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Multiple choice question for engineering

Set 1

1. Which of the following is not the operating characteristics of Dc generator?
c) External characteristics
d) Internal characteristics

Answer: d [Reason:] The relationship between various parameters has to be presented graphically because of the magnetic saturation effect. Four characteristics of importance are the following: 1) No load characteristics 2) Load characteristics 3) External characteristics 4) Armature characteristics.

2. Characteristics drawn at Ia = 0 is also called as ____________
a) Magnetization characteristics
b) Non-magnetization characteristics
c) Anti-magnetization characteristics
d) Cannot be determined

Answer: a [Reason:] With Ia = 0 (no load) at constant n, it is the presentation of Vt (=Ea) vs If. This is the most important characteristic as it reveals the nature of the magnetization of the machine. It is easy to determine as the generator is on no load and so only low rated prime mover will serve the purpose. It is commonly called the open–circuit/magnetization characteristic.

3. Open circuit characteristics (OCC) is generally drawn across __________
a) Ea vs If, Ia=constant (not equal to rated)
b) Ea vs If, Ia=0
c) Ea vs If, Ia=constant
d) Ea vs If, Ia=constant (rated)

Answer: b [Reason:] Open circuit characteristics is also called as no-load characteristics or magnetization characteristics. No load clearly states that armature current will equal to 0. Thus, OCC is drawn at Ea vs If, Ia=0.

4. Characteristics of a DC generator drawn across Vt vs If at rated armature current and constant speed, is called as ____________
c) External characteristics
d) Armature characteristics

Answer: a [Reason:] Since we have Ia value which is equal to rated i.e. non-zero, it is indeed not a no-load characteristic. Axes given are Vt and If, hence it is not an armature characteristic. Thus, it’s called as load characteristic or magnetization characteristic on load.

5. In an OCC at If =0, graph starts from origin.
a) True
b) False

Answer: b [Reason:] As the machine would have been previously subjected to magnetization, a small residual voltage would be present with field unexcited. As will be seen practically, this is necessary for generator to self-excite. So, graph will start from just above the origin on Voc axis.

6. While conducting OCC, in order to avoid hysteresis loop, in which direction If should be increased?
a) -ve direction
b) +ve direction
c) In any direction
d) In both direction there exists hysteresis loop

Answer: b [Reason:] In conducting the OCC test, If must be raised gradually only in the forward direction otherwise the curve would exhibit local hysteresis loops. In OCC at If =0 there exists small residual voltage shown by non-zero Voc.

7. Air gap line is drawn at iron _________
a) Saturated
b) Unsaturated
c) Moderately saturated
d) Variable saturation

Answer: b [Reason:] The extension of the liner portion of the magnetization curve, is known as the air-gap line as it represents mainly the magnetic behaviour of the machine’s air-gap, the iron being unsaturated in this region consumes negligible ampere-turns; in any case the effect of iron is also linear here.

8. If suppose OCC is conducted at speed n1, where n1< nrated, OCC will lie ____________
a) Above OCC at nrated
b) On OCC at nrated
c) Below OCC at nrated
d) Can’t comment by only speed information

Answer: c [Reason:] For a less speed than the rated one, residual voltage appearing at terminal call Voc will also be less than Voc at rated value, it will vary in parallel manner but will never intersect OCC at rated speed.

9. Ea can be determined using __________
c) Cannot be determined
d) Above OCC

Answer: a [Reason:] Under load conditions Ea cannot be determined from the OCC for If in the saturation region because of the demagnetizing effect of armature reaction. We must therefore determine experimentally the equivalent demagnetizing ampere-turns ATd due to armature reaction under actual load conditions.

10. If load characteristics are drawn on OCC itself, we get curve ________
a) Above OCC
b) On OCC
c) Below OCC
d) Intersecting OCC

Answer: c [Reason:] Since on load operation of a DC machine, we’ll get terminal voltage less than the terminal voltage obtained in OCC, graph will start from below OCC. On load, the effect of armature reaction will draw load characteristics parallel to the OCC below it, causing no intersection.

11. Load characteristics drawn at Ra =0 and Ra not equal to 0, will lie _____
a) Above
b) On
c) Below
d) Intersecting

Answer: c [Reason:] Load characteristics with at Ra =0 will lie below the load characteristics drawn at Ra not equal to 0. To the load characteristic we add IaRa drop to get Ea induced emf with load. Thus, it will lie above.

12. OCC is drawn at two different speeds both less than rated speed. OCC drawn at speed N1 lies below OCC drawn at speed N2. Which of the following relation is correct?
a) N2 = N1
b) N2 < N1
c) N2 >> N1
d) Can’t comment

Answer: c [Reason:] As a speed on which OCC is taken decreases, the residual voltage appearing on Voc axis also decrease and OCC starts from below, compare to first one. Thus, N1 is less comparatively, as its OCC lies below than the OCC drawn at other speed.

13. Why No-load or load characteristics are also called as magnetization characteristics?
a) Ea α If
b) Ea α φ
c) If α φ
d) Cannot be determined

Answer: b [Reason:] As the generated voltage in the armature in the case of DC generator is proportional to terminal voltage, which also proportional to magnetic flux, as seen by residual voltage appearing at 0 field current. No-load and load characteristics are called as magnetization curves.

Set 2

1. In a DC generator the ripples in the direct emf generated can be reduced by________
a) Using conductor of annealed copper
b) Using commutator with large number of segments
c) Using carbon brushes of superior quality
d) Using equalizer rings

Answer: c [Reason:] Brushes carry current to/from rotating parts from/to stationary part. Ripples can be avoided if brushes are maintained. Else, brushes will have some voltage drop in it and we’ll not get simple repeating part in emf.

2. The drop in the voltage for which of the following types of brush can be expected to be least?
a) Graphite brushes
b) Carbon brushes
c) Metal graphite brushes
d) Cannot be determined

Answer: c [Reason:] Metal graphite brushes are ideal for a variety of applications because of their low resistivity. Thus, drop will be less in metal graphite brushes. Metal graphite brushes are used on commutators of plating generators where low voltage and high brush current densities are encountered.

3. What is the requirement of the good commutation?
a) Brushes should be of proper grade, size and material
b) Brushes should smoothly run in the holders
c) Smooth, concentric commutator properly undercut
d) Brushes should be of proper grade, size, material, run smoothly in the holders and concentric commutator properly undercut

Answer: d [Reason:] Brushes are in contact with commutator. So, for good commutation brushes must be of superior quality so that brushes will give/receive appropriate current to and from commutator. Also, the contact between brushes and commutator must be smooth for proper commutation process.

4. How to avoid grooves in the commutation of DC machine with the help of brush?
a) Brushes of opposite polarity should track each other
b) Brushes of same polarity should track each other
c) Brush position has no effect on the commutator grooving
d) Brushes should not track each other

Answer: a [Reason:] Brushes are located such that they are displaced 900 electrically from the axes of main poles. The two positive and two negative brushes are respectively connected in parallel for feeding the external circuit.

5. Reason behind the rapid wear of brushes is __________
a) Abrasion from dust
b) Excessive spring pressure
c) Rough commutator bars
d) Abrasion from dust, excessive spring pressure and rough commutator bars

Answer: d [Reason:] Brushes undergo various forces due to their location in a DC machine, they are in contact with rotating and stationary part of the machine. Hence, rough contact between commutator and brushes, inappropriate pressure on brush to rotating part may affect quality of commutation process.

6. For both lap and wave windings, what is the number of commutator bars equal to?
a) Slots
b) Armature conductors
c) Winding elements
d) Poles

7. Spacing between the brushes for a 4-pole machine in terms of commutator segments for 12 conductor segments is _____
a) 48
b) 3
c) 2
d) 6

Answer: b [Reason:] The spacing between adjacent brushes in terms of the commutator segment is ratio of number of commutator segments with poles for a given DC machine. C/P= 12/4= 3. It may also be noted that C/P need not necessarily be an integer.

8. Spacing between the brushes for a 4-pole machine in terms of commutator segments is equal to 6. What will be the number of armature slots?
a) 48
b) 3
c) 24
d) 6

Answer: c [Reason:] The spacing between adjacent brushes in terms of the commutator segment which is also equal to armature slots is ratio of number of commutator segments with poles for a given DC machine. C= P*Spacing= 4*6= 24.

9. What is the range of the brush friction coefficients for medium category?
a) 0.40 and above
b) 0.22 to 0.40
c) 0.11 to 0.22
d) 0.08 to 0.11

Answer: b [Reason:] Brush friction is influenced by many variables including brush temperature, spring force, current, atmospheric conditions, mechanical conditions, ring or commutator materials, surface films, speed and other factors. Brush friction is of medium category when, coefficient of friction lies in between 0.22 to 0.44.

10. Specific resistance for a brush is given by _________
a) R = (E * W * T) / (I * L)
b) R = (E * W * I) / (T * L)
c) R = (E * W * I) * (T * L)
d) Doesn’t depend on E, W, T, I, L.

Answer: a [Reason:] Specific resistance is measured in the length direction of the brush, since resistance in the direction of width or thickness may be considerably different. For, E = voltage drop over length L, I = amps of current passed through the sample, W = width of sample, T = thickness on sample, L = that portion of the length, over which the voltage drop E is measured, R is calculated by R = (E * W * T) / (I * L).

11. How many poles to be used in DC machine if brushes are placed 4 commutator segments apart for 16 commutator segments?
a) 8
b) 12
c) 2
d) 4

Answer: d [Reason:] Number of poles can be found by dividing the total commutator segments to spacing between brushes. Hence, number of poles = 16 commutator segments/ 4 commutator spacing= 4 poles.

12. DC generators are disconnected/connected from/to the busbars only under the floating condition because ____________
b) To avoid burning of all switch contacts
c) To avoid mechanical jerk to the shaft
d) To avoid sudden loading of the prime mover, burning of all switch contacts and avoid mechanical jerking to the shaft

Answer: d [Reason:] Brushes are the medium between rotating and non-rotating part of the DC machine. If sudden change in connections are done, whole machine undergoes change in all electrical quantities, which may damage machine. Thus, machines are connected and disconnected only at floating condition.

Set 3

1. To achieve spark less commutation brushes of a DC generator are placed ________
a) just ahead of magnetic neutral axis
b) in magnetic neutral axis
c) just behind the magnetic neutral axis
d) can be placed anywhere

Answer: a [Reason:] Brushes collect the current due to the induced emf in the armature coils. When a brush is at any particular commutator segment, it shorts out that particular coil and draws current from the rest of the coils and fed to the commutator. To achieve all positive outcomes, we place them just ahead of MNS.

2. If in the DC machine, the reversal of current in the coil is faster than ideal or linear commutation then the commutation is said to be __________ commutation.
a) Retarded
b) Curvilinear
c) Accelerated
d) Under

Answer: c [Reason:] Speed of the commutation is dependable on change in an induced current direction. When reversal of current in coil is faster then, obviously change of coils are taking place at faster rate. Thus, commutation is said to be accelerated commutation.

3. For a DC machine, in a commutator ________
a) Copper is harder than mica
b) Mica and copper are equally hard
c) Mica is harder than copper
d) Cannot be determined

Answer: c [Reason:] Due to its mechanical strength and insulating properties mica is a satisfactory material. However, mica is much harder than the copper segments, so during manufacturing it requires to under-cut the mica by sawing slots between the adjacent segments of the commutator.

4. The insulating material used between the commutator segments is normally _______
a) Graphite
b) Paper
c) Air gap
d) Mica

Answer: d [Reason:]s: Each conducting segment of the commutator is insulated from adjacent segments. Mica is a good electric insulator and good thermal conductor as well. Its applications in electric fields are derived from its unique mechanical properties. Thus, it allows mica to be ductile enough for its appropriate space of application.

5. Why interpoles are tapered in a DC machine?
a) Simpler design
b) Reduction in the weight
c) Increase in acceleration of commutation
d) Cannot be determined

Answer: c [Reason:] To speed up the commutation process, the reactance voltage must be neutralized by injecting a suitable polarity dynamical (speed) voltage into the commutating coil. In order that this injection is restricted to commutating coils, narrow interpoles are provided in the interpolar region.

6. The main function of interpoles in a loaded DC machine is to minimize _______between the brushes and the commutator.
a) Friction
b) Sparking
c) Current
d) Wear and tear

Answer: b [Reason:] Interpoles are introduced in a DC machine in order to speed up the commutation process, sp that sparking will be minimised. As, sparking arises at end of commutation period when commutation is not completed in given time.

7. Which of the following is different component?
a) Commutating poles
b) Compoles
c) Interpoles
d) Compensating winding

Answer: d [Reason:] Interpoles are located in interpolar region in a DC machine, called as interpoles. They raise up the speed of voltage commutation. So, they are also called as commutating poles. Compensating winding though used for reducing armature reaction, performs different function compare to interpoles.

8. How many coils under an adjoining pole pairs is/are connected between adjacent commutator segments in lap winding?
a) 1/2
b) 2
c) 1
d) 1/4

Answer: c [Reason:] No. of parallel paths in a lap winding are equal to P (No. of poles). Thus for 1 pole pir there will be exact 1 coil, connected to adjacent commutator segments in lap winding. In wave winding there will be half coils connected between adjacent commutator segments.

9. In wave winding, P/2 coils are connected under the influence of “x” pole-pairs which connect adjacent segments. X is ____
a) P
b) P/2
c) 2P
d) P/4

Answer: b [Reason:] One coil each under an adjoining pole-pair is connected between adjacent commutator segments in a lap wound DC armature, while in a wave-wound armature the only difference is that P/2 coils under the influence of P/2 pole-pairs are connected between adjacent segments.

10. The process of current reversal takes place when the coil is passing through the interpolar region.
a) True
b) False

Answer: a [Reason:] The process of current reversal called commutation takes place when the coil is passing through the interpolar region (q-axis) and during this period the coil is shorted via the commutator segments by the brush located (electrically) in the interpolar region.

11. How many coil sets undergo commutation simultaneously in a wave winding?
a) 1
b) 2
c) 4
d) 3

Answer: b [Reason:] Commutation takes place simultaneously for P coils in a lap-wound machine (it has P brushes) and two coil sets of P/2 coils each in a wave-wound machine (electrically it has two brushes independent of P).

12. Which coil is shorted in commutation process?
a) Coil under north pole
b) Coil under south pole
c) Coil lying in an interpolar region
d) Cannot be determined

Answer: c [Reason:] During the commutation period, the coil is shorted via the commutator segments by the brush. These brushes are located in interpolar regions electrically, and magnetically in neutral region. Thus, interpolar coil gets shorted.

13. Ideal commutation can be shown graphically by _____________
a) Straight line passing through origin
b) Straight line not passing through origin but with +ve slope
c) Straight line not passing through origin but with -ve slope
d) Curve increasing towards +ve t axis

Answer: c [Reason:] Ideal Commutation (also called straight-line commutation) is that in which the current of the commutating coils changes linearly from + Ic to – Ic in the commutation period. Thus, it will form a straight line with -ve slope.

Set 4

1. Flux density in the interpolar region drops down because of ______
a) ATa (peak)
b) Large air gap
c) Absence of magnetic poles
d) Depends on other parameters

Answer: b [Reason:] The exact way to find the flux density owing to the simultaneous action of field and armature ampere-turns is to find the resultant ampere-turn distribution ATresultant(∅) = ATf(∅) + ATa(∅). The flux density of ATa(∅) which, because of large air-gap in the interpolar region, has a strong dip along the q-axis even though ATa(peak) is oriented along it.

2. Resultant ampere-turn distribution of a DC machine is given by _________
a) ATresultant(∅) = ATf (∅) – ATa(∅)
b) ATresultant(∅) = – ATf (∅) + ATa(∅)
c) ATresultant(∅) = -ATf (∅) – ATa(∅)
d) ATresultant(∅) = ATf (∅) + ATa(∅)

Answer: d [Reason:] The exact way to find the flux density owing to the simultaneous action of field and armature ampere-turns is to find the resultant ampere-turn distribution ATresultant(∅) = ATf (∅) + ATa(∅), where ∅ is the electrical space angle.

3. Which axis undergo shifting as a result of armature reaction?
a) GNA
b) MNA
c) Both GNA and MNA
d) Remains fixed

Answer: b [Reason:] Apart from distortion of the resultant flux density wave, its MNA also gets shifted from its GNA by a small angle α so that the brushes placed in GNA are no longer in MNA as is the case in the absence of armature current.

4. Armature reaction in a machine is demagnetizing due to _________
a) Machine is designed with iron which is slightly saturated
b) Machine is designed with iron which is unsaturated
c) Depends on the application where machine is being is used
d) Can’t tell

Answer: a [Reason:] The armature reaction in a DC machine is cross-magnetizing causing distortion in the flux density wave shape and a slight shift in MNA. It also causes demagnetization because a machine is normally designed with iron slightly saturated.

5. Which of the following are effects of armature reaction?
a) Increase in iron losses
b) Commutation problems
c) Possibility of commutator sparking
d) Increase in iron losses, commutation problems and commutator sparking

Answer: d [Reason:] Armature reaction in a DC machine is a result of distortion of main field flux distribution by armature current, which produces its own mmf called armature mmf. Directly or indirectly armature reaction is the problem occurring in DC machine as it causes various effects, which reduce machine efficiency.

6. A 250 kW, 400 V, 6-pole dc generator has 720 lap wound conductors. Armature current is ____
a) 625A
b) 6.25A
c) 62.5A
d) 0.625A

Answer: a [Reason:] Armature current multiplied by the armature voltage is called as rating of a DC generator. Thus, 250 kW is the given rating while 400 V is the armature voltage. So, armature current is equal to 250*1000/400 = 625A.

7. What is the total ampere conductors/pole (in SI) if 600 lap wound conductors carry 120A current through conductors (P=4)?
a) 18000
b) 9000
c) 4500
d) 13500

Answer: a [Reason:] Ampere-conductors/pole =ZIc/P= Zla/AP. Ampere conductors per pole is calculated by multiplying total no. of conductors with the current carried by them divided by the total no. of poles. Ampere-conductors/pole = 600*120/4 =18000.

8. What is the total ampere turns/pole (in AT/pole) if 600 lap wound conductors carry 120A current through conductors (P=4)?
a) 18000
b) 9000
c) 4500
d) 13500

Answer: b [Reason:] Ampere-conductors/pole =ZIc/P= Zla/AP. Ampere turns per pole is calculated by multiplying total no. of conductors with the current carried by them divided by the twice the total no. of poles. Ampere-turns/pole = 600*120/8 =18000/2= 9000 .

9. If total ampere turns per pole is equal to 6000 A-turns, peak ampere turns for a 4-pole machine is _____
a) 24000
b) 3000
c) 1500
d) 4500

Answer: c [Reason:] Peak flux density in terms of total flux density is given by ATa (peak) = ATa (total) /P. Thus, for a 4-pole machine, ATa (total)= 6000 and P=4. Thus, Peak flux density is equal to 6000/4= 1500.

10. What is the total ampere turns per pole for 720 lap wounded conductors with carrying armature current equal to 625A in a 6-pole machine?
a) 6252 AT/pole
b) 625.2 AT/pole
c) 62.52 AT/pole
d) 8252 AT/pole

Answer: a [Reason:] For a given machine number of parallel paths is equal to 6. So, conductor current will be equal to armature current divide by no. of parallel paths i.e. 625/6. Conductor current = 104.2 A. Total armature ampere-turns, ATa = ½(720*104.2/6)= 6252 AT/pole.

11. For 6252 AT/Poles, if brush shift is of 2.50 mech. Degrees, what will be the demagnetizing ampere-turns per pole for a 6-pole DC machine?
a) 521
b) 5731
c) 5231
d) 571

Answer: a [Reason:] From given mech. Degrees shift we need to find electrical degrees shift. Electrical shift= mechanical shift*(P/2). Thus, electrical shift is equal to 7.50. Demagnetizing ampere-turns is given by 6250*(2*7.5/180) = 521 AT/Pole.

12. For 6252 AT/Poles, if brush shift is of 2.50 mech. Degrees, what will be the cross-magnetizing ampere-turns per pole for a 6-pole DC machine?
a) 521
b) 5731
c) 5231
d) 571

Answer: b [Reason:] For calculations, from given mech. Degrees shift we need to find electrical degrees shift. Electrical shift= mechanical shift*(P/2). Thus, electrical shift is equal to 7.50. Cross-magnetizing ampere-turns is given by 6250*(1-2*7.5/180) = 5731 AT/Pole.

Set 5

1. Net field current in a compound motor is given by ______________
a) If (net)= If + (Nse/Nf) Ia
b) If (net)= If – (Nse/Nf) Ia
c) If (net)= If + (Nf/Nse) Ia
d) If (net)= If – (Nf/Nse) Ia

Answer: a [Reason:] Net flux in a compound motor is determined by addition of shunt field flux and series field flux. Thus, for calculating effective flux, net field current is find out. If is the constant value given for shunt field, while If net takes the presence of series field also.

2. With increase in armature current, speed of the cumulative compound motor _________
a) Reduces slowly compare to shunt motor
b) Reduces sharply compare to shunt motor
c) Remains constant
d) Increases in proportionality with armature current

Answer: b [Reason:] As armature current increases, the back emf decreases similar to shunt motor. By the same time series field resistance carries more armature current producing more amount of flux which is inversely proportional speed.

3. Speed current characteristic of cumulative DC compound motor at larger values of armature current is similar to that of _____________
a) DC shunt motor
b) Induction motor
c) DC differential compound motor
d) DC series motor

Answer: d [Reason:] At higher values of current speed of the DC cumulative compound motor starts decreasing due to effect of series field flux, which is also seen in DC series motor. Thus series motor and DC cumulative motor follow same characteristics.

4. Differential compound motor is used in large number of applications.
a) True
b) False

Answer: b [Reason:] A differentially compound motor has flux/pole φ = (φse – φf). It is seen that on over-load φ reduces sharply and so the motor acts like a series motor on no load. This is why the differential compound motor is not used in practice.

5. Where speed-current characteristic of DC shunt motor lies with respect to DC cumulative compound motor (assume current greater than full load current)?
a) Can be represented by same curve
b) Above
c) Below
d) Cannot be determined

Answer: b [Reason:] Because of φse increasing with Ia, the speed of the compound motor falls much more sharply than the shunt motor. Therefore, the n – armature current characteristic of the compound motor lies above that of the shunt motor for Ia < Ia ( fl) and lies below for Ia > Ia ( fl).

6. Where speed-current characteristic of DC shunt motor lies with respect to DC cumulative compound motor (assume current smaller than full load current)?
a) Can be represented by same curve
b) Above
c) Below
d) Cannot be determined

Answer: c [Reason:] At start, speed of DC cumulative motor is more than the speed of shunt motor because of series characteristic effect. Because of φse increasing with Ia beyond full load current, the speed of the compound motor falls much more sharply than the shunt motor.

7. Only motor listed below whose speed increases with increase in armature current is ___________
a) DC series motor
b) DC cumulative compound motor
c) DC shunt motor
d) DC differential compound motor

Answer: d [Reason:] Shunt motor speed though decreases by slight value as armature current increases, it is assumed to be constant. But series motor and cumulative motor speed decreases as armature current increases. Only differential motor flux reduces with increase in armature current thus, speed increases.

8. Torque of the cumulative compound DC motor is _____________
a) Addition of Ia and Ia2 term
b) Subtraction of Ia from Ia2 term
c) Subtraction of Ia2 from Ia term
d) Addition of -Ia and -Ia2 term

Answer: a [Reason:] DC cumulative compound motor is the addition of best of series and best of shunt field motor. For shunt type torque is proportional to armature current while in series motor torque is proportional to square of armature current.

9. Torque-current characteristic of DC cumulative compound motor _____________
a) Lies above shunt motor characteristic
b) Lies below shunt motor characteristic
c) Lies above of series and shunt motor characteristic both
d) Lies between of series and shunt motor characteristic both

Answer: c [Reason:] Starting from zero armature current the characteristic of cumulative motor lies above the series characteristic and shunt characteristic, because it is the sum of series and shunt characteristic.

10. For strong series field, speed-torque characteristic of cumulative compound motor lies __________
a) Above series motor characteristic if current is less than full load current
b) Above shunt motor characteristic if current is less than full load current
c) Below series motor characteristic if current is more than full load current
d) Below series motor characteristic if current is less than full load current