#### Updated Armature Windings ( DC Machines ) MCQs – DC Machines MCQs

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**DC Machines MCQs – Armature Windings ( DC Machines ) MCQs**

The most occurred **mcqs of Armature Windings ( )** in past papers. **Past papers of Armature Windings ( DC Machines )** Mcqs. **Past papers of** ** Armature Windings ( DC Machines )** Mcqs . Mcqs are the necessary part of any competitive / job related exams. The Mcqs having specific numbers in any written test. It is therefore everyone have to learn / remember the related ** Armature Windings ( DC Machines )** Mcqs. The Important series of ** Armature Windings ( DC Machines ) Mcqs** are given below:

# Armature Winding and Commutator

**1. Armature winding is mounted on a __________**

a) Stator**b) Rotor**

c) Can be mounted anywhere on stator or rotor

d) Not required**Answer: b****Explanation: A DC machine is a heteropolar structure with stationary poles and the rotating armature. Field coil is mounted on stator. In an AC machine armature winding is located at rotor while field coil is located at stator. An ‘armature’ is a moving part as it a ‘rotor’.**

**2. In a DC machine, how coil-side emf varies towards the outer side of poles?****a) Decreases**

b) Remains same

c) Increases

d) First increases the decreases**Answer: a****Explanation: Coil side current pattern is the same as the emf pattern. Only difference is that while the coil-side emf reduces towards the outer side of poles, the current remains the same in all the coil-sides except for alterations from pole to pole, while the coil side current reverses, the current exchanged with external circuit must be unidirectional.**

**3. Commutator performs rectification so that output of the machine is unidirectional.****a) True**

b) False**Answer: a****Explanation: Since it is a DC machine the generated output must be DC. Any electrical machine works on induced emf concept which is AC in nature. Commutator and brush assembly of the DC machine performs the mechanical rectification process so; induced AC is converted into DC(Unidirectional).**

**4. What is the difference of DC voltages in the adjoining Brushes?**

a) Depends on the Shaft speed**b) Zero**

c) Non-zero

d) Depends on the various other parameters**Answer: b****Explanation: Brushes are located electrically in the magnetically neutral region. Due to their location adjoining brushes are at constant DC voltage and the coil in series between the constitute one parallel path.**

**5. What is the effect of armature coils at points where brushes are located?**

a) Induces positive emf

b) Induces negative emf**c) Induces zero emf**

d) Depends on the speed of rotor**Answer: c****Explanation: Brushes are at magnetically neutral region hence, induced emf due to armature coils at brushes will be equal to zero. As in the magnetically neutral region change in flux will be equal to the zero, emf will not be induced (Faraday’s law).**

**6. As the armature rotates, the number of coils in series tapped by the brush pairs_________****a) Remains same**

b) Increases

c) Decreases

d) Depends on rotor speed and direction of torque**Answer: a****Explanation: Brushes are located electrically in magnetically neutral region. Adjoining brush pairs are at constant DC voltage and the number of coils tapped by brush pairs also remain constant. Their disposition relative to the poles is the same.**

**7. Coil span for 4-pole, 12-slot armature winding is_______**

a) 24

b) 48

c) 8**d) 3****Answer: d****Explanation: Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Slots in the armature winding= 12, Number of poles= 4.****Y _{CS}= 12/4= 3.**

**8. What is the nature of the coils when Y _{CS} value is non-integral?**

a) Long-pitched

b) Medium-pitched

**c) Short-pitched**

d) Can’t be determined by Y

_{CS}value

**Answer: c**

**Explanation: Coil-side voltages around the coil are additive most of the time (except when coil-sides lie near the magnetic neutral region). Thus Y**

_{CS}= Nearest lower integer, which means that for non-integral S/P, the coils are short-pitched.**9. For a 2-pole DC machine with coil span equal to 6, what are the number of commutator segments?**

a) 3**b) 12**

c) 4

d) 8**Answer: b****Explanation: Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Number of poles= 2, Slots in the armature winding= S= P* Y _{CS}. Here, Y_{CS}= 6, P=2. Thus S=12. C=S=Number of commutator segments.**

**10. When coil sides are pole pitch apart, the DC armature winding is called as ___________**

a) Multiplex

b) Fractional-pitch**c) Full-pitch**

d) Pole-pitch**Answer: c****Explanation: Pole pitch is called as center to center distance between two adjacent poles. When measured in electrical degrees one pole itch is equal to 1800. Coil span is simply a peripheral distance between two sides of a coil. If the coil span is equal to the pole pitch, then the armature winding is said to be full pitched coil.**

**Lap Winding**

**1. Resultant pitch in the lap winding is__________**

a) Depends on Y_{b} value

b) Depends on Y_{f} value

c) Depends on Y_{b} and Y_{f} value**d) Always equal to 2****Answer: d****Explanation: In a lap winding the “finish” of one coil is connected to “start” of the adjoining coil. The coil side displacement of the front-end connection is called the front-pitch. The coil side displacement of the back-end connection is called the back-pitch. Resultant-pitch is equal to difference between Y _{b} and Y_{f} which is equal to 2, irrespective of Y_{b} and Y_{f} value.**

**2. What is the condition of retrogressive winding?**

a) Y_{b} > Y_{f}**b) Y _{b} < Y_{f}**

c) Y

_{b}= Y

_{f}

d) No condition in bterms of Y

_{b}and Y

_{f}

**Answer: b**

**Explanation: The coil side displacement of the front-end connection is called the front-pitch. The coil side displacement of the back-end connection is called the back-pitch. The direction in which the winding progresses depends upon which is more, Yb or Yf. For retrogressive winding Yb < Yf .**

**3. What is the value of Yb for a lap winding with a 4-pole, 12-slot armature with two coil sides/slot. Assume single-turns coils.**

a) 3

b) 5**c) 7**

d) 9**Answer: c****Explanation: Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Slots in the armature winding= 12, Number of poles= 4.****Y _{CS}= 12/4= 3.**

**Y**

_{b}= 2_{YCS}+1= 7.

**4. What is the value of Yf for a lap winding with a 4-pole, 12-slot armature with two coil sides/slot? (Assume single-turns coils – progressive winding)**

a) 3**b) 5**

c) 7

d) 9**Answer: b****Explanation: Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Slots in the armature winding= 12, Number of poles= 4.****Y _{CS}= 12/4= 3.**

**Y**

_{b}= 2Y_{CS}+1= 7**Y**

_{f}=Y_{b}– 2= 7-2= 5.**5. Equalizer rings are needed in lap winding.**

a) False**b) True****Answer: b****Explanation: Each parallel path in lap winding is under the influence of one pair of poles, so if a machine consists of multiple pairs of poles then dissimilarities occurs, due to which unequal voltages may be induced in the paths and a circulating current may flow. In wave winding each path is under the influence of all poles, so voltages are induced in each path causing no such dissimilarities like lap winding. Equalizers in lap windings are used to remove this dissimilarity, they’re not needed in wave winding.**

**6. What is the symmetry requirement of lap winding?**

a) 2C/P= 0**b) 2C/P= integer**

c) 2C/P= non-integer

d) Can’t express mathematically**Answer: b****Explanation: To avoid no-load circulating currents and certain consequential commutation problems, all the parallel paths must be identical so as to have the same number of coil-sides. Symmetry thus requires ratio of 2C/P is equal to the integer. Also, US/P equal to integer represents the same.**

**7. What is the relation between number of parallel paths(A) and number of poles(P)?****a) A = P**

b) A < P

c) A > P

d) No relation exists**Answer: a****Explanation: Complex winding can be divided into different parallel paths lying under different pole pairs. It is, therefore, concluded that the number if parallel paths is equal to the number of poles. In wave winding number of parallel paths is equal to 2.**

**8. Current flowing through the armature conductors Ic is related to total current Ia by_______**

a) Ic = A Ia**b) Ic = Ia/A**

c) Ic = A^{2} Ia

d) Ic = A/Ia**Answer: b****Explanation: Two positive and two negative brushes are respectively connected in parallel for feeding the external circuit. As per the ring diagram Ia splits into the number of poles equally. Poles = Parallel paths. Thus, Ic = Ia /A.**

**9. Value of commutator pitch in lap winding is_____**

a) +2**b) +1 or -1**

c) -2

d) Different for different parameters**Answer: b****Explanation: Two ends of coil are connected across the adjacent commutator segments. Depending on the type of winding that is, retrogressive or progressive, we have two values for commutator pitch.****For progressive winding, commutator pitch = +1. For retrogressive winding, commutator pitch = -1.**

**10. What is the value of Yf for a lap winding with a 4-pole, 12 commutator segments, with two coil sides/slot.?****(Assume single-turns coils -retrogressive winding).****a) 9**

b) 3

c) 11

d) 5**Answer: a****Explanation: Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Slots in the armature winding= Number of commutator segments= 12, Number of poles= 4.****Y _{CS}= 12/4= 3.**

**Y**

_{b}= 2Y_{CS}+1= 7**Y**

_{f}=Y_{b}+ 2= 7+2= 9.**Wave Winding**

**1. What will be the value of “Y _{f} + Y_{b}” for a wave winding?**

a) Equal to Y

_{c}

b) Half of the Y

_{c}value

**c) Double of the Y**

_{c}valued) Four times Y

_{c}value

**Answer: c**

**Explanation: In the wave winding, as the number of coil-sides is double the number of segments, the top coil-side of the second coil will be numbered as (1+2*Y**

_{c}). After numbering other coil sides,**1 + 2*Y**

_{c}– Y_{f}= 1+ Y_{b}**So Y**

_{f}+ Y_{b}= 2Y_{c}.**2. For a progressive wave winding Y _{c} = ______**

a) 2C/P

**b) 2(C+1)/P**

c) 2(C-1)/P

d) 2C/(P+1)

**Answer: b**

**Explanation: Starting at segment 1 and after going through P/2 coils or Yc (P/2) segments, the winding should end in segment 2 for progressive winding or segment (C) for retrogressive winding. That is mathematically,**

**Y**

_{c}(P/2) = (C+1)**Y**

_{c}= 2(C+1)/P**3. Number of parallel paths in wave winding are ______**

a) Equal to P

b) Equal to P/2**c) 2**

d) Depends on other parameters**Answer: c****Explanation: In wave winding all coils are divided into 2 groups- all coils carrying clockwise current are series connected and so are all coils with counter-clockwise current- and these 2 groups are in parallel because the winding is closed. Thus, a wave winding has 2 parallel paths irrespective of number of poles.**

**4. What is the spacing between the brushes for a wave winding when a machine is 6-pole DC armature with 16 slots having 2-coil sides per slot and single-turn coils.**

a) 4 segments**b) 8 segments**

c) 16 segments

d) 12 segments**Answer: b****Explanation: Only 2 brushes are required in this case as the number of poles in wave winding is equal to 2. So, spacing between the brushes is equal to total number of segments i.e. total slots divided by 2. Spacing between brushes = C/A = 16/2 = 8 segments.**

**5. What is the relation between conductor current and armature current in wave winding?**

a) Ic = Ia

b) Ic = 2Ia

c) Ic = 4Ia**d) Ic = Ia/2****Answer: d****Explanation: the number of parallel paths in the in a wave winding is equal to 2. So, armature current will get divided equally into total number of conductors/paths. Conductor current in a wave wounded machine is half of the Ia.**

**6. For a conductor current equal to 4mA, Current carried by a particular brush in a 2-pole machine will be _____**

a) 16mA**b) 8mA**

c) 2mA

d) 10mA**Answer: b****Explanation: Conductor current in a wave wounded machine is half of the Ia. So, Ia= 8mA. All positive and all negative brushes are respectively connected in parallel to feed the external circuit. Thus, IBRUSH = Ia /(P/2). Solving we get Brush current = 8mA.**

**7. Equalizer rings are needed in the wave winding.**

a) True**b) False****Answer: b****Explanation: The armature coil forms 2 parallel paths under the influence of all pole-pairs so that the effect of the magnetic circuit asymmetry is equally present in both the parallel paths resulting in equal parallel-path voltages. Thus, equalizer rings are not needed in wave winding.**

**8. For a wave winding when a machine is 6-pole DC armature with 16 slots having 2-coil sides per slot and single-turn coil, Yf value is ____****a) 5**

b) 3

c) 2

d) 7**Answer: a****Explanation: Y _{cs} = 16/6 = 2 slots (nearest lower integral value)**

**Y**

_{b}= 2*2+1 = 5**Y**

_{c}= 2(16-1)/6 = 5 segments**Y**

_{f}= 2Y_{c}– Yb = 5.**9. Wave winding machines are used in ______ currents applications.**

a) High

b) Moderate**c) Low**

d) Can be used anywhere**Answer: c****Explanation: Lap winding machine has the advantage of large number of parallel paths and lower conductor current and is therefore used in low voltage and high current applications. Wave winding has fixed number of parallel paths so, wave wounded machine is used in low currents application.**

**10. For a wave wounded machine number of brushes for small, large machines respectively is ________ _________**

a) 2, 2

b) 4, 2**c) 2, P**

d) Both values depend on the given conditions**Answer: c****Explanation: For a small wave wounded machine number of parallel paths are 2, thus 2 brushes are used. For a large machine total number of brushes is equal to the total number of poles. The spacing between adjacent brushes is C/P commutator segments.**