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Multiple choice question for engineering

Set 1

1. Hopkinson’s test of D.C. machines is conducted at _______________
a) No-load
b) Part load
c) Full-load
d) Overload

View Answer

Answer: c [Reason:] Unlike Swinburne’s test Hopkinson’s test is carried out at loaded condition. Thus, we get stray load loss also, while finding out the efficiency. Hence, efficiency is not over-estimated like Swinburne’s test.

2. Hopkinson’s test requires ____________
a) One DC machine on which test is carried out
b) Two different DC machines
c) Two identical DC machines
d) Can be worked with one or two machines

View Answer

Answer: c [Reason:] This is a regenerative test in which two identical dc shunt machines are coupled mechanically and tested simultaneously. One of the machines is used as a motor driving while the other one acts as a generator which supplies electric power to motor.

3. In Hopkinson’s test, two machines are connected in ______________
a) Series
b) Parallel
c) Can be connected in parallel or series
d) Two machines are not required

View Answer

Answer: b [Reason:] The two machines are made parallel by means of switch S after checking that similar polarities of the machine are connected across the switch. Here, one machine is driving another machine.

4. What will happen if field current of generator in Hopkinson’s test is increased?
a) Current through motor armature will increase
b) Current through motor armature will decrease
c) Current through motor armature will remain constant
d) Motor armature current cannot be determined

View Answer

Answer: a [Reason:] If field current through generator is increased, back emf of generator will increase thus it’ll become greater than back emf of a motor, so to compensate this effect armature current in generator will increase thus, motor armature current will also increase.

5. What will happen if field current of motor in Hopkinson’s test is decreased?
a) Current through motor armature will increase
b) Current through motor armature will decrease
c) Current through motor armature will remain constant
d) Motor armature current can’t be determined

View Answer

Answer: a [Reason:] If field current through motor is decreased, speed of the motor will increase due to inverse proportionality, back emf of generator will increase thus it’ll become greater than back emf of a motor, so to compensate this effect armature current in generator will increase thus, motor armature current will also increase.

6. Hopkinson’s test is a regenerative test.
a) True
b) False

View Answer

Answer: a [Reason:] Hopkinson’s test is a regenerative test, because the power drawn from the mains is only that needed to supply losses. The test is, therefore, economical for long duration test like a “heat run”.

7. For carrying out load test on Hopkinson’s test setup _________________
a) Actual load is needed
b) By changing field currents in two machines load can be changed
c) Can’t carry out
d) By changing the armature current test is carried out

View Answer

Answer: b [Reason:] There is no need to arrange for actual load (loading resistors) which apart from the cost of energy consumed, would be prohibitive in size for large-size machines. By merely adjusting the field currents of the two machines, the load can be easily changed and a load test conducted over the complete load range in a short time.

8. Hopkinson’s test gives _______________
a) Combined iron losses of two machines which can be separated
b) Combined iron losses of two machines which can’t be separated
c) Doesn’t include iron losses
d) Depends on actual setup

View Answer

Answer: b [Reason:] From Hopkinson’s test both machines are not loaded equally and this crucial in small machines. Thus, it is important to know the separate iron losses for given machines. But, test gives combined losses which are different for different machines as excitation differs.

9. Hopkinson’s test is suitable for ______________
a) Small machines only
b) Small and medium machines
c) All machines
d) Only large machines

View Answer

Answer: d [Reason:] A large variation of field currents is required for small machines, the full-load set speed is usually higher than the rated speed and the speed varies with load. The full load in small machines cannot obtained by cutting out all the external resistances present in the generator field. Sufficient reduction in the motor field current is necessary to achieve full-load conditions resulting in speeds greater than the rated value.

10. Why field test is conducted even if Hopkinson’s test is present?
a) Instability of an operation
b) Possibility of run-away speed
c) Both instability and possibility of run-away speed
d) Field test is not conducted

View Answer

Answer: c [Reason:] Regenerative test on two identical series motors is not feasible because of instability of such an operation and the possibility of run-away speed. Therefore, we have toconduct a loading test.

11. In field’s test generator field and motor field are connected in ______________
a) Series
b) Parallel
c) Alternatively, series and parallel
d) Not connected

View Answer

Answer: a [Reason:] The generator field is connected in series with motor field circuit. The generator is thus separately excited and its excitation is identical to that of motor at all loads. This ensures that the iron-loss of both the machines are always equal.

Set 2

1. Resultant pitch in the lap winding is__________
a) Depends on Yb value
b) Depends on Yf value
c) Depends on Yb and Yf value
d) Always equal to 2

View Answer

Answer: d [Reason:] In a lap winding the “finish” of one coil is connected to “start” of the adjoining coil. The coil side displacement of the front-end connection is called the front-pitch. The coil side displacement of the back-end connection is called the back-pitch. Resultant-pitch is equal to difference between Yb and Yf which is equal to 2, irrespective of Yb and Yf value.

2. What is the condition of retrogressive winding?
a) Yb > Yf
b) Yb < Yf
c) Yb = Yf
d) No condition in bterms of Yb and Yf

View Answer

Answer: b [Reason:] The coil side displacement of the front-end connection is called the front-pitch. The coil side displacement of the back-end connection is called the back-pitch. The direction in which the winding progresses depends upon which is more, Yb or Yf. For retrogressive winding Yb < Yf .

3. What is the value of Yb for a lap winding with a 4-pole, 12-slot armature with two coil sides/slot. Assume single-turns coils.
a) 3
b) 5
c) 7
d) 9

View Answer

Answer: c [Reason:] Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Slots in the armature winding= 12, Number of poles= 4. YCS= 12/4= 3. Yb = 2YCS +1= 7.

4. What is the value of Yf for a lap winding with a 4-pole, 12-slot armature with two coil sides/slot? (Assume single-turns coils – progressive winding)
a) 3
b) 5
c) 7
d) 9

View Answer

Answer: b [Reason:] Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Slots in the armature winding= 12, Number of poles= 4. YCS= 12/4= 3. Yb = 2YCS +1= 7 Yf =Yb – 2= 7-2= 5.

5. Equalizer rings are needed in lap winding.
a) False
b) True

View Answer

Answer: b [Reason:] Each parallel path in lap winding is under the influence of one pair of poles, so if a machine consists of multiple pairs of poles then dissimilarities occurs, due to which unequal voltages may be induced in the paths and a circulating current may flow. In wave winding each path is under the influence of all poles, so voltages are induced in each path causing no such dissimilarities like lap winding. Equalizers in lap windings are used to remove this dissimilarity, they’re not needed in wave winding.

6. What is the symmetry requirement of lap winding?
a) 2C/P= 0
b) 2C/P= integer
c) 2C/P= non-integer
d) Can’t express mathematically

View Answer

Answer: b [Reason:] To avoid no-load circulating currents and certain consequential commutation problems, all the parallel paths must be identical so as to have the same number of coil-sides. Symmetry thus requires ratio of 2C/P is equal to the integer. Also, US/P equal to integer represents the same.

7. What is the relation between number of parallel paths(A) and number of poles(P)?
a) A = P
b) A < P
c) A > P
d) No relation exists

View Answer

Answer: a [Reason:] Complex winding can be divided into different parallel paths lying under different pole pairs. It is, therefore, concluded that the number if parallel paths is equal to the number of poles. In wave winding number of parallel paths is equal to 2.

8. Current flowing through the armature conductors Ic is related to total current Ia by_______
a) Ic = A Ia
b) Ic = Ia/A
c) Ic = A2 Ia
d) Ic = A/Ia

View Answer

Answer: b [Reason:] Two positive and two negative brushes are respectively connected in parallel for feeding the external circuit. As per the ring diagram Ia splits into the number of poles equally. Poles = Parallel paths. Thus, Ic = Ia /A.

9. Value of commutator pitch in lap winding is_____
a) +2
b) +1 or -1
c) -2
d) Different for different parameters

View Answer

Answer: b [Reason:] Two ends of coil are connected across the adjacent commutator segments. Depending on the type of winding that is, retrogressive or progressive, we have two values for commutator pitch. For progressive winding, commutator pitch = +1. For retrogressive winding, commutator pitch = -1.

10. What is the value of Yf for a lap winding with a 4-pole, 12 commutator segments, with two coil sides/slot.?
(Assume single-turns coils -retrogressive winding).
a) 9
b) 3
c) 11
d) 5

View Answer

Answer: a [Reason:] Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Slots in the armature winding= Number of commutator segments= 12, Number of poles= 4. YCS= 12/4= 3. Yb = 2YCS +1= 7 Yf =Yb + 2= 7+2= 9.

Set 3

1. Which DC motor is generally preferred for cranes and hoists?
a) Series motor
b) Shunt motor
c) Cumulatively compounded motor
d) Differentially compounded motor

View Answer

Answer: a [Reason:] DC series motor is used where high starting torque is required, thus in cranes and hoists DC series motor is used widely. Torque-speed characteristics of DC series motor suits perfectly to these applications.

2. Which of the following motor has the poorest speed regulation?
a) Shunt motor
b) Series motor
c) Differential compound motor
d) Cumulative compound motor

View Answer

Answer: b [Reason:] If the load is applied to the any DC motor or machine, the speed of the motor decreases automatically. Thus, in order to maintain the constant speed, the difference between no load and full load speed called as speed regulation should be maintained very less. In series motor this difference is almost equal to infinity.

3. Buses, trains, trolleys, hoists, cranes make use of DC series motor because __________
a) It provides low starting torque
b) Cost efficient
c) It provides high starting torque
d) It requires minimum power compare to other motors

View Answer

Answer: c [Reason:] DC series motor provides very high starting torque, and at higher torques speed of the DC series motor is also maintained to constant. Both of these properties are used in applications of DC series motor.

4. DC motor is to drive a load which is having certain minimum value for most of the time and some peak value for short duration. We will select the _______
a) Series motor
b) Shunt motor
c) Compound motor
d) Cannot be determined

View Answer

Answer: a [Reason:] When load is minimum, we will get very high speed at shaft in DC series motor, but it is advisable that DC series motor should not be used on no-load application. For the application stated in question DC series motor is perfect option.

5. Which DC motor has got maximum self-relieving property?
a) Series motor
b) Shunt motor
c) Cumulatively compounded motor
d) Differentially compounded motor

View Answer

Answer: a [Reason:] DC series motor develops large starting torque initially and slow down with increase in load and so get automatically relived from heavy excessive load. Thus, DC series motor is called as self-reliving machine.

6. Which motor should not be started on no-load?
a) DC Series motor
b) DC Shunt motor
c) Synchronous motor
d) Induction motor

View Answer

Answer: a [Reason:] No-load speed of the DC motor is very high, it’ll create a dangerous situation and the centrifugal forces will destroy the armature and may harm the personnel. As, flux is inversely proportional to speed, speed will be infinity by calculations.

7. Which of the following is the correct equation for DC series motor ________
a) [ Vt / kφ]- [ Rse +Ra / kφ] Ia
b) [ Vt / kφ]- [ Ra/ kφ2] Ia
c) [ Vt / kφ2]- [Rse +Ra/ kφ2] Ia
d) [ Vt / kφ2] + [ Ra/ kφ2] Ia

View Answer

Answer: a [Reason:] Speed-torque characteristic equation is obtained by back emf = (Vt -IaRa-IaRse) and back emf= kφn. Thus, by combining these equations and by assuming all constants equal to k we get, the final equation shown above.

8. Actual speed obtained at any value of armature current comparing to speed obtained by assuming linear magnetic assumption in DC series motor is _______
a) Same
b) Higher
c) Lower
d) Cannot say

View Answer

Answer: b [Reason:] From speed current characteristics of a DC series motor is easily seen that, initial value of speeds in both of the cases is same or rather infinite, but as a armature current or load current increases speed starts dropping down relatively more in the case of linear magnetization.

9. Torque of e DC series motor is __________
a) Directly proportional to armature current
b) Inversely proportional to armature current
c) Directly proportional to square of armature current
d) Inversely proportional to square of armature current

View Answer

Answer: c [Reason:] Torque in a DC series motor, in the case of linear magnetization varies directly with armature current square. Practically at higher values of armature current it tends to settle at constant value.

10. Practically, at higher values of armature current torque does not follow its relationship with armature current.
a) True
b) False

View Answer

Answer: a [Reason:] At higher values of armature current or load current, saturation and demagnetization due to armature reaction becomes prominent, and DC machine shows its effect on torque. Thus, torque tends to remain at constant value.

11. What will happen to a torque, if a DC series motor is accidentally connected to single phase ac supply voltage?
a) Pulsating and unidirectional
b) Steady and unidirectional
c) Oscillating
d) Cannot be determined

View Answer

Answer: a [Reason:] In DC series motor, the AC currents through the field and armature winding will be made unidirectional by the function of commutator and brushes. So, torque will be unidirectional but it will be pulsating also due to AC.

Set 4

1. Which winding has large number of turns?
a) Shunt field
b) Series field
c) Both have same number of turns
d) Depends on requirement

View Answer

Answer: a [Reason:] The shunt field winding is provided with a large number (hundreds or even thousands) of turns of thin wire and is excited from a voltage source. The series field winding has a few turns of thick wire and is excited from armature current by placing it in series with armature.

2. Which winding contains wire with higher thickness?
a) Shunt field
b) Series field
c) Both have same number of turns
d) Depends on requirement

View Answer

Answer: b [Reason:] Series field winding is used when wire is thick. Thus, by making minimum number of turns it can be used to excite a DC machine. For a given field current, control of this field is achieved by means of a diverter, a low resistance connected in parallel to series winding.

3. Which winding have higher resistance?
a) Shunt field
b) Series field
c) Both have same number of turns
d) Depends on requirement

View Answer

Answer: a [Reason:] The shunt field winding is provided with a large number (hundreds or even thousands) of turns of thin wire and is excited from a voltage source. The field winding, therefore, has a high resistance and carries a small amount of current. It is usually excited in parallel with armature circuit.

4. Which is more practical method used in control of series field?
a) Diverter
b) Tappings
c) Switch relay
d) Cannot be determined

View Answer

Answer: b [Reason:] For a given field current, control of series field is achieved by means of a diverter, a low resistance connected in parallel to series winding. A more practical way of a series field control is changing the number of turns of the winding by suitable tappings which are brought out for control purpose.

5. In compound excitation, which winding/s is/are excited?
a) Shunt
b) Series
c) Both
d) Cannot be determined

View Answer

Answer: c [Reason:] In compound excitation, both series and shunt windings are connected to the armature. For compound excitation both of these windings are excited. Two types are cumulative excitation and differential excitation.

6. In which type of excitation air gap flux increases with armature current?
a) Differential compound
b) Cumulative compound
c) Differential and Cumulative
d) Cannot be determined

View Answer

Answer: b [Reason:] In compound excitation both shunt and series field are excited. If the two field aid each other (their ampere-turn is additive), the excited is called cumulative compound. The shunt field is much stronger than the series field. The air gap flux increases with armature current.

7. In differential compound excited machine, what is the variation in air gap flux per pole with respect to armature current?
a) Increases
b) Decreases
c) Remains constant
d) Always varies

View Answer

Answer: b [Reason:] If the two fields oppose each other, the excitation is called differential compound. The air gap flux/pole decreases with armature current. The series field is so designed that the increase or decrease in flux/pole is to a limited extent.

8. Which winding in compound excitation is responsible for change in air gap flux per pole?
a) Series
b) Parallel
c) Interconnected
d) No coil is responsible

View Answer

Answer: a [Reason:] In a compound excited machine, both series and parallel windings are connected with armature circuit. The series winding is specially designed for increasing or decreasing flux per pole in DC machine up to certain extent.

9. How short shunt and long shunt compound winding is selected?
a) Mechanical considerations
b) Switch reversal
c) Performance
d) Mechanical considerations and switch reversal

View Answer

Answer: d [Reason:] In long shunt compound, the shunt field is connected across terminals. In short shunt compound, the shunt field is connected directly across the armature. There is no significant difference in machine performance for the two types of connections. The choice between them depends upon mechanical consideration or the reversing switches.

10. If a DC compound machine connected as a motor is about to use as a generator, we reverse the series field connections.
a) True
b) False

View Answer

Answer: a [Reason:] If a dc compound machine connected as a generator is run as a motor, the series field connections must be reversed as the armature current reverses. The motoring action as cumulative/ differential would then be preserved (same as in the generator). This equally applies vice versa – motor to generator.

11. For a long-shunt compound motoring, which of the following equation is correct?
a) Vt= Ea+ Ia(Ra+ Rse)
b) Vt= Ea– Ia(Ra+ Rse)
c) Vt= Ea+ Ia(Ra– Rse)
d) Vt= -Ea+ Ia(Ra+ Rse)

View Answer

Answer: a [Reason:] For a long shunt compound motor Rse is connected in series with armature, while in short shunt compound motors Rse is connected in series with terminal voltage. IL is supplied through the terminals which split into If and Ia.

12. Vt= Ea– Ia(Ra+ Rse) is the equation for _______________
a) Short shunt compound motoring mode
b) Short shunt compound generating mode
c) long shunt compound motoring mode
d) Cannot be determined

View Answer

Answer: b [Reason:] In a DC machine, for a long shunt compound motor Rse is connected in series with armature, while in short shunt compound motors Rse is connected in series with terminal voltage. IL is supplied to the terminals which is equal to Ia – If.

13. How shunt field is controlled?
a) Diverter resistor in parallel
b) Tapped field winding
c) Series regulating resistance
d) Other methods

View Answer

Answer: c [Reason:] Control of Excitation: 1) Shunt field: by a series regulating resistance.2) Series field: For small armature by a diverter resistance connected in parallel with series field. For large armature by tapped field winding so the winding turns can be changed.

14. The generator is called flat compounded if _____________
a) The series field ampere turns are such as to produce the same voltage at rated load as at no load
b) The series field turns are such as that the no load voltage is smaller than the rated load voltage
c) The rated voltage is less than the no load voltage
d) Cannot be determined

View Answer

Answer: a [Reason:] According to the operating characteristics of a DC compound generator, if series field mmf produces same voltage at rated load as that of no load then it is called as flat compounded generator.

Set 5

1. In three-point starter, as a starting handle is rotated __________
a) The resistance is added into armature circuit
b) The resistance is removed from field circuit
c) The resistance is added into field circuit
d) Resistance is neither added nor removed

View Answer

Answer: c [Reason:] The starting resistance is arranged in steps between conducting raised studs. As the starting handle is rotated about its fulcrum, it moves from one stud to the next, one resistance step is cut out, and it gets added to the field circuit.

2. How much torque is ensured by resistance adding arrangement in 3-point starter?
a) Low and non-zero
b) Infinite
c) High finite
d) Zero

View Answer

Answer: c [Reason:] As a starting handle is rotated one resistance step is added into field circuit. There is a short time wait at each stud for the motor to build up speed. This arrangement ensures a high average starting torque.

3. The resistance of NVC is _______
a) Small
b) Large
c) Infinite
d) Zero

View Answer

Answer: a [Reason:] The resistance of no volt coil (NVC) is small. NVC resistance forms a part of field resistance when resistor rotating handle of three-point starter is moved from min. to max. position. Starting resistance is also added to the field circuit.

4. NVC will release the handle electromagnetically when ________
a) In the case of failure of field current
b) If the resistance is very high
c) At the end of each rotation
d) At the start of each rotation

View Answer

Answer: a [Reason:] In case of failure of field current (due to accidental or otherwise open circuiting) NVC coil releases the handle (held electromagnetically), which goes back to the OFF position under the spring action.

5. Over-load coil performs the function when __________
a) In the case of failure of field current
b) If the resistance is very high
c) If armature current increases beyond certain value
d) At the start of each rotation

View Answer

Answer: c [Reason:] The contact of this relay at armature current above a certain value (over load/ short circuit) closes the NVC ends, again bringing the handle to OFF position. NVC and OL release are protections incorporated in 3-point starter.

6. In 4-point starters the resistance is added in series with NVC because __________________
a) To increase field current
b) To increase armature current
c) To limit the NVC current
d) To limit armature current

View Answer

Answer: c [Reason:] To overcome the problem caused when the field current is low, NVC is connected across the two lines, one line connected to F terminal through the starter and other directly to the second line from another L terminal of the starter. To limit the NVC current a protective resistance R is connected in series with it.

7. What will be the γ value for starter taking 4 steps, where ratio of resistance at maximum allowable current to armature resistance is equal to 1.8?
a) 1.2164
b) 1.8
c) 2.2468
d) 0.8220

View Answer

Answer: a [Reason:] γ is defined as the ratio of upper current limit to the lower current limit in starters of DC machine. γ n-1 = ratio of resistance at maximum allowable current to the armature resistance. Substituting values for n=4, we get γ=1.2164.

8. Maximum allowable current for a 240-V DC shunt motor is equal to 65 A. Minimum allowable current for same is equal to 40 A. What will be the value of γ?
a) 0.6153
b) 1.265
c) 1.625
d) 2.652

View Answer

Answer: c [Reason:] γ is defined as the ratio of upper limit current value to the lower limit current value. So, γ will be the ratio of 65/40. From calculations, we get γ= 1.625. Note that γ is unitless quantity.

9. For a certain machine having γ = 1.8, we are using starter with 5 steps. What will be the resistance at step 3 if step 2 resistance is equal to 2 Ω?
a) 2.111
b) 1.111
c) 3.6
d) 10.8

View Answer

Answer: b [Reason:] γ is defined so as to calculate step resistance from the given maximum limit of current and minimum limit of the same. This γ = rn/rn-1. So, for calculating resistance at step 3 we’ll substitute the corresponding values in the equation, which will give step 3 resistance as 1.1111Ω.

10. Which of the following is the correct formula for calculating step resistance?
a) γ = rn/rn-1
b) γ = rn/rn+1
c) γ = rn*rn-1
d) γ = rn*rn+1

View Answer

Answer: a [Reason:] For given values of armature currents upper and lower, corresponding equivalent resistances are calculated. So, by the induction formula we calculate the value of γ and the step resistance as well, where n denotes the number of steps.

11. Which of the following starter can sufficiently start the DC series motor?
a) 3-point starter
b) 4-point starter
c) 2-point starter
d) Cannot be determined

View Answer

Answer: c [Reason:] 2-point starter is enough to start the DC series motor. Since armature and field winding are in series already high armature current will not flow. No requirement of 3 or 4-point starter in DC series motor.