DC Machines Multiple Choice Questions on “Lap Winding”.
1. Resultant pitch in the lap winding is__________
a) Depends on Yb value
b) Depends on Yf value
c) Depends on Yb and Yf value
d) Always equal to 2
Answer: d
Clarification: 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 Yb and Yf which is equal to 2, irrespective of Yb and Yf value.
2. What is the condition of retrogressive winding?
a) Yb > Yf
b) Ybf
c) Yb = Yf
d) No condition in bterms of Yb and Yf
Answer: b
Clarification: 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 Ybf .
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
Clarification: 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.
YCS= 12/4= 3.
Yb = 2YCS +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
Clarification: 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.
YCS= 12/4= 3.
Yb = 2YCS +1= 7
Yf =Yb – 2= 7-2= 5.
5. Equalizer rings are needed in lap winding.
a) False
b) True
Answer: b
Clarification: 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
Clarification: 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
Clarification: 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 = A2 Ia
d) Ic = A/Ia
Answer: b
Clarification: 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
Clarification: 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
Clarification: 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.
YCS= 12/4= 3.
Yb = 2YCS +1= 7
Yf =Yb + 2= 7+2= 9.