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TECHNOLOGICAL INSTITUTE OF THE PHILIPPINES Cubao, Quezon City
ELECTROMECHANICAL ENERGY CONVERSION
Assignment No. 2
Chapter 3
“DIRECT CURRENT DYNAMOS, CONSTRUCTION CONSTRUCTION AND ARMATURE WINDINGS”
Submitted by: Ezekiel M. Brizuela EC41FA1
Engr. Jurieve Bagay Instructor
January 14, 2012 Date
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1. Define the term dynamo.
The term dynamo is a symbol of power. In the field of electrical, dynamo is defined as rotating. In the field of electrical, a dynamo is defined as a rotating electrical machine that converts mechanical energy into electrical energy and even an electrical energy into mechanical energy.
2. Name the various parts of the dynamo and and indicate which parts are stationary and which rotate.
A dynamo divided into two sections, namely, stator and rotor. A stator is a portion which is stationary and a rotor is a portion which rotates. The most important part of the stator is the field. It also has field poles, yoke, bearings and brush rigging. In rotor, it has steel core, slot, shaft, armature winding, armature coil, commutator and carbon brush.
3. From what material is each part listed in Question 2 made?
The materials that comprises the parts of rotor and stator are: in the field poles, usually bolted to a field yoke or frame, to which are also fastened the end bells with their bearings and brush riggings. The yoke may have a base with feet or a supporting bracket upon which the whole structure rests. The rotor is built up of a laminated steel core, slotted to receive the insulated copper armature winding. A spring tension is applied to the brushes so that good uniform contact is made between them and the commutator.
4. What is meant by pole core? pole shoe?
Field pole core is built up of stack of steel laminations, about 0.025 in. thick per laminations, having good magnetic qualities; rivets are driven through holes in the sheets to fasten together a stack of such laminations equal to the axial length of the armature core. Pole Shoe is the spread-out portion that permits the flux to spread out over a wider area where the flux enters the armature core.
5. Why are laminations used in constructing the armature core?
The construction of armature core uses lamination to reduced cross section that permits the use of less copper wire for the field poles. Also, to increase the area of the pole shoe that reduces the reluctance of the air gap between the pole face and
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the armature core. And, it permits the entire pole core and windings to be assembled before the former is bolted to the yoke frame. 6. Why are laminations used in armature core?
Like the field core, the armature core is a stack of steel laminations, but it is circular in section. The circumferential edge is slotted to a convenient depth to receive the copper armature winding, the number of slots being very carefully selected.
7. Why are pole cores constructed with a shoe?
The construction of pole cores uses a shoe to reduce cross section that permits the use of less copper wire for the field poles and to increase the area of the pole shoe that reduces the reluctance of the air gap between the pole face and the armature core.
8. What two kinds of field windings are used for generators and motors? Describe the construction of each.
The two kinds of field windings which are used for generators and motors are shunt field in which there are many turns of fine wire and series field in which there are comparatively few turns of heavy wire and the series motors provides the starting torque required for moving large loads while Shunt motors provides starting torque that varies with the load applied and good speed regulation by controlling the shunt field voltage.
9. What is the purpose of dipping and baking of field coils?
The purpose of dipping in an insulating varnish and baking in an oven of field coils is to add stiffness, a mechanical strength and good insulating properties to the winding.
10. What is the yoke? What purpose does it serve? How it is usually constructed?
Yoke serves as the frame of the machine and located at the outside of the rotor. The yoke is a circular steel ring, which supports the field, poles mechanically and provides the necessary magnetic path between the poles. The yoke can be solid or laminated. To construct a yoke, it should noted that feet for motor support and an outlet box have been welded to yoke.
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11. Why is cast steel or rolled steel more desirable than cast iron for the yoke material of dynamos?
Cast steel or rolled steel is more desirable to use because the steel material makes it to be possible in order to reduce the weight by as much as 60% without any increase in the reductance of the magnetic circuit.
12. Describe a commutator construction?
Commutator is a built-up group of hard-drawn copper bars, wedge-shaped in section when viewed on end, and having V-shaped grooves at each end. It consists of a number of segments normally equal to the number of slots. The segments or commutator bars are made of silver bearing copper and are separated from each other by mica insulation.
13. What purpose does the commutator serve in a generator? In a motor?
The purpose of the commutator in a dc generator is to replace the slip rings of the ac generator. With this, it mechanically reverses the armature loop connections to the external circuit. In motor, commutator reverses the armature current at the moment when unlike poles of the armature and field are facing each other, hence, reversing the polarity of the armature field.
14. What is the function of the armature winding in a generator? In a motor?
The function of the armature winding in a generator is to generate voltage while the torque speed is developed in motor.
15. Under what conditions is it necessary necessa ry to use brush rigging in which each arm contains several brush holders and brushes?
Using brush rigging, it is essential to use in which brush arm may have several brush holders for a large machines so that the brush rides freely without chattering on the commutator.
16. What keeps the brushes in good contact with the commutator?
To keep the brushes in good contact, a carbon or copper-graphite brush must have a spring tension to have good uniform contact between the brush and the commutator.
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17. How is the brush surface made to conform with the commutator surface?
The brush surface made to conform with the commutator must be grounded or sandpapered smooth that perfect contact exists between them.
18. What purpose is served by the brush pigtails?
The purpose of the brush pigtails is the flexible copper pigtails serves good electrical connection between the insulated brush arms and the respective brushes.
19. Which terminal is positive on a generator? On a motor?
In multipolar machines, alternate sets are electrically connected together. One of the junction becomes positive armature terminal while the other becomes negative armature winding terminal. In a generator, the positive feeds current to the load while in the motor it is the terminal that is connected to the positive but the source.
20. Name the two general types of armature winding.
The two general types of the armature winding are lap winding and the wave winding.
21. Distinguish between a simplex-lap sim plex-lap and simplex-wave simp lex-wave winding with regard re gard to construction; with regard to the number of parallel paths.
The coil ends are connected to adjacent commutator segments in simplex-lap, while in simplex-wave windings, the coil ends are connected to commutator segments, very nearly but never exactly, equal to the distance between poles of the same polarity, that is, alternate poles. It should be emphasized that the distance between the two coil sides is exactly the same for both types of winding if the machines have the same number of poles.
22. In general, what is a frog-leg winding?
A Frog-leg winding is a combination of a lap and wave winding.
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23. What is the important requirement with regard to the coil span of all types of armature winding?
The important aspect of all types of winding that applies not only to all DC windings but also to most different phase and some single-phase AC windings is that the distance between the two sides of a coil must be equal to the distance between the two adjacent poles. The coil span must be 180 electrical degrees or approximately so.
24. Explain what would happen if the coil span were 360 electrical degrees in a generator; in a motor.
In a motor, the force exerted by one side of a coil must be clockwise, for example, at the same instant that the force exerted by the other side of the same coil is also clockwise if both forces are to developed torque rotation in the same direction. In a generator, the voltage generated in one side of a coil must be away from the observer at the same instant that the voltage generated in the other side of the same coil is toward the observer if the voltages generated in the two coil sides are to aid on another. This rule can only mean that if one coil side is under the center of north pole the other side must be very close to the center of a south pole, 180 electrical degrees.
25. State the formula to determine the coil span
for an armature winding?
= coil spitch, in slots total armature of armature slots = number of poles any part of S/P that is subtracted to make
an integer
26. What is the commutator pitch in the simplex-lap winding?
The commutaor pitch in the simplex-lap winding ends of coils of simplex-lap windings are connected to adjacent commutator segments. Since this applies to every one of the simplex-lap coils, they are all connected together in succession at successive commutator segments.
27. What is meant by the term reentrancy ?
Reentrancy or reenter, the winding is said to close upon itself.
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28. Under what condition is the reentrancy of a duplex-lap winding, single? Double?
If the commutator has an even number of segments, a winding is doubly reentrant. On the other hand, a duplex-lap winding on armature having an odd number of segments will be singly reentrant.
29. Under what condition is the reentrancy of a triplex-lap winding, single? Triple?
If the triplex-lap windings would have the first coil ends then it is connected to segments 1 and 4. The degree of reentrancy depends upon the relation between the number of commutator segments and the “plex” of the winding.
30. What is the purpose of multiplex-lap windings?
Multiplex-Lap windings are generally restricted to low-voltage high-current machines because it is desirable to limit the current per path to values no greater than about 250-300 Ampere.
31. State the general rule for determining the reentrancy of a multiplex-lap winding?
The general rule for determining the reentrancy of a multiplex-lap winding is the degree of reentrancy of lap windings must equal to the highest common factor between the number of commutator segments and the “plex” windings.
32. What general rule can be used to determine determ ine the number of parallel paths in a multiplex-lap winding?
When the current passes through any armature winding, it always divides into an even number of parallel paths. In general, parallel paths is equal to m multiply to P in which m represents the multiplicity of the windings.
33. To what values of machine ratings are multiplex-lap windings restricted?
The values of machine ratings are multiplex-lap winding is restricted to low voltage and high current machines.
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34. Why must the ends of wave coils never be connected to the commutator exactly 360 electrical degrees apart?
The commutator pitch is comparatively large number because the coil ends must be joined to segments approximately 360 electrical degrees because a wave winding is impossible if the coil ends are joined to segments exactle 360 electrical degrees.
35. State the general rule for determining determi ning the commutator com mutator pitch pit ch in a simplex si mplex-wave -wave winding.
The commutator pitch in a simplex-wave winding requires that the commutator pitch Yc be so chosen with respect to the number of segments that after tracing the winding once around the commutator, the last coil end arrives one segment behind or one segment ahead of the starting segment.
36. Explain why only two t wo brush sets need to be used in a machine the h which the th e armature is wave wound.
The use of two set of brushes in the multipolar machines is often very convenient, particularly in installations in which brushes cannot be replaced without difficulty. Furthermore, in a wave wound machine equipped with as many brushes sets as poles, if one or more of the brushes sets develop poor contact with the commutator satisfactory operation is still possible.
37. How are the conductors in each path of a lap winding windi ng distributed distribute d around the armature?
The conductors in one path may be cutting fluxes under stronger poles than may the conductors of some other path. Under this condition, current must flow from the path in which the generated voltage is higher to the one in which the generated voltage is lower. Since this circulating current must pass across the brush contacts, sparkling is likely to result.
38. How are the conductors in each ea ch path of a wave winding distributed d istributed around a round the armature?
Each of the two parallel paths contains conductors distributed completely around the entire circumference, in wave armatures, each of the poles parallel path contains conductors lying under two poles only.
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39. How are the conductors in each path of a lap winding wind ing distributed distribute d around the th e armature?
The two sets of brushes is often very suitable, particularly in setting up in which the crushes cannot be restored without any having trouble.
40. Under what conditions cond itions is it desirable d esirable to use many man y brush sets as poles in machines having wave-wound armatures?
The two sets of brushes is often very suitable, particularly in setting up in which the crushes cannot be restored without any having trouble.
41. What are multiplex-wave windings?
The multiples-wave windings are duplex-wave, triplex-wave and quadruplex-wave winding. It is a type of armature winding with two or more parallel paths.
42. Under what circumstances would it be desirable to use multiplex-wave windings?
For generators and motors of up to about 75-kW capacity with voltages that result in total armature currents of no more than 600 ampere, the current per path will not exceed 300 ampere.
43. Why is it possible to have circulating currents in lap-wound armatures?
In lap-wound armatures must have current flow from the path in which the generated voltage is higher to the one in which the generated voltage is lower. Since the circulating current must pass across the brush contacts, sparkling is likely the result.
44. Why is it possible to have circulating currents flowing in wave-wound armatures?
There are no circulating currents flowing in the wave-wound armature because only lap-winding armatures can flow current because the conductors of each paths are not distributed completely around the circumference.
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45. State the rule for determining the commutator pitch in a multiplex-wave winding.
The commutator pitch, which is symbolized by Y c, which designates the coil end connections to the commutator, is equal to the plex of lap-wound armature windings. Thus, Yc is equal to 1, 2, 3, 4 etc., respectively.
46. In tracing a simplex-wave winding once around the commutator, com mutator, at what segment should one arrive with respect to the starting point?
The multiples-wave windings are duplex-wave, triplex-wave and quadruplex-wave winding. It is a type of armature winding with two or more parallel paths.
47. Answer Question Questio n 46 for a duplex-wave duplex-wa ve winding; triplex-wave winding; multiplexm ultiplexwave winding.
In a duplex-wave winding, the last coil end arrives two segments behind or ahead of the starting point .In a triplex-wave winding, the last coil end arrives three segments behind or a head the star in point. In a multiplex-wave winding, the last coil end arrives m segments behind or a head the starting point.
48. Why is it usually more m ore satisfactory s atisfactory to construct cons truct armatures a rmatures that have ha ve more commutator segments than slots?
When the number of commutator segments is increased, the voltage between those that is adjacent to each other decreases and the number of turns of wire in the coil or coils. When the number of slots is reduced, the armature core teeth become mechanically stronger, so that, from the standpoint of handling in manufacture, there is less damage to laminations and coils.
49. Are pitch calculations and affected when an armatures that have more commutator segments than slots?
Pitch calculations Y s and Yc is not affected when an armatures have more commutator segments than slots. They are made exactly the same way, regardless of whether the number of slots and segments is the same or not.
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50. What is meant by a double-element coil? A triple-element coil ? When are they necessary?
Double-element coil is when an armature has twice as many commutator segments as slots, each of the completely formed coils is a sort of double-element coil, in the sense that it serves double capacity. This is necessary because each slot must hold as many wires as are ordinarily held by two slots when the number of slots equals the number of segments. Triple-element coil is when the armature has three (3) times as many segments as the slots.
51. Under what condition condit ion is it desirable to wind an armature before the commutator commutat or is pressed on the shaft?
It is desirable to wind an armature before the commutator is pressed on the shaft to insert the coils in the slots more easily because there is an ordinarily very little space between the core and the commutator.
52. What is meant by a “dead” or “dummy” element?
When the arrangement states, “the number of segment is one less than 2, 3, 4, etc., times the number of slots” is followed then the dummy element will exist. Dead or dummy element is the unconnected element.
53. When does an armature winding have a “dummy” element?
An armature winding have a “dummy element” - When the winding is connected, one element is left open in the armature, that was the dummy element.
54. What purpose is served by a “dummy” element?
Dummy serves only to keep the revolving structure balanced mechanically.
55. Under what conditions do circulating currents flow in lap-wound armatures?
Circulating currents flow in lap-wound armatures when the machines are encountering trouble due to the air gap between stationary poles and rotating armatures.
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56. Why are circulating currents detrimental to good operation?
The circulating currents detrimental to good operation not only tend to heat the armature above the temperature caused by the normal load current, but also caused an undue amount of sparking at the brushes , since such current must circulate from one path to another through the contacts between the brushes and the commutator.
57. What is an equalizer connection?
Equalizer connection is used to overcome the detrimental effects resulting from the circulating currents. It is constructed with low resistance copper wires that connected together in the point of the armature winding which should, under ideal conditions, be at exactly the same potential at all times but which, because of mechanical and electrical difference.
58. What two important functions fun ctions are served ser ved by equalizer connections in i n lap-wound lap -wound armatures?
They relieve the brushes of the circulating current load by causing the latter to be bypassed. They create a magnetic effect that actually reduces the flux under those poles where there is too much magnetism and increases the flux under those poles where there is too little magnetism.
59. State the general rule ru le for determining determi ning the number of equalizer equal izer connections in lapl apwound armatures?
The total number of coils in an armature winding must be divisible by half the number of poles .
60. Exactly why does a frog-leg winding eliminate the necessity for using equalizer connections? Explain Carefully.
A frog-leg winding eliminate the necessity for using equalizer connections because wave winding are self-equalizing virtue of the fact that each path is made up of a series of coils lying under all the poles at the same time.
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61. Is it possible to place a lap winding on top of a properly designed wave winding and have this combination of windings perform as satisfactorily as a frog-leg winding? Explain.
It is not possible to place a lap winding on top of a property designed wave winding because they are non-potential generating wires that carries equalizing currents.
62. What is the origin of the term frog-leg winding ?
The term frog-leg winding eliminate the necessity for using equalizer connections because wave winding are self-equalizing virtue of the fact that each path is made up of a series of coils lying under all the poles at the same time. It indicates the similarity between the type of coil and the legs of the frog.
63. If the lap section of a frog-leg frog -leg winding windi ng is always simplex, what must be the multiplicity of the wave section?
If the lap section of a frog-leg winding is always simples, then the multiplicity of the
wave section is equal to .
64. How many parallel paths are there in a P-pole frog-leg winding?
The multiplex section provides P circuits, the total being 2P paths in parallel. For the frog-leg winding, it has as many circuits in parallel as duplex-lap winding because the simplex-lap portion supplies P circuits.
