This set of Basic Electrical Machines Questions and Answers focuses on “Armature Windings”.
a) identical magnetic axis
b) two magnetic axis
c) no magnetic axis
d) physical spacing
Explanation: Concentrated winding have all turns on one magnetic axis.
2. DC machines have ________ windings and synchronous machines use ______ windings.
b) open, closed
c) open, open
d) closed, closed
Explanation: All the commutator machines use closed winding as it has nothing to deal with reactance. In ac machines, the winding can be converted
to star to delta and vice versa.
3. We can place closed windings in
a) ac commutator machine
b) stepper motor
c) ac machine
d) dc machine
Explanation: Closed windings are placed only in commutator as it does not need any reconnections later.
4. Which of the following machines can be used to place open slot winding?
a) ac machine
b) ac commutator machine, ac machine
c) dc machines
d) all of the mentioned
Explanation: Only ac machines are used for open type of winding inside the machine.
5. The simplex lap winding has the range of winding pitch of
c) more than 2
d) less than 1
Explanation: For a progressive winding, forward pitch – backward pitch = winding pitch
yb= coil sides/P +or-1 (m)
Here m is such that to make yb an odd integer.
a) 1 and -1
d) 2 to -2
Explanation: It is either 1 or -1.
7. A 200V dc machine has 4 poles and 40 coils, having simplex lap winding. The number of commutator segments which required in the given machine will be
Explanation: It is a simplex winding and so one segment of commutator will be needed for each coil.
8. Commutator segments in a DC shunt machine is equal to the number of
a) coil sides
Explanation: Commutator segments are placed as per the coil sides in the machine.
9. We can employ dummy coils in a DC machine to
a) compensate reactance voltage
b) reduce armature reaction
c) provide mechanical balance to the armature
d) improve the waveforms generated inside the commutator
Explanation: Dummy coils are placed inside so that to provide mechanical support to dc machine and they do not take part in the emf development.
10. In AC machines we should prefer double layer winding over single layer windings because
a) it requires identical coils
b) it is economical to use
c) it offers lower leakage reactance
d) all of the mentioned
Explanation: Double layer winding enables more usage of the space and lesser air gaps so the leakage flux also reduces and efficiency improves.
11. For an electrical machine with C number of coils and P poles, the distance between the coils connected by an equalizer ring is
Explanation: backward pitch for an equalizer ring = number of coils/pole pairs = C/P/2=2C/P.
Explanation: Current per brush (lap winding) = total armature current/Number of pole pairs.
13. For a given dc machine it is advised to use the dummy windings for a stable operation. But if it has be replaced as it unused component in the machine, then we must
a) S/P/2 should not be an integer
b) S/P/2 should be integer
c) S/P should be integer
d) S/P should not be an integer
Explanation: If slots per pole pair is not integer than all the winding will be completed without adding any dummy coil.
14. While doing regular checks on the dc machine with the lap connected winding, it is reported to have ammeter fluctuations, this can be due to
a) different air gaps under poles
b) variable reluctances in the core
c) irregular design deformations
d) all of the mentioned
Explanation: In a lap winding configuration, if the machine as air gap variations due to the design aspects, it will give circulating current in the windings.
15. A 6-pole lap wound DC generator has a developed power of P watts and brush voltage of E volts.
Three adjacent brushes of the machine had been found worn out and got open circuited.
If the machine is operated with the remaining brushes, voltage and power that could be obtained from the machine are
Explanation: Taking Kirchoff’s law into study here, the voltage across the winding will remain same.
But the current will reduce in the overall winding as the resistance has increased, so power will decrease by 1/3.
16. Consider a dc machine having its armature wound in lap fashion having 12 coils and each resistance 0.1 ohms.
If it is measured resistance between two adjacent commutator segments, the result will be
a) 0.092 ohms
b) 1.1 ohms
c) 1.8 ohms
d) 0.92 ohms
Explanation: There are 11 resistance which are in parallel to one resistance. So taking the equivalent resistance of 11 series connected
resistance and the one single resistance of the commutator segment.
R(eq) = 0.1||1.1 = 0.0917
Sanfoundry Global Education & Learning Series – Electrical Machines.
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