This set of DC Machines Multiple Choice Questions & Answers (MCQs) focuses on “Commutation Process – 2”.
1. To achieve spark less commutation brushes of a DC generator are placed ________
a) just ahead of magnetic neutral axis
b) in magnetic neutral axis
c) just behind the magnetic neutral axis
d) can be placed anywhere
View Answer
Explanation: Brushes collect the current due to the induced emf in the armature coils. When a brush is at any particular commutator segment, it shorts out that particular coil and draws current from the rest of the coils and fed to the commutator. To achieve all positive outcomes, we place them just ahead of MNS.
2. If in the DC machine, the reversal of current in the coil is faster than ideal or linear commutation then the commutation is said to be __________ commutation.
a) Retarded
b) Curvilinear
c) Accelerated
d) Under
View Answer
Explanation: Speed of the commutation is dependable on change in an induced current direction. When reversal of current in coil is faster then, obviously change of coils are taking place at faster rate. Thus, commutation is said to be accelerated commutation.
3. For a DC machine, in a commutator ________
a) Copper is harder than mica
b) Mica and copper are equally hard
c) Mica is harder than copper
d) Cannot be determined
View Answer
Explanation: Due to its mechanical strength and insulating properties mica is a satisfactory material. However, mica is much harder than the copper segments, so during manufacturing it requires to under-cut the mica by sawing slots between the adjacent segments of the commutator.
4. The insulating material used between the commutator segments is normally _______
a) Graphite
b) Paper
c) Air gap
d) Mica
View Answer
Explanation: Each conducting segment of the commutator is insulated from adjacent segments. Mica is a good electric insulator and good thermal conductor as well. Its applications in electric fields are derived from its unique mechanical properties. Thus, it allows mica to be ductile enough for its appropriate space of application.
5. Why interpoles are tapered in a DC machine?
a) Simpler design
b) Reduction in the weight
c) Increase in acceleration of commutation
d) Cannot be determined
View Answer
Explanation: To speed up the commutation process, the reactance voltage must be neutralized by injecting a suitable polarity dynamical (speed) voltage into the commutating coil. In order that this injection is restricted to commutating coils, narrow interpoles are provided in the interpolar region.
6. The main function of interpoles in a loaded DC machine is to minimize _______between the brushes and the commutator.
a) Friction
b) Sparking
c) Current
d) Wear and tear
View Answer
Explanation: Interpoles are introduced in a DC machine in order to speed up the commutation process, sp that sparking will be minimised. As, sparking arises at end of commutation period when commutation is not completed in given time.
7. Which of the following is different component?
a) Commutating poles
b) Compoles
c) Interpoles
d) Compensating winding
View Answer
Explanation: Interpoles are located in interpolar region in a DC machine, called as interpoles. They raise up the speed of voltage commutation. So, they are also called as commutating poles. Compensating winding though used for reducing armature reaction, performs different function compare to interpoles.
8. How many coils under an adjoining pole pairs is/are connected between adjacent commutator segments in lap winding?
a) 1/2
b) 2
c) 1
d) 1/4
View Answer
Explanation: No. of parallel paths in a lap winding are equal to P (No. of poles). Thus for 1 pole pir there will be exact 1 coil, connected to adjacent commutator segments in lap winding. In wave winding there will be half coils connected between adjacent commutator segments.
9. In wave winding, P/2 coils are connected under the influence of “x” pole-pairs which connect adjacent segments. X is ____
a) P
b) P/2
c) 2P
d) P/4
View Answer
Explanation: One coil each under an adjoining pole-pair is connected between adjacent commutator segments in a lap wound DC armature, while in a wave-wound armature the only difference is that P/2 coils under the influence of P/2 pole-pairs are connected between adjacent segments.
10. The process of current reversal takes place when the coil is passing through the interpolar region.
a) True
b) False
View Answer
Explanation: The process of current reversal called commutation takes place when the coil is passing through the interpolar region (q-axis) and during this period the coil is shorted via the commutator segments by the brush located (electrically) in the interpolar region.
11. How many coil sets undergo commutation simultaneously in a wave winding?
a) 1
b) 2
c) 4
d) 3
View Answer
Explanation: Commutation takes place simultaneously for P coils in a lap-wound machine (it has P brushes) and two coil sets of P/2 coils each in a wave-wound machine (electrically it has two brushes independent of P).
12. Which coil is shorted in commutation process?
a) Coil under north pole
b) Coil under south pole
c) Coil lying in an interpolar region
d) Cannot be determined
View Answer
Explanation: During the commutation period, the coil is shorted via the commutator segments by the brush. These brushes are located in interpolar regions electrically, and magnetically in neutral region. Thus, interpolar coil gets shorted.
13. Ideal commutation can be shown graphically by _____________
a) Straight line passing through origin
b) Straight line not passing through origin but with +ve slope
c) Straight line not passing through origin but with -ve slope
d) Curve increasing towards +ve t axis
View Answer
Explanation: Ideal Commutation (also called straight-line commutation) is that in which the current of the commutating coils changes linearly from + Ic to – Ic in the commutation period. Thus, it will form a straight line with -ve slope.
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