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Multiple choice question for engineering

Set 1

1. Which of the following is the best braking method?
a) Friction
b) Electromechanical action
c) Eddy-currents
d) Electric braking

View Answer

Answer: d [Reason:] Braking methods based on friction, electromechanical action, eddy-currents, etc. are independent of the motor but sometimes electric braking is better justified owing to its greater economy and absence of brake wear.

2. DC motor is still widely used in tractions due to its excellent braking properties.
a) True
b) False

View Answer

Answer: b [Reason:] Dc motor is used in tractions because of its excellent braking characteristics and ability of smooth transition from the motor to the generator mode and vice versa. Also, characteristics suit perfectly for traction application.

3. Which of the following is not the method of electrical braking?
a) Plugging or counter-current
b) Dynamic or rheostatic
c) Regenerative
d) Eddy current

View Answer

Answer: d [Reason:] Eddy current is the electrical effect or response of the system, which is reflected mechanically at brakes to reduce the speed of the motor. Thus, eddy current is not an electrical brake, it is mechanical one.

4. Which of the following is the plugging method of braking?
a) Reversal of field connections
b) Reversal of armature connections
c) Addition of equal and opposite field
d) Removal of field circuit from current machine circuit

View Answer

Answer: b [Reason:] Plugging is method where connections are reversed at a given instant. Because of the problem of interrupting highly inductive field current and the time needed for the field current to build up in opposite direction, it is a common practice to reverse armature connections.

5. Which of the following is correct formula for braking torque in plugging?
a) n (ka2/Rb)
b) n2 (ka2/Rb)
c) n-1 (ka2/Rb)
d) (ka2/Rb)

View Answer

Answer: a [Reason:] Braking torque is equal to braking power divided by speed of the motor. T= [(nka )2/Rb]/n. As, Braking power is equal to Ea2/Rb/n. By solving for the braking torque from the above equation, we get n (ka2/Rb).

6. Electrical braking of any variety becomes less effective as ________________
a) Speed increases
b) Speed decreases
c) Independent of speed
d) Depends on supply voltage

View Answer

Answer: b [Reason:] Braking torque of the DC machine is given by n (ka2/Rb). Here, braking torque is directly proportional to the speed of the motor, so as the speed decreases the efficiency of electrical brakes which is dependent on braking torque decreases.

7. Plugging is applied in a motor, if we don’t make the switch OFF what will happen?
a) Motor will come to rest as a result of plugging
b) Motor will come to rest and will start rotating in another direction
c) Motor will burn
d) Nothing will happen

View Answer

Answer: b [Reason:] If the switch is kept ON near to zero speed, motor will have braking torque acting in opposite direction greater than the electromechanical torque. Thus, motor will come to rest and for the next instant motor will start rotating in opposite direction.

8. Plugging is used in ____________
a) Small motors only
b) Small and medium powered
c) Only in large heavy machines
d) Everywhere

View Answer

Answer: a [Reason:] Plugging is used in small scale applications only. The large initial current and high mechanical stress restrict the application of plugging in large machines. So, in order to balance stress this method is used in small machines only.

9. Which of the following is dynamic braking?
a) Reversal of field connections
b) Reversal of armature connections
c) Addition of equal and opposite field
d) Removal of armature circuit from current machine circuit

View Answer

Answer: d [Reason:] Reversal of the connections of armature is the method called plugging. In dynamic braking we remove the armature circuit and connect it to different resistor, with field circuit still connected to the external supply.

10. Braking time in the dynamic braking is the function of _____________
a) System inertia
b) Load torque
c) Motor rating
d) All- system inertia, load torque and motor rating

View Answer

Answer: d [Reason:] In dynamic braking, when brakes are applied the armature is disconnected from machine circuit and connected to the braking resistor. Now, at this point motor is driven by kinetic energy gained earlier, dissipating power in braking resistor.

11. In dynamic braking, when braking is applied system acts as ___________
a) Freely running machine
b) Motor with slow speed
c) Generator
d) Motor with same speed in opposite direction

View Answer

Answer: c [Reason:] The armature is disconnected from the supply and then a braking resistor Rb is immediately connected across it. The motor acts as a generator, driven by the inertia and stored kinetic energy dissipating power in Rb. This is a simple method of bringing a motor nearly to a standstill.

12. In which of the following electrical braking method, energy is supplied back to the supply?
a) Plugging
b) Dynamic braking
c) Regenerative braking
d) In all electrical braking

View Answer

Answer: c [Reason:] In plugging energy is wasted in braking resistance which is equal to starting resistance while running as a motor. In dynamic braking energy is generated but it is not fed back to supply. In regenerative method energy is sent back for reuse.

Set 2

1.The armature in DC machines is always placed on rotor because _______
a) Otherwise commutation will not be possible
b) Otherwise there will not be any induced emf
c) Otherwise current will not flow
d) Can be placed anywhere

View Answer

Answer: a [Reason:] A DC machine is a heteropolar structure with stationary poles and the rotating armature. The armature winding of a DC machine is placed on the rotor to improve commutation i.e. to convert the alternating voltage produced in the winding into direct voltage at the brushes.

2. In a DC machine, rectification provided with commutator is ___________
a) Half wave rectification
b) Full wave rectification
c) Semi controlled rectification
d) Uncontrolled rectification

View Answer

Answer: b [Reason:] In any electromagnetic machine the voltage generated is always alternating one as per Faraday’s law. For a DC machine the output must be unidirectional. This is carried out by a commutator. Hence, commutator provides full wave rectification.

3. Commutator performs rectification so that output of the machine is bi-directional.
a) True
b) False

View Answer

Answer: b [Reason:] Commutator and brush assembly of the DC machine performs the mechanical rectification process so induced AC is converted into DC (Unidirectional). Commutation process provides full wave rectification.

4. Which of the following method is used to connect commutator segments to armature conductors?
a) Brazing
b) Normal wires
c) Insulation pads
d) Copper lugs

View Answer

Answer: d [Reason:] Commutator in connected to the armature using lugs. Generally, they are made with copper. They are tightly bolted to the armature in order to prevent the centrifugal forces from causing the segments to fly away.

5. In D.C. generators, rapid brush ware causes due to _________
a) Severe sparking
b) Rough commutator surface
c) Imperfect contact
d) Severe sparking, rough commutator surface or imperfect contact

View Answer

Answer: d [Reason:] Brushes are the parts in a DC machine which are always in contact with rotating and stationary parts. Thus, imperfect contact, rough surfaces, sparking all these may reduce the life of brushes.

6. What are the number of the brushes in the lap winding?
a) Double the number of poles
b) Same as the number of poles
c) Half the number of poles
d) Two

View Answer

Answer: b [Reason:] In a lap winding, the number of parallel paths, A, is always equal to the number of poles, P, and also to the number of brushes. In wave windings, the number of parallel paths, a, is always two (2), and there may be two or more brush positions.

7. When Copper brushes are used in DC machine?
a) Where low voltage and high currents are involved
b) Where high voltage and small currents are involved
c) Cannot be determined
d) Where low voltage and low currents are involved

View Answer

Answer: a [Reason:] Due to various limitations, copper brushes are used in low voltage applications. For various other voltage ratings, different carbon-graphite proportions are used in manufacturing of brush materials.

8. In DC generators, current is fed up to the external circuit from armature through _______
a) Commutator
b) Direct wire
c) Slip rings
d) Cannot be determined

View Answer

Answer: a [Reason:] In any rotary machine current is induced in Sine wave format, according to Faraday’s law. For DC machine commutator provides mechanical rectification so that output is in the unidirectional format.

9. What are the number of the brushes in the wave winding?
a) Same or more than the number of poles
b) Same or less than the number of poles
c) Exactly half the number of poles
d) Depends on other parameters

View Answer

Answer: a [Reason:] In wave windings, the number of parallel paths, A, is always two (2), and there may be two or more brush positions. When two adjacent commutator bars make contact with a particular brush, p/2 coils are shorted by the brush in the wave winding.

10. Which conductors are in point of contact with brushes?
a) Lie under south pole
b) Lie under north pole
c) Lie in inter polar region
d) Are farthest from the poles

View Answer

Answer: c [Reason:] Because of the diamond shape of coils, the brushes which are physically opposite the pole centres are electrically connected to coil-sides lying close to the interpolar region. Thus, electrically the brushes are displaced 90° elect. From the axes of the main poles.

11. In case of DC machine winding, number of commutator segments is equal to ______
a) Number of armature coils
b) Number of armature coil sides
c) Number of armature conductors
d) Number of armature turns

View Answer

Answer: a [Reason:] Armature current is induced in a DC machine, which is fed up to the external circuit. Thus, it needs to be unidirectional. So, for converting bidirectional current to unidirectional commutators which are equal in number of armature coils need to be used.

12. How total number of brushes in a commutator are determined in a given DC machine?
a) Speed of armature
b) Type of winding
c) Voltage
d) Amount of current to be collected

View Answer

Answer: d [Reason:] Brushes are in contact with rotating part and stationary part. Thus, if more amount of current is to be carried, it requires more number of brushes. Hence brush number depends directly on the amount of current that needs to be collected and fed up in or out.

Set 3

1. Which of the following method is adopted for controlling a DC motor?
a) Resistance control
b) Motor-generator set
c) Inverter Rectifier
d) DC chopper

View Answer

Answer: d [Reason:] All methods listed above are the methods which can be used to control DC motor, but resistive method makes lots of wastage of energy, while motor-generator set and inverter rectifier set are bulky and costly.

2. Which of the following method is employed when regenerative braking is necessary?
a) DC chopper
b) Variable resistor
c) Inverter rectifier
d) Motor-generator

View Answer

Answer: a [Reason:] A chopper is essentially an electronic switch that turns on the fixed-voltage DC source for short time intervals and applies source potential to motor terminals in series of pulses. This process controls the average voltage applied to the motor.

3. Which of the following statement is not true regarding to DC chopper?
a) Cheap
b) Fast response
c) Regeneration
d) AC to DC control

View Answer

Answer: d [Reason:] The chopper provides stepless and efficient control of DC voltage and it is less expensive. It is having another advantage of faster response and regeneration. The chopper is relatively new technology and equivalent to DC-DC transformer.

4. Which of the following correctly suits with chopper?
a) Thyristor in series with load
b) Thyristor in parallel with load
c) Thyristor switch in series with load
d) Thyristor switch in parallel with load

View Answer

Answer: c [Reason:] A chopper is essentially a thyristor switch in series with the load. A shunting diode is provided across the load because free-wheeling the load current when thyristor is off. The thyristor can be shown enclosed by a dotted square can be turned-off over a time period and the cycle is repeated.

5. The average value of chopper output waveform is given by _____________
a) αV
b) V/α
c) α2V
d) V/α2

View Answer

Answer: a [Reason:] Here the average value of the chopper output voltage waveform is given by Va=V tON/(tON+tOFF) = V (tON/T) =αV. So, by varying duty cycle α one can vary the average voltage at output.

6. How output voltage of chopper can be varied?
a) By using constant frequency system
b) By using variable frequency system
c) By using constant and variable frequency systems
d) By using constant or variable frequency systems

View Answer

Answer: c [Reason:] The average voltage of the chopper output depends on α, thus, α can be varied by either changing the complete time period or by changing an on time or off time. These two methods are named as constant and variable frequency system.

7. What is the formula for step up chopper?
a) V(1-α)
b) V/(1-α)
c) αV
d) v/α

View Answer

Answer: b [Reason:] With the help of conventional chopper one can reduce the average voltage, so by doing slight modification in the circuit one can use chopper method to increase the average voltage applied, by the formula V/(1-α).

8. Which of the following is not the method of auxiliary commutation?
a) Voltage commutation
b) Resistance commutation
c) Current commutation
d) Load commutation

View Answer

Answer: b [Reason:] Thyristor in a chopper has to be turned off by auxiliary commutation. This can be done by two methods-Forced commutation and Load commutation. Forced commutation can also be done by two ways, current and voltage.

9. In voltage commutation, in chopper circuit we use _____________
a) 2 auxiliary thyristors
b) 2 diodes
c) 1 auxiliary thyristor
d) Many diodes

View Answer

Answer: a [Reason:] Two auxiliary thyristors are connected in the chopper circuit along with one diode, one inductor, one capacitor in series with the voltage supply which is then connected in parallel with diode and DC motor armature.

10. In current commutation, in chopper circuit we use _____________
a) 2 auxiliary thyristors
b) 2 diodes
c) 1 auxiliary thyristor
d) 2 auxiliary thyristors and 2 diodes

View Answer

Answer: d [Reason:] Two auxiliary thyristors are connected in the chopper circuit along with two diodes, one inductor, one capacitor in series with the voltage supply which is then connected in parallel with diode and DC motor armature.

Set 4

1. What is Self-excitation in DC shunt generator?
a) Field winding is connected in series of armature
b) Field winding is connected in parallel of armature
c) Field winding is not connected to the armature
d) Field Winding is not excited

View Answer

Answer: b [Reason:] Rather than arranging a separate dc source for excitation purposes, practical generators are always excited from their own armature terminals, this method of excitation being known as self-excitation. A self-excited generator with such connection known as a shunt generator.

2. For a self-excited DC shunt generator Ia = _____________
a) IL – If
b) IL + IL
c) – IL – IL
d) – IL + IL

View Answer

Answer: b [Reason:] For a self-excited DC shunt generator field winding is connected in parallel with armature winding so that, armature current splits into load current and field current. Load current will be given out as the machine is working as a generator.

3. For self-excited DC generator, field circuit curve in the OCC will be __________
a) Straight line not passing through the origin with +ve slope
b) Straight line passing through the origin
c) Straight line not passing through the origin with -ve slope
d) Increasing curve

View Answer

Answer: b [Reason:] Field is connected such that this current increases the field mmf and therefore the induced emf, the machine will continuously build up. For the field circuit V = If *Rf, which is a straight-line relationship, called the Rf -line, in V-If plot.

4. Intersection of Rf line and magnetization curve will give _____________
a) No load terminal voltage
b) Field current at no load
c) Both a and b
d) Not specific data

View Answer

Answer: c [Reason:] When OCC is plotted against terminal voltage and field current where, field resistance line is also drawn, an intersection point will arise when OCC almost becomes constant due to saturation effect. So, corresponding value field current and no-load voltage can be observed from x and y axis resp.

5. What will happen if field resistance is decreased?
a) No load voltage will decrease
b) No load voltage will increase
c) No change in no load voltage
d) Field current will decrease

View Answer

Answer: b [Reason:] When field resistance is decreased, it will increase field current by Ohm’s law. Thus, field resistance line will lie below the older line of field resistance. Eventually, it’ll intersect OCC afterwards causing increase in no-load voltage.

6. No-load voltage can’t be defined where _____________
a) Field resistance line lies below the magnetization linear line
b) Field resistance line intersects the magnetization linear line
c) Field resistance line coincides the magnetization linear line
d) Defined everywhere

View Answer

Answer: c [Reason:] We get no-load voltage when magnetization curve intersects with field resistance line. If that intersection is not occurring, we’ll not get no load voltage. Thus, when both lines coincide we get undefined value of no-load voltage.

7. Machine does not excite to an appreciable value when ___________
a) Field resistance is less than critical resistance
b) Field resistance is more than critical resistance
c) In both cases a and b
d) Depends on other parameters

View Answer

Answer: b [Reason:] The no-load voltage is undefined for a field resistance whose line coincides with the linear portion of the magnetization curve. With field resistance even slightly more than this value, the machine does not excite to any appreciable value.

8. No-load voltage observed at two different field resistance values, is V1 < V2. What will be the relation in field resistance values?
a) Rf1 < Rf2
b) Rf1 = Rf1
c) Rf1 > Rf1
d) Can’t determine from no-load voltage

View Answer

Answer: c [Reason:] When field resistance value increases the line of field resistance shifts towards more +ve slope, proving that field current decreases. This line will now intersect OCC at lower regions indicating lower values of terminal voltage.

9. No-load voltage observed at two different field resistance values, is V1 < V2. What will be the relation in speed if field resistance is kept same for both cases?
a) N1 < N2
b) N1 = N1
c) N1 > N1
d) Can’t determine from no-load voltage

View Answer

Answer: a [Reason:] For Field resistance kept constant, field resistance line will intersect OCC at different points as speed variation may shift OCC up or down. When speed is increased OCC shifts up indicating that no-load voltage will also rise up.

10. DC generator will not self-excite in __________
a) Positive feedback
b) Negative feedback
c) Zero Feedback
d) Doesn’t depend on feedback

View Answer

Answer: b [Reason:] The field connection to the armature is such that the induced emf due to the residual magnetism tends to destroy the residual magnetism, is called negative feedback. In the absence of residual magnetism machine will fail to self-excite.

11. Which of the following is not a reason, behind failing of self-exciting machine?
a) Residual magnetism is absent
b) Negative feedback
c) Field resistance value is more than critical resistance value
d) Armature resistance is non-zero

View Answer

Answer: d [Reason:] In case a and b, machine will tend to destroy the residual magnetism present in a core, which will not excite machine. When field resistance value is more than critical value as seen from OCC, machine will not excite to appropriate value.

12. By changing the polarity of the armature of failed self-exciting machine, machine can be started.
a) True
b) False

View Answer

Answer: a [Reason:] By changing the polarity of field connections to the armature negative feedback can be turned into positive feedback, thus it will help to build up the voltage and machine will act good in self-excitation mode.

13. Which of the following will be useful to generate positive feedback?
a) Increasing the field resistance
b) Increasing the armature resistance
c) Decreasing the armature resistance
d) Changing the shaft direction

View Answer

Answer: d [Reason:] Negative feedback can be remedied simply by reversing the field connection to the armature or reversing the direction of rotation. For large dc generators with permanent connections and a fixed direction of rotation, the problem is overcome by temporarily exciting the field from a battery source.

Set 5

1. Commutation is delayed due to __________
a) Leakage reactance
b) Effect of armature reaction
c) Leakage reactance and armature reaction
d) Because of other factors

View Answer

Answer: c [Reason:] The leakage inductance Lc of the coil undergoing commutation has induced in it reactance voltage Lc (dic/dt) which opposes the change in current thereby delaying commutation. The effect of armature reaction causes shift in MNA, delaying the whole commutation process ultimately.

2. Why brushes shifting method is not employed in practical commutation?
a) Expensive
b) Construction problems
c) Causes demagnetization
d) Used practically

View Answer

Answer: c [Reason:] Brushes are located at GNA’s, a small voltage is induced in the commutating coil. It opposes current commutation as the commutating coil is cutting the flux which has the same sign as that of the pole being left behind. It could be partially remedied by shifting the brushes towards MNA but that causes direct demagnetization and is therefore not employed in practice.

3. Which are the methods for getting an ideal commutation?
a) Resistance commutation
b) Voltage commutation
c) Current commutation
d) Resistance and voltage commutation

View Answer

Answer: d [Reason:] Adding resistance between commutator segments and brushes, thus, reducing L/R and consequently getting faster commutation is one of the method. In voltage commutation we, introduce narrow poles called as interpoles to fasten the process.

4. In resistance commutation method we add resistance between __________________
a) Brushes and external circuit
b) Commutator and armature winding
c) At field winding
d) Commutator and brush

View Answer

Answer: d [Reason:] High contact resistance between commutator segments and brushes, achieved by using carbon brushes, adds resistance to the circuit of the commutating coil thereby reducing the time constant (L/R) of the current transient (ic(t)), helping it to change faster in the desired direction.

5. What is the effect on time constant of transient current, as a result of resistance commutation?
a) Remains same
b) Decreases
c) Increases
d) Increases then decreases

View Answer

Answer: d [Reason:] We add high contact resistance between commutator segments and brushes thus, it adds resistance to the circuit of the commutating coil thereby reducing the time constant (L/R) of the current transient (ic(t)), helping it to change faster in the desired direction.

6. Which voltage is neutralized in voltage commutation process?
a) Armature
b) Reactance
c) Field
d) Cannot be determined

View Answer

Answer: b [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.

7. Interpoles are excited with ____________
a) Armature current
b) Field current
c) Separate supply
d) Mains current

View Answer

Answer: a [Reason:] These interpoles apply a local correction to the air-gap flux density wave such that a pip of appropriate flux density exists over the commutating coil to induce in it a voltage of the same sign as that of coil current after commutation. Hence, they are excited with armature current.

8. Interpoles are excited by keeping them in ____________ with armature.
a) Series
b) Parallel
c) Anywhere
d) Not kept with armature

View Answer

Answer: a [Reason:] In voltage commutation method we use interpoles to speed up the commutation process. For neutralization of reactance voltage at all loads, the interpoles must be excited by armature current by connecting them in series with armature.

9. Polarity of interpoles is one pole ahead in the direction of armature rotation in _____________
a) Motor
b) Generator
c) Always ahead
d) Always behind

View Answer

Answer: b [Reason:] polarity of an interpole is that of the main pole ahead in the direction of armature rotation for the generating mode and that of the main pole left behind with respect to the direction of rotation for motoring mode.

10. What is the size of interpolar air gap compare to main pole air gap?
a) Small
b) Same
c) More
d) Can be of any size

View Answer

Answer: c [Reason:] The interpolar air-gap is kept larger than that of the main pole so that their magnetic circuit is linear resulting in cancellation of the reactance voltage (a linear derivative term) at all loads. Large air-gap results in greater amount of leakage flux which is accommodated by tapering the interpoles with a wider base.

11. Formula for cancellation of reactance voltage on an average basis using interpoles ________
a) 2[Bi(av)liva] Nc = Lc (dic/dt) = Lc (2lc/tc)
b) [Bi(av)liva] Nc = Lc (dic/dt) = Lc (2lc/tc)
c) 2[Bi(av)liva] Nc = Lc (dic/dt) = Lc (lc/tc)
d) Cannot be determined

View Answer

Answer: a [Reason:] 2[Bi(av)liva] Nc = Lc (dic/dt) = Lc (2lc/tc). With Bi determined by above equation, the ampere-turns needed to cancel the armature reaction ampere-turns and then to create the necessary flux density are given by, ATi = ATa (peak)+(Bi0 )lgi

12. For a given 4-pole machine, carrying armature current 56.82 A, with 846 conductors. The mean flux density in the air gap under the interpoles is 0.5 Wb/m2 on full load and radial gap length is 0.3 cm. Ampere-turns required for an interpole is ____________
a) 3198
b) 2099
c) 4198
d) 6297

View Answer

Answer: c [Reason:] Required ampere-turns are given by ATi = ATa (peak) +(Bi0 )lgi . Substituting the ATi =[(56.82*846)/ (2*2*4 )] +(0.5/4π*10-7 )* 0.3*10-2 =4198. Turns can be found by dividing with armature current.

13. A 440 V, 4-pole, 25 kW, dc generator has a wave-connected armature winding with 846 conductors. The mean flux density in the air-gap under the interpoles is 0.5 Wb/m2 on full load and the radial gap length is 0.3 cm. Number of turns required on each interpole is _________
a) 74
b) 84
c) 64
d) 54

View Answer

Answer: a [Reason:] Corresponding value of ampere-turns for an interpole ATi = ATa (peak) +(Bi0) lgi. = Ia* Z/2*A*P + (Bi0) lgi. Assuming Ia = Iline Ia = 25*103/440= 56.82 A. ATi = [(56.82*846)/ (2*2*4)] +(0.5/4π*10-7 )* 0.3*10-2 =4198. Ni= 4198/56.82= 74.