Multiple choice question for engineering
Set 1
1. The brushes are placed
a) along geometrical neutral axis
b) perpendicular to geometrical neutral axis
c) along magnetic neutral axis
d) perpendicular to magnetic neutral axis
Answer
Answer: a [Reason:] The brushes are placed along geometrical neutral axis so that in ideal conditions commutations take place without sparking. Also placing brushes along the magnetic neutral axis requires consistent variations as load varies which is not desirable.
2. The brushes should be shifted in _______ direction in generator for the satisfactory operation of commutation.
a) forward
b) reverse to rotation
c) any of the mentioned
d) none of the mentioned
Answer
Answer: a [Reason:] For the generator, the electromagnetic torque produced is in the opposite direction and so the induced current. So to eliminate the sparking the brushes should be shifted in the forward direction of rotation.
3. With no commutating poles used the brushes are given backward lead in the dc motor.
a) True
b) False
Answer
Answer: a [Reason:] The armature reaction flux at the trailing edge weakens the flux and at the leading edge is strengthens the flux in the dc motor.
4. If the critical commutation time is 2 ms but it was observed that the practical commutation took 2.5 ms. This is the case of
a) under commutation
b) over commutation
c) critical commutation
d) none of the mentioned
Answer
Answer: a [Reason:] Since the time taken is more than the prescribed time limit for the commutation, it is under commutation.
5. The coil under going commutation lies along _____ while the coil getting short circuited lies along ____ for a practical dc machine.
a) gna, mna
b) mna, gna
c) gna, gna
d) mna, mna
Answer
Answer: a [Reason:] In a practical dc machine, the coil undergoing commutation lies along GNA so as the brushes and the coils which actually get short circuited lies along mna. This lead to sparking as a voltage difference is created at the coils.
6. The principle contributors to magneto motive force is/are
a) field current and armature current
b) field excitation
c) electromagnetic torque
d) all of the mentioned
Answer
Answer: a [Reason:] The interaction between the field excitation and armature supply create the mmf and the interconnected behaviour creates the emf.
7. The absence of compensating winding lead to
a) statically induced emf in armature
b) fluctuations in supply
c) reduction in the flux
d) any of the mentioned
Answer
Answer: a [Reason:] The static emf induced inside gets reduced due to the armature reaction in large machines when there is no compensating winding present.
8. The number of compensating conductors/pole faces is _______ where Z is the number of active armature conductors/pole; A is number of parallel paths;
(Ia is the armature current)
a) Z/A*Ia
b) 2*A*Ia/Z
c) Z*Ia/A
d) Z/2A*Ia
Answer
Answer: a [Reason:] Compensating conductors per pole face is Z/A*Ia.
9. A 400-V, 1000-A, lap wound dc machine has 10 poles, 860 armature conductors. The number of conductors in the pole face to give full compensation if pole face covers 70% of pole span is
a) 3010
b) 4300
c) 2400
d) 2800
Answer
Answer: a [Reason:] AT/pole for the compensating winding = 0.7*(Z*I/2P)
= 0.7*(860*100/2*10)
= 3010 AT.
10. Each of the following is valid for interpoles except
a) they are connected in parallel with the armature so that they carry part of armature current
b) they are small yoke fixed poles spaced in between main poles
c) their polarity, in case of generator is same as that of main poles ahead
d) they automatically neutralize not only reactance voltage but cross-magnetization also
Answer
Answer: a [Reason:] All the options are valid for the interpoles except a because they are connected in series not in parallel.
11. Mark the most incorrect. In dc generator, commutation can be improved by
a) using interpoles
b) using carbon brushes
c) shifting brush axis in direction of armature rotation
d) none of the mentioned
Answer
Answer: a [Reason:] All the given measures are used to eliminate the effect of armature reaction and hence improves commutation also.
12. The commutation process involves basically reversal of current in armature coil as it crosses MNA.
a) True
b) False
Answer
Answer: a [Reason:] By reversing the current direction in the MNA the current can be limited in the coils to avoid sparking.
13. The armature reaction is produced mainly by
a) load current in armature
b) load current in field
c) any of the mentioned
d) none of the mentioned
Answer
Answer: a [Reason:] The armature reaction is initiated by the loaded conditions of the machine because an unloaded dc machine does not armature flux to get distorted.
14. The compensating winding in dc generator is provided to
a) neutralize cross-magnetizing flux
b) neutralize demagnetizing flux
c) neutralize armature mmf
d) maintain magnetizing mmf
Answer
Answer: a [Reason:] The compensating winding functions such that to nullify the cross magnetization effects in the armature.
15. If the magnetic circuit of the dc machine is in the saturation region, the armature reaction
a) does not affect flux/pole
b) increases the flux/pole
c) decreases flux/pole
d) affects the flux/pole only when armature current is small
Answer
Answer: a [Reason:] In the saturation region the flux will be constant.
Set 2
1. The d-axis reactance is determined by
a) OCC & SCC
b) OCC
c) Slip test
d) SCC
Answer
Answer: a [Reason:] Both the OCC as well as short circuit test are used to calculate the d-axis reactance.
2. The slip test is used to determine
a) Xq
b) Xd
c) Xd and Xq
d) None of the mentioned
Answer
Answer: a [Reason:] Slip test is used to determine the q-axis reactance.
3. During slip test it would be observed that
i) Swing of ammeter is wide
ii) Swing of voltmeter is wide
a) i is true, ii is false
b) i is false, ii is true
c) both true
d) both false
Answer
Answer: a [Reason:] The impedance during slip test is low so the voltage drop will be less and the current will be more.
4. Slip test must be conducted at low armature terminal voltage. This is due to
a) avoiding error due to large slip
b) avoiding error due to small slip
c) avoiding heating
d) all of the mentioned
Answer
Answer: a [Reason:] Slip test is conducted at low terminal voltage to avoid large slip in the synchronous machines.
5. A 3 Phase 400, 100 MVA alternator is connected to infinite bus bar. If the mechanical power input is more than the maximum reluctance power, the reluctance generator will lose synchronizm if
a) field is open circuited
b) field is short-circuited
c) load is removed
d) damper is removed
Answer
Answer: a [Reason:] If the excitation itself is made zero, then machine will stop.
6. If a cylindrical rotor synchronous generator, is connected to the IBB, if the field is made zero, then it behaves as
a) induction generator for a short time
b) induction motor
c) synchronous motor
d) synchronous generator
Answer
Answer: b [Reason:] It will act as induction motor as there will be difference in the speed.
7. When a 3–phase alternator is suddenly short–circuited at its terminals, the initial value of the short–circuit current is limited by which one of the following ?
a) x”d
b) x’d
c) xs
d) Sum of x”d, x’d and xs
Answer
Answer: a [Reason:] When suddenly the terminals of the synchronous machine will be short circuited, at that immediate instant it will be subtransient reactance.
8. An alternator with higher value of SCR has
a) better voltage regulation and higher stability limit
b) poor voltage regulation and lower stability limit
c) poor voltage regulation and higher stability limit
d) better voltage regulation and lower stability limit
Answer
Answer: a [Reason:] SCR is directly proportional to voltage regulation. Even stability limit is directly proportional to SCR.
SCR = 1/X.
9. The phasor diagram of a synchronous machine connected to an infinite bus is shown in the figure.
The machine is acting as
a) generator and operating at leading pf
b) generator and operating at lagging pf
c) motor and operating at leading pf
d) motor and operating at lagging pf
Answer
Answer: a [Reason:] The induced voltage leads the bus voltage, which means that it is at leading pf and then excitation magnitude is also more than terminal bus voltage.
10. If the excitation of a 3-phase alternator operating on infinite bus bars is changed, which one of the following will get affected?
a) Active real power of machine
b) Reactive power of machine
c) Terminal voltage of machine
d) Frequency of machine
Answer
Answer: b [Reason:] Excitation is coupled with the reactive power of the machine majorly.
11. In which one of the following is reluctance power developed?
a) Salient pole alternator
b) Non-salient pole alternator
c) Squirrel cage rotor type induction motor
d) All of the mentioned
Answer
Answer: a [Reason:] Because there is variable air gap due to saliency of the poles.
12. While conducting measurement for Xd and Xq, the information obtained is below
Id max = 10 A; Id min = 5 A
Vd max = 30 V; Vd min = 25 V
Which one of the following is correct?
a) Xd = 3, Xq = 2.5
b) Xd = 6, Xq = 3.86
c) Xd = 3, Xq = 3.86
d) Xd = 6, Xq = 2.5
Answer
Answer: d [Reason:] Xd=V(max)/I(min) = 30/5 = 6 ohms;
Xq=V(min)/I(max) = 25/10=2.5 ohms.
13. A synchronous motor is operated at a bus voltage of 1.5 pu and drawing 1.0 pu at zpf leading current. Its synchronous reactance is 0.4 pu. What is the excitation emf of the motor?
a) 2.0 pu
b) 1.5 pu
c) 1.0 pu
d) 1.1 pu
Answer
Answer: d [Reason:] E = V-I*X = 1-0.5=0.5 pu.
14. Consider a 3-phase alternator with negligible armature resistance and high rotational inertia delivering power to an isolated load when the armature terminals got short circuited. After sometime, the mechanical
power input to the shaft of the machine is terminated at time t0. The short-circuit current circulating in the armature will change till the machine comes to halt at ‘ts’. Which one of the curves depicts this phenomena?
a)
b)
c)
d)
Answer
Answer: a [Reason:] The short circuit current will be a constant value till it flows.
15. A stationary alternator is connected to the infinite bus. It should not be connected because
a) it will get short circuited
b) it will operate as induction motor
c) it will reduce bus bar voltage
d) all of the mentioned
Answer
Answer: a [Reason:] When a stationary synchronous machine is connected to the infinite bus, due to lack of synchronism there will be short circuit in the machine as the impedance reached very negligible.
16. Which of the options given will have no effect on the working of the synchronous motor if ignored?
a) It has comparable reactance
b) It requires dc excitation
c) It can not be started under no load
d) It has tendency to hunt
Answer
Answer: a [Reason:] The medium reactance is not any disadvantage to the synchronous machine.
17. When synchronous motor is running at synchronous speed, the damper winding produces
a) no torque
b) eddy current torque
c) damping torque
d) torque aiding the developed torque
Answer
Answer: a [Reason:] As the relative speed will be zero in the damper winding and the torque too.
18. Slip test is performed to obtain
a) direct axis reactance and quadrature axis reactance
b) slip
c) positive and negative sequence reactance
d) sub transient reactance
Answer
Answer: a [Reason:] Slip test is used to determine the direct axis and quadrature axis reactance.
Set 3
1. For the given traction application using a dc series motor for a starting time ’t’ is applied. If this method is replaced by a series-parallel control, giving 50% time for each series and parallel. The saving in the starting energy of
a) 25%
b) zero
c) 50%
d) 75%
Answer
Answer: a [Reason:] Initially let the energy utilized was 100% for time t.Then for 50% duration for series, energy used is 50%. For parallel operation for the same circuit, the resistance gets halved assuming the same machine. So, for 50% of the parallel operation, 25% of energy is only utilized. Hence total saving is 25%.
2. If two series motors are identically coupled. One is running as motor while other as generator. For this combination,the iron losses and frictional losses are found identical for
a) identical speed and excitation
b) identical speed
c) identical rating and construction
d) none of the mentioned
Answer
Answer: a [Reason:] For same iron and frictional losses, same frequency should be there. For that same speed must be run.
3. Hysteresis loss in a dc machine is _____ of rate of flow of air and _____ on frequency of _____
a) independent, dependent, magnetic reversal
b) independent, dependent, operation
c) dependent, independent, magnetic reversal
d) none of the mentioned
Answer
Answer: a [Reason:] Hysteresis loss in a dc machine is independent of rate of flow of air and depends on frequency of magnetic reversal.
4. In dc machine iron losses cause
a) heating in core
b) loss in efficiency
c) rise in temperature of ventilating air
d) all of the mentioned
Answer
Answer: a [Reason:] The iron losses cause heating of the core which causes reduction inefficiency and cooling air gets heated up.
5. For squirrel cage and slip ring induction motor, cooling methods is efficient in
a) squirrel cage induction motor
b) slip ring induction motor
c) both of the motors
d) none of the mentioned
Answer
Answer: a [Reason:] As the squirrel cage induction motor has more ventilation and more space as compares to slip ring induction motor, there will be more efficient ways to cool down the squirrel cage motor.
6. If one of the phases of the supply breaks down, then the connected three phase induction motor
a) continues to run as a single phase induction motor, provided load does does not increase beyond 57.7%
b) stops operating after few seconds
c) continues to run as single phase induction motor
d) continues to run as two phase induction motor
Answer
Answer: a [Reason:] When one of the phases breaks down then the other two phases will supply the rated current but the load should be reduced to 57.7%.
7. If one of the phases of RYB supply gets broken, then the temperature rise of the induction motor
a) remains same as before
b) reduces as compared to the normal operating temperature
c) reduces to temperature rise of corresponding single phase induction motor
d) increases
Answer
Answer: a [Reason:] The current flowing in the armature will remain same so to maintain the flux requirements of the machine.Hence the temperature rise will also remain same as the current remains same.
8. A three phase induction motor is connected to the infinite bus operating at the normal conditions. There occurs an unbalancing in the supply, leading to
a) unequal heating losses
b) stopping of motor
c) increase in lower
d) none of the mentioned
Answer
Answer: a [Reason:] The unbalancing in the supply phase will create an unbalanced distribution of the current in the phases, thereby unequal heating.
9. A peaky voltage supply is given to the 3 phase power transformer of the connection power system. These results into
a) reduction in iron losses
b) reduction in copper losses
c) increase in iron losses
d) reduction in noise
Answer
Answer: a [Reason:] The flux wave will be sinusoidal in nature as the emf is peaky in nature. Sine wave has least losses when compared to other wave forms.
10. The iron losses in a saturated three phase alternator is lesser than the non-saturated three phase alternator.
a) True
b) False
Answer
Answer: a [Reason:] The non-saturated reactance is greater than saturated reactance. So the reactive losses or iron losses will reduce due lesser contribution of the same.
11. The hydroelectric plants in the industry are best suited with
a) closed circuit air cooling
b) hydrogen gas
c) direct water
d) any of the mentioned
Answer
Answer: a [Reason:] The closed circuit is so used to reuse the air in the system , so that to save water again in the cooling system.
Set 4
1. Power required during the open circuit and short circuit test is
a) losses incurring in the transformer
b) executing the power requirements by measuring instruments
c) power for the core losses only
d) all of the mentioned
Answer
Answer: a [Reason:] The power measured while conducting the tests is the winding losses and the core losses.
2. The open circuit test results in finding which of the following parameters
I. core losses
II. shunt branch parameters
III. turns ratio of transformer
a) I,II,III
b) I,II
c) II,III
d) I,III
Answer
Answer: a [Reason:] OC test gives the shunt branch parameters as well as the turns ratio by connecting a voltmeter at open circuited secondary terminals.
3. The open circuit test results in finding which of the following parameters
I. core losses
II. shunt branch parameters
III. series parameters
a) I,II,III
b) I,II
c) II,III
d) I,III
Answer
Answer: b [Reason:] Series parameters are obtained by short circuit test results.
4. Which of the following informations are obtained from short-circuit test?
I. Ohmic losses at rated current
II. Equivalent resistance and leakage reactance
III. Core losses
IV. Voltage regulation
a) I,II,IV
b) II,III
c) I,II,IV
d) II,III,IV
Answer
Answer: a [Reason:] Core losses are found from OC test .
5. Which of the below estimations require results of both open circuit test and short circuit test?
a) Efficiency
b) Equivalent impedance of one side of the winding
c) Voltage regulation for exact circuit
d) All of the mentioned
Answer
Answer: a [Reason:] All the estimations asked will need results of both the tests.
6. To conduct the open circuit test, test is conducted on the
a) l.v. side
b) h.v. side
c) primary
d) secondary
Answer
Answer: a [Reason:] OC test is performed on the l.v. side of the transformer because the l.v. winding will have lower stress on the insulation and no damage will occur.
7. To conduct the short circuit test, test is conducted on the
a) l.v. side
b) h.v. side
c) primary
d) secondary
Answer
Answer: b [Reason:] To circulate the rated current in the winding, we should opt for lower value of the current so that winding will not damage.
8. A single phase transformer of 2200/220 V having rated l.v. current of 150 A has to undergo open circuit test on h.v. side.Which of the below instruments range should be used?
a) 6A,200V
b) 150A,22V
c) 60A,220V
d) 6A,20V
Answer
Answer: a [Reason:] Open circuit test is conducted on l.v. side, so the measuring instruments will be on the h.v. side.
So the current in the h.v. side will be around 3-6% of the rated.
9. A single phase transformer of 2000/200 V having rated l.v. current of 100 A has to undergo short circuit test on l.v. side. Which of the below instruments range should be used?
a) 200V,10A
b) 20V, 10A
c) 300V,100A
d) 200V,50A
Answer
Answer: a [Reason:] Short circuit test is conducted on h.v. side, so the measuring instruments will be on the l.v. side.
Rated current on the h.v. will be 10A and voltage will be around 5-12% to account for winding losses.
10. A single phase transformer of 2200/220 V having rated l.v. current of 150 A has to undergo open circuit test on h.v side. The instruments used are voltmeter
of 200V and ammeter of 1A. Then the results
a) will be wrong
b) will be accurate
c) of ammeter will burn
d) none of the mentioned
Answer
Answer: c [Reason:] The current in the h.v. winding will be around 10 A but the ammeter is of 1 A rating. So, it will burn off.
11. Which of the following conditions have to ensured for a short-circuit test?
A. L.v. is short circuited
B. It helps in calculation of voltage regulation
C. It is performed at rated voltage
a) A,B
b) A,B,C
c) B,C
d) A,C
Answer
Answer: a [Reason:] For conducting short circuit test, l.v. winding is short circuited and it is not performed at rated voltage.
12. Which of the following conditions have to ensured for a short-circuit test?
A. h.v. winding is short-circuited
B. It helps in calculation of voltage regulation
C. It is performed at rated voltage
D. l.v. winding is short-circuited
a) B,D
b) A,B,C
c) B,C,D
d) A,C
Answer
Answer: a [Reason:] For conducting short circuit test, l.v. winding is short circuited and it is not performed at rated voltage.
13. Transformers with high leakage impedance is used in
a) arc welding
b) power distribution
c) power generating terminals
d) none of the mentioned
Answer
Answer: a [Reason:] The feature of the high impedance is extracted in the arc welding applications.
14. Which of the following conditions have to ensured for a open-circuit test?
A. Performed on L.V side
B. Leakage impedance can be obtained
C. It is performed at rated voltage
D. It gives magnetizing impedance
a) B,D
b) A,B,C
c) B,C,D
d) A,C,D
Answer
Answer: d [Reason:] Leakage impedance is not found from open-circuit test.
15. Which of the following conditions have to ensured for a open-circuit test?
A. Performed on L.V side
B. Leakage impedance can be obtained
C. It is performed at 10-12% of rated voltage
D. It gives magnetizing impedance
a) B,D
b) A,B,C
c) B,C,D
d) A,D
Answer
Answer: a [Reason:] Leakage impedance is not found from open-circuit test. And it is performed on the rated voltage to account for core losses.
16. Which of the following informations are not obtained from short-circuit test?
I. Ohmic losses at rated current
II. Equivalent resistance and leakage reactance
III. Core losses
IV. Voltage regulation
a) I,II
b) II,III
c) I,II,IV
d) II,III,
Answer
Answer: d [Reason:] Core losses are found from OC test and the voltage regulation is not obtained from one single test here.
Set 5
1. Which of the following characteristics reveal about the magnetization nature of the machine?
a) No-load characteristics
b) Load characteristics
c) Armature characteristics
d) Both no-load and load characteristics
Answer
Answer: d [Reason:] Both no-load as well as load characteristics are required to predict the magnetization of the machine.
2. Choose the most inappropriate out of the following for the no-load characteristics of the dc generator.
a) It is the open circuit characteristic of the machine
b) It is magnetization characteristic of the machine
c) It is conducted on the unloaded machine
d) None of the mentioned
Answer
Answer: d [Reason:] All the mentioned nature of the characteristics are correct.
3. The external characteristic is plotted between
a) terminal voltage vs armature current at constant excitation
b) terminal voltage vs field current at constant armature current
c) induced armature emf vs armature current at constant excitation
d) none of the mentioned
Answer
Answer: a [Reason:] The external characteristic is plotted between terminal voltage and armature current at fixed excitation.
4. A student forgot to mark the x-y axes in his experiments but he just noted down the cause and the effect for each. How will he conclude about the armature
characteristic out of the all plotted graphs?
a) By marking graph for constant terminal voltage
b) By marking graph for constant field current
c) By marking graph for constant armature current
d) By marking graph for constant speed
Answer
Answer: a [Reason:] Armature characteristic is observed at constant terminal voltage for emf vs armature current.
5. Armature characteristic is also known as
a) regulation characteristic
b) magnetization characteristic
c) external characteristic
d) load characteristic
Answer
Answer: a [Reason:] Armature characteristic is also known as regulation characteristic. It is so called due to the fact that the difference in the terminal voltage helps to determine the voltage deviation.
6. The air gap line represents
a) magnetic behaviour of the air gap of the dc machine
b) magnetic behaviour of the air gap of the induction machine
c) magnetic behaviour of the iron core
d) all of the mentioned
Answer
Answer: a [Reason:] Air gap line in the magnetization curve represents the ideal nature of the machine considering no saturation for the dc machine.
7. Identify the armature characteristic of the dc generator.
a) 
b) 
c) 
d) 
Answer
Answer: a [Reason:] At low values of the armature conductor, the increase in the field current is very small to provide IaRa drop. At large armature current,field current increases sharply to compensate for the voltage drop caused by armature reaction.
8. For a given dc generator, the external characteristic is plotted . Without using further plots, how can we obtain internal characteristic?
a) By adding the IaRa drop to the plot
b) By adding armature reaction
c) By reducing IaRa drop
d) All of the mentioned
Answer
Answer: a [Reason:] Adding the armature resistance drop we can obtain the external characteristic for a dc generator.
9. Identify the machines by observing their external characteristics for (i) and (ii) respectively.
(i) 
(ii) 
a) Separately excited dc generator, shunt generator
b) Shunt generator, separately excited dc generator
c) Differentially compound dc generator, separately excited
d) Series dc generator, shunt generator
Answer
Answer: a [Reason:] In dc shunt generator, voltage drops off much more rapidly with load due to fal in field current with terminal voltage.
10. The voltage drop in terminal voltage from no-load to full load in a shunt generator can be compenated using
a) aiding series field
b) long-shunt, diferential field
c) aiding shunt field
d) any of the measures
Answer
Answer: a [Reason:] By aiding the series field the armature reaction drop can be compensated.
11. Mark the correct order for the external characteristics of compound generators from the below graph.
a) 1: over compound, 2: level compound, 3: under compound, 4: differential compound
b) 1: under compound, 2: level compound, 3: over compound, 4: differential compound
c) 1: differential compound, 2: level compound, 3: under compound, 4: over compound
d) 1: over compound, 2: differential compound, 3: under compound, 4: level compound
Answer
Answer: a [Reason:] Over compounding provides maximum voltage difference at no load while the differential compunding has least.
12. The external characteristic for the dc series generator is
a) 
b) 
c) 
d) 
Answer
Answer: a [Reason:] There is a linear behaviour till the saturation.
13. Why is armature of a dc machine made of silicon steel stampings?
a) To reduce hysteresis loss
b) To reduce eddy current loss
c) For the ease with which slots can be created
d) To achieve high permeability
Answer
Answer: a [Reason:] The silicon steel has very high permeability and makes the flux past consistent. Thereby making lesser losses.
14. What losses occur in the teeth of dc generator?
a) To reduce hysteresis loss
b) To reduce eddy current loss
c) To reduce eddy current as well as hysteresis losses
d) To achieve high permeability
Answer
Answer: c [Reason:] Teeth of the machine has both eddy current losses as well as hysteresis losses.
15. For a 220-V level compound generator the terminal voltage at the half load is
a) more than 220-V
b) same as no-load voltage
c) more than no-load voltage
d) lesser than no-load voltage
Answer
Answer: b [Reason:] In a level compound dc generator the terminal voltage remains same as no load voltage at all the loading.
