-axis 3. Field current on Y-axis and Power factor on X-axis 4. Field current on X-axis and Power factor on Y-axis Hide Explanation Answer.4. Field current on X-axis and Power factor on Y-axis Explanation  The Inverted V Curve is a graph showing the relation of power factor as a function of field current.  In the case of a synchronous generator, the inverted V curve is drawn between field current on X-axis and Power factor on Y-axis.  Similar to V Curve, its purpose is to show variation in power factor as the field current of the machine is varied. Ques.50. A 10 pole 25 Hz alternator is directly coupled to and is driven by 60 Hz synchronous motor then the number of poles in a synchronous motor is 1. 24 poles 2. 12 poles 3. 6 poles 4. 3 poles Hide Explanation Answer.1. 24 poles Explanation Number of poles of alternator Pa = 10 F = 25 Hz (alternator) F = 60 Hz (motor) Then the number of poles of motor Pm =? Since the synchronous motor is directly coupled hence The synchronous speed of an alternator = Synchronous speed of the motor (120 x 25)/10 = (120 x 60)/ Pm Pm = 24 poles Ques 51. Alternators are usually designed to generate which type of ac voltage? 1. With fixed frequency 2. With variable frequency 3. Fixed current 4. Fixed power factor Hide Explanation Answer.1. With fixed frequency Explanation:  In an alternating current electric power system, synchronization is the process of matching the speed and frequency of a generator or other source to a running network. An AC generator cannot deliver power to an electrical grid unless it is running at the same frequency as the network. If two segments of a grid are disconnected, they cannot exchange AC power again until they are brought back into exact synchronization.  Hence alternators are usually designed with the fixed frequency of AC voltage. Ques 52. Two alternators rated 40 MVA and 60 MVA respectively are working in parallel and supplying a total load of 80 MW. Speed regulation of both the alternator is 5%. The load sharing between them will be 1. 30 MW, 50 MW 2. 32 MW, 48 MW 3. 36 MW, 44 MW 4. 40 MW each Hide Explanation Answer. 2. 32 MW, 48 MW Explanation:- Change in frequency from No load to full load f = 50 x 0.05 = 2.5Hz Full load frequency ffl1 = 50 – 2.5 = 47.5Hz Since both alternators have same speed regulation then ffl1 = ffl2 From the above diagram For machine 1 (50 – f)/(80 – x) = (50 – 47.5)/40 x – 16f = 80 – 16 x 50 x- 16f = -720—–1 For machine 2 (50 – f)/(80 – x) = (50 – 47.5)/60 x – 24f = -1200 ——-2 From equation (1) and (2) x= 48 MW f = 60 Hz So, machine A operates at a load of 48 MW While machine B will operate at a load of 80 – 48 = 32 MW Ques 53. An alternator has a per unit impedance of 0.9 p.u. to a base of 20 MVA, 33 kV, Then the p.u. impedance to the base of 50 MVA and 11 kV. 1. 20.25 2. 15.75 3. 25.78 4. 10.50 Hide Explanation Answer.1 20.25 Explanation:- The per unit impedance is given as \begin{array}{l}{Z_{pu\;new}} = {Z_{pu\;old}}\left( {\dfrac{{MV{A_{base\;new}}}}{{MV{A_{base\;old}}}}} \right){\left( {\dfrac{{K{V_{base\;old}}}}{{K{V_{base\;new}}}}} \right)^2}\\\\ = 0.9 \times \dfrac{{50}}{{20}} \times {\left( {\dfrac{{33}}{{11}}} \right)^2=20.25}\end{array}Zpunew=Zpuold(MVAbaseoldMVAbasenew )(KVbasenewKVbaseold)2=0.9×2050×(1133)2=20.25 Ques 54. In an alternator, the effect of armature reaction is minimum at the power factor of 1. 0.5 Lagging 2. 0.866 Lagging 3. 0.866 Leading 4. Unity Hide Explanation Answer.4. Unity Explanation:- At unity p.f., the effect of armature reaction is merely to distort the main field; there is no weakening of the main field and the average flux practically remains the same. At zero p.f. lagging, armature reaction is directly demagnetizing and the armature reaction weakens the main flux. This causes a reduction in the generated e.m.f. At zero p.f. leading, armature flux is now in the same direction as the field flux and, therefore the armature reaction strengthens the main flux. This causes an increase in the generated voltage. Ques 55.The armature leakage reactance is a parameter that accounts for that part of the ______ produced by the armature 1. Magnetic flux 2. Armature Flux 3. Magnetic and Armature flux 4. None of the above Hide Explanation Answer.1. Magnetic Flux Explanation:- The armature leakage reactance is a parameter that accounts for that part of the magnetic flux produced by the armature that does not cross the air gap to link with the rotor winding. The flux leakage is formed by endwinding leakage, slot leakage, and tooth tip leakage fluxes. Ques 56. Turbo alternators have rotors of 1. Small diameter and long axial length 2. Large diameter and long axial length 3. Large diameter and small axial length 4. Small diameter and axial length Hide Explanation Answer. 1. Small diameter and long axial length Explanation:- Smooth Cylindrical Type or Non-Salient Pole alternator rotor is used for steam driven alternator i.e turbo alternator which runs at very high speed.  This type of Rotor is used for steam-driven alternator i.e turbo alternator which runs at very high speed.  The Rotor is made up of smooth solid forgings of alloy steel cylinder having the number of slots along the outer periphery.  The field windings of cylindrical type rotor are connected in series to the slip rings through which they are excited by the DC exciter.  The top portion of the slot is covered with the help of steel or manganese wedges and the unslotted portion of the cylinder acts as the poles of an alternator.  The field windings are arranged in such a way that its flux density is maximum on the polar central line.  In the cylindrical rotor, the pole doesn’t project out from the smooth surface of the rotor hence they maintain the uniform air gap between the stator and rotor.  Since the steam turbine runs at very high speed, therefore, they required less number of poles hence the diameter of the rotor is small, and axial or rotor length is large. Ques 57. The steady-state fault current during a 3 phase terminal fault on a generator is determined by: 1. The transient reactance of the generator 2. The Sub-transient reactance of the generator 3. The synchronous reactance of the generator 4. DC offset during the instant of the fault Hide Explanation Answer.3. The synchronous reactance of the generator Explanation:- 1. Sub-transient (xd”). Subtransient reactance is a value used to determine the short circuit current during the first few cycles after a short-circuit occurs. This is the short-circuit current value to be used in all short-circuit studies. 2. Transient reactance (Xd’). Transient reactance is a value used to determine the short-circuit current from the first few cycles up to about 30 cycles after the short-circuit occurs (depending upon the design of the machine). This value is often used in voltage regulation studies. 3. Synchronous reactance (Xd). Synchronous reactance is a value used to determine the short-circuit current when the steady-state condition has been reached. Steady-state is reached several seconds afier the shortcircuit occurs. This value is often used to determine the setting of the generator backup overcurrent relays. Ques 58. To reduce the peripheral speed of an alternator, the diameter of the rotor is 1. Increased 2. Decreased 3. Increased or decreased 4. Kept the same Hide Explanation Answer 2. Decreased Explanation:- Peripheral speed refers to the actual linear speed of a tooth or point of the circumference of an alternator when it is revolving under power. Peripheral speed is thus the product of the circumference of the alternator and the number of revolutions per minute. The peripheral speed of the alternator Ps = π × D × RPM The peripheral speed depends on the speed as well as the diameter of the rotor. So to reduce the peripheral speed of the alternator the diameter of the rotor should be decreased. Ques 59. The positive, negative and zero sequence impedances of 3-phase synchronous generator are j 0.5 pu, j 0.3 pu, and j 0.2 pu respectively. When the symmetrical fault occurs on the machine terminals. Find the fault current. The generator neutral is grounded through reactance of j0.1 pu 1. -j 3.33 pu 2. -j 1.67 pu 3. -j2.0 pu 4. -j 2.5 pu Hide Explanation Answer. 2. -j 1.67 pu For symmetrical fault, the fault current is given as If = E/(Zi +Zn) Where E = Pre fault voltage Which is equal to 1 Zi = poisitive impedance Zn = Neutral Impedance Zi = 0.5j & Zn = 0.1 j If = 1/(0.5j + 0.1j) If = -j 1.67 Ques 60. The emf induced per phase in a three-phase star-connected synchronous generator having the following data Distribution factor = 0.955 Coil-span factor = 0.966 Frequency = 50 Hz Flux per pole = 25 mwb Turns per phase = 240, then emf per phase is 1. 2128.36 Volts 2. 1228.81 Volt 3. 869.46 Volts 4. 1737.80 Volts Hide Explanation Answer. 2. 1228.81 volt Explanation:- E.M.F Equation of an alternator is given as E = Kc Kd √2π f Φ Np Or E = 4.44 Kc Kd f Φ Np ………….. (since √2π = 4.44) Where Kc = Coilspan factor Kd = Distribution factor Φ = Flux per pole f = frequency Np = Turns per phase E = 4.44 × 0.955 × 0.966 × 50 × 25 × 10-3 × 240 E = 1228.1 volts