TESTs MCQs PDF

Chose the correct answer: 1) The field current is directly proportional to ………… a) EA b) Vt c) the magnetomotive force d) all answers. Answer c 2) In field resistance control, the lower the field current in a shunt (or separately excited) dc motor……….. a) the f...

Chose the correct answer: 1) The field current is directly proportional to ………… a) EA b) Vt c) the magnetomotive force d) all answers. Answer c 2) In field resistance control, the lower the field current in a shunt (or separately excited) dc motor……….. a) the faster it turns. b) the lower it turns. c) the lower Vt it comes. d) answer b,c. Answer a 3) If in DC Shunt Motor the flux is reduced suddenly to zero, then a) the motor will come to stop. b) the motor will attain dangerously high speed. c) the speed of motor will be reduced. d) the armature current will reduce. Answer b 4) Eddy current loss will depends on a) Frequency. b) Flux density. c) Thickness. d) All of the above. Answer d 5) No-load speed of which of the following motor is highest? a) Differentially compound motor b) Cumulative compound motor c) Series Motor d) Shunt Motor Answer c 6) Which of the following rule is used to determine the direction of rotation of D.C motor? a) Columb’s Law b) Lenz’s Law c) Fleming’s Right-hand Rule 1  d) Fleming’s Left-hand Rule Answer d 7) Which part of the DC motor can sustain maximum temperature rise? a) Armature Winding b) Field winding c) Slip Ring d) Commutator Answer d 8) The ratio of starting torque to full load torque is least in a) Differential Compound Motor b) Shunt motor c) Series Motor d) Cumulative compound motor Answer a 9) The efficiency of the DC motor at maximum power is a) 90% b) 100% c) Around 80% d) Less than 50% Answer d 10) In which of the following applications DC series motor is used? a) Centrifugal Pump b) Motor Operation in DC and AC a) Water pump drive b) Starter for car Answer c 11) In the DC machine, the fractional pitch winding is used a) To reduce the Harmonic in generated EMF b) Improve Cooling c) Increase EMF d) To reduce the copper losses Answer a 12) A three-point starter is suitable for 2  a) Shunt Motor b) Series Motor c) Shunt & Compound Motor d) Shunt, Series, and compound motor Answer c 13) Nowadays DC motor is widely used in a) Electric Traction b) Air compressor c) Centrifugal Pump d) Machine shop Answer a 14) Which DC motor has the least percentage increase in Input current for the same percentage increase in torque? a) Separately excited motor b) Series motor c) Shunt motor d) Compound motor Answer b 15) By looking at which particular part of the motor we can Identify a “DC motor”? a) Shaft b) Field winding c) Commutator d) Armature winding Answer c 16) Counter EMF of the DC motor is a) Less than the applied voltage b) More than the applied voltage c) Equal to the applied voltage d) None of the above Answer a 17) If the back EMF of DC motor vanishes then a) The motor continues to run b) Motor will stop 3  c) Armature will burn d) The motor continues to run in slow speed Answer c 18) Why is the speed of DC shunt motor dependent on Back EMF? a) Because flux is proportional to the armature current b) Because armature drop is negligible c) Because Back EMF is equal to armature current d) Because flux is constant in DC shunt motor Answer d 19) What will happen when the field of a DC shunt motor gets opened while the motor is running? a) Continue to run at the same speed b) Speed of motor will be reduced c) The motor will attain dangerous high speed d) Armature current will be reduced Answer c 20) Which of the following DC motor have the tendency of load instability? a) Cumulative compound motor b) Shunt Motor c) Series motor d) Differentially compound motor Answer d 21) The application of Differentially compound motor is a) Frequent on-off cycle b) Low starting torque c) High starting torque d) Variable speed Answer b 22) Which DC motor is preferred for Elevator? a) Differentially compound motor b) Series motor c) Shunt Motor d) Cumulative compound motor Answer d 23) Which of the following application requires high starting torque? a) Air blower b) Elevator c) Locomotive d) Centrifugal Pump 4  Answer c 24) Which DC motor is preferred for constant speed? a) Compound motor b) Series motor c) Shunt motor d) Differential motor Answer c 25) The direction of rotation of D.C series motor can be changed by a) Interchanging field terminal b) Interchanging supply terminal c) Both a & b d) None of the above Answer a 26) What will happen when the D.C motor is connected to A.C supply then? a) D.C motor will run at rated speed b) D.C motor will burn c) D.C motor will run at slow speed d) Both 2 & 3. Answer d 27) Which D.C motor is preferred for machine tools? a) Series motor b) Cumulative compound motor c) Differentially compound motor d) Shunt motor Answer d 28) When the speed of D.C motor is increased a) Back E.M.F increases and current drawn decreases b) Back E.M.F decreases and current drawn increases c) Back E.M.F and current drawn both increases d) Back E.M.F and current drawn both decreases Answer a 29) The starting resistance of D.C motor is generally a) High b) Infinite c) Low d) None of the above Answer c 5 30) Which of the following will decrease when the supply voltage of a D.C motor is increased? a) Operating speed b) Starting Torque c) Full-Load Current d) No load current Answer c 31) Sparking of commutator at D.C motor result in a) Increase in power consumption b) Damage to commutator insulation c) Damage of commutator segments d) All of the above Answer d 32) Which DC motor is preferred for crane and hoist? a) Series motor b) Cumulative compound motor c) Shunt motor d) Differentially compound motor Answer a 33) What will happen if the 220 volts DC series motor is connected to 220 Volt Ac supply? a) The motor will not run b) The armature winding of the motor will burn c) The motor will run with less efficiency and more sparking d) Either 2 & 3 Answer d 34) Ta be the armature torque and Ia be the armature current then which of the following relation is valid for DC series motor before saturation? a) Ta ∝ Ia b) Ta ∝ Ia2 c) Ta ∝ 1/Ia d) Ta ∝ 1/Ia2 Answer b 35) The reason for providing field winding of DC motor with thick wire is a) To provide large flux b) To carries large load current c) To reduce eddy current d) To increase the flux Answer b 6 36) In D.C motor the percentage of residual magnetism is a) 2 – 3% b) 10 -20% c) 50% d) 80 – 90% Answer a 37) Why is the DC motor preferred over AC motor? a) High-speed operation b) Low-speed operation c) Rated speed operation d) Variable speed operation Answer d 38) Which of the following is not the function of pole shoe in DC motor? a) To reduce eddy current loss b) To reduce reluctance c) To spread out flux d) To support field coil Answer a 39) The condition for maximum power in D.C motor is a) Back emf (Eb) = Supply voltage (Vs) b) Eb = 1/2 Vs c) 1/2 Eb = Vs Answer b 40) DC machine windings are a) Short pitched b) Full pitched c) Any of the above d) None of the above Answer b 41) The function of the Dummy coil is to a) Reduce eddy current b) Reduce hysteresis loss c) Increase flux d) Mechanical Balancing Answer d 42) The flux density under leading pole tip in DC motor tends to a) Decrease b) Remain same c) Increase d) None of the above Answer c 7 43) The flux density under trailing pole tip in DC motor tends to a) Decrease b) Remain same c) Increase d) None of the above Answer a 44) When the speed of the DC motor is Increases its Armature current a) Increases b) Decreases c) Remain Constant d) Any of the above Answer b 45) The amount of back e.m.f. of a shunt motor will increase when a) When the Load is Increased b) When the Field is Weakened c) When the Field is strengthed d) None of the above Answer b 46) The shaft torque (Tsh) in a d.c motor is less than total armature torque (Ta) because of ……………. in the motor. a) Copper losses b) Field Losses c) Iron and Friction losses d) None of the above Answer c 47) The torque developed by a d.c motor is directly proportional to a) Flux per pole × Armature current b) Armature resistance × Applied voltage c) Armature Resistance × Armature current d) Square of armature resistance Answer a 48) Armature reaction in a DC motor is increased a) When the armature current decreases b) When the armature current Increases c) When the field current increases d) By Interpoles Answer b 8 49) In a d.c. motor, the brushes are shifted from the mechanical neutral plane in a direction opposite to the rotation to a) Decrease Speed b) Increase Speed c) Reduce Sparking d) Increase torque Answer c 50) In very large d.c. motors with severe heavy duty, armature reaction effects are corrected by a) Using interpoles Only b) Using compensating winding in addition to interpoles c) Shifting the brush position d) None of the above Answer b 51) In flux control method the speed of DC motor is obtained _____ a) Above Base speed b) Below base speed c) Both above and below base speed d) Remain constant Answer a 52) The method of speed control of DC Shunt motor is used for applications where a very wide range sensitive speed control is required is a) Ward-Leonard system b) Multiple Voltage Control c) Tapped field control d) Rheostatic Control Answer a 53) The compensating winding in the DC machine is a) Located in the armature slot for the compensation of armature reaction b) Located on commutating poles for Improving the commutation c) Located on pole shoes for avoiding the flashover at the commutator surface. d) Located on the pole shoe to avoid the sparking at the brushes Answer c 9 54) For an application that requires smooth and precise speed control over the wide range, the motor is preferred is a) Squirrel cage Induction Motor b) Synchronous Motor c) DC motor d) Wound Rotor Induction Motor Answer c 55) The most commonly used method of speed control of a D.C motor is by varying the a) The voltage applied to the motor b) Field Strength c) Armature circuit Resistance d) Effective number of conductors in series Answer b 56) The deciding factor in the selection of a d.c. motor for a particular application is its …………….. characteristic. a) Speed-Torque b) Speed -Armature current c) Torque -Armature current d) None of the above Answer a 57) The Brushes on DC machine are always placed on—————, in order to achieve sparkles commutation? a) MNA b) GNA c) Either GNA or MNA d) None of the above Answer a 58) If a DC series motor is made to run on AC supply. It will a) Have no effect on efficiency b) Spark in excess but will not affect the efficiency c) Spark excessively and have poor efficiency d) Have poor efficiency Answer c 59) The demand for a large increase in torque of a d.c. shunt motor is met by a) Large decrease in speed b) Large increase in speed c) Large increase in current d) Small increase in current 11  Answer c 60) ……………….. motors never use belt-connected loads. a) Series b) Shunt c) Cumulatively compounded d) Differentially compounded Answer a 61) A cumulatively compounded motor does not run at dangerous speed at light loads because of the presence of a) Compensating Winding b) Interpoles c) Shunt winding d) Series winding Answer c 62) ……………….. motor is most suitable for punch presses. a) Shunt b) Series c) Differentially compounded d) Cumulatively compounded Answer d 63) The friction and windage losses in a d.c. motor depends upon ……………….. a) Speed b) Flux c) Armature current d) Field and armature resistance Answer a 64) The greatest percentage of power loss in a d.c. motor is due to ……………… a) Windage loss b) Copper loss c) Core loss d) Friction loss Answer b 65) A motor takes a large current at starting because a) Shunt field is producing the weak field b) The armature resistance is high c) Back e.m.f. is low d) None of the above Answer c 11 66) Excessive sparking at the brushes may be caused due to a) Unequal Space b) Dirt in commutator c) Overload d) All of the above Answer d 67) Excessive motor vibration is caused by a) Too much brush tension b) Worn bearings c) Open armature coil d) Bent shaft Answer b 68) Two series motors are mechanically coupled. One machine is run as a motor and the other as a generator. The iron and friction losses of the machines will be identical when a) Their speeds are identical b) Their speeds and excitations are identical c) Their speeds are equal and back e.m.f.s are half the supply voltage. d) Their ratings and armature sizes are equal Answer b 69) In DC compound Motor the shunt field and series field winding is made up of _________ & ____________ respectively a) High Resistance, Low Resistance b) Low Resistance, High Resistance c) Equal Resistance d) None of the above Answer a 70) If the pole flux of a dc motor approaches zero, its speed will a) Approach zero b) Approach infinity c) Not change d) Approach a stable value between zero and infinity Answer b 71) All d.c. machines are characterized by a) Electric brushes b) Armature c) Commutator d) Magnetic poles Answer c 12 72) Armature voltage control is suitable if d.c. machine is driven at constant a) Torque b) Magnetic field c) Speed d) Current Answer a 73) In all electric machines, basic action taking place is a) Motor action b) Generator action c) Both (a) and (b) d) All of these Ans: (c) 74) In case of a motor Only motor action takes place b) Motor action precedes generator action c) Generator action precedes motor action d) Both take place precedes simultaneously a) Ans: (b) 75) All rotating machines are basically a) D.C. machines b) A.C. machines c) Electro-mechanical convertors d) Heat converters Ans: (c) 76) All machines have the structure such that a) Armature is rotating and field is fixed b) Field is rotating and armature is fixed c) Either (a) or (b) d) None of these Ans: (a) 77) All electrical machines have poles with a) A hetro-polar structure b) A horse shoe structure c) (a) above (b) 13  d) None of these Ans: (a) 78) Maximum number of brushes which can be used in a machine is a) 2 b) 4 c) 6 d) Number of poles in the machine or 2 Ans: (d) 79) oltage applied across the shunt motor has to a) Overcome the back emf b) Supply armature ohmic drop c) Supply fied ohmic drop d) Both (a) and (b) Ans: (d) 80) If back emf of d.c. motor is doubled while its speed is also doubled, then torque developed by the machine will a) Remain same b) Become four times c) Double d) Becomes half Ans: (a) 81) What is the standard direction of a motor? a) Clockwise b) Anti-clockwise c) None of these d) Either (a) or (b) Ans: (b) 82) Left hand rule is applicable to a) Motor b) Generator c) Transformer 14  d) Mercury are rectifier Ans: (a) 83) A dc series motor is running with a diverter connected across its field winding. If the diverter resistance is increased, then the speed of the motor a) Decreases/increase b) Increase c) Remains unchanged d) Becomes very high Ans: (a) 84) In a dc shunt motor, field excitation is kept at maximum value during starting to a) Increase acceleration time b) Reduce armature heating c) Prevent voltage dip in the supply mains d) Decrease starting torque Ans: (b) 85) Two dc series motors connected in series are driving the same mechanical load. If now the motors are connected in parallel, the speed becomes a) Slightly more than double b) Slightly less than double c) Slightly more than half d) Slightly less than half Ans: (a) 86) Which of the following dc motors has the least reduction in speed from no load to rated load? a) Shunt motor with commutating poles b) Series motor with commutating poles c) Series motor without commutating poles d) Compound motor without commutating poles Ans: (a) 15  87) Which of the following statements about a series motor is correct a) It can run easily without load. b) It has poor torque. c) It has an almost constant speed. d) Its field winding consists of small number of thick wires. Ans: (d) 88) The output power of any electrical motor is taken from a) The armature b) The conductors c) The coupling mounted on the shaft d) The poles Ans: (c) 89) Torque of a motor is a) Force on N-m acting on the rotor b) Product of tangential force on the rotor and its radius c) The electrical power in kW d) Power given to the load Ans: (b) 90) If the back emf of a dc motor suddenly vanishes a) The motor will run faster b) The motor will start hunting the efficiency of the motor will increase c) The motor will start hunting d) The motor will burn out Ans: (d) 91) The brush voltage drops in dc in / of the order of a) 2 V b) 10 V c) 20 V 16  d) 40 V Ans: (a 92) If the load current and flux of a dc motor are held constant and voltage applied across are held constant and voltage applied across its armature is increased by 10 per cent, its speed will a) Decrease by about 10 per cent b) Remain unchanged c) Increase by about 10 per cent d) Increase by 20 per cent Ans: (c) 93) The following method is best suited for finding the voltage regulation of an alternator a) Synchronous impedance method b) Politer triangle method c) MMF method d) None of these Ans: (b) 94) Power factor of an alternator driven by constant prime mover input can be changed by changing its a) Speed b) Load c) Field excitation d) Phase sequence Ans: (c) 95) Salient pole machines have a) Large number of poles b) Small number of poles c) Small diameters 17  d) Long cores Ans: (a) 96) The stator of modern alternators are wound for a) 60° phase groups b) 120° phase groups c) 180° phase groups d) 240° phase groups Ans: (a) 97) The armature reaction of an alternator influences a) Windage losses b) Operating speed c) Generated voltage per phase d) Waveform of generated voltage Ans: (c) 98) When an alternator is supplying unity power factor load, the armature reaction will produce a) Magnetization of the main field b) Demagnetization of the main field c) Distortion of the main field d) None of these Ans: (c) 99) Alternators are generally designed to generate a) Fixed frequencies b) Variable frequencies c) Fixed currents d) Fixed power factors Ans: (a) 18 100) The maximum current that can be supplied by an alternator depends on a) Number of poles b) Speed of the exciter c) Exciter current d) Strength of the magnetic field Ans: (d) 101) Unbalanced 3-phase stator currents in an alternator cause a) Heating of rotor b) Vibrations c) Double frequency currents in the rotor d) All of these Ans: (d) ******************************************************** 19 Generators 1- The field coils of D.C. generator are usually made of A. Mica  B. Copper C. Cast iron D. Carbon 2- A cumulatively compounded long shunt generator when operating as a motor would be A. Cumulatively compounded long shunt  B. Differentially compounded long shunt C. Cumulatively compounded short shunt D. Differentially compounded short shunt 3- If brushes of a D.C. generator are moved in order to bring these brushes in magnetic neutral axis, there will be A. Demagnetization only B. Cross magnetization as well as magnetization  C. Cross-magnetization as well as demagnetizing D. Cross magnetization only 4- Two D.C. shunt generators, each with armature resistance of 0.02 ohm and field resistance of 50 ohm run inparallel and supply a total current of 1000 amperes to the load circuit. If their e.m.fs. are 270 V and 265 V, their busbar voltage will be A. 270 V  B. 267.5 V 21 C. 265 V D. 257.4 V 5- Which of the following generators will be preferred if they are required to be run in parallel?  Shunt generators B. Series generators C. Compound generators D. None of the above 6- In a D.C. generator the critical resistance can be increased by A. Increasing its field resistance B. Decreasing its field resistance  C. Increasing its speed D. Decreasing its speed 7- In a D.C. machine, short-circuited field coil will result in A. odour of barning insulation B. unbalanced magnetic pull producing vibrations C. reduction of generated voltage for which excitation has to be increased to maintain the voltage  D. all above 8- Eddy currents are induced in the pole shoes of a D.C. machine due to A. Oscillating magnetic field B. Pulsating magnetic flux  C. Relative rotation between field and armature D. All above 21 9- Welding generator will have  Lap winding B. Wave winding C. Delta winding D. Duplex wave winding 10- The demagnetizing component of armature reaction in a D.C. generator  Reduces generator e.m.f. B. Increases armature speed C. Reduces interpoles flux density D. Results in sparking trouble 11- Equalizer rings are required in case armature is A. Wave wound  B. Lap wound C. Delta wound D. Duplex wound 12- According to Fleming's right-hand rule for finding the direction of induced e.m.f., when middle finger points in thedirection of induced e.m.f., forefinger will point in the direction of A. Motion of conductor  B. Lines of force C. Either of the above D. None of the above 22 13- Following energized winding of a D.C. machine should not be opened as it would produce high inductive voltagewhich may be dangerous to personnel and may cause its own insulation failure. A. Series field B. Compensating field C. Inter pole field  D. Shunt field 14- The essential condition for parallel operation of two D.C. generators is that they have A. same kW rating B. the same operation r.p.m.  C. the same drooping voltage characteristics D. same percentage regulation 15- A D.C. generator works on the principle of A. Lenz's law B. Ohm's law  C. Faraday's law of electromagnetic induction D. None of the above 16- In a D.C. generator in case the resistance of the field winding is increased, then output voltage will A. Increase  B. Decrease C. Remain unaffected D. Fluctuate heavily 23 17- In case of D.C. machines, mechanical losses are primary function of A. Current B. Voltage  C. Speed D. None of above 18- Which of the following generator will have negligible terminal voltage while running on no-load?  Series generator B. Shunt generator C. Compound generator D. Separately excited generator 19- To achieve sparkless commutation brushes of a D.C. generator are rockedm ahead so as to bring them  just ahead of magnetic neutral axis B. in magnetic neutral axis C. just behind the magnetic neutral axis 20- In over compounded generator, full load terminal voltage is A. Almost zero B. Less than no load terminal voltage  C. More than no load terminal voltage D. Equal to no-load terminal voltage 21- In a level compounded D.C. generator, full load terminal voltage is A. Negligibly low  B. Equal to no-load terminal voltage 24 C. More than no-load terminal voltage D. Less than no-load terminal voltage 22- In D.C. generators the pole shoes are fastened to the pole core by A. Rivets  B. Counter sunk screws C. Brazing D. Welding 23- Which generator would you prefer for feeding long D.C. transmission lines? A. Series generator B. Shunt generator  C. Over compound generator D. Flat compound generator 24- In D.C. generators, lap winding is used for A. High voltage, high current  B. Low voltage, high current C. High voltage, low current D. Low voltage, low current 25- Copper brushes in D.C. machine are used  Where low voltage and high currents are involved B. Where high voltage and small currents are involved C. In both of the above cases D. In none of the above cases 25 26- A shunt generator running at 1000 r.p.m. has generated e.m.f. as 200 V. If the speed increases to 1200 r.p.m., the generated e.m.f. will be nearly. A. 150 V B. 175 V  C. 240 V D. 290 V 27- For both lap and wave windings, there are as many commutator bars as the number of A. Slots B. Armature conductors  C. Winding elements D. Poles 28- Armature reaction of an unsaturated D.C. machine is  Cross-magnetizing B. Demagnetizing C. Magnetizing D. None of above 29- Two generators A and B have 6 poles each. Generator A has wave wound armature while generator B has lapwound armature. The ratio of the induced e.m.f. is generator A and B will be A. 2 : 3  B. 3 : 1 C. 3 : 2 D. 1 : 3 26 30- A shunt generator can self-excite  Only if the resistance of the field circuit is less than critical value B. Only if the resistance of the field circuit is greater than critical value C. Irrespective of the value of the resistance in the field circuit D. None of the above 31- A series generator can self-excite A. Only if the load current is zero  B. Only if the load current is not zero C. Irrespective of the value of load current D. None of the above 32- The armature of D.C. generator is laminated to A. reduce the bulk B. provide the bulk C. insulate the core  D. reduce eddy current loss 33- The material for commutator brushes is generally A. Mica B. Copper C. Cast iron  D. Carbon 34- Armature coil is short circuited by brushes when it lies  Along neutral axis B. Along field axis 27 C. In any of the above positions D. In none of the above positions 35- Satisfactory commutation of D.C. machines requires A. Brushes should be of proper grade and size B. Brushes should smoothly run in the holders C. Smooth, concentric commutator properly undercut  D. All of the above 36- A D.C. generator can be considered as A. Rectifier B. Prime mover  C. Rotating amplifier D. Power pump 37- In case of a flat compounded generator A. Voltage generated is less than the rated voltage B. Generated voltage is proportional to the load on the generator  C. Voltage remains constant irrespective of the load D. Speed varies in proportion to the load on the generator 38- The e.m.f. generated in a D.C. generator is directly proportional to A. Flux/pole B. Speed of armature C. Number of poles  D. All of the above 28 39- A 220 V D.C. generator is run at full speed without any excitation. The open circuit voltage will be A. Zero  B. About 2 V C. About 50 V D. 220 V 40- If B is the flux density, I the length of conductor and v the velocity of conductor, then induced e.m.f. is given by  Blv B. Blv² C. Bl²v D. Bl²v² 41- Fleming's right-hand rule regarding direction of induced e.m.f., correlates A. Magnetic flux, direction of current flow and resultant force  B. Magnetic flux, direction of motion and the direction of e.m.f. induced C. Magnetic field strength, induced voltage and current D. Magnetic flux, direction of force and direction of motion of conductor 42- Compensating windings are used in D.C. generators A. Mainly to reduce the eddy currents by providing local short-circuits B. To provide path for the circulation of cooling air  C. To neutralize the cross-magnetizing effect of the armature reaction D. None of the above 43- Flashing the field of D.C. generator means 29 A. Neutralizing residual magnetism  B. Creating residual magnetism by a D.C. source C. Making the magnetic losses of forces parallel D. Increasing flux density by adding extra turns of windings on poles 44- In a separately excited generator supplying rated load the armature reaction:  Is always present B. Is always absent C. May be sometimes present D. None of the above 45- If a D.C. generator fails to build up the probable cause could not be A. Imperfect brush contact  B. Field resistance less than the critical resistance C. No residual magnetism in the generator D. Faulty shunt connections tending to reduce the residual magnetism 46- D.C. generators are connected to the bus-bars or disconnected from them only under the floating condition A. To avoid sudden loading of the prime mover B. To avoid mechanical jerk to the shaft C. To avoid burning of switch contacts  D. All above 47- Two generators are running in parallel. One of the generators may run as motor for which of the followingreasons? A. The direction of that generator is reversed 31 B. The speed of that generator is increased C. The field of that generator is weakened  D. That generator takes large share of loads 48- The function of pole shoes in the case of D.C. machine is A. To reduce the reluctance of the magnetic path B. To spread out the flux to achieve uniform flux density C. To support the field coil  D. To discharge all the above functions 49- In case of a 4 pole D.C. generator provided with a two layer lap winding with sixteen coils, the pole pitch will be A. 4  B. 8 C. 16 D. 32 50- For the parallel operation of two or more D.C. compound generators, we should ensure that A. Voltage of the incoming generator should be same as that of bus bar B. Polarity of incoming generator should be same as that of bus bar C. All the series fields should be run in parallel by means of equalizer connection  D. Series fields of all generators should be either on positive side or negative side of the armature 51. For a D.C. generator when the number of poles and the number of armature conductors is fixed, then which winding will give the higher e.m.f.? A. Lap winding 31 B. Wave winding C. Either of (A) and (B) above D. Depends on other features of Answer: Option B 52. In any rotating machine that part which houses the conductors and in which e.m.f. induced is to be utilised is called A. rotor B. stator C. field D. armature Answer: Option D 53. In any rotating machine that part which houses the conductors and in which e.m.f. induced is to be utilized is called A. Rotor B. Stator C. Field D. Armature Answer: Option D 54. The resistance of armature winding depends on A. Length of conductor B. Cross-sectional area of the conductor C. Number of conductors D. All of the above Answer: Option D 32 55. A D.C. welding generator has A. Lap winding B. Wave moving C. Duplex winding D. Any of the above Answer: Option A 56. Iron losses in a D.C. machine are independent of variations in A. Speed B. Load C. Voltage D. Speed and voltage Answer: Option B 57. In a shunt generator the voltage build up is generally restricted by A. speed limitation B. armature heating C. insulation restrictions D. saturation of iron Answer: Option D 58. The series field of a short-shunt D.C. generator is excited by A. external current 33 B. armature current C. shunt current D. load current Answer: Option D 59. D.C. series generator is used A. To supply traction load B. To supply industrial load at constant voltage C. Voltage at the toad end of the feeder D. For none of the above purpose Answer: Option C 60. Which of the following D.C. generators will be in a position to build up without any residual magnetism in the poles ? A. Series generator B. Shunt generator C. Compound generator D. None of the above Answer: Option D 61. The e.m.f. generated by a shunt wound D.C. generator is E. Now while pole flux remains constant, if the speed of the generator is doubled, the e.m.f. generated will be A. E/2 B. 2E 34 C. Slightly less than E D. E Answer: Option B 62. In the case of lap winding resultant pitch is A. Multiplication of front and back pitches B. Division of front pitch by back pitch C. Sum of front and back pitches D. Difference of front and back pitches Answer: Option D 63. Armature reaction in a generator results in A. Demagnetization of leading pole tip and magnetization of trailing pole tip B. Demagnetization of trailing pole tip and magnetization of leading pole tip C. Demagnetizing the center of all poles D. Magnetizing the center of all poles Answer: Option A 64. Which of the following statement about D.C. generators is false? A. Compensating winding in a D.C. machine helps in commutation B. In a D. C. generator interpoles winding is connected in series with the armature winding C. Back pitch and front pitch are both odd and approximately equal to the pole pitch 35 D. Equalizing bus bars are used with parallel running of D.C. shunt generators Answer: Option D 65. Following D.C. generator will be in a position to build up without any residual magnetism in the poles A. Series generator B. Shunt generator C. Compound generator D. Self-excited generator Answer: Option D 66. The following constitute short-circuit in the armature winding. A. Insulation failure between two commutator bars B. Insulation failure between two turns of a coil C. Two of more turns of the same coil getting grounded D. All of the above Answer: Option D 67. In a D.C. generator the actual flux distribution depends upon A. size of air gap B. shape of the pole shoe C. clearance between tips of the adjacent pole shoes D. all of the above Answer: Option D 36 68. In D.C. generators, the cause of rapid brush wear may be A. Severe sparking B. Rough commutator surface C. Imperfect contact D. Any of the above Answer: Option D 69. In a D.C. generator the number of mechanical degrees and electrical degrees will be the same when A. r.p.m. is more than 300 B. r.p.m. is less than 300 C. Number of poles is 4 D. Number of poles is 2 Answer: Option D 70. In a commutator A. Copper is harder than mica B. Mica and copper are equally hard C. Mica is harder than copper D. None of the above Answer: Option C 71. In a D.C. generator A. External resistance = internal characteristic armature reaction 37 B. Internal characteristic = magnetization characteristic ohmic drop C. External characteristic = magnetization characteristic ohmic drop armature reaction D. Magnetization characteristic = external characteristic Answer: Option C 72. The terminal voltage of a series generator is 150 V when the load current is 5 A. If the load current is increased to 10 A, the terminal voltage will be A. 150 V B. Less than 150 V C. Greater than 150 V D. None of the above Answer: Option C 73. The e.m.f. induced in the armature of a shunt generator is 600 V. The armature resistance is 0.1 ohm. If the armature current is 200 A, the terminal voltage will be A. 640 V B. 620 V C. 600 V D. 580 V Answer: Option D 74. Shunt generators are most suited for stable parallel operation because of their A. rising voltage characteristics 38 B. identical voltage characteristics C. drooping voltage characteristics D. linear voltage characteristics Answer: Option C 75. Permeance is the reciprocal of A. Flux density B. Reluctance C. Ampere turns D. Resistance Answer: Option B 76. The critical resistance of the D.C. generator is the resistance of A. Field B. Brushes C. Armature D. Load Answer: Option A 77. In D.C. generators, the brushes on commutator remain in contact with conductors which A. Lie under South Pole B. Lie under North Pole C. Lie under interpolar region D. Are farthest from the poles Answer: Option C 39 78. In a D.C. generator the magnetic neutral axis coincides with the geometrical neutral axis, when A. There is no load on the generator B. The generator runs on full load C. The generator runs on overload D. The generator runs on designed speed Answer: Option A 79. With a D.C. generator which of the following regulation is preferred? A. 100% regulation B. Infinite regulation C. 50% regulation D. 1% regulation Answer: Option D 80. A separately excited generator as compared to a self-excited generator A. Is amenable to better voltage control B. Is more stable C. Has exciting current independent of load current D. Has all above features Answer: Option D 81. The purpose of providing dummy coils in a generator is A. To reduce eddy current losses 41 B. To enhance flux density C. To amplify voltage D. To provide mechanical balance for the rotor Answer: Option D 82. Brushes of D.C. machines are made of A. Carbon B. Soft copper C. Hard copper D. All of above Answer: Option A 83. Lap winding is composed of A. Any even number of conductors B. Any odd number of conductors C. That even number which is exact multiple of poles + 2 D. That even number which is exact multiple of poles Answer: Option A 84. Wave winding is composed of A. Any even number of conductors B. Any odd number of conductors C. That even number which is exact multiple of poles + 2 D. That even number which is exact multiple of poles 41 Answer: Option C 85. The commutator segments are connected to the armature conductors by means of A. Copper lugs B. Resistance wires C. Insulation pads D. Brazing Answer: Option A 86. Which of the following components of a D.C, generator plays vital role for providing direct current of a D.C. generator? A. Dummy coils B. Commutator C. Eye bolt D. Equalizer rings Answer: Option B 87. Open circuited armature coil of a D.C. machine is A. Identified by the scarring of the commutator segment to which open circuited coil is connected B. Indicated by a spark completely around the commutator C. Both (A) and (B) D. None of the above Answer: Option C 88. In lap winding, the number of brushes is always A. Double the number of poles 42 B. Same as the number of poles C. Half the number of poles D. Two Answer: Option B 89. Which of the following generating machine will offer constant voltage on all loads? A. Self-excited generator B. Separately excited generator C. Level compounded generator D. All of the above Answer: Option C 90. The number of brushes in a commutator depends on A. Speed of armature B. Type of winding C. Voltage D. Amount of current to be collected Answer: Option D 91. D.C. generator generally preferred for charging automobile batteries is A. Series generator B. Shunt generator C. Long shunt compound generator D. Any of the above Answer: Option C 92. To avoid formation of grooves in the commutator of a D.C. machine 43 A. The brushes of opposite polarity should track each other B. The brushes of same polarity should track each other C. Brush position has no effect on the commutator grooving D. All of the above Answer: Option A 93. The main factor which leads to unstable parallel operation of flat and over compounded generators is A. their rising voltage characteristics B. unequal number of turns in their series field windings C. unequal speed regulation of their primemovers D. unequal series field resistances Answer: Option A 94. The terminal voltage of a D.C. shunt generator drops on load because of all of the following reasons except A. Armature reaction B. Armature resistance drop C. Field weakening due to armature reaction and armature D. Commutation Answer: Option D 95. For a D.C. machines laboratory following type of D.C. supply will be suitable A. Rotary converter 44 B. Mercury are rectifier C. Induction motor D.C. generator set D. Synchronous motor D.C. generator set Answer: Option C 96. When two D.C. series generators are running in parallel, an equalizer bar is used A. To increase the speed and hence generated e.m.f. B. To increase the series flux C. So that two similar machines will pass approximately equal currents to the load D. To reduce the combined effect of armature reaction of both machines Answer: Option C 97. When two D.C. generators are running in parallel an equalizer bar is used A. To increase the series flux B. To increase the generated e.m.f. C. To reduce the combined effect of armature reaction of both the machines D. So that the two identical machines will pass approximately equal currents to the load Answer: Option D 98. The armature core of a D.C. generator is usually made of A. Silicon steel 45  B. Copper C. Nonferrous material D. Cast-iron Answer: Option A 99. In a D.C. generator the ripples in the direct e.m.f. generated are reduced by A. Using conductor of annealed copper B. Using commutator with large number of segments C. Using carbon brushes of superior quality D. Using equalizer rings Answer: Option C 100. Magnetic field in a D.C. generator is produced by A. Electromagnets B. Permanent magnets C. Both (A) and (B) D. None of the above Answer: Option A 101.Laminations of core are generally made of A. Case iron B. Carbon C. Silicon steel D. Stainless steel Answer: Option C 102.In D.C. generators on no-load, the air gap flux distribution in space is 46  A. Sinusoidal B. Triangular C. Pulsating D. Flat topped Answer: Option D 103.A sinusoidal voltage of 5 Hz is applied to the field of a shunt generator. The armature voltage wave A. will be zero B. will be of 5 Hz C. will be of 5 xiVHz D. will be of v Hz 5 Answer: Option B 104.The essential condition for parallel operation of two D.C. generators is that they have A. Same kW rating B. The same operation r.p.m. C. The same drooping voltage characteristics D. Same percentage regulation Answer: Option C 105.Which of the following could be lamina proximately the thickness of laminations of a D.C. machine? A. 0.005 mm B. 0.05 mm C. 0.5 m D. 5 m Answer: Option C 106.The rapid wear of brushes takes place due to A. Abrasion from dust B. Excessive spring pressure C. Rough commutator bars D. All of the above factors Answer: Option D 107.In case of D.C. machine winding, number of commutator segments is equal to A. Number of armature coils B. Number of armature coil sides C. Number of armature conductors 47  D. Number of armature turns Answer: Option A 108.The insulating material used between the commutator segments is normally A. Graphite B. Paper C. Mica D. Insulating varnish Answer: Option C 109.The bearings used to support the rotor shafts are generally A. Ball bearings B. Bush bearings C. Magnetic bearings D. Needle bearings Answer: Option A 110.If a self excited D.C. generator after being installed, fails to build up on its first trial run, the first thing to do is to A. reverse the field connections B. increase the field resistance C. increase the speed of primemover D. check armature insulation resistance Answer: Option A 111.If residual magnetism is present in a D.C. generator, the induced e.m.f. at zero speed will be A. Zero B. Small C. The same as rated voltage D. High Answer: Option A 112.In a D.C. generator in order to reduce sparking at brushes, the self-induced e.m.f. in the coil is neutralized by all of the following except A. Interpoles B. Dummy coils C. Compensating winding D. Shifting of axis of brushes Answer: Option B 48 113.The open circuit voltage of a compound generator is 250 V. At full load the terminal voltage A. Will be less than 250 V B. Will always be 250 V C. May be greater or less than 250 V D. None of the above Answer: Option D 114.While applying Fleming's right-hand rule to and the direction of induced e.m.f., the thumb points towards A. Direction of induced e.m.f. B. Direction of flux C. Direction of motion of the conductor if forefinger points in the direction of generated e.m.f. D. Direction of motion of conductor, if forefinger points along the lines of flux Answer: Option D 115.In a D.C. generator the critical resistance refers to the resistance of A. Brushes B. Field C. Armature D. Load Answer: Option B 116.As a result of armature reaction, the reduction in the total mutual air gap flux in a D.C. generator is approximately A. 40 percent B. 25 percent C. 10 percent D. 5 percent Answer: Option D 117.An exciter for a turbo generator is a A. Separately excited generator B. Shunt generator C. Series generator D. Compound generator Answer: Option B 118.In D.C. generators the polarity of the interpoles 49  A. Is the same as that of the main pole ahead B. Is the same as that of the immediately preceding pole C. Is opposite to that of the main pole ahead D. Is neutral as these poles do not play part in generating e.m.f Answer: Option A 119.Inter-pole flux should be sufficient to A. Neutralize the commutating self-induced e.m.f. B. Neutralize the armature reaction flux C. Neutralize both the armature reaction flux as well as commutating e.m.f. induced in the coil D. Perform none of the above functions Answer: Option C 120.The number of armature parallel paths in a two-pole D.C. generator having duplex lap winding is A. 2 B. 4 C. 6 D. 8 Answer: Option B 121.In D.C. generators, current to the external circuit from armature is given through A. Commutator B. Solid connection C. Slip rings D. None of above Answer: Option A 122.In a four-pole D.C. machine A. All the four poles are north poles B. Alternate poles are north and south C. All the four poles are south poles D. Two north poles follow two south poles Answer: Option B 51 123.The voltage drop for which of the following types of brush can be expected to be least? A. Graphite brushes B. Carbon brushes C. Metal graphite brushes D. None of the above Answer: Option C 124.In a D.C. machine stray loss is the sum of A. Total copper loss and mechanical loss B. Armature copper loss and iron loss C. Shunt field copper loss and mechanical loss D. Iron loss and mechanical loss Answer: Option D 125.Number of tapings for each equalizer ring is equal to A. Number of pole pairs B. Number of poles C. Number of parallel paths D. Number of commutator segments Answer: Option A ************************************************************* 51 Transformer Multiple Choice Questions 1) A transformer a. Steps up or down dc voltages b. Changes ac to dc c. Steps up or down ac voltages d. Changes dc to ac 2) The primary winding of a transformer has a 120 V ac supply. What is the value of secondary voltage if the turn ratio is 10? a. 120 V b. 12 V c. 12000 V d. 1200 V 3) Power transformers are designed to have maximum efficiency at a. No-load b. 60 % load c. 80 % load d. Full load 4) If the supply frequency (f) of a transformer decreases, the effect of frequency on the transformer's secondary output voltage? a. Remain the same b. Decreases c. Increases d. All of these 52  5) During the open circuit test of a transformer The secondary is supplied rated KVA. a. Primary is supplied with no-load current. b. Primary is supplied current at high voltage. c. Primary is supplied rated voltage. 6) Lamination of the transformer core is made up of a. Aluminum b. Iron c. Steel d. Silicon steel 7) A power system has 3 synchronous generators. The turbine-control characteristics equivalent to the generators are X 1 =60(60 - f), X2 = 120(70 - f), X3 = 140(80 - f) Where, f denotes the system frequency in Hz, and X 1, X2, X3 are the power outputs of the turbines in MW. Considering the generators and transmission network to be lossless, the system frequency for a load of 800 MW is a. 65 Hz b. 70 Hz c. 80 Hz d. 50 Hz 8) Transformer core are laminated in order to a. Reduce copper loss b. Minimize eddy current loss c. Reduce eddy current and hysteresis loss d. Reduce hysteresis loss 53  9) A transformer has a primary coil with 1600 loops and a secondary coil with 1200 loops. If the current in the primary coil is 6 Ampere, then what is the current in the secondary coil of a transformer. a. 78 Ampere b. 98 Ampere c. 68 Ampere d. 58 Ampere 10) The short circuit test in the transformer is performed on a. High voltage side b. light voltage side c. Both d. Either option a or b. 11) The secondary voltage is 440 Volt, and primary voltage is 220 Volt, then a comparison of the secondary coil and primary coil is a. 4/1 b. 5/2 c. 2/1 d. 7/5 12) Which test determines the efficiency of two identical transformers under load conditions? a. Low voltage test b. Full - load test c. Back-to-Back (Sumpner's Test) d. Open circuit test. 13) The transformer ratings are expressed in terms of 54 a. KW (Kilo-Watt) b. Volts c. KVAR (Kilo-Volt-Ampere-Reactive) d. KVA (Kilo-Volt-Ampere) 14) A transformer has 6 windings in its primary core and 3 in its secondary coil. If the primary voltage is 440 V, find the secondary voltage. a. 175 V b. 185 V c. 155 V d. 165 V 15) Oil is provided in an oil-filled transformer for a. Lubrication b. Cooling c. Insulation d. Both cooling and Insulation 16) A Buchholz relay can be installed on a. Oil cooled transformers b. Autotransformers c. Welding transformers d. Air-cooled transformers 17) The noise of the transformer primarily due to a. Mechanical vibration b. Cooling fan c. Magnetostriction in an iron core d. All of the above 18) If a transformer is connected to a direct current supply, what happened? 55 a. Damage the transformer b. No effect c. Operate with low frequency d. Operate with high frequency 19) Cruciform shape is used in the transformer core to a. Reduce core reluctance b. Reduce core loss c. Reduce copper winding d. All of the above 20) Which of the following does not change in an ordinary transformer? a. Current b. Frequency c. Voltage d. All of the above 21) The dielectric strength of transformer oil is expected to be a. 3 kV b. 33 kV c. 330 kV d. 100 kV 22) In the transformer, the function of a conservator is to a. Protect the transformer form damage when oil expands due to heating. b. It provides air for cooling the transformer. c. It provided colling oil to transformer when transformer needed. d. None of the above 23) The efficiency of a transformer is maximum when a. Eddy current loss = iron loss b. Hysteresis loss = copper loss. 56  c. Eddy current loss = hysteresis loss d. Iron loss = Copper loss. 24) Which winding in a transformer has a greater number of turns a. Constant voltage winding b. Low voltage winding c. Secondary winding d. High voltage winding 25) In a transformer, tappings are usually provided. a. High voltage side b. Low voltage side c. Primary side d. Both option a and b 26) The highest voltage for transmitting electrical power in India is a. 132 kV b. 400 kV c. 33 kV d. 66 kV 27) The purpose of a breather in a transformer to a. Filter the transformer oil b. Provide cold air in the transformer c. Absorb moisture of air during breathing d. None of the above 28) The necessary condition for the parallel operation of two single-phase transformers is that they must have the same. a. Turn Ratio b. Polarity c. KVA Rating 57  d. Both option a & b 29) The primary purpose of performing a short circuit test in a transformer is to measure its a. Core loss b. Iron loss c. Insulation loss d. Copper loss 30) The flashpoint of transformer oil must be greater than a. 100 degree b. 125 degree c. 140 degree d. 160 degree 31) Which of the given test determine the iron loss of the transformer? a. Short circuit test b. Back-to-back test c. Open circuit test d. Both option a & b 32) The frictional loss in a transformer is a. 10 % b. 50 % c. 0 % d. More than 50 % 33) Which type of winding is used in the three-phase shell type transformer a. Square type b. Circular type 58  c. Sandwich type d. Cylindrical type 34) Which of the given winding of the transformer has less cross-sectional area? a. Primary winding b. Low voltage winding c. High voltage winding d. Secondary winding 35) The secondary winding of which of the given transformer is always kept closed? a. Voltage transformer b. Current transformer c. Step-up transformer d. Power transformer 36) Which of the given losses changes with the load in the transformer? a. Copper loss b. Iron loss c. Core loss d. None of these 37) A transformer transforms a. Voltage b. Frequency c. Current and voltage d. Current 38) A transformer has negative voltage regulation, only if its load power factor is a. Leading 59  b. Unity c. Lagging d. None of these 39) In an autotransformer, the primary and secondary are....... Coupled. a. Magnetically coupled b. Electrically coupled c. Both magnetically and electrically coupled. d. None of these 40) Crushed rocks are provided in the substation to a. To avoid growing plants and weeds. b. To provide insulation c. To avoid fire accidents during leakage of transformer oil. d. All of the above 1- the full load copper loss of a transformer is 1600 w.at half load, the copper loss will be a-6400w b- 1600W C-800W d -400W 2-the size of a transformer core will depend on A-frequency. B-area of the core. C- flux density of the core material. D-(A) and (B) both. 3-a transformer cannot raise or lower the voltage of a DC. Supply because. A-there is no need to change the DC voltage. B-a DC circuit has more losses. 61 C-faraday's laws of electromagnetic induction are not valid since the rate of change of flux is zero. D- none of the above. 4- the purpose of providing an iron core in a transformer is to ? A-provide support to windings. B- reduce hysteresis loss. C- decrease the reluctance of the magnetic path. D- reduce eddy current losses -5- a good voltage regulation of a transformer means. A-output voltage fluctuation from no load to full load is least. B-output voltage fluctuation with power factor is least. C- difference between primary and secondary voltage is least. D- difference between primary and secondary voltage is maximum. 6-An ideal transformer will have maximum efficiency at a load such that ? A-copper loss =iron loss B- copper loss< iron loss C- copper loss >iron loss D –none of the above 7-which of the following loss in a transformer is zero even at full load? A-core loss. B-friction loss. C- eddy current loss. D-hysteresis loss. 61 – 8-part of the transformer which is most subject to damage from overheating is. A-iron core. B-copper winding. C-winding insulation. D-frame or case. 9-The no-load current by transformer is usually what per cent of the full-load current ? A-0.2 to 0.5 per cent B-2 to 5 per cent C-12 to 15 per cent D- 20 to 30 per cent. 10- two transformers are connected in parallel. these transformers do not have equal percentage impedance.this is likely to result in. A - Short circuiting of the second Aries. B -power factor of one of the transformers is leading while that of the other lagging. C - Transformer having higher copper losses will have negligible core losses. D-loading of the transformers not in proportion to their KVA ratings. 11.The no-load current drawn by transformer is usually what per cent of the full-load current? A.0.2 to 0.5 per cent 62 B.2 to 5 per cent C.12 to 15 per cent D.20 to 30 per cent Answer: Option B 12.The chemical used in breather for transformer should have the quality of A.ionizing air B.absorbing moisture C.cleansing the transformer oil D.cooling the transformer oil Answer: Option B 13.The function of conservator in a transformer is A.to project against internal fault B.to reduce copper as well as core losses C.to cool the transformer oil D.to take care of the expansion and contraction of transformer oil due to variation of temper-ature of sur-roundings Answer: Option D 14.The function of breather in a transformer is A.to provide oxygen inside the tank B.to cool the coils during reduced load C.to cool the transformer oil 63 D.to arrest flow of moisture when outside air enters the transformer Answer: Option D 15.transformers are connected in parallel. These transformers do not have equal percentage impedance. This is likely to result in A.short-circuiting of the secondaries B.power factor of one of the transformers is leading while that of the other lagging C.transformers having higher copper losses will have negligible core losses D.loading of the transformers not in proportion to their kVA ratings Answer: Option D 17. In a given transformer for given applied voltage, losses which remain constant irrespective of load changes are A. friction and windage losses B. copper losses C. hysteresis and eddy current losses D. none of the above Q18. The changes in volume of transformer cooling oil due to variation of atmospheric temperature during day and night is taken care of by which part of transformer A. conservator B. breather C. bushings D. buchholz relay 64 Q19. Star/ star transformers satisfactorily when A. load is unbalanced only B. load is balanced only C. on balanced and unbalanced loads D. none of the above Q20. Delta/star transformer works satisfactorily when A. load is balanced only B. load is unbalanced only C. on balanced and unbalanced loads D. none of the above Q21. In a transformer the resistance between its primary and secondary should be A. zero B. 10 ohms C. 1000 ohms D. infinity Q22. Gas is usually not liberated due to dissociation of transformer oil unless the oil temperature exceeds A. 50° C B. 80° C C. 100° C D. 150° C 65 Q23. For given applied voltage, with the increase in frequency of the applied voltage A. eddy current loss will decrease B. eddy current loss will increase C. eddy current loss will remain unchanged D. none of the above Q24. In an actual transformer the iron loss remains practically constant from noload to fullload because A. value of transformation ratio remains constant B. permeability of transformer core remains constant C. core flux remains practically constant D. primary voltage remains constant Q25. The chemical used in breather is A asbestos fiber B. silica sand C. sodium chloride D. silica gel Q26. In case of double cage induction motor, the inner cage has A. high inductance arid low resistance B. low inductance and high resistance C. low inductance and low resistance D. high inductance and high resistance 66 Q27. If the supply frequency to the transformer is increased, the iron loss will A not change B. decrease C. increase D. any of the above Q28. The term "cogging" is associated with A three phase transformer B. compound generators C. series motors D. induction motors Q29. Minimum voltage regulation occurs when the power factor of the load is A unity B. lagging C. leading D. zero Q30. The highest voltage for transmitting electrical power in India is A 33 kV B. 66 kV C. 132 kV D. 400 kV 67 31-The secondary winding of which of the following transformers is always kept closed? A.Step-up transformer B.Step-down transformer C.Potential transformer D.Current transformer Answer: Option D 32-If a transformer is switched on to a voltage more than the rated voltage A.its power factor will deteriorate B.its power factor will increase C.its power factor will remain unaffected D.its power factor will be zero Answer: Option A 33-The transformer oil should have _____ volatility and _____ viscosity. A.low,low B.high,high C.low,high D.high,low Answer: Option A 34-The degree of mechanical vibrations produced by the laminations of a transformer depends on A.tightness of clamping B.gauge of laminations 68 C.size of laminations D.all of the above Answer: Option D 35-The thickness of laminations used in a transformer is usually A.0.4 mm to 0.5 mm B.4 mm to 5 mm C.14 mm to 15 mm D.25 mm to 40 mm Answer: Option A 36-The core used in high frequency transformer is usually A.copper core B.cost iron core C.air core D.mild steel core Answer: Option C 37-A transformer transforms A.voltage B.current C.power D.frequency Answer: Option C 38-In a transformer the resistance between its primary and secondary is 69 A.zero B.1 ohm C.1000 ohms D.infinite Answer: Option D 39-A transformer can have regulation closer to zero A.on full-load B.on overload C.on leading power factor D.on zero power factor Answer: Option C 40. A transformer cannot work on the DC supply because __________________ a) There is no need to change the DC voltage b) A DC circuit has more losses c) Faraday’s laws of electromagnetic induction are not valid since the rate of change of flux is zero d) Cannot be determined Answer : c 41. An ideal transformer has infinite primary and secondary inductances. a) True b) False Answer :b 42. In a transformer the resistance between its primary and secondary is ______________ a) Zero b) Very small 71 c) Cannot be predicted d) Infinite Answer: d 43. Identify the correct statement relating to the ideal transformer. a) no losses and magnetic leakage b) interleaved primary and secondary windings c) a common core for its primary and secondary windings d) core of stainless steel and winding of pure copper metal Answer: a 44. An ideal transformer will have maximum efficiency at a load such that _____________ a) copper loss = iron loss b) copper loss < iron loss c) copper loss > iron loss d) cannot be determined Answer: a 45. Which of the following statement regarding an ideal single-phase transformer is incorrect? Transformer is having a turn ratio of 1: 2 and drawing a current of 10 A from 200 V AC supply is incorrect? a) It’s a step-up transformer b) Its secondary voltage is 400 V c) Its rating is 2 kVA d) Its secondary current is 20 A Answer: d 46. Ideal transformer core has permeability equal to _____ a) Zero b) Non-zero finite c) Negative d) Infinite Answer: d 47.Turns ratio of the transformer is directly proportional to ____________ a) Resistance ratio b) Currents ratio c) Voltage ratio 71 d) Not proportional to any terms Answer: c 48. Which of the following statement is correct regarding turns ratio? a) Current ratio and turns ratio are inverse of each other b) Current ratio is exactly same to the voltage ratio c) Currents ratio is exactly same to the turns ratio d) Voltage ratio and turns ratio are inverse of each other Answer: a 49. Which of the following is the expression for emf induced in primary with voltage applied to primary of an ideal transformer? a) e=V b) V= √2*e*cos ωt c) e= √2*V*cos ωt d) Cannot say Answer: c 50. Which of the following is the wrong expression? a) i1N1=i2N2 b) i1v1=i2v2 c) i1N2=i2N1 d) v2N1=v1N2 Answer: c 51. For transformer given, turns ratio is equal to a, what will be the impedance of primary with respect to secondary? a) a2 times the secondary impedance b) a times secondary impedance c) secondary impedance/a d) secondary impedance/a2 Answer: d 52. Power transformed in the ideal transformer with turns ratio a is _______ a) a2 times primary b) a times primary c) primary power/ a2 d) primary power Answer:d 72 53. For a transformer with primary turns 100, secondary turns 400, if 200 V is applied at primary we will get ___________ a) 80 V at secondary b) 800 V at secondary c) 1600 V at secondary d) 3200 V at secondary Answer: b 54. For a transformer with primary turns 400, secondary turns 100, if 20A current is flowing through primary, we will get ___________ a) 80A at secondary b) 5A at secondary c) 800A at secondary d) 40A at secondary Answer: a 55. What is the no-load current drawn by transformer? a) 0.2 to 0.5 per cent b) 2 to 5 per cent c) 12 to 15 per cent d) 20 to 30 per cent Answer: b 56. Purpose of no-load test on a transformer is ___________ a) Copper loss b) Magnetising current c) Magnetising current and loss d) Efficiency of the transformer Answer : c 57. No-load current in a transformer ________________ a) Lags behind the voltage by about 75° b) Leads the voltage by about 75° c) Lags behind the voltage by about 15° d) Leads the voltage by about 15° Answer : a 58. Which of the following statement is true for no-load current of the transformer? 73 a) has high magnitude and low power factor b) has high magnitude and high power factor c) has small magnitude and high power factor d) has small magnitude and low power factor Answer : d 59. In no-load test we keep secondary terminals __________ a) Shorted b) Shorted via fixed resistor c) Open d) Shorted via variable resistors Answer: c 60. For a linear B-H relationship, which option is correct? a) The exciting current is equal to core loss current b) The exciting current is equal to magnetizing current c) The exciting current is equal to de-magnetizing current d) The exciting current is equal to cross-magnetizing current Answer : b 61.Third harmonic current in transformer at no-load is ______________ a) 3% of exciting current b) 10% of exciting current c) 25% of exciting current d) 35% of exciting current Answer : d 62. Ii in no-load test is responsible for ______________ a) Production of flux b) Reactive power drawn from the supply c) Active power drawn from the supply d) No significance Answer : c 63. The parallel circuit model is drawn because _________________ a) Conductance Gi accounts for core-loss current b) Inductive susceptance Bm accounts for magnetizing current c) Gi for core – loss current and Bm for magnetizing current d) Cannot say Answer : c 74 64. In a Transformer , The primary flux is always _________ the secondary ( flux). a) Greater then b) Smaller then c) Equal d) Equal in both step up and Step down Transformer Answer: d 65. In a transformer the resistance between its primary and secondary is ______________ a) Zero b) Very small c) Cannot be predicted d) Infinite Answer: d 66. What will happen if the primary of a transformer is connected to D.C supply? a) Transformer will operate with low efficiency b) Transformer will operate with high efficiency c) No effect d) Transformer may start to smoke and burn Answer: d 67. What would happen if a power transformer designed for operation on 50 Hz (frequency) were connected to a 5 Hz (frequency) source of the same voltage? a) Current will be too much low b) Transformer may start to smoke c) Eddy Current and Hysteresis loss will be excessive d) No effect Answer: b 68. A Step Up transformer _____________. a) Step Up the level of Voltage b) Step down the level of current c) Step up level the power d) Step up the level of Frequency e) 1 and 2 only Answer: e 75 69) Transformer works on the principle of a) Self induction b) Mutual induction c) Faraday’s law of electromagnetic induction d) Self and mutual induction both Answer: (d) 70)The power factor in a transformer a) is always unit b) is always leading c) is always lagging d) depends on the power factor of load Answer: (d) 71) A transformer does not change the following a) Waveform b) Frequency c) Voltage d) Both frequency and waveform Answer: (d) 72) An ordinary transformer works on a) A.C b) D.C c) Both a.c. and d.c. d) Pulsating d.c. Answer: (a) 73) An ideal transformer is one which has a) A common core for its primary and secondary winding b) Core of stainless steel and winding of pure copper wire c) No losses and magnetic leakage 76 d) Interleaved primary and secondary windings Answer: (c) **************************************************************** 77 True or false: 1) In a separately excited generator, the field flux is derived from a separate power source independent of the generator itself. Answer T 2) A shunt dc motor is a motor whose field circuit gets its power directly across the armature terminals of the motor. Answer T 3) In the DC series motor, the flux is directly proportional to the armature current. Answer T 4) positive voltage regulation means a drooping characteristic, and a negative voltage regulation means a rising characteristic. Answer T 5) DC series motors are used where operating speed is variable and starting torque is high. Answer T 6) If a DC motor is connected across AC supply it may run. Answer T 7) The direction of rotation for a DC series motor may be reversed by interchanging only the field leads Answer T 8) In a cumulative compound motor, reversing the series field connections will change the direction of rotation Answer F 9) A two pole motor has its field windings series connected, and then paralleled with the armature. This indicates a series motor. 78  Answer: False, parallel with the armature means a shunt motor 10) A series DC motor may be connected as a shunt motor, and a shunt motor may be connected as a series motor, if necessary Answer F 11) Differential connected motors have lower starting torque than cumulative connected compound motors. Answer T 12) The series motor has better speed regulation than a cumulative compound motor. Answer F 13) Less current through the field of a shunt motor will cause a speed increase. Answer T 14) A differential motor started under heavy load may start up in a direction opposite to that desired. Answer T 15) Series motor cannot be started on no load Answer T 16) Differential compound motors has positive speed regulation Answer F 17) Field weakening control method is used for the above rated speed Answer T 18) Armature resistance control method is used for the below rated speed Answer T 19) Field weakening method is also called as constant power control Answer T 79 20) Diverters are used only in series motors Answer T 21) Friction and windage losses represent the rotating losses of machine Answer T 22) Armature copper loss are constant losses Answer F 23) Maximum efficiency will occur, when copper loss is equal to iron loss Answer T 24) If field current is decreased in shunt dc motor, the speed of the motor decreases. Answer F 25) In Ward-Leonard system, the lower limit of the speed imposed by Residual magnetism of the generator. Answer T 26) Ward-Leonard control is basically an Armature voltage control method. Answer T 27) For very sensitive and wide speed control, the preferable control method is Ward-Leonard control. Answer T 28) Eddy current loss will depends on flux desity , thickness and Frequency Answer T 29) Thin laminations are used in a machine in order to reduce Copper losses Answer F 81 30) In a dc machine 4 pole lap winding is used. The number of parallel paths are 8 Answer F 31) Dynamic braking is very effective for separately excited motors. Answer T 32) Counter emf of a DC motor helps in energy conversion. Answer T 33) Rotating part of DC motor is known as Armature. Answer T 34) Eb/Vt ratio of a DC motor is an indication of efficiency. Answer T 35) In DC shunt motor if load is increased, the speed reduce slightly. Answer T 36) Direction of rotation of DC motor is reversed by Adding resistance to field circuit. Answer F 37) With the increase in speed of a DC motor back emf increases but line current falls. Answer T 38) If the back emf in DC motor vanishes suddenly the motor will burn. Answer T 39) The output power of any electrical motor is taken from the Field. Answer F 81 40) The speed of a DC motor is directly proportional to back emf and inversely proportional flux 41) The direction of rotation of conductor of a DC motor can be determined by Ampere law. Answer F 42) If the temperature of armature resistance is increased, the speed of a series motor would decrease. Answer T 43) A small value of leakage reactance will increase the maximum power output of motor. Answer T 44) Field winding of a DC series motor is usually provided with thick wire as it carries large load current. Answer T 45) In case of DC Shunt Motor electromagnetic torque developed is Proportional to the armature current. Answer T 46) For a given torque, reducing diverter resistance of a DC series motor causes increase in speed demanding more armature current. Answer T 47) Field divertor method of speed control of a DC series motor gives speed above rated one due to reduction of Field current. Answer T 48) The ratio of starting torque to full load torque is minimum in Cumulative compound motor. 82  Answer ( False ) : Differential compound motor. Armature voltage. 1. Interpoles connected in series with the armature so that the strength of the interpole matches the strength of the armature field. True 2. The series generator is a practical machine. False 3. Field rheostat is used in the shunt field circuit to control generator voltage True 4. Line drop can be compensated for by using a generator that is over- compound. True 5. An increase in mechanical load on the shaft of a motor causes motor speed to 0. False 6. At standstill, the counter emf of a motor is 0. True 7. Field poles provide the necessary magnetic field. True 8. The two effects of armature reaction on the operation of a generator are weakening of the main field, Exvessive brush sparking. False 9. A practical method of operating below base speed involves controlling the voltage applied to the armature. True 10. The two basic types of armature windings are Lap and Wave. True 11. Whenever a conductor carries current in the presence of a magnetic field, a force will act on the conductor at right angles to the field. False 12. Torque is directly proportional to the product of Flux and Speed. False 13. DC machines use sleeve or ball bearings bearings. True 14. The condition under which the series field adds to the shunt field is called cumulative compound. True 83 15. Differential compound field windings are made of a low number of turns of large gauge wire. False 16. The two ways in which field coils are connected in self-excited generators are series and shunt (parallel). True 17. A generator characteristic curve shows the effect of connected load on terminal voltage. True 18. Suitable brush pressure is usually between sleeve or ball bearings psi. False 19. The emf induced in armature conductors as they cut the magnetic field is called counter emf. True 20. A locked rotor usually means that the motor will bearing friction and windage. False 21. Efficiency is the ratio of power out to power in. True 22. Armature reaction is a distortion of the main field flux caused by current flowing in armature conductors.. True 23. The most common solution for minimizing the effects of armature reaction involves the use of a rheostat in the field circuit. (field rheostat) False 24. A reduction in counter emf causes armature current to bearing friction and windage. False 25. The value of field resistance that will limit field current to a value insufficient for voltage build-up is called critical field resistance. True 26. The starting current limited to a safe value in large DC motors by laminating the core materials. False 27. T o operate a motor below base speed, it is necessary to decrease armature current. True 28. With the increase in speed of a DC motor both back emf as well as line current increase. False 84 29. The build-up of voltage in a self-excited generator requires residual magnetism. True 30. The rotating part of a DC generator is called the no-load terminal voltage. False 31. At start-up, the current drawn by a DC motor is limited only by armature resistance. True 32. Voltage build-up occurs until the generator reaches its no-load terminal voltage. True 33. In a self-excited generator, voltage build-up requires the presence of effective voltage. False 34. Interpole coils consist of a small number of turns of large gauge wire. True 35. Direction of rotation of motor is determined by Fleming’s left-hand rule. True 36. The types of power loss in a generator are Lap and Wave. False 37. Torque is a twisting or turning force that is capable of producing rotation about an axis. True 38. The persistence of high starting current may damage the motor. True 39. Rotational losses include Sleeve or ball bearings. False 40. The counter emf of a motor is proportional to flux and speed. True 41. In the case Supply voltage = 2 x Ea we will get maximum power. True 42. Gross power is mentioned on a name plate of a motor. False 43. An electric motor is having constant output power. So, motor will have a torque speed characteristic rectangular hyperbola. True 44. Starting torque will decrease if supply voltage is increased. False 45. Sometimes motor has to be de-rated. True 46. Sparking is discouraged in a DC motor. False 85  47. Armature reaction in a D.C. Generator tends to cause sparking at the brushes and reduction of generated voltage True 48. The rising voltage characteristic is used to offset the decrease in terminal voltage associated with shunt field excitation in a compound generator. True 49. The current drawn by the armature of DC motor is directly proportional to the voltage across the terminals False 50. Commutator will surely tell that given motor is DC motor and not an AC type. True Transformers - True or False:- 1-A transformer responds only to dc. False 2-in the transformer If the voltage is stepped up, the current is stepped down, and vice versa. True 3- A step-down transformer has a turns ratio less than 1. True 4- The primary is the winding connected to the source, and the secondary is the winding connected to the load. True 5- A transformer consists of two or more cores that are electrically coupled on a common core. False 86 6- A reflective load is a load as it appears to the source in the primary of a transformer. True 7- A step-up transformer steps up power. False 8- A transformer changes ac to dc. False 9- A center tap (CT) is a connection at the midpoint of the secondary winding of a transformer. True 10- An autotransformer is one in which the primary and secondary are automatically coupled. False 11- If voltage is stepped-up to the secondary, current will be stepped-down by the same amount. True 12- In the given circuit, the secondary voltage is in phase with the primary voltage. True 13- The reflected load value that the source sees will always be larger than the load value itself. 87 False 14- An open secondary winding of a transformer will cause the primary current to be nearly zero. True 15- The secondary voltages produced by a center-tapped transformer are equal in amplitude but 180° out of phase with respect to the center tap. True 16- A transformer that has 700 turns in the primary and 35 turns in the secondary has a turns ratio of 20:1. True 17- Regardless of whether the transformer is step-up or step-down, power in the primary equals the power in the secondary if there is 100% efficiency. True 18- In the given circuit, Maximum power is being transferred to the load. False 19- For mutual inductance to occur, the two coils must be at right angles to each other. False 88 20- If the current drawn in the secondary of a transformer increases, the power in the primary should remain the same. False 21-An idea transformer has infinite values of primary and secondary inductances. The statement is False 22-A transformer is an electrical device that works on the principle of self-induction. False 23- Ohm Meter is used to test normal resistance of transformer winding. True 24-the efficiency of transformer is independent of power factor. False 25- The tapping in the transformer are always provided in the low voltage side False 89