Q.1 What purpose is served by pose shoe in a d.c. machine?
Ans. The pole shoes serve the following purposes:
1. They spread out the flux in the air gaps.
2. Since they are of larger cross-sections, the reluctance of the magnetic path is reduced.
3. They support the field coils.
Q.2 What are the advantages and disadvantages of carbon brushes?
Ans. Advantages
1. They lubricate and polish the commutator.
2. If sparking occurs, they damage the commutator less than with the copper brushes.
3. They provide good commutation.
Disadvantages
1. The contact resistance is high and causes a loss of about 2 V. Hence they are unsuited for low voltage machines in which this forms a larger percentage loss.
2. Due to high voltage drop, the commutator must be made larger than copper brushes.
3. Their low current density necessitates large brush holders.
Q. 3. Why is the armature of a dc? machine made of silicon steel?
Ans. The most important function of an armature is to provide a low reluctance path for the flux through the armature from the N-pole-shoes to the S ones. It is, therefore, made of a material having high permeability. e.g. silicon steel. An additional advantage of silicon steel is that it has low hysteresis loss.
Q. 4. Why do we use slotted armature in a d. c. machine?
Ans. The slotted armature has the following advantages :
(i) The air gap can be reduced to a value required for mechanical clearance. This reduces the reluctance of the magnetic circuit.
(ii) The conductors can be properly placed so that there is little fear of them being displaced. The drag on the conductors is greatly reduced so that their insulation is in no danger of damage by pressure.
Q. 5. Why is armature winding placed on the rotor of a DC machine?
Ans. The armature winding of a d.c. the machine is placed on the rotor to facilitate commutation i.e., to convert the alternating voltage produced in the winding into a direct voltage at the brushes.
Q. 6. what factors determine the number of poles in a d.c. machine?
Ans. The number of poles in a d.c. the machine is governed by
(i) frequency (ii) current per brush arm
The upper limit to the number of poles is imposed by a frequency that is 20-30 Hz for large machines and not exceeding 50 Hz for small machines. The lower e is due to current per brush arm which should not exceed 400 A otherwise an excessive commutator will be necessary.
Q. 7. Why is the armature winding of a dc machine always double layer winding?
Ans. When the coil sides are placed in one layer, it is difficult to arrange their ends so that a will pass each other. In order to avoid this difficulty, the coils are generally arranged in; layers. Each coil has one of its sides at the top of the slot and the other at the bottom of so, the other slots. The coil ends and Will then lies side by side.
Q. 8. What is the essential difference between lap and wave windings?
Ans. The essential difference between a lap winding and a wave winding is in the com connections. In a lap winding, the commutator pitch is 1 whereas for a wave winding twice the pole pitch.
Q. 9. The armature winding of a d.c. the machine is a single closed loop. How do we get Await paths?
Ans. Whether the winding he lap or wave, when the brushes are lifted, the armature circuit is a single closed loop. However, when brushes are lowered onto the commutator, two. or more parallel paths are formed upon the type of winding. Each path consists of the same number of coils in series.
1. with a lap winding the number of parallel paths formed is equal to the number of poles.
2. With a wave winding, the number of parallel paths formed is 2 irrespective of the number of poles.
Q. 10. why is lap winding used in large multipolar dc generators?
Ans. For a given number of armature conductors, a lap winding can carry, a heavier current than a wave winding because it has more parallel paths. Therefore, lap winding is used in hugs multipolar d.c. generators to avoid having conductors of large cross-sections.
Q. 11. Why are “equalizers” not used in wave winding?
Ans. There is no need for equalizers. in a wave-wound machine because conductors in each of the two parallel paths pass under all the N and S poles successively.
Q. 12. What is the need for multiplex winding?
Ans. With a simple wave winding, there are only 2 parallel paths whereas, for a lap winding, the number of parallel paths is equal to the number of poles. Thus in as of a 10-pole machine, using a simple winding, the designer is restricted to either two parallel circuits (wave) or ten Parallel circuits (lap). However, sometimes, it is desired to increase the number of Parallel Paths, enabling them to carry a large current and at the same time reducing the conductor current. For this purpose, multiplex windings are used. Thus with duplex wave winding, the above 10-pole machine will have 4 parallel paths. Likewise, with duplex lap winding, the machine will have 20 parallel paths.
Q. 13. What are the advantages of using a large number of Poles in a d.c. machine?
Ans. The advantages of using a large number of poles in a d.c. machine are :
(i) The flux per pole is reduced. This reduces the thickness of the armature core and yo and hence the weight of the machine.
(ii) The length of the magnetic circuit is reduced, resulting in the reduction of field ampere-turns.
(iii) The length of the armature end connections is reduced.
(iv) The current per brush arm is reduced. This reduces the length of the commutator.
(v) Too small a number of poles means a large number of armature ampere-turns per pol and a high value of armature reaction, which requires a longer air gap.
Q.14. What are the disadvantages of using a large number of poles in a d.c. machine?
Ans. The disadvantages of a large number of poles in a d.c. machine are :
(i) More commutator segments are needed or alternatively, there must be more volts per segment for a given commutator diameter and thickness of the segment.
(ii) The brush arms are closer together, increasing the danger of flash-over.
(iii) Frequency is proportional to the number of poles and core loss depends upon frequency.
(iv) The armature diameter is increased.
Q. 15. What decides the upper and lower limit to the number of poles in a dc machine?
Ans. The upper limit to the number of poles is imposed by a frequency which is of the order of 20-30 Hz for large machines and not exceeding 50 Hz for small ones. The lower limit is decided by the current per brush arm which should not exceed 400 A otherwise an excessively long commutator will be necessary. These limits usually restrict the choice to about two alternatives.
Q. 16. What are the advantages and disadvantages of a separately excited generator over a self-excited generator?
Ans. The advantages of a separately excited generator over a self-excited generator are :
1. The field ampere-turns can be independently controlled.
2. It will operate in a stable condition with any field excitation.
3. We can have a wide range of output voltages.
The disadvantages of a separately excited generator as compared to a self-excited generator are :
1. A separate d.c. the source is required.
2. It is an inconvenient arrangement.
Q. 17. What are the factors that affect the voltage build-up in d.c. shunt generator?
Ans. The voltage of a shunt generator may fail to build up for one or the following reasons:
- The field connections are i is too high.
2. The resistance of the field circuit.
3. The speed is too low.
4. There is insufficient residual magnetism.
Q. 18. Why is the series generator not generator used?
Ans. The external characteristic of a series generator shows that during the increase in load. This is unstable. curve, the terminal voltage increases For this reason, a series generator has very limited applications.
Q. 19. Upon what factors the armature resistance (Ra) of a d.c. Does the machine depend?
Ans. The armature resistance of a d.c. the machine depends upon:
- Number, size, and connections of armature coils.
2. Contact resistance between the carbon brush and the copper commutator.
