DC Machines Construction Questions and Answers

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DC Machines Construction Questions and Answers by Manish 4-5 minutes

This set of DC Machines Multiple Choice Questions & Answers (MCQs) focuses on “Construction”. 1. Where is field winding mounted in a DC machine? a) Stator b) Rotor c) Absent d) Anywhere on stator or rotor View Answer Answer: a Explanation: The field winding (concentrated type) is mounted on salient-poles on the stator and the armature winding (distributed type) is wound in slots on a cylindrical rotor. In AC machines field winding is mounted on rotor. 2. What are the materials used for brushes in dc machines? a) Iron b) Carbon c) Aluminum d) Steel

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View Answer Answer: b Explanation: On some extent carbon brush can act as a self-lubricating brush. On moment, polishes the commutator segments. Damage to the commutators is less when copper brushes are used on occurrence of sparkover. 3. Function of yoke is to provide the return path for magnetic flux. a) True b) false View Answer Answer: a Explanation: The function of yoke is that it protects the entire machine from dust and dirt. It also provides mechanical for the magnetic poles. It acts as the return path for the magnetic flux. 4. The angle (electrical) made by brushes with axes of ading filed poles is ______________ a) 450 b) 1800 c) 900 d) 300 View Answer Answer: c Explanation: Brushes in a DC machine are normally placed electrically in the interpolar regions and therefore make an angle of 900 electrical with the axes of ading filed poles.

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5. In a DC machine, rectification process is carried out in order to get unidirectional output (DC). This rectification process is carried out by ______________ a) Half wave rectifier b) Full wave rectifier c) Mechanical rectification d) Centre tapped rectifier View Answer Answer: c Explanation: In a DC machine electronic rectification is not used. Instead they use mechanical rectification with the help of commutator-brush assembly. 6.Which of the following part is used in construction of DC machine but not in AC machine? a) Armature Winding b) Field winding c) Commutator d) Shaft View Answer Answer: c Explanation: Commutator is used in mechanical rectification process, to convert induced AC to output DC. In AC machine, we don’t need rectification process. 7. In a DC machine fractional pitch winding is used to _______________ a) To improve cooling b) To reduce sparking c) To reduce copper losses 3 of 5

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d) To increase generated EMF View Answer Answer: b Explanation: Due to poor performance of brush, poor undercutting of commutator, incorrect spring pressure sparking at brush faces happen. To overcome this sparking fractional pitch winding is used. 8. In normal dc machines operating at full-load conditions, the most powerful electromagnet is _________ a) Field winding b) Interpole Winding c) Interpole and compensating winding together d) Armature winding View Answer Answer: a Explanation: Electromagnet is more powerful when its MMF is high. At full-load condition, field winding contains maximum ampere turns, hence it is most powerful electromagnet in a DC machine. 9. If a DC motor is connected to AC supply what will happen then? a) Not run b) Burn c) Run at normal speed d) Run at extremely low speed View Answer Answer: b Explanation: If a DC motor is connected to AC supply, an 4 of 5

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alternating current through the brushes and commutator to the armature winding, while it es through the commutator it is converted into DC so that the group of conductors under successive field poles carry current in same direction. So, the flux per pole will remain constant and not vary. There will be production of heat due to flow of eddy current in field winding and the motor will be burned. 10. The armature of DC motor is laminated to ____________ a) To reduce mass b) To reduce hysteresis loss c) To reduce eddy current loss d) To reduce inductance View Answer Answer: c Explanation: The armature is built up in a cylindrical or drum shape high grade silicon steel in form of lamination. By using laminations, the circular path of eddy currents is terminated. Hence heating and ultimately damage to the armature can be reduced by lamination. Sanfoundry Global Education & Learning Series – DC Machines. To practice all areas of DC Machines, here is complete set of 1000+ Multiple Choice Questions and Answers.

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DC Machines Applications Questions and Answers


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DC Machines Applications Questions and Answers by Manish 7-8 minutes

This set of DC Machines Multiple Choice Questions & Answers (MCQs) focuses on “Applications of DC Machines”. 1. Why are the DC motors preferred for traction applications? a) Torque and speed are inversely proportional to armature current b) Torque is proportional to armature current c) Torque is proportional to square root of armature current d) The speed is inversely proportional to the torque and the torque is proportional to square of armature current View Answer Answer: d Explanation: DC motors are used for traction as, according to the characteristics of DC motors speed is inversely proportional to torque and square of armature current as well, if linear magnetization is concerned. Thus, DC motors are perfectly suitable for traction.

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2. Which of the following load application normally needs starting torque more than the rated torque? a) Blowers b) Conveyors c) Air compressors d) Centrifugal pumps View Answer Answer: b Explanation: Conveyors need high starting torque initially, and constant torque later. Thus, DC series motor is used in conveyors as it provides very high starting torque, which is practically 5 times the rated torque. 3. Which of the following motors can be used to drive the rotary compressor? a) DC shunt motor b) DC series motor c) Universal motor d) Synchronous motor View Answer Answer: d Explanation: Rotary compressor generally demand constant speed operation throughout the load. Sometimes, DC machines are not able to produce constant speed throughout the process hence, synchronous machine is used. 4. Which DC motor is used with flywheel for intermittent light and heavy loads? a) Series motor 2 of 8

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b) Shunt motor c) Cumulatively compounded motor d) Differentially compounded motor View Answer Answer: c Explanation: Cumulative compound DC motor is used with flywheel carrying peaks and so to smooth out the load on the motor as well as to reduce peaks on power system. Without flywheel the motor construction will be much larger. 5. Separately excited DC generators are still used in _________________ a) Thermal power plants b) Ward Leonard speed control system c) Hydro power plant d) In all fields View Answer Answer: b Explanation: Separately excited DC generators are still used in wide output voltage control like in Ward Leonard speed control. In all power plants today, generally AC generators are used due to low cost and less maintenance required. 6. In world today, around 25% of the motors are manufactured are DC motors. a) True b) False View Answer Answer: a 3 of 8

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Explanation: For a dc machine, of course, the main attraction lies in its flexibility, versatility and ease of control. This explains why in spite of its rather heavy initial investment it still retains its charm in strong competitive industrial applications. 7. Maximum torque in a DC machine is limited by ______________ a) Commutation b) Heating c) Losses other than heating d) Stability View Answer Answer: a Explanation: While for all other motors maximum torque is restricted to certain value as various losses in other motors lead to heating of the core materials. In DC machines for maximum torque commutation time will obviously decrease and beyond some point commutation process can’t be fastened. 8. Which of the following motor can replace DC series motor? a) DC shunt motor b) Cumulative compound motor c) Wound-rotor induction motor d) Synchronous motor View Answer Answer: c Explanation: DE series motor’s closest rival is the wound4 of 8

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rotor induction motor with a rotor resistance control. But ultimately the availability and economics of a dc power is the deciding factor rather than the motor characteristic. 9. Which motor has almost replaced DC shunt motor from its applications? a) Wound-rotor induction motor b) Differential compound motor c) Air motor d) Squirrel caged induction motor View Answer Answer: d Explanation: Owing to the relative simplicity, cheapness and ruggedness of the squirrel cage induction motor, the shunt motor is less preferred for constant-speed drives except at low speeds. At high or medium speed applications we use induction motor, mostly squirrel caged. 10. DC shunt motor is still used instead of synchronous motor in _____________ a) High speed applications b) Low speed applications c) Medium and high-speed applications d) Everywhere View Answer Answer: b Explanation: At low speeds, DC shunt motors are comparable with synchronous motors. The outstanding feature of a DC shunt motor however is its superb wide range flexible speed control above and below the base 5 of 8

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speed using solid-state controlled rectifiers. 11. Which type motors are preferred for lathes? a) DC shunt motors b) Squirrel Cage induction motor c) Synchronous motor d) DC Shunt Motors or Squirrel cage induction motor View Answer Answer: d Explanation: Lathes machines requires uniform torque which is provided with squirrel cage induction or DC shunt motors. Hence, they are preferred for lathes. DC shunt motor and induction motor of squirrel cage type follow same shunt characteristics. 12. When an electric train is moving down a hill, the DC motor will operate as ____________ a) DC series motor b) DC series generator c) DC shunt motor d) DC shunt generator View Answer Answer: b Explanation: Normally in electric traction purposes DC series motors are employed. At above condition the back emf is greater than supply voltage hence, it will operate as series generator which will provide energy back to the supply. 13. Nowadays, DC motor is widely used only in ____________ 6 of 8

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a) Pumping sets b) Air compressors c) Electric traction d) Machine shops View Answer Answer: c Explanation: DC machine is used in tractions, because its characteristics suit perfectly with application. Also, its smooth braking capacity and ability to transfer from one mode to other is very good. 14. Which motor is preferred in highly explosive atmosphere? a) Air motor b) Shunt motor c) Series motor d) Battery operated motor View Answer Answer: a Explanation: A pneumatic motor (Air motor) is a motor which does mechanical work by expanding compressed air. Air motors generally convert the compressed air energy to mechanical work through either linear or rotary motion, and are preferred in highly explosive atmosphere. 15. For which of the following operations a DC motor is preferred over an AC motor? a) Low speed operation b) High speed operation c) Variable speed operation 7 of 8

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d) Fixed speed operation View Answer Answer: c Explanation: Due to the various speed control techniques available and with the help of electrical brakes various speed levels can be obtained in DC motor. While in AC motor for same purpose we require frequency change which requires another complex circuitry. Sanfoundry Global Education & Learning Series – DC Machines. To practice all areas of DC Machines, here is complete set of 1000+ Multiple Choice Questions and Answers.

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Brushes & Commutator Assembly - DC Machine...


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Brushes & Commutator Assembly - DC Machines Questions and Answers by Manish 6-7 minutes

This set of DC Machines Multiple Choice Questions & Answers (MCQs) focuses on “Brushes and Commutator Assembly – 1”. 1.The armature in DC machines is always placed on rotor because _______ a) Otherwise commutation will not be possible b) Otherwise there will not be any induced emf c) Otherwise current will not flow d) Can be placed anywhere View Answer Answer: a Explanation: A DC machine is a heteropolar structure with stationary poles and the rotating armature. The armature winding of a DC machine is placed on the rotor to improve commutation i.e. to convert the alternating voltage produced in the winding into direct voltage at the brushes. 2. In a DC machine, rectification provided with commutator 1 of 6

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is ___________ a) Half wave rectification b) Full wave rectification c) Semi controlled rectification d) Uncontrolled rectification View Answer Answer: b Explanation: In any electromagnetic machine the voltage generated is always alternating one as per Faraday’s law. For a DC machine the output must be unidirectional. This is carried out by a commutator. Hence, commutator provides full wave rectification. 3. Commutator performs rectification so that output of the machine is bi-directional. a) True b) False View Answer Answer: b Explanation: Commutator and brush assembly of the DC machine performs the mechanical rectification process so induced AC is converted into DC (Unidirectional). Commutation process provides full wave rectification. 4. Which of the following method is used to connect commutator segments to armature conductors? a) Brazing b) Normal wires c) Insulation pads d) Copper lugs 2 of 6

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View Answer Answer: d Explanation: Commutator in connected to the armature using lugs. Generally, they are made with copper. They are tightly bolted to the armature in order to prevent the centrifugal forces from causing the segments to fly away. 5. In D.C. generators, rapid brush ware causes due to _________ a) Severe sparking b) Rough commutator surface c) Imperfect d) Severe sparking, rough commutator surface or imperfect View Answer Answer: d Explanation: Brushes are the parts in a DC machine which are always in with rotating and stationary parts. Thus, imperfect , rough surfaces, sparking all these may reduce the life of brushes. 6. What are the number of the brushes in the lap winding? a) Double the number of poles b) Same as the number of poles c) Half the number of poles d) Two View Answer Answer: b Explanation: In a lap winding, the number of parallel paths, A, is always equal to the number of poles, P, and also to 3 of 6

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the number of brushes. In wave windings, the number of parallel paths, a, is always two (2), and there may be two or more brush positions. 7. When Copper brushes are used in DC machine? a) Where low voltage and high currents are involved b) Where high voltage and small currents are involved c) Cannot be determined d) Where low voltage and low currents are involved View Answer Answer: a Explanation: Due to various limitations, copper brushes are used in low voltage applications. For various other voltage ratings, different carbon-graphite proportions are used in manufacturing of brush materials. 8. In DC generators, current is fed up to the external circuit from armature through _______ a) Commutator b) Direct wire c) Slip rings d) Cannot be determined View Answer Answer: a Explanation: In any rotary machine current is induced in Sine wave format, according to Faraday’s law. For DC machine commutator provides mechanical rectification so that output is in the unidirectional format. 9. What are the number of the brushes in the wave winding? 4 of 6

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a) Same or more than the number of poles b) Same or less than the number of poles c) Exactly half the number of poles d) Depends on other parameters View Answer Answer: a Explanation: In wave windings, the number of parallel paths, A, is always two (2), and there may be two or more brush positions. When two adjacent commutator bars make with a particular brush, p/2 coils are shorted by the brush in the wave winding. 10. Which conductors are in point of with brushes? a) Lie under south pole b) Lie under north pole c) Lie in inter polar region d) Are farthest from the poles View Answer Answer: c Explanation: Because of the diamond shape of coils, the brushes which are physically opposite the pole centres are electrically connected to coil-sides lying close to the interpolar region. Thus, electrically the brushes are displaced 90° elect. From the axes of the main poles. 11. In case of DC 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 5 of 6

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d) Number of armature turns View Answer Answer: a Explanation: Armature current is induced in a DC machine, which is fed up to the external circuit. Thus, it needs to be unidirectional. So, for converting bidirectional current to unidirectional commutators which are equal in number of armature coils need to be used. 12. How total number of brushes in a commutator are determined in a given DC machine? a) Speed of armature b) Type of winding c) Voltage d) Amount of current to be collected View Answer Answer: d Explanation: Brushes are in with rotating part and stationary part. Thus, if more amount of current is to be carried, it requires more number of brushes. Hence brush number depends directly on the amount of current that needs to be collected and fed up in or out. Sanfoundry Global Education & Learning Series – DC Machines. To practice all areas of DC Machines, here is complete set of 1000+ Multiple Choice Questions and Answers.

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DC Machines Interview Questions and Answers


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DC Machines Interview Questions and Answers by Manish 6-8 minutes

This set of DC Machines Interview Questions and Answers focuses on “Brushes and Commutator Assembly – 2”. 1. In a DC generator the ripples in the direct emf generated can be 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 View Answer Answer: c Explanation: Brushes carry current to/from rotating parts from/to stationary part. Ripples can be avoided if brushes are maintained. Else, brushes will have some voltage drop in it and we’ll not get simple repeating part in emf. 2. The drop in the voltage for which of the following types of brush can be expected to be least? a) Graphite brushes b) Carbon brushes

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c) Metal graphite brushes d) Cannot be determined View Answer Answer: c Explanation: Metal graphite brushes are ideal for a variety of applications because of their low resistivity. Thus, drop will be less in metal graphite brushes. Metal graphite brushes are used on commutators of plating generators where low voltage and high brush current densities are encountered. 3. What is the requirement of the good commutation? a) Brushes should be of proper grade, size and material b) Brushes should smoothly run in the holders c) Smooth, concentric commutator properly undercut d) Brushes should be of proper grade, size, material, run smoothly in the holders and concentric commutator properly undercut View Answer Answer: d Explanation: Brushes are in with commutator. So, for good commutation brushes must be of superior quality so that brushes will give/receive appropriate current to and from commutator. Also, the between brushes and commutator must be smooth for proper commutation process. 4. How to avoid grooves in the commutation of DC machine with the help of brush? a) Brushes of opposite polarity should track each other 2 of 7

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b) Brushes of same polarity should track each other c) Brush position has no effect on the commutator grooving d) Brushes should not track each other View Answer Answer: a Explanation: Brushes are located such that they are displaced 900 electrically from the axes of main poles. The two positive and two negative brushes are respectively connected in parallel for feeding the external circuit. 5. Reason behind the rapid wear of brushes is __________ a) Abrasion from dust b) Excessive spring pressure c) Rough commutator bars d) Abrasion from dust, excessive spring pressure and rough commutator bars View Answer Answer: d Explanation: Brushes undergo various forces due to their location in a DC machine, they are in with rotating and stationary part of the machine. Hence, rough between commutator and brushes, inappropriate pressure on brush to rotating part may affect quality of commutation process. 6. For both lap and wave windings, what is the number of commutator bars equal to? a) Slots b) Armature conductors c) Winding elements 3 of 7

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d) Poles View Answer Answer: c Explanation: 7. Spacing between the brushes for a 4-pole machine in of commutator segments for 12 conductor segments is _____ a) 48 b) 3 c) 2 d) 6 View Answer Answer: b Explanation: The spacing between adjacent brushes in of the commutator segment is ratio of number of commutator segments with poles for a given DC machine. C/P= 12/4= 3. It may also be noted that C/P need not necessarily be an integer. 8. Spacing between the brushes for a 4-pole machine in of commutator segments is equal to 6. What will be the number of armature slots? a) 48 b) 3 c) 24 d) 6 View Answer Answer: c 4 of 7

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Explanation: The spacing between adjacent brushes in of the commutator segment which is also equal to armature slots is ratio of number of commutator segments with poles for a given DC machine. C= P*Spacing= 4*6= 24. 9. What is the range of the brush friction coefficients for medium category? a) 0.40 and above b) 0.22 to 0.40 c) 0.11 to 0.22 d) 0.08 to 0.11 View Answer Answer: b Explanation: Brush friction is influenced by many variables including brush temperature, spring force, current, atmospheric conditions, mechanical conditions, ring or commutator materials, surface films, speed and other factors. Brush friction is of medium category when, coefficient of friction lies in between 0.22 to 0.44. 10. Specific resistance for a brush is given by _________ a) R = (E * W * T) / (I * L) b) R = (E * W * I) / (T * L) c) R = (E * W * I) * (T * L) d) Doesn’t depend on E, W, T, I, L. View Answer Answer: a Explanation: Specific resistance is measured in the length direction of the brush, since resistance in the direction of 5 of 7

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width or thickness may be considerably different. For, E = voltage drop over length L, I = amps of current ed through the sample, W = width of sample, T = thickness on sample, L = that portion of the length, over which the voltage drop E is measured, R is calculated by R = (E * W * T) / (I * L). 11. How many poles to be used in DC machine if brushes are placed 4 commutator segments apart for 16 commutator segments? a) 8 b) 12 c) 2 d) 4 View Answer Answer: d Explanation: Number of poles can be found by dividing the total commutator segments to spacing between brushes. Hence, number of poles = 16 commutator segments/ 4 commutator spacing= 4 poles. 12. DC generators are disconnected/connected from/to the busbars only under the floating condition because ____________ a) To avoid sudden loading of the prime mover b) To avoid burning of all switch s c) To avoid mechanical jerk to the shaft d) To avoid sudden loading of the prime mover, burning of all switch s and avoid mechanical jerking to the shaft View Answer

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Answer: d Explanation: Brushes are the medium between rotating and non-rotating part of the DC machine. If sudden change in connections are done, whole machine undergoes change in all electrical quantities, which may damage machine. Thus, machines are connected and disconnected only at floating condition. Sanfoundry Global Education & Learning Series – DC Machines. To practice all areas of DC Machines for Interviews, here is complete set of 1000+ Multiple Choice Questions and Answers.

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PMDC Motors - DC Machines Questions and A...


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PMDC Motors - DC Machines Questions and Answers by Manish 5-6 minutes

This set of DC Machines Multiple Choice Questions & Answers (MCQs) focuses on “PMDC Motors”. 1. In PMDC motors we use magnetic material with ___________ a) Low residual flux b) Low coercivity c) High residual flux d) All magnetic materials View Answer Answer: c Explanation: In market magnetic materials with various properties are available, but for DC motor application we need magnetic materials with high residual flux in order to get high no load voltage. Along with residual flux, coercivity of material should be high. 2. In PMDC magnetic material is placed instead of _____________ a) Armature

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b) Rotor c) Stator d) Can be placed anywhere View Answer Answer: c Explanation: In PMDC motors we keep magnetic material or magnetic poles in the place of field winding of original DC machine. That is we are changing stator windings with two permanent magnetic poles bounded on casing. 3. PMDC motors are _________________ a) Smaller than corresponding rated field wound motor b) Larger than corresponding rated field wound motor c) Almost same size that of corresponding rated field wound motor d) Can be smaller or greater than corresponding rated field wound motor View Answer Answer: a Explanation: PMDC motors are always of smaller size compare to field wound motors of same rating. Here, field winding is replaced by small block of magnet so that power output remains same. PMDC motors are costly compare to DC motor of same rating. 4. PMDC motor offers ___________ a) Cumulative compound type characteristic b) Differential compound type characteristic c) Series characteristic d) Shunt characteristic 2 of 6

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View Answer Answer: d Explanation: The stator is made of permanent magnet in the PMDC motor hence, there is no need for separate field excitation for stator. So, PMDC acts as a shunt motor with fixed field. This can be seen from the construction also. 5. Which speed control method is employed in PMDC? a) Field control method b) Armature control method c) Armature Field control method d) Cannot be determined View Answer Answer: b Explanation: Unlike in DC motors, field is provided by permanent magnets in PMDC. This implies that field provided in PMDC motor is permanent and can’t be altered unless whole construction is modified. Thus, armature voltage control method is employed here. 6. What is the air gap flux density for PMDC motor? a) Bg= µ0(tm/lg) Hm b) Bg= – µ0Hm c) Bg= – µ0(tm/lg) Hm d) Bg= – µ0(lg/tm) Hm View Answer Answer: c Explanation: Since, flux crosses the air gap length lg two times a thickness of permanent magnet two times tm. We get, 2*lg* Hg + 2*tm Hm = 0 and we know that air gap flux 3 of 6

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density is equal to µ times the magnetic field intensity. 7. Which of the following is most favoured choice for PMDC motor? a) Neodymium-iron-boron b) Iron-nickel c) Nickel-neodymium d) Cannot be given View Answer Answer: a Explanation: Obvious choice of PMDC motor is neodymiumiron-boron which has high coercivity and high retentivity. Its characteristic is almost a straight line. Hence, neodymiumiron-boron combination is preferred over the other materials. 8. Speed-torque characteristic of PMDC motors is _______________ a) Starting from zero increasing continuously b) Starting from some positive value and remaining constant c) Starting from some positive value and decreasing exponentially d) Starting from some positive value and decreasing on straight line View Answer Answer: d Explanation: The material chosen for PMDC motor is such that speed torque characteristic of a given motor remains straight line for wider range of torque, for wider range of 4 of 6

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armature voltage. The straight line is such that it makes positive intercepts on both axes. 9. PMDC produces high torque on ______________ a) At low speeds b) Only at maximum speed c) Torque is constant d) Cannot say View Answer Answer: a Explanation: PMDC produces high torque even at very low speeds. In DC shunt motor torque produced at very low speeds is lower than the torque produced at the same speeds in PMDC motor. 10. PMDC gives better speed regulation than DC shunt motor. a) True b) False View Answer Answer: a Explanation: PMDC motor exhibits better speed regulation and efficiency than dc shunt motor. The main problem of dc shunt motor is going to run-away when the field terminals are opened. But in PMDC there is no run-away problem, so it gives practical benefit to the industry applications. 11. If higher terminal voltage is applied to PMDC what will happen? a) Speed at 0 torque will be reduced b) Torque at 0 speed will be reduced 5 of 6

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c) Torque at 0 speed will be increased d) Torque will remain constant View Answer Answer: c Explanation: The straight-line characteristic of PMDC is such that it produces positive intercepts on both axes. If terminal voltage is increased both of these intercepts increases, slope remaining constant. Thus, speed and torque at initial conditions both will increase. Sanfoundry Global Education & Learning Series – DC Machines. To practice all areas of DC Machines, here is complete set of 1000+ Multiple Choice Questions and Answers.

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