McqMate
These multiple-choice questions (MCQs) are designed to enhance your knowledge and understanding in the following areas: Mechanical Engineering .
| 51. |
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 |
| Answer» C. regenerative braking | |
| Explanation: 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. | |
| 52. |
connected across AC supply? |
| A. | will run at normal speed |
| B. | will not run |
| C. | will run at lower speed |
| D. | burn due to heat produced in the field winding |
| Answer» D. burn due to heat produced in the field winding | |
| Explanation: in case of parallel field connection, it won’t rotate at all and will start humming and will create vibrations, as a torque produced by positive and negative cycle will cancel out each other. dc motor will be heated up and it may burn. | |
| 53. |
What will happen if the back emf of a DC motor vanishes suddenly? |
| A. | the motor will stop |
| B. | the motor will continue to run |
| C. | the armature may burn |
| D. | the motor will run noisy |
| Answer» C. the armature may burn | |
| Explanation: if back emf vanishes suddenly, motor circuit will try to retain back emf by drawing more current from supply. if supplying unit didn’t trip down by this time, excess current in armature may heat up the armature. | |
| 54. |
is DC motor and not an AC type? |
| A. | winding |
| B. | shaft |
| C. | commutator |
| D. | stator |
| Answer» C. commutator | |
| Explanation: all other parts except brushes and commutator are same in ac machine when outer looks are only taken in consideration. commutator is used only in dc machine for providing mechanical rectification and not in ac machine. | |
| 55. |
can sustain the maximum temperature rise? |
| A. | field winding |
| B. | commutator |
| C. | slip rings |
| D. | armature winding |
| Answer» A. field winding | |
| Explanation: maximum temperature rise can be sustained by field winding, as it is not involved in rotary parts. field winding is present away from rotary parts of the machine, so temperature rise in the machine will not produce any effect in machine rotations. | |
| 56. |
Direction of rotation of motor is determined by |
| A. | faraday’s law |
| B. | lenz’s law |
| C. | coulomb’s law |
| D. | fleming’s left-hand rule |
| Answer» D. fleming’s left-hand rule | |
| Explanation: flemings laws can be summarized as whenever, a current carrying conductor comes under a magnetic field, there will be a force acting on the conductor and on the other hand, if a conductor is forcefully brought under a magnetic field, there will be an induced current in that conductor. | |
| 57. |
of a motor? |
| A. | gross power |
| B. | power drawn in kva |
| C. | power drawn in kw |
| D. | output power available at the shaft |
| Answer» D. output power available at the shaft | |
| Explanation: name plate of the motor shows rated values i.e. rated speed, rated current, rated voltage. it also shows output power available at shaft when all other quantities are set to rated values. | |
| 58. |
An electric motor is having constant output power. So, motor will have a torque speed characteristic |
| A. | circle about the origin. |
| B. | straight line parallel to the speed axis. |
| C. | straight line through the origin. |
| D. | rectangular hyperbola |
| Answer» D. rectangular hyperbola | |
| Explanation: in case of dc motor for the constant output power, ea×ia = t×ω. as t×ω | |
| 59. |
In which of the following case we will get maximum power? |
| A. | ea = 2 x supply voltage |
| B. | ea = supply voltage |
| C. | supply voltage = 2 x ea |
| D. | supply voltage = 4 x ea |
| Answer» C. supply voltage = 2 x ea | |
| Explanation: for a motor, from power equation it is known that, | |
| 60. |
Sometimes motor has to be de-rated. |
| A. | true |
| B. | false |
| Answer» A. true | |
| Explanation: derating refers to the operation of equipment at reduced capacity/power or speed. derating in motors can be caused due to the following reasons- frequency, voltage, ambient temperature, altitude. | |
| 61. |
able to withstand |
| A. | bending moment due to weight of the armature. |
| B. | any unbalanced magnetic pull on the armature core. |
| C. | twisting stains due to transmission of torque. |
| D. | bending moment, unbalanced magnetic pull and twisting stains |
| Answer» D. bending moment, unbalanced magnetic pull and twisting stains | |
| Explanation: the armature shaft must be able to withstand any unbalanced magnetic pull on the armature core, bending moment due to weight of the armature and commutator, twisting stains due to transmission of torque, for a good and long run application of motor. | |
| 62. |
present. The order of residual magnetism is |
| A. | 2 to 3 per cent |
| B. | 10 to 15 per cent |
| C. | 20 to 25 per cent |
| D. | 50 to 75 per cent |
| Answer» A. 2 to 3 per cent | |
| Explanation: in a dc machine residual magnetism is present and it plays very important role in starting of any dc machine. it’s present because of the previous application on the same motor, it also provides some no-load voltage. | |
| 63. |
Sparking is discouraged in a DC motor. |
| A. | true |
| B. | false |
| Answer» B. false | |
| Explanation: sparking at brushes and commutator segments lead to damage of commutators, which is the main distinguishable component in a dc machine. so, it is advisable to fasten the commutation speed and avoid sparking. | |
| 64. |
construction of single phase induction motor? |
| A. | 3 |
| B. | 4 |
| C. | 5 |
| D. | 6 |
| Answer» C. 5 | |
| Explanation: there are 5 steps in the construction of the single phase induction motor. they are stator, stator windings, rotor, starting switches, electrolytic capacitor. | |
| 65. |
What is the lamination used for the stator? |
| A. | cast iron |
| B. | die cast aluminium alloy frame |
| C. | cast iron or die cast aluminium alloy frame |
| D. | cast iron and die cast aluminium alloy frame |
| Answer» C. cast iron or die cast aluminium alloy frame | |
| Explanation: the stator is made up of a block of laminations. the block of laminations are made up of cast iron or die cast aluminium alloy frame. | |
| 66. |
What type of coils are used for winding the single phase induction motor generally? |
| A. | rectangular coils |
| B. | square coils |
| C. | cruciform coils |
| D. | circular coils |
| Answer» D. circular coils | |
| Explanation: the slots house the starting and running windings. the single phase induction motors are generally wound with concentric coils. | |
| 67. |
How many kinds of single phase windings are present? |
| A. | 2 |
| B. | 3 |
| C. | 4 |
| D. | 5 |
| Answer» B. 3 | |
| Explanation: there are basically 3 kinds of single phase windings. they are concentric, progressive and skein. | |
| 68. |
How are the poles and pitches in the concentric windings? |
| A. | single pole, different pitches |
| B. | different pole, different pitches |
| C. | different pole, single pitch |
| D. | single pole, single pitch |
| Answer» A. single pole, different pitches | |
| Explanation: the concentric windings have a single pole for a common centre. they have different pitches for each individual coil. | |
| 69. |
What is the form of the progressive windings? |
| A. | double layer diamond coil windings |
| B. | single layer diamond coil windings |
| C. | multi layer diamond coil windings |
| D. | three layer diamond coil windings |
| Answer» B. single layer diamond coil windings | |
| Explanation: the progressive windings is one kind of the stator windings. they are in the form of the single layer diamond coil windings. | |
| 70. |
When is the skein winding made use of? |
| A. | when small amount of relatively small size wire is used |
| B. | when large amount of relatively small size wire is used |
| C. | when large amount of relatively large size wire is used |
| D. | when small amount of relatively large size wire is used |
| Answer» A. when small amount of relatively small size wire is used | |
| Explanation: skein winding is one of the 3 kinds of single phase windings used. it is used when small amount of relatively small size wire is used. | |
| 71. |
winding made use of? |
| A. | maximum horse power single phase induction motor |
| B. | fractional horse power single phase induction motor |
| C. | minimum horse power single phase induction motor |
| D. | zero horse power single phase induction motor |
| Answer» B. fractional horse power single phase induction motor | |
| Explanation: the skein winding is one of the 3 kinds of single phase induction motor. the skein winding is used when fractional horse power single phase induction motor is used. | |
| 72. |
Which winding is mostly used winding in the single phase induction motor? |
| A. | circular winding |
| B. | concentric winding |
| C. | progressive winding |
| D. | skein winding |
| Answer» B. concentric winding | |
| Explanation: the concentric winding is the most widely used winding. it is also the most flexible winding of the windings used in the single phase induction motor. | |
| 73. |
What is/are the advantages of the skein winding? |
| A. | low cost to wind |
| B. | low cost to insert |
| C. | permits some freedom of choice of distribution |
| D. | low cost to wind, low cost to insert, permits some freedom of choice of distribution |
| Answer» D. low cost to wind, low cost to insert, permits some freedom of choice of distribution | |
| Explanation: the skein winding is the low cost to wind and to insert. it also permits some freedom of choice of distribution. | |
| 74. |
What material is used in the tunnel of the rotor of the single phase induction motor? |
| A. | aluminium |
| B. | copper |
| C. | steel |
| D. | wood |
| Answer» A. aluminium | |
| Explanation: the rotor consists of a block of slotted laminations. the slots form a series of tunnels that are filled with aluminium in its molten state. | |
| 75. |
What type of operations are used in the starting switches? |
| A. | mechanical operation |
| B. | electrical operation |
| C. | centrifugal operation and mechanical operation |
| D. | centrifugal operation |
| Answer» C. centrifugal operation and mechanical operation | |
| Explanation: the starting switch is used to cut the auxillary winding when the motor attains 75% of the full load speed. the switches operate in both the centrifugal as well as mechanical operation. | |
| 76. |
The ac electrolytic capacitor is formed by winding two sheets of etched aluminium foil. |
| A. | true |
| B. | false |
| Answer» A. true | |
| Explanation: modern capacitor start motors employ ac electrolytic capacitors. the ac electrolytic capacitor is formed by winding two sheets of etched aluminium foil, separated by two layers of insulating paper, into a cylindrical shape. | |
| 77. |
The electrolytic capacitor and insulator unit is impregnated using ethylene glycol or a derivative. |
| A. | true |
| B. | false |
| Answer» A. true | |
| Explanation: the electrolytic capacitor and insulator unit is impregnated using the | |
| 78. |
What is the range of the power factor of electrolytic capacitors? |
| A. | 2-4 |
| B. | 4-6 |
| C. | 6-8 |
| D. | 7-9 |
| Answer» C. 6-8 | |
| Explanation: the minimum power factor of the electrolytic capacitor is 6. the maximum power factor of the electrolytic capacitor is 8. | |
| 79. |
Why starters are required in a DC motor? |
| A. | back emf of these motors is zero initially |
| B. | these motors are not self-starting |
| C. | these motors have high starting torque |
| D. | to restrict armature current as there is no back emf at starting |
| Answer» D. to restrict armature current as there is no back emf at starting | |
| Explanation: at the time of starting (n=0), the induced emf of a motor is zero such that current drawn by armature, from rated voltage supply would be ia= v/ra. since armature resistance is very low, armature current drawn is very high and will damage the machine. | |
| 80. |
For which motors DOL starter can be used? |
| A. | up to 5 h.p |
| B. | up to 10 h.p |
| C. | up to 15 h.p |
| D. | up to 20 h.p |
| Answer» A. up to 5 h.p | |
| Explanation: dol starters are limited to the small rating motors where distribution system (mains supply) can withstand high starting currents without excessive voltage dips. for a large rating motor, ranging from 5 hp to 25 hp, oil immersed dol starters are used | |
| 81. |
A three-point starter is used for |
| A. | shunt motors |
| B. | shunt as well as compound motors |
| C. | shunt, compound and series motors |
| D. | not for dc motors |
| Answer» B. shunt as well as compound motors | |
| Explanation: 3-point starters are used only for shunt and compound motors, they are not used for series motors. three-point starter is employed where motor field current can be varied in a narrow range and so does the motor speed. | |
| 82. |
The starting resistance of a DC shunt motor is generally |
| A. | low |
| B. | around 0.5 kΩ |
| C. | around 5 kΩ |
| D. | infinitely large |
| Answer» A. low | |
| Explanation: starting resistance of a dc shunt motor and dc compound motor is low. well, that’s the reason why we use starters in a dc motors, in order to limit the armature current flowing through the armature and to protect machine circuitry. | |
| 83. |
without starter? |
| A. | heavy sparking at brushes |
| B. | it’ll start smoothly |
| C. | will not start at all |
| D. | depends on load |
| Answer» A. heavy sparking at brushes | |
| Explanation: it would cause intolerably heavy sparking at the brushes which may destroy the commutator and brush-gear. sudden development of large torque will cause mechanical shock to the shaft, reducing its life. such heavy current cannot be generally permitted to be drawn from the source of supply. | |
| 84. |
Motor will start quickly when used without starter. |
| A. | true |
| B. | false |
| Answer» A. true | |
| Explanation: the only thing in favour of direct starting must be mentioned here. since the torque of the motor with direct start is much higher, the motor starts much more quickly. as a consequence, the joule input | |
| 85. |
In shunt and compound motor starting the shunt field should be made on with full starting resistance in |
| A. | series with field |
| B. | parallel with field |
| C. | series with armature |
| D. | parallel with armature |
| Answer» C. series with armature | |
| Explanation: in shunt and compound motors starting the shunt field should be switched on with full starting resistance in armature circuit. a short time delay in this position allows the field current to build up to the steady value of the inductive field transients. | |
| 86. |
motor. The maximum allowable current is 55 A and the minimum current is about 35 A. The armature resistance of the motor is 0.4 Ω. What will be the number of sections of starter resistance required? |
| A. | 5 |
| B. | 4 |
| C. | 6 |
| D. | 8 |
| Answer» C. 6 | |
| Explanation: i1=55 a, i2 =35 a | |
| 87. |
γ is given as 1.585. Resistance at maximum allowable current is given equal to 4 Ω, what is the 5th step resistance? |
| A. | 0.235 |
| B. | 0.370 |
| C. | 1.476 |
| D. | 2 |
| Answer» A. 0.235 | |
| Explanation: γ is given as 1.585. so γ-1 is equal to 0.631. r1 is provided and it is equal to 4 Ω. | |
| 88. |
Four-point starter is used when |
| A. | motor field current is varied in narrow range |
| B. | motor speed is varied in small range |
| C. | motor field current is varied over wide range |
| D. | can be used anywhere |
| Answer» C. motor field current is varied over wide range | |
| Explanation: three-point starter is employed where motor field current can be varied in a narrow range and so does the motor speed while four-point starter is used when motor field current can vary over a wide range and so does the motor speed. | |
| 89. |
starting in an induction motor as well as dc motor. |
| A. | induction motor has to control starting torque whereas in dc motor, it is done to avoid large current |
| B. | to limit starting current in both the machines |
| C. | to limit starting speed |
| D. | all of the mentioned |
| Answer» A. induction motor has to control starting torque whereas in dc motor, it is done to avoid large current | |
| Explanation: for both the machines the purpose is different. | |
| 90. |
A 100 hp, 250 V, 350 A shunt dc motor with an armature resistance of 0.05 ohms. To limit maximum starting current to twice the rated of its value, what will be the number of stages of starting resistances? |
| A. | 3 |
| B. | 2 |
| C. | 4 |
| D. | 5 |
| Answer» A. 3 | |
| Explanation: n = log(ra/rt)/log(imin/imax); rt = vt/imax = 250/700 = 0.357 ohms | |
| 91. |
is/are |
| A. | 3-point and 4-point starter |
| B. | 5-point starter |
| C. | 4-point starter |
| D. | 5-point and 3-point starter |
| Answer» A. 3-point and 4-point starter | |
| Explanation: both 3-point and 4-point starters can be used. | |
| 92. |
The direct-on-line starter is used to start a small dc motor because it limits initial current drawn by armature circuit. |
| A. | true |
| B. | false |
| Answer» B. false | |
| Explanation: the direct-on-line starter can not limit the speed by limiting the current. | |
| 93. |
over DOL starter due to |
| A. | lesser losses |
| B. | controlled direction |
| C. | least resistance offered |
| D. | all of the mentioned |
| Answer» A. lesser losses | |
| Explanation: a thyristor controlled method | |
| 94. |
full load efficiency of 85% has armature resistance of 0.25 ohms. Calculate the value of starting resistance in ohms for a current 1.5 times of the full load current. |
| A. | 2.788 |
| B. | 3.038 |
| C. | 2.688 |
| D. | 2.588 |
| Answer» A. 2.788 | |
| Explanation: full load current = 7460/(200*0.85) = 43.88 a | |
| 95. |
The effect of fringing increases as we |
| A. | increase air gap |
| B. | decrease air gap |
| C. | increase in flux density |
| D. | introduce more ferric core material |
| Answer» A. increase air gap | |
| Explanation: fringing is the effect in which the magnetic flux lines bulge out of the flux. and it is introduced at the air gaps. | |
| 96. |
is |
| A. | main field distortion |
| B. | shift in mna |
| C. | reduction in main field |
| D. | none of the mentioned |
| Answer» D. none of the mentioned | |
| Explanation: there is no such post effect of armature reaction. the armature reaction phenomena affect the working condition only. | |
| 97. |
If the students give a forward shift of 10° to the dc generator, then it |
| A. | reduces flux per pole |
| B. | improves flux per pole |
| C. | increases the flux density in core |
| D. | none of the mentioned |
| Answer» A. reduces flux per pole | |
| Explanation: a generator given a forward brush shift will get its flux density reduced at the pole as the flux will be in the opposite direction. | |
| 98. |
A dc machine is run at rated speed in forward direction and then in backward direction. It is observed that, speeds of the rotation are different, then it leads to the conclusion of |
| A. | incorrect brush placement |
| B. | incorrect pole and core alignment |
| C. | incorrect field supply |
| D. | all of the mentioned |
| Answer» A. incorrect brush placement | |
| Explanation: given at the same speed of operation, if the speeds are different then the brushes are placed in not aligned manner. | |
| 99. |
For a dc machine, its commutator has a diameter of 50 c rotating at 1000 rpm. For a brush width of 1 cm, the time commutation taken by the machine will be |
| A. | 0.382 ms |
| B. | 0.456 ms |
| C. | 0.573 ms |
| D. | 0.312 ms |
| Answer» A. 0.382 ms | |
| Explanation: time of commutation = brush width/peripheral velocity | |
| 100. |
An induction motor can be said analogous to |
| A. | transformer |
| B. | synchronous motor |
| C. | universal motor |
| D. | stepper motor |
| Answer» A. transformer | |
| Explanation: an induction motor has similar operation as a transformer. | |
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