McqMate
| 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. | |
| 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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