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Multiple choice question for engineering

Set 1

1. Which of the following motor is used where high starting torque and wide speed range control is required?
a) All motors
b) Induction motor
c) Synchronous motor
d) DC motor

View Answer

Answer: d [Reason:] DC series motor gives the maximum starting torque at starting. With the help of Ward Leonard speed control method various speeds are also available. It provides one mode to othe transmission also.

2. The armature voltage control of DC motor will provide ___________
a) Constant power drive
b) Constant voltage drive
c) Constant current drive
d) Constant torque drive

View Answer

Answer: d [Reason:] By changing the armature voltage we can run motor to give various speeds at constant torque. While changing the field is also another idea to change speed but it’ll be variable torque constant power operation.

3. As there is no back emf at the instant of starting a DC motor, in order to prevent a heavy armature current from flowing though the armature circuit ___________
a) Series resistance is connected with armature
b) Parallel resistance is connected to the armature
c) armature is temporarily open circuited
d) a high value resistor is connected across the field winding

View Answer

Answer: a [Reason:] To prevent large armature current from damaging the armature circuit we connect series resistance with armature and in order to avoid losses we reconnect the same resistance with field circuit once machine is started, i.e. the exact principle of starters.

4. What will happen to torque if back emf and speed of the DC motor is doubled?
a) Remain unchanged
b) Reduce to one-fourth value
c) Increase four folds
d) Be doubled

View Answer

Answer: a [Reason:] Torque depends on armature current, so as long as armature current remains constant torque will not change. Speed depends directly with back emf thus, it will definitely become double when back emf is doubled.

5. At the instant of starting, when a DC motor is put on supply, it will behave like ______________
a) Highly resistive circuit
b) Low resistance circuit
c) Capacitive circuit
d) Inductive circuit

View Answer

Answer: b [Reason:] At the starting when back emf is very low, machine will fetch large amount of armature current from supply acting as a low resistive circuit. In order to avoid such case, we use starters in DC machine which add some resistance to armature at the starting.

6. All others are advantages of DC motor over AC motor except _______________
a) Low cost
b) Wide speed range
c) Stability
d) High starting torque

View Answer

Answer: a [Reason:] Dc machine is very useful over AC when operation is concerned but its superior speed control system itself requires 2 more machines. DC machine requires high maintenance due to commutator and brushes so it is an expensive machine.

7. If a DC motor designed for 45°C ambient temperature is to be used for 55°C ambient temperature, then the motor ___________
a) Of lower HP should be selected
b) Of higher HP should be selected
c) Can be used for 50°C ambient temperature also
d) Is to be de-rated by a factor recommended by manufacturer and select the next higher H.P. motor

View Answer

Answer: d [Reason:] The ambient temperature is one of a factor in derating. If the ambient temperature given is high, the insulation may reach its maximum temperature limit quickly. Hence, the motor may have to be de-rated, and for application next motor must be used.

8. Torque developed by a DC motor depends upon _____________
a) magnetic field
b) active length of the conductor
c) current flow through the conductors
d) Current, active length, no. of conductors, magnetic field all

View Answer

Answer: d [Reason:] Torque produced by a DC motor is directly proportional to an armature current flowing through the machine. While it also depends on some constant values like active length, no. od armature conductors and magnetic field which are constant for a given machine.

9. For very sensitive and wide speed control, which of the following method is preferred?
a) Armature control
b) Ward-Leonard control
c) Multiple voltage control
d) Field control

View Answer

Answer: b [Reason:] In ward-Leonard method, by using big circuitry, very fine speed control over the whole range from zero to normal speed and that too in both directions can be obtained. The motor-generator set can provide speed both below and above the rated speed and in both direction, by this method.

10. Field weakening control method is used for the speed _________
a) Above rated speed
b) Below rated speed
c) Can be used anywhere
d) Can’t tell

View Answer

Answer: a [Reason:] By reducing of field current, flux will also reduce, because flux is directly proportional to the field current passing through the winding. As speed is inversely proportional to flux, so this method can be used for only above rated speed.

11. Armature resistance control method is used for the speed _____________
a) Above rated speed
b) Below rated speed
c) Can be used anywhere
d) Can’t tell

View Answer

Answer: b [Reason:] In armature circuit resistance method armature resistance can be only increased, by adding another resistance in series. As a result, the back emf will decrease. Speed is directly proportional to back emf so, it will also decrease. Therefore, this method is used for below rated speed.

12. Which function is performed by counter emf of a DC motor?
a) Exceeds supply voltage
b) Aids applied voltage
c) Helps in energy conversion
d) Regulates its armature voltage

View Answer

Answer: c [Reason:] Back emf or counter emf makes the DC a self-regulating machine i.e. it makes the DC motor to draw as much armature current as it is just enough to develop the required load torque. Thus, it helps in energy conversion and it is always less than applied voltage.

13. The output power of any electrical (AC or DC) motor is taken from the ______________
a) Field
b) Coupling mounted on the shaft
c) Armature
d) Motor frame

View Answer

Answer: b [Reason:] The output power of any electrical motor is always taken from the coupling mounted on the shaft because the motor converts input electrical power into output mechanical power. So, we get mechanical power at shaft coupling.

14. Why field winding of a DC series motor is provided with thick wire?
a) As it carries large load current
b) To provide large flux
c) In order to reduce eddy current to provide large flux
d) To reduce the of insulting materials

View Answer

Answer: a [Reason:] In DC series motor, the cross-sectional area of wire used for field coils has to be large because the in this case field coils carry the load or armature current. As the cross-sectional area is large, the resistance is minimum and series voltage drop is also minimum.

Set 2

1. Differentially compound DC motors are used in applications requiring _______
a) High starting torque
b) Low starting torque
c) Variable speed
d) Frequent on-off cycles

View Answer

Answer: b [Reason:] Compound motor shows combine effect of shunt and series field windings. Differential compound series motor gives low starting torque, examined by torque current characteristic. Hence, applications with low starting torque are called in differentially compound DC motor.

2. Which DC motor is more preferred for elevators?
a) Shunt motor
b) Series motor
c) Differential compound motor
d) Cumulative compound motor

View Answer

Answer: d [Reason:] Cumulative wound DC motors give high starting torque like a series motor and reasonable good speed regulation at high speeds like a shunt dc motor. As this type of motor offers the best of both series and shunt motor, it is practically suitable for most common applications like elevators.

3. Which DC motor is used in shears and punches?
a) Shunt motor
b) Series motor
c) Cumulative compound motor
d) Differential compound motor

View Answer

Answer: c [Reason:] Cumulative compound DC motor provides high starting torque which is required at instant of starting in shears and punches. It also provides good speed regulation in order to vary pressure. So, this motor suits well to shears and punches application.

4. Which DC motor has got maximum self-loading property?
a) Series motor
b) Shunt motor
c) Cumulative compound motor
d) Differential compound motor

View Answer

Answer: d [Reason:] A differentially compound DC motor, flux reduces so sharply at small increase in load at higher values of load. It is advisable that motor should not be used beyond some load value, as it may damage itself by self-loading.

5. For the same H.P. rating and full load speed, which of the following motor has poor starting torque?
a) Series motor
b) Shunt motor
c) Cumulative compound motor
d) Differential compound motor

View Answer

Answer: d [Reason:] In differential compound motor, series field opposes shunt field. It has poor starting torque as the resultant flux is minimized by this opposition. The flux starts decreasing with increase in load. The decrease in flux cause the starting torque to be less than any other DC motor.

6. Unlike DC series motor, DC compound motor can be started on no-load.
a) True
b) False

View Answer

Answer: a [Reason:] Cumulative motor takes best from shunt motor and series motor as well. Due to shunt field flux component, DC cumulative motor can be started on no load. No-load speed of DC cumulative motor is greater than that of DC shunt motor but it’s not equal to infinity like series motor.

7. Flywheel is used with DC compound motor to reduce the peak demand by the motor, compound motor has to be _____________
a) Level compounded
b) Under compounded
c) Cumulatively compounded
d) Differentially compounded

View Answer

Answer: c [Reason:] In case of cumulative compound DC motor, whenever a mechanical load is subject to fluctuations for short durations, a flywheel is generally provided for carrying peaks. Flywheel will smooth out the load on the motor and will also reduce peaks on power system.

8. What will happen if shunt field suddenly opens, in a differential compound DC motor?
a) Motor will first stop and then run in opposite direction as series motor
b) Motor will not work and come to stop
c) Motor will work as series motor and run at infinitely high speed
d) Motor will work as series motor and run at slow speed

View Answer

Answer: a [Reason:] If shunt field is suddenly opened, flux due to shunt field will become 0. As, net flux is equal to shunt field flux – series field flux, the direction of flux will be reversed and thus motor will run in opposite directions.

9. What will happen if the series field of a differential compound motor is short circuited?
a) Net flux will decrease
b) Speed will increase
c) Speed will remain constant
d) Speed will decrease

View Answer

Answer: d [Reason:] As the series field is short circuited, current through series field becomes zero and MMF due to series field becomes zero. Hence, net MMF in motor becomes greater than previous case, which results in total increase in the flux created and decrease in speed.

10. What will happen if the series field of a cumulative compound motor is short circuited?
a) Net flux will increase
b) Speed will increase
c) Speed will remain constant
d) Speed will decrease

View Answer

Answer: b [Reason:] As the series field is short circuited now, current through series field becomes zero and MMF due to series field becomes zero. Hence, total MMF in motor becomes less than previous case, which results in total decrease in the flux created and increase in speed.

11. DC motor is to a drive a load which is almost zero for certain part of the load cycle and peak value for short duration. We will select __________
a) Series motor
b) Shunt motor
c) Compound motor
d) Any DC motors

View Answer

Answer: c [Reason:] We can’t use series motor as our load is almost zero at some points. Thus, we’ll use compound motor which can work on no load also. Cumulative compound motor is provided with flywheel so that this machine can deal with peak value.

Set 3

1. DC series motors are used _______________
a) Where load is constant
b) Where load changes frequently
c) Where constant operating speed is needed
d) High starting speed

View Answer

Answer: d [Reason:] DC series motors are used in those applications where high starting speed is required at its initial phase. Since no load operation speed of DC series motor is very high, it is always operated on some non-zero load.

2. Where compensating winding is provided in case of conductively compensated DC series motor?
a) As separately wound unit
b) In series with armature winding
c) In parallel with armature winding
d) In parallel with field winding

View Answer

Answer: b [Reason:] Compensating winding is always excited with armature current in a given DC machine. So, compensating winding is provided in case of conductively compensated DC series motor in series with the armature winding.

3. No-load speed of which of the following DC motor will be highest?
a) Shunt motor
b) Series motor
c) Cumulative compound motor
d) Differentiate compound motor

View Answer

Answer: b [Reason:] At no load, armature current tends to zero, flux φ tends to zero, where speed is inversely proportional to the flux, speed will tend to infinity. Thus, no load speed of DC series motor is highest.

4. The direction of rotation of a DC series motor shaft can be changed by _____________
a) Interchanging supply terminals
b) Interchanging field terminals
c) Not possible
d) Cannot be determined

View Answer

Answer: b [Reason:] From speed current characteristics for a DC series motor, we get that speed is inversely proportional to flux. If flux direction is reversed, same flux will be available with negative sign in vectors, thus, speed will remain same but with the other direction.

5. Which of the following DC motor can be used at conveyors?
a) Series motor
b) Shunt motor
c) Differentially compound motor
d) Cumulative compound motor

View Answer

Answer: a [Reason:] A conveyor requires high torque with steady speed, which provided by DC series motor. From torque speed characteristic of a DC series motor, it is easily stated that speed of the series motor practically remains constant at high torques.

6.Which statement is correct about DC series motor?
a) Has its field winding consisting of thick wire and more turns
b) Has a poor torque
c) Can be started easily without load
d) Has its field winding consisting of thick wire and less turns

View Answer

Answer: d [Reason:] When field winding is connected in series it has to be of made by thick wire. When field winding is connected in parallel it has to be of made by thin wire. So, thick wire is used with less number of turns while thin one is used with more number of turns.

7. In which of the following applications DC series motor is invariably tried?
a) Starter for a car
b) Drive for a water pump
c) Fan motor
d) Home appliances

View Answer

Answer: a [Reason:] Starter of the car simply implies start of operation at maximum load. Thus, series motor can be used in this application. For higher torques, since speed remains constant car starter is one of the best application of DC series motor.

8. The speed of a DC series motor is ___________
a) Proportional to field current
b) Proportional to the square of the armature current
c) Proportional to the armature current
d) Inversely proportional to the armature current

View Answer

Answer: d [Reason:] Speed of DC series motor is inversely proportional to the armature current because, as armature current increases the flux produced also increases due to the series combination. Similarly if armature current is reduced flux is reduced which will increase speed.

9. In a DC series motor, if the armature current is halved, the torque of the motor will be equal to
a) 100% of the previous value
b) 50% of the previous value
c) 25% of the previous value
d) 10% of the previous value

View Answer

Answer: c [Reason:] Torque in the case of linear magnetization of DC series motor is directly proportional to square of the armature current. So, armature current is made 1/2th of the original value, then torque will be 1/4th of the original value.

10. Applications demanding large applications at starting and small torque while running use DC series motor.
a) True
b) False

View Answer

Answer: a [Reason:] Large starting torque, infinite speed at no-load, low running torque for steady speed operation are called as Series characteristics. This are shown by DC series motor, so applications like cranes, traction, etc. use DC series motor.

Set 4

1. For machine tools, which DC motor can be used?
a) DC Series motor
b) DC Shunt motor
c) DC cumulative compound motor
d) DC differential compound motor

View Answer

Answer: b [Reason:] Shunt characteristics is the speciality of DC shunt motor. The speed of the motor almost remains constant on various loads, thus it suits perfectly to the application, where speed requirement is constant like in machine tools.

2. In a DC shunt motor, speed is related to armature current as _____________
a) Directly proportional to the armature current
b) Proportional to the square of the current
c) Independent of armature current
d) Inversely proportional to the armature current

View Answer

Answer: d [Reason:] When armature reaction is ignored in a DC shunt motor, flux almost remains constant but the speed of the motor decreases according to the increase armature current, increasing Ia*Ra drop. Hence, inverse proportionality.

3. In a DC shunt motor for zero armature current we get speed ______
a) Non-zero and minimum
b) Zero
c) Non-zero and maximum
d) Doesn’t depend on armature current

View Answer

Answer: c [Reason:] For zero armature current we get some non-zero value, indicated by positive intercept on speed characteristics. As armature current is increased speed of DC shunt motor starts decreasing due to increase in voltage drop at armature resistance.

4. What will be the effect of opening of field of a DC shunt motor while motor is running?
a) The speed of motor will be reduced
b) The armature current will reduce
c) The motor will attain dangerously high speed
d) The motor will continue to constant speed

View Answer

Answer: c [Reason:] In a DC shunt motor if supply for the field winding is cut down, the speed would dangerously increase in order to maintain the back emf of the motor. For a constant back emf, flux is inversely proportional to the speed of DC shunt motor. So, if flux drops to zero theoretically speed will tend to infinity.

5. What will be the effect of reducing load on DC shunt motor?
a) Speed will increase abruptly
b) Speed will increase in proportion to reduction in load
c) Speed will remain almost constant
d) Speed will reduce

View Answer

Answer: c [Reason:] For DC shunt motor, speed-armature current characteristics is called as a shunt characteristic as speed almost remains constant. Thus, by reducing load speed will increase negligibly, thus remaining almost constant.

6. How speed of the DC shunt motor can be increased?
a) Decreasing the field current
b) Increasing the field current
c) Decreasing the load current
d) Increasing the armature current

View Answer

Answer: b [Reason:] For a constant load, load current will remain constant. Decreasing armature current will help in increasing speed for DC shunt motor. Since load current is addition of armature current and field current we’ll get less armature current for more field current.

7. Practical reason behind speed of DC shunt motor is proportional to back emf only is ____________
a) Back emf is equal to armature drop
b) Flux is proportional to field current
c) Flux is proportional to armature current
d) Flux is practically constant in DC shunt motors

View Answer

Answer: d [Reason:] The field winding in DC shunt motor is connected in parallel to the armature winding and the supply. If we assume that the supply voltage for motor is constant then flux also becomes constant. At the rated speed the back emf also becomes almost constant if the load is same.

8. In a DC shunt motor, what will be the armature current at maximum load?
a) Almost negligible
b) Rated full-load current
c) Less than full-load current
d) More than full-load current

View Answer

Answer: d [Reason:] As the load will increase in rapid manner, speed change in DC shunt motor will be visible. As load increase, speed will decrease, though by some small value. From, current armature characteristic armature current will be more than the earlier case.

9. In which of the following motor, ratio of starting torque to full-load torque will be least?
a) DC series motors
b) DC shunt motors
c) DC compound motors
d) Synchronous motors

View Answer

Answer: b [Reason:] From torque current characteristic, we get that torque at no load or starting torque is equal to zero. As load increases, speed decreases and armature current increases so as torque also increases linearly, if effect of armature reaction is neglected.

Set 5

1. For connecting two generators in parallel, they should have __________________
a) Same current rating
b) Same ohmic rating
c) Same voltage rating
d) All ratings must be same

View Answer

Answer: c [Reason:] For connecting two DC generators in parallel we equal voltage ratings. As the generator voltage is easily adjustable in a range, so the condition stated above is not a must. But, it is desirable condition.

2. While connecting two DC generators in parallel, which of the following is not a desirable condition?
a) Same voltage rating
b) Same percentage voltage regulation
c) Same percentage speed regulation of the prime movers
d) Same current rating

View Answer

Answer: d [Reason:] Same voltage rating, same percentage voltage regulation, same percentage speed regulation are the desirable conditions for connecting two generators in parallel, though these conditions are not a must.

3. We connect two generators in parallel _____________
a) For large DC load
b) For small DC load
c) For any DC load
d) For any AC or DC load

View Answer

Explanation: a Answer: We connect two generators in parallel for supplying large DC load. It is desirable to use more than one generator in parallel. This arrangement provides the security that if one generator gives way, the other(s) can feed part load.

4. Load sharing of two generators connected in parallel is determined by __________
a) Internal characteristics
b) External characteristics
c) Both internal and external characteristics
d) It doesn’t depend on load sharing

View Answer

Answer: b [Reason:] Two generators are connected in parallel such that, summation of current carried by both generators is equal to load current. In such cases we need to see external characteristics on loaded condition.

5. For a parallel operation of 2 DC shunt generators, we get net external characteristics ___________
a) Starting from the same no-load point and between generator 1 and generator 2
b) Starting from the same no-load point and below generator 1 and generator 2
c) Starting from the same no-load point and above generator 1 and generator 2
d) Can’t be determined

View Answer

Answer: c [Reason:] The load sharing by these generators is determined by addition of external characteristics of both of generators. Thus, at no load generators will have common point, on y- axis so parallel characteristic will start from same point and will lie above of both.

6. When two compound generators are connected in parallel, when load current in generator 1 is increased __________
a) Both generator will share same load
b) Generator 1 will start running as motor
c) Generator 2 will be overloaded
d) Generator 2 will start running as motor

View Answer

Answer: d [Reason:] If load current in G1 increases, load current in G2 decreases. Series excitation and internal voltage in G1 increases and for G2, these quantities decrease. Finally, all load shifts, which may turn to run G2 as a motor. All this leads to heavy overloading of G1.

7. While running two compound motors in parallel, we connect equalizer ring between ___________
a) Two armatures
b) Two fields
c) Two load points
d) Anywhere

View Answer

Answer: a [Reason:] A low-resistance equalizer connection is made directly between the two armatures before the series fields. Any emf variations of the armatures causes equalizing circulating current which do not affect the current through the series windings. Thereby the parallel operation is stabilized.

8. When two DC series motors are connected in parallel, the resultant speed is __________
a) More than the normal speed
b) Loss than the normal speed
c) Normal speed
d) Zero

View Answer

Answer: a [Reason:] When two DC series motor are connected in parallel then the resultant speed is increase the normal speed. If the voltage across each motor is assumed to be V, then current through each motor in parallel connection will be I/2. So, speed α (voltage/current) α (V/Ia/2). Hence the resultant speed is more than normal speed.