McqMate
These multiple-choice questions (MCQs) are designed to enhance your knowledge and understanding in the following areas: Mechanical Engineering .
| 1. |
Which motor is preferred for Jaw crushers? |
| A. | wrim |
| B. | scim |
| C. | belt slip ring im |
| D. | dc shunt motor |
| Answer» C. belt slip ring im | |
| Explanation: belt slip ring im is preferred for the operation of jaw crushers. a high starting is provided by the belt ring im. slip rings are used to add external resistance. | |
| 2. |
A high starting torque is offered by the Belt Conveyors. |
| A. | true |
| B. | false |
| Answer» A. true | |
| Explanation: the belt conveyors can provide continuous-flow transportation. the amount of material transported on a conveyor system depends upon belt speed, belt width. | |
| 3. |
Which starting method is the worst method in Induction motor? |
| A. | direct online starting |
| B. | autotransformer starting |
| C. | reactance starting |
| D. | star-delta starting |
| Answer» C. reactance starting | |
| Explanation: the reactance starting method is the worst method in an induction motor. it uses the concept of the potential divider. the losses are very high in case of reactance starting method. | |
| 4. |
Explanation: Intermittent duty cycle has frequent on load and off-loads period. The on-load period and off-loads periods are too |
| A. | frequency changer |
| B. | frequency remover |
| C. | time period remover |
| D. | dc machine |
| Answer» A. frequency changer | |
| Explanation: the induction motor is also known as frequency changer. the stator frequency can be changed using the slip concept. the rotor frequency of im is slip×(stator frequency). | |
| 5. |
The capacity of the Crane is expressed in terms of tonnes. |
| A. | true |
| B. | false |
| Answer» A. true | |
| Explanation: cranes are used to carry or lift the heavy loads. the capacity of a crane is expressed in terms of tonnes. 1 tonne is equal to 1000 kg. | |
| 6. |
1800 second rated motors are used for |
| A. | heavy duty cranes |
| B. | light duty cranes |
| C. | medium duty cranes |
| D. | intermittent duty cranes |
| Answer» C. medium duty cranes | |
| Explanation: medium duty crane motor is used to develop high starting torque. they provide the output for a specified interval of time without exceeding a specified temperature. | |
| 7. |
Calculate the frequency of the rotor side of the IM if the value of slip is 0.34 and the supply frequency is 70 Hz. |
| A. | 23.8 hz |
| B. | 22.7 hz |
| C. | 24.5 hz |
| D. | 23.1 hz |
| Answer» A. 23.8 hz | |
| Explanation: the frequency of the rotor side is low as compared to supply frequency. the rotor frequency is slip×(supply frequency)=.34×70=23.8 hz. | |
| 8. |
frequency is 2 Hz. |
| A. | 50 hz |
| B. | 49 hz |
| C. | 45 hz |
| D. | 51 hz |
| Answer» A. 50 hz | |
| Explanation: the frequency of the stator side is high as compared to the rotor side. the stator frequency is (rotor frequency) ÷ slip = 2÷.04 = 50 hz. | |
| 9. |
values of the capacitor and inductor are 4 F and 4 H. |
| A. | .25 rad/sec |
| B. | .26 rad/sec |
| C. | .28 rad/sec |
| D. | .29 rad/sec |
| Answer» A. .25 rad/sec | |
| Explanation: during resonance condition xl=xc. the value of the resonant frequency is 1÷√lc=1÷√16=.25 rad/sec. the voltage across the capacitor and inductor becomes equal. | |
| 10. |
Calculate the value of the angular acceleration of the Heavy duty crane using the given data: J = 5 kg-m2, load torque = 30 N-m, motor torque = 80 N-m. |
| A. | 10 rad/s2 |
| B. | 20 rad/s2 |
| C. | 30 rad/s2 |
| D. | 50 rad/s2 |
| Answer» A. 10 rad/s2 | |
| Explanation: using the dynamic equation of motor j×(angular acceleration) = motor torque – load torque: 5×(angular acceleration) = 80-30=50, angular acceleration=10 rad/s2. | |
| 11. |
motor torque = 2 N-m. |
| A. | 45 rad/s2 |
| B. | the machine will fail to start |
| C. | 30 rad/s2 |
| D. | 51 rad/s2 |
| Answer» B. the machine will fail to start | |
| Explanation: using the dynamic equation of motor j×(angular acceleration) = motor torque – load torque: .541×(angular acceleration) = 2-10=-8, angular acceleration=-14.78 rad/s2. the load torque is greater than the motor torque. the motor will fail to start. | |
| 12. |
The direction of the single-phase IM can be reversed by using which one of the method? |
| A. | by interchanging the supply terminals |
| B. | by removing the capacitor |
| C. | by changing the direction of auxiliary winding current |
| D. | by removing the main winding |
| Answer» A. by interchanging the supply terminals | |
| Explanation: the direction of the single- phase im can be reversed by changing the direction of auxiliary winding or main winding current.the net torque developed in the im is imiasin(Φ). | |
| 13. |
Which motor is preferred for overhead traveling cranes? |
| A. | intermittent periodic motor |
| B. | continuous duty motor |
| C. | slow speed duty motor |
| D. | short time rated motor |
| Answer» A. intermittent periodic motor | |
| Explanation: intermittent periodic motor is preferred for overhead traveling cranes. | |
| 14. |
conditions where electric motors are being used? |
| A. | thermography |
| B. | geography |
| C. | seismography |
| D. | anthropology |
| Answer» A. thermography | |
| Explanation: thermography is used to detect heat patterns. it uses an infrared rays camera. this test is used to determine the conditions where electric motors are being used. | |
| 15. |
What is the range of size of the motor (kW) for efficiency nearly equals to 90%? |
| A. | 15-150 |
| B. | 0-2 |
| C. | 3-15 |
| D. | 150-250 |
| Answer» A. 15-150 | |
| Explanation: as the size of the motor increases its speed decreases. the range of size of the motor (kw) for efficiency nearly equals to 90 % is 15-150. the efficiency increases as the size increases. | |
| 16. |
Calculate the heat loss in the electric motor for time interval 0-5 sec using the following data: Ia=2 A, Ra=.8Ω. |
| A. | 16 j-sec |
| B. | 14 j-sec |
| C. | 12 j-sec |
| D. | 10 j-sec |
| Answer» A. 16 j-sec | |
| Explanation: the heat loss can be calculated using the joule law of heating effect. the | |
| 17. |
sec. |
| A. | .641 |
| B. | .334 |
| C. | .293 |
| D. | .417 |
| Answer» C. .293 | |
| Explanation: one british thermal unit is equal to .293 watt. 1 btu/h is the amount of energy requires to increase the temperature of one pound of water by 1℉. | |
| 18. |
Overloading can be prevented using |
| A. | over-current protection |
| B. | speed protection |
| C. | over frequency protection |
| D. | oversize protection |
| Answer» A. over-current protection | |
| Explanation: overloading can be prevented using over-current protection. it will detect the over-current and interrupt the supply. | |
| 19. |
Full form of IDMT is |
| A. | inverse definite minimum time |
| B. | inverter definite minimum time |
| C. | inverter definite maximum time |
| D. | insert definite minimum time |
| Answer» A. inverse definite minimum time | |
| Explanation: idmt stands for inverse definite minimum time over-current relay. it has both the characteristics of inverse over- current and definite time over-current relay. | |
| 20. |
The most common cause of motor failure is |
| A. | overloading |
| B. | low resistance |
| C. | contamination |
| D. | over-heating |
| Answer» B. low resistance | |
| Explanation: the most common cause of motor failure is low resistance. overheating, corrosion can damage the insulation of the windings which leads to low resistance. | |
| 21. |
Full form of THD is |
| A. | total harmonic diverter |
| B. | total harmonic digital |
| C. | total harmonic distortion |
| D. | total harmonic discrete |
| Answer» C. total harmonic distortion | |
| Explanation: thd stands for total harmonic distortion. it measures the harmonic distortion in the signal. according to ieee-1992, only 5% of thd is valid. | |
| 22. |
Calculate the compensator rating required for a sec(Φ)=1. |
| A. | 0.64 p.u |
| B. | 0.21 p.u |
| C. | 0.57 p.u |
| D. | 0 p.u |
| Answer» D. 0 p.u | |
| Explanation: the compensator rating can be calculated using the relation qp.u=√1- | |
| 23. |
Calculate the value of Voltage ripple factor if the Form factor is 1.1. |
| A. | 45.8 % |
| B. | 46.2 % |
| C. | 47.5 % |
| D. | 49.9 % |
| Answer» A. 45.8 % | |
| Explanation: voltage ripple factor measures the harmonics on the dc side of the converter. the value of voltage ripple factor is √f.f2-1=45.8 %. the form factor is vr.m.s÷vavg. | |
| 24. |
Calculate the value of THD if the |
| A. | 76 % |
| B. | 75 % |
| C. | 74 % |
| D. | 73 % |
| Answer» B. 75 % | |
| Explanation: thd measures the harmonics distortion in the signal. the value of thd is | |
| 25. |
Full form of VRF. |
| A. | voltage ripple factor |
| B. | voltage revert factor |
| C. | volume ripple factor |
| D. | volume revert factor |
| Answer» A. voltage ripple factor | |
| Explanation: voltage ripple factor measures the harmonics on the dc side of the converter. the value of voltage ripple factor is √f.f2-1. the form factor is vr.m.s÷vavg. | |
| 26. |
-Φ fully controlled rectifier is |
| A. | 5th |
| B. | 7th |
| C. | 3rd |
| D. | 2nd |
| Answer» B. 7th | |
| Explanation: only 6k±1 order harmonics are present in 3-Φ fully controlled rectifier. the second lowest order harmonic is 7th harmonic for k=1. third order harmonics are absent in | |
| 27. |
to protect them from dust and contamination. |
| A. | enclosures |
| B. | open environment |
| C. | moisturizing environment |
| D. | hot environment |
| Answer» A. enclosures | |
| Explanation: electric motors are placed in enclosures to protect them from dust and contamination. there are seven most common types of enclosures. for example- open drip proof. | |
| 28. |
voltage of a dc machine is a function of its magneto-motive force. |
| A. | non-linear |
| B. | linear |
| C. | constant |
| D. | inverse |
| Answer» A. non-linear | |
| Explanation: the flux and induced emf are non linear function of its mmf. | |
| 29. |
compound dc motor is shown below. What conclusions can be made? |
| A. | this is an unstable machine |
| B. | there is regenerative increment in the speed |
| C. | this is impractical to be used |
| D. | all of the mentioned |
| Answer» D. all of the mentioned | |
| Explanation: this is basically an unstable scenario which makes the machine unable to be used. | |
| 30. |
It is impossible to start a differential compounded dc motor. |
| A. | true |
| B. | false |
| Answer» A. true | |
| Explanation: it is so because of the unstable speed and torque characteristics. at starting, armature current and series field current are high. since the series flux subtracts from shunt flux, series flux actually reverses the magnetic polarity. this motor will typically be still or turn slowly in wrong direction while burning up because of excessive current. | |
| 31. |
compound motor and he has been asked to make it start. How will he try? |
| A. | by shorting series field at start |
| B. | to run as shunt motor at start |
| C. | by making rated current at start |
| D. | all of the mentioned |
| Answer» D. all of the mentioned | |
| Explanation: at starting, the flux should be rated so that there is no abnormal situation arise. | |
| 32. |
with compensating winding has a field current of 5 A to produce a voltage of 250 V at 1200 rpm. What will be the shunt field current of this machine at no load? |
| A. | 5 a |
| B. | 5.6 a |
| C. | 4 a |
| D. | 0 a |
| Answer» A. 5 a | |
| Explanation: at no load, armature current is zero, so the internal generated voltage is 250 | |
| 33. |
many types? |
| A. | three |
| B. | two |
| C. | four |
| D. | five |
| Answer» B. two | |
| Explanation: load torques can have two types. they are active and passive load torques. active load torques are able to run the motor under equilibrium conditions and their sign remains the same even if the motor rotation changes but passive load torques always opposes the motion by changing their sign with the change in rotation of the motor. | |
| 34. |
Rolling mills exhibit what type of load torque characteristics? |
| A. | constant torque characteristics |
| B. | linearly rising torque characteristics |
| C. | non-linearly rising torque characteristics |
| D. | non-linearly decreasing torque characteristics |
| Answer» D. non-linearly decreasing torque characteristics | |
| Explanation: rolling mills are an example of non-linearly decreasing torque characteristics because torque and speed exhibits inversely proportional relationships and power are constant. | |
| 35. |
Torque inversely varies with the speed in the windage load torque component. |
| A. | true |
| B. | false |
| Answer» B. false | |
| Explanation: torque varies with a square of speed in the windage load torque component whereas in coulomb torque component torque is constant. | |
| 36. |
What type of force handles for active torques? |
| A. | strong nuclear forces |
| B. | weak nuclear forces |
| C. | gravitational forces |
| D. | electrostatic forces |
| Answer» C. gravitational forces | |
| Explanation: gravitational forces are responsible for active torques. active torques due to gravitational forces can be obtained in the case of hoists, lifts or elevators and railway locomotives operating on gradients. | |
| 37. |
Passive torques always oppose the motion of the driven machine. |
| A. | true |
| B. | false |
| Answer» A. true | |
| Explanation: passive torques are due to friction or shear and deformation in elastic bodies. they always oppose the motion, restricting the motion of the machine. | |
| 38. |
Among the following which one exhibits linearly rising load torque characteristics? |
| A. | elevators |
| B. | rolling mills |
| C. | fan load |
| D. | separately excited dc generator connected to the resistive load |
| Answer» D. separately excited dc generator connected to the resistive load | |
| Explanation: separately excited dc generator connected to the resistive load is an example of linearly rising load torque characteristics as the torque increases linearly with an increase in speed. | |
| 39. |
What is the condition for the steady-state operation of the motor? |
| A. | load torque > motor torque |
| B. | load torque <<<< motor torque |
| C. | load torque = motor torque |
| D. | load torque < motor torque |
| Answer» C. load torque = motor torque | |
| Explanation: according to the dynamic equation of motor, load torque must be equal to motor torque so that motor should run at a uniform speed. if load torque is greater than motor torque, the motor will fail to start and if load torque is less than motor torque, the motor will run at a higher speed which can damage the shaft of the motor. | |
| 40. |
Choose the correct one. (* stands for multiplication, J represents the moment of inertia, w represents angular speed). |
| A. | j*d(w)/dt = load torque – motor torque |
| B. | j*d(w)/dt = load torque + motor torque |
| C. | j*d(w)/dt = motor torque – load torque |
| D. | j*d(w)/dt = load torque * motor torque |
| Answer» C. j*d(w)/dt = motor torque – load torque | |
| Explanation: j*d(w)/dt = motor torque – load torque is the dynamic equation of the motor. motor torque will try to aid the motion of the motor, but load torque will oppose the motion of motor that’s why it subtracts in the equation. | |
| 41. |
Which of the following is the best braking method? |
| A. | friction |
| B. | electromechanical action |
| C. | eddy-currents |
| D. | electric braking |
| Answer» D. electric braking | |
| Explanation: braking methods based on friction, electromechanical action, eddy- currents, etc. are independent of the motor but sometimes electric braking is better justified owing to its greater economy and absence of brake wear. | |
| 42. |
DC motor is still widely used in tractions due to its excellent braking properties. |
| A. | true |
| B. | false |
| Answer» B. false | |
| Explanation: dc motor is used in tractions because of its excellent braking characteristics and ability of smooth transition from the motor to the generator | |
| 43. |
Which of the following is not the method of electrical braking? |
| A. | plugging or counter-current |
| B. | dynamic or rheostatic |
| C. | regenerative |
| D. | eddy current |
| Answer» D. eddy current | |
| Explanation: eddy current is the electrical effect or response of the system, which is reflected mechanically at brakes to reduce the speed of the motor. thus, eddy current is not an electrical brake, it is mechanical one. | |
| 44. |
Which of the following is correct formula for braking torque in plugging? |
| A. | n (ka2/rb) |
| B. | n2 (ka2/rb) |
| C. | n-1 (ka2/rb) |
| D. | (ka2/rb) |
| Answer» A. n (ka2/rb) | |
| Explanation: braking torque is equal to braking power divided by speed of the motor. t= [(nka )2/rb]/n. as, braking power is equal to ea2/rb/n. by solving for the braking torque from the above equation, we get n (ka2/rb). | |
| 45. |
Electrical braking of any variety becomes less effective as |
| A. | speed increases |
| B. | speed decreases |
| C. | independent of speed |
| D. | depends on supply voltage |
| Answer» B. speed decreases | |
| Explanation: braking torque of the dc machine is given by n (ka2/rb). here, braking torque is directly proportional to the speed of the motor, so as the speed decreases the efficiency of electrical brakes which is dependent on braking torque decreases. | |
| 46. |
Plugging is applied in a motor, if we don’t make the switch OFF what will happen? |
| A. | motor will come to rest as a result of plugging |
| B. | motor will come to rest and will start rotating in another direction |
| C. | motor will burn |
| D. | nothing will happen |
| Answer» B. motor will come to rest and will start rotating in another direction | |
| Explanation: if the switch is kept on near to zero speed, motor will have braking torque acting in opposite direction greater than the electromechanical torque. thus, motor will come to rest and for the next instant motor will start rotating in opposite direction. | |
| 47. |
Plugging is used in |
| A. | small motors only |
| B. | small and medium powered |
| C. | only in large heavy machines |
| D. | everywhere |
| Answer» A. small motors only | |
| Explanation: plugging is used in small scale applications only. the large initial current and high mechanical stress restrict the application of plugging in large machines. so, in order to balance stress this method is used in small machines only. | |
| 48. |
Which of the following is dynamic braking? |
| A. | reversal of field connections |
| B. | reversal of armature connections |
| C. | addition of equal and opposite field |
| D. | removal of armature circuit from current machine circuit |
| Answer» D. removal of armature circuit from current machine circuit | |
| Explanation: reversal of the connections of armature is the method called plugging. in dynamic braking we remove the armature circuit and connect it to different resistor, with field circuit still connected to the external supply. | |
| 49. |
Braking time in the dynamic braking is the function of |
| A. | system inertia |
| B. | load torque |
| C. | motor rating |
| D. | all- system inertia, load torque and motor rating |
| Answer» D. all- system inertia, load torque and motor rating | |
| Explanation: in dynamic braking, when | |
| 50. |
In dynamic braking, when braking is applied system acts as |
| A. | freely running machine |
| B. | motor with slow speed |
| C. | generator |
| D. | motor with same speed in opposite direction |
| Answer» C. generator | |
| Explanation: the armature is disconnected from the supply and then a braking resistor rb is immediately connected across it. the motor acts as a generator, driven by the inertia and stored kinetic energy dissipating power in rb. this is a simple method of bringing a motor nearly to a standstill. | |
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