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TECHNOLOGICAL INSTITUTE OF THE PHILIPPINES Cubao, Quezon City
ELECTROMECHANICAL ENERGY CONVERSION
Assignment No. 5 (FINALS)
“SINGLE-PHASE TRANSFORMER ”
Submitted by: Ezekiel M. Brizuela EC41FA1
Engr. Jurieve Bagay Instructor
February 6, 2012 Date
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1. What is meant by transformer action? Under what conditions will it take place? Under what conditions will it not take place?
Transformer action is when two coils of wire are inductively coupled; the flux that passes through one of them also passes entirely or in part through the other. This fact implies that the coils have a magnetic circuit that is common to both.
2. Explain why transformer action can take place in a d-c circuit?
If the flux will created by a varying current through one coil changes, the mutual flux will change; under this condition, there will be created an induced voltage in the second coil. The secondary induced voltage results because the flux changes through the coil, although this flux change is occasioned in the first place by the current change in the first or primary coil.
3. If alternating current is impressed on one coil, what will be the frequency of the induced voltage in another coil with which it is coupled?
The frequency of the induced voltage in the second coil is exactly the same as the frequency of the induced voltage of the primary.
4. Define a static transformer?
Static transformer is a device that most commonly utilizes the principle of the transformer section.
5. Why are transformers more efficient than rotating electric machines?
Transformers are more efficient than the rotating electric machines because the losses in transformers are low, consisting only of copper and iron losses, the efficiency is extremely high compared with the rotating electrical machines.
6. Distinguish between core-type and shell-type transformers.
Core type is the coils surrounded a considerable part of the magnetic part. The primary and secondary coils are wrapped around the core sides, with low-voltage coil leads at the top and the high voltage leads at the bottom. Shell type is the magnetic circuit surrounds a considerable portion of the windings. All the primary and secondary coils are assembled, after which the entire coil assembly is dipped in an insulating varnish and baked. The properly
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cut laminations are then pushed through the coil opening and are buttoned to pieces surrounding the entire coil. 7. To what circuit is the primary of a transformer connected? The secondary connected?
The primary transformer connected in a a-c system circuit while the secondary transformer connected to a d-c system circuit.
8. Describe how the core and windings of a transformer are assembled.
Rigid bracing is important in transformer assemblies. The laminations and coils will vibrate with the current changes and thus cause objectionable noise – a humming sound – and even insulation failure.
9. What advantages are claimed for the transformer that is constructed by winding steel strips spirally through the openings and around the coils?
The advantages claimed are: a more rigid core; smaller size for a given kilovoltampere rating; reduction in strains in the iron, normally set up by clamps; lower iron losses at higher flux densities and reduction in the cost of manufacture.
10. Why are transformer windings divided into several coils?
A transformer generally consists of one or more coils (winding) of conducting wire, wound on a former (bobbin) that surrounds the centre limb (sometimes all limbs) of a circuit of magnetic material (core). The winding wires are insulated and the core is made from thin sheet steel plates known as laminations (this reduces 'eddy current' losses). The assembly is held together by metal cheeks known as clamps, these clamps are held in place by long screws that are insulated from the rest of the structure (again to limit eddy currents). The winding wires are either made off to terminals mounted on the clamps or the wire may leave the coil by 'flying leads'.
11. What is meant by leakage flux?
Any flux that does not do in the transformer action where it exists when flux – mutual flux – couples both primary and secondary.
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12. Why are the first few turns of high-voltage transformer coils especially well insulated?
The first few turns – the ended turns – are the ones that are subjected to voltages high enough to break down the insulation. It is for the reason that the end turns are more widely separated and more strongly insulated than are the other turns.
13. Why do transformers hum? How can this hum be minimized?
The laminations and coils will vibrate with the current changes and this cause hum or noise. Transformers are generally placed inside tightly fitted sheet-metal cases developed so that it serves both to insulate the windings and through circulation to keep the windings reasonably cool.
14. What purposes are served by placing transformers in oil-filled tanks?
Oil filled transformer fluid promotes longevity in more power driven transformers; and with larger applications, dependability is a priority. Oil filled transformer fluid is usually a very refined mineral oil that is stable at higher temperatures, and in bigger applications can prevent equipment breakdown or overheating.
15. What properties should good transformer oil possess?
Oil (usually high quality mineral oils) cools the transformer. Oil filled transformer fluids also provide insulation between internal parts of the transformer. Because the oil stays stable when large machinery and equipment operate large applications for long periods of time, there is less chance of production shutting down due to overheating in equipment. Oil filled tanks in the transformer may have radiators, which circulate the oil keeping internal temperatures around 55 degrees Celsius.
16. List several common insulating materials for transformers.
Several common insulating materials for transformers are: paper, pressboard, cloth, mica, asbestos and impregnating compounds.
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17. Why are the tanks of some large transformers corrugated?
In larger size, tanks are corrugated or finned or have external tubes and in very large units the tanks must be supplied with external radiators through which the oil circulates by natural convection, owing to differences in temperature in the liquid. This method can be employed for units of any size or voltage rating, although large-capacity units become rather expensive and bulky. The common liquid employed is an insulating oil.
18. What is meant by oil sludging? How is it caused?
Sludging is simply the decomposition of oil with continued use. It is caused principally by exposure to oxygen during heating and results in the formation of heavy deposits of dark, heavy matter that will eventually clog the cooling ducts in the transformer.
19. Describe how transformer coils are constructed.
Transformers are constructed with different types of metal enclosing structures to meet the requirements of different conditions of installation. One type of enclosure is designed for mounting on poles, either directly or with hanger irons, for use in overhead distribution work. Another type of enclosure, called the platform type is suitable for installations in which the transformer stands upon its own base.
20. What current flows in the primary of a transformer when the secondary is open circuited? What function has this current?
If the secondary circuit is open, no current can flow in it, but if it is closed, a certain current, proportional to the impedance of the secondary circuit, will flow. This current, because of the direction of the emf generated in the secondary, will be in such a direction that the magnetic flux produced in the core by it will oppose the flux due to the primary winding. It will therefore decrease the effective or resultant flux in the core by a small amount which will decrease the counter-emf of the primary winding and permit more current to flow into the primary winding
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21. Upon what factors does the induced voltage in the primary of a transformer depend? In the secondary?
Since the primary and secondary windings are on the same core, the magnetic flux generated by the magnetizing or exciting current flowing in the primary winding also cuts the turns of the secondary winding and generates an emf in them. This emf will be, in accordance with a well-known law, opposite in direction to that impressed on the primary.
22. What relation exists between primary and secondary voltages and turns?
The voltage of both primary and secondary will change with the value of the current of the system. Such transformers are used for instrument transformers and in some series street-lighting installations. The currents in the primary and the secondary windings will be, very closely, inversely proportional to the ratio of the primary and secondary voltages because, if the small losses of transformation are disregarded, the power put into a transformer will equal the power delivered by it.
23. What is meant by the ratio of transformation? How can this be determined experimentally?
The voltage ratio of a constant-voltage transformer, the ratio of primary to secondary voltage, depends primarily upon the ratio of the primary to the secondary turns. The voltage ratio will vary slightly with the amount and power factor of the load. For general work the voltage ratio can be taken as equal to the turn ratio of the windings.
24. What relation exists between the primary and secondary currents and turns?
The turns ratio Np:Ns are both proportional to the inversely current ratio Is:Ip.
25. What relation exists between the primary and secondary voltage and currents?
It will result to Power of both primary and secondary are the same.
26. Distinguish between step-up and step-down transformers?
A step-up transformer is a constant-voltage transformer so connected that the delivered voltage is greater than the supplied voltage.
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A step-down transformer is one so connected that the delivered voltage is less than that supplied; the actual transformer may be the same in one case as in the other, the terms step-up and step-down relating merely to the application of the apparatus.
27. Why are some transformers constructed with the primary and secondary windings divided into two halves?
It is necessary merely to select any two points on the continuous winding such that one-half of the total number of turns is included between them.
28. When a winding is divided into two halves, why are the center leads crossed before they are brought out?
If an ordinary two-coil transformer had been used, the circuits would have been, while the required constructive material would have been approximate I.
29. Distinguish between power and distribution transformers.
For the transformation of a large amount of power, more than 500 kVA, are called power transformers. Transformers for general constant-voltage power transformation, whose rating is 500 kVA or less, are called distribution transformers.
30. Why are distribution transformers tapped?
Distribution transformers generally are liquid immersed, self-cooled. Power and distribution transformers are normally of the standard type with the windings insulated from each other, although those with autotransformer construction can be obtained for special applications in which the voltage ratio is small. Power transformers are always of the platform type. Distribution transformers are made with tanks for pole and platform mounting and with tanks of the subway and vault types.
31. Explain carefully how the primary current increases automatically in direct proportion to the current delivered by the secondary.
The primary is connected to a constant-voltage source. The secondary voltage varies proportionally with the load, while the secondary current remains constant.
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The primary current and kilovolt-amperes will be constant for all loads, but the kilowatt input and power factor will vary with the load. 32. Why does the principle of transformer action require that the primary and secondary ampere-turns be equal? The ratio of the number of turns in the primary winding to the number of turns in the secondary winding determines the ratio of the primary to the secondary voltage. 33. What is meant by voltage regulation of a transformer?
The regulation of a transformer is the change in secondary voltage from no load to full load. The regulation depends upon the design of the transformer and the power factor of the load. Although with a noninductive load such as incandescent lamps, the regulation of transformers is within about 3 percent, with an inductive load the drop in voltage between no load and full load increases to possibly about 5 percent.
34. What is the effect of the primary and secondary resistances upon the regulation of a transformer?
. If the motor load is large and fluctuating and close lamp regulation is important, it is desirable to use separate transformers for the motors.
35. Distinguish between the primary and secondary leakage fluxes.
The primary leakage flux is the flux does not link with the secondary winding while the secondary leakage flux is the flux does not link to primary winding.
36. What effect have the leakage fluxes upon the regulation of a transformer?
The direction of the emf generated in the secondary, will be in such a direction that the magnetic flux produced in the core by it will oppose the flux due to the primary winding. It will therefore decrease the effective or resultant flux in the core by a small amount which will decrease the counter-emf of the primary winding and permit more current to flow into the primary winding.
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37. What is meant by the term leakage-reactance drop?
The leakage-reactance voltage drop is latter of leakage flux that result to poorly voltage regulation.
38. Upon what do the various voltage drops in a transformer depend?
The various voltage drops depends upon the leakage flux and its reactance.
39. Referring to fig. 184, explain why the induced voltage lags behind the flux by 90 degrees.
Referring to the figure, the induced voltage lags behind the flux by 90 degrees because the time d the rate of change flux is again zero. The induced emf will be zero due to point d’ on the voltage wave represents the voltage at time d.
40. Why is it necessary to treat the various voltage in a transformer vectorially?
It is necessary to treat the various voltage because to yield the developed secondary emf, although, depending upon the magnitude and character which is lagging and leading of the load power factor, the leakage-reactance drops may attempt to reduce or raise the terminal voltage.
41. Assuming a constant impressed primary voltage, how does the secondary terminal voltage tend to change when the power factor is unity? Is lagging? Is leading at a very low value?
At a constant impressed voltage the constant Vp tends to change the power unity due to leakage drop, is lagging behind the flux by exactly 90 degrees, while is leading the power factor to minimum value.
42. What is meant by the equivalent resistance of a transformer? How may it be calculated in primary terms? In secondary terms?
In equivalent resistance it is the secondary resistance added by the primary resistance divided by squared of a. While, in primary terms, the equivalent resistance is squared of a multiplied by the secondary resistance added with primary resistance.
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43. What is meant by the equivalent reactance of a transformer? How may it be calculated in primary terms? In secondary terms?
Equivalent reactance in primary terms is; squared of a multiplied by reactance od secondary added with the reactance od primary. The secondary terms is equivalent reactance is equaled to secondary reactance added with primary reactance divided by a squared of a.
44. How can the equivalent impedance of a transformer be determined?
The equivalent impedance of a transformer can be determined by; the squared of equivalent resistance plus the equivalent reactance.
45. If the transformer is represented by an equivalent circuit like that shown in fig. 188, what is neglected? Is this neglect permissible? Explain. In Fig 188, neglecting the no-load current, it is permissible to use the equivalent resistance and equivalent reactance. Merely acts like an impedance voltage drop. 46. Outline carefully the procedure for performing the short-circuit test. The short-circuited, the impressed voltage on the other winding must be very low, usually about 5 per cent of rated value. This implies that the load terminals are short-circuited, so that Vs = 0; under this condition the impressed voltage emf Vp/a must merely overcome the full-load and voltage drops . 47. What useful information is obtained from the short-circuit test?
Useful information might do a short circuit test to make sure you don't have a direct connection between two nodes that need some resistance between them.
48. Outline carefully the procedure for performing the open-circuit test.
The open-circuit test is made to obtain the values of the exciting-admittance, conductance, and susceptance. In the case of power transformers or constantvoltage transformers operating at one specified frequency, the open-circuit test consists in the application of rated voltage at rated frequency usually to the lowvoltage winding with the high-voltage winding open circuited. Measurements are made by means of indicating instruments (voltmeter, ammeter, and wattmeter) of voltage, current, and power.
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49. What useful information is obtained from the open-circuit test?
For all practical purposes, therefore, a wattmeter used to measure the power input under the condition of open circuit will register the core loss, and safer when it is done at the lower voltage.
50. What are the two components of the core loss?
The two components of the core loss are: hysteresis and eddy current loss.
51. How is the hysteresis loss affected by a change in flux density?
A hysteresis loss; in this scenario, the components in the core material contract and expand in a way that causes the flow of energy to dissipate. When this type of dissipation takes place, the energy is converted to heat. The loss of energy will escalate as the equipment continues to run through additional cycles and the magnetic field becomes increasingly unbalanced.
52. Assuming a constant frequency, how does a voltage change affect the hysteresis loss? The eddy-current loss?
The core material resists the flow of currents, creating a conversion of energy into heat. This effectively reduces the flow of energy through an inductor. Laminating the core material can help to reduce the potential for this particular type of core loss. Another solution is to make use of core components that are constructed with materials that are less conductive and less likely to be resistant to the flow of the eddy current.
53. Assuming a constant impressed voltage, how does a frequency change affect the hysteresis loss? The eddy-current loss?
There is change in eddy-current due to directly proportionality of square of the impressed emf.
54. While the hysteresis loss no frequency change since the impressed voltage is constant. How do the copper losses vary with the load?
The copper losses vary with the winding of primary and secondary that causes the power to load change.
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55. List the losses in a static transformer. How can these be determined experimentally?
The copper losses in the primary and secondary windings and the hysteresis and eddy-current losses in the laminated core. The losses determined the copper losses measured by the wattmeter when the short-circuit test is performed, and the core losses are measured by the wattmeter when the open-circuit test is performed.
56. Explain why the core losses are unaffected by the load.\
The core losses are due to the EMF causing hysteresis and eddy currents to flow in the core materials the load may reduce the EMF on the windings due to losses, and this could cause small reductions in losses due to hysteresis and eddy currents
57. Under what condition does the maximum efficiency occur in a transformer?
The efficiency of a transformer is, as with any other device, the ratio of the output to input or, in other words, the ratio of the output to the output plus the losses
58. Why are distribution transformers frequently designed to develop maximum efficiency at loads that are somewhat lower than rated value?
Network transformers are distribution transformers specially constructed and equipped with attached auxiliaries such as junction boxes and switches for disconnecting and grounding the high-voltage cable to meet the requirements of transformers for supplying low-voltage networks. A network transformer is designed for use in a vault to feed a variable capacity system of interconnected secondaries.
59. Under what condition will the maximum efficiency occur at rated load?
When the power output and power input are almost same the maximum efficiency will occur.
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60. Why is it desirable to have a power transformer operate at maximum efficiency when it is delivering rated load?
For a given transformer maximum efficiency occurs under the condition that the load impedance equals the internal impedance of the transformer. It is desirable to have a power transformer operate at maximum efficiency when it is delivering rated load because; Presumably the transformer will be operating at rated load more total hours than it will be operating at something other than rated load. If this is true then it would be most economical to have the transformer`s best operating efficiency be while operating at its rated load.
61. Define all-day efficiency.
All-day efficiency is what determines operating cost of the transformer.
62. Why is all-day efficiency a more reasonable basis of comparison than the conventional efficiency?
All-day efficiency is more reasonable basis of comparison than the conventional efficiency because it is ready at all time than the conventional efficiency.
63. What is an autotransformer?
An autotransformer sometimes called autostep down transformer is an electrical transformer with only one winding. The auto prefix refers to the single coil acting on itself rather than any automatic mechanism. In an autotransformer portions of the same winding act as both the primary and secondary. The winding has at least three taps where electrical connections are made. An autotransformer can be smaller, lighter and cheaper than a standard dual-winding transformer however the autotransformer does not provide electrical isolation.
64. What advantages are possessed by autotransformers? Autotransformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 138 kV to 66 kV for transmission. Another application is in industry to adapt machinery built such as 480 V supplies to operate on a 600 V supply. In audio applications, tapped autotransformers are used to adapt speakers to constant-voltage audio distribution systems, and for impedance matching such as between a lowimpedance microphone and a high-impedance amplifier input.
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65. List the disadvantages of autotransformers.
An autotransformer does not provide electrical isolation between its windings as an ordinary transformer does; if the neutral side of the input is not at ground voltage, the neutral side of the output will not be either. A failure of the insulation of the windings of an autotransformer can result in full input voltage applied to the output. Also, a break in the part of the winding that is used as both primary and secondary will result in the transformer acting as an inductor in series with the load (which under light load conditions may result in near full input voltage being applied to the output). These are important safety considerations when deciding to use an autotransformer in a given application.
66. What is meant by the terms transformed power and conducted power when they refer to an autotransformer? The transformed power is the power output multiplied with the voltage ratio, while the conducted power is the difference of power factor and power transformed. 67. Under what assumption can an autotransformer be considered the equivalent of a two-winding transformer?
The primary and secondary windings of a two winding transformer have induced emf in them due to a common mutual flux and hence are in phase. The currents drawn by these two windings are out of phase by 180 ◦. This prompted the use of a part of the primary as secondary. This is equivalent to fusing the secondary turns into primary turns. The fused section need to have a cross sectional area of the conductor to carry ampere. This ingenious thought led to the invention of an auto transformer.
68. Make a sketch showing how a two-winding transformer may be connected to operate as: (a) step-up autotransformer; (b) step-down autotransformer.
Step-up autotransformer
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Step-down autotransformer
69. How is it possible to use an autotransformer to obtain a neutral for a three-wire system?
The autotransformer may be used to obtain the neutral of three-wire 230 or115volt services in much the same manner as is done on the secondary side of conventional two windings transformer. The autotransformer in this case is merely a 230-volt reactance coil with a center tap.
70. What are current transformers? Potential transformers?
The current transformation and impedance conversion aspects of a two winding transformer are retained but with lesser material (and hence lesser weight) used. The losses are reduced increasing the efficiency. Reactance is reduced resulting in better regulation characteristics. All these benefits are enhanced as the voltage ratio approaches unity.
71. What special precaution must be taken when a current transformer is used?
Current transformers are commonly used in electricity meters to facilitate the measurement of large currents which would be difficult to measure more directly. Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary as in this circumstance a very high voltage would be produced across the secondary. Current transformers are often constructed with a single primary turn either as an
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insulated cable passing through a toroidal core, or else as a bar to which circuit conductors are connected. 72. What instrument ranges are generally used in conjunction with instrument transformers?
These are transformers that are used in conjunction with measuring instruments and are related to the field of instrumentation. These are mainly used for measurement of current, voltage, energy, power etc. in high power A.C. lines. Here the values of these quantities are so high that they can’t be measured directly by ammeters, voltmeters etc because of the range limitations of these instruments.
73. How is the ratio of transformation of a current transformer specified? What is the significance of this notation? The current transformer ratio of 100:5 means that the voltage stepped down, while for stepped up the ratio should be 20:1. 74. Describe the clamp-on type of a ammeter. What advantages does it possess?
Clamp on ammeter is an instrument used to measure the electric current in circuit. The clamp on meter is a tool which understands the electricity. In electrical engineering a clamp on ammeter is an electrical device having two jaws on both sides which open to allow clamping around an electrical conductor. Clamp on ammeter is much better and safer than ammeter that is inserted in-line less disturbing to circuit, and too much convenient. For contact less measurement with clamp ring the ammeter uses a Hall Effect device to measure the axial magnetic field generated in the wire by current flowing through it.
75. Explain why the accuracy of a current transformer is impaired if, after its secondary is accidentally open-circuited, it is used again.
The ampere-turns are no longer available to react with the primary ampere-turns, with the result that primary load current is completely magnetizing current; the core therefore saturates.
76. Referring to fig. 195, 198, and 199, explain why circuits should be grounded.
It should be grounded to serves and protect the operator from the high voltage hazard should come in contact with the wiring.
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77. To what does the polarity of a transformer refer?
Transformer Polarity refers to the relative direction of the induced voltages between the high voltage terminals and the low voltage terminals. During the AC half-cycle when the applied voltage (or current in the case of a current transformer) is from H1 to H2 the secondary induced voltage direction will be from X1 to X2.
78. What names are given to standard polarities of transformers?
The names that ate given to standard polarities of transformers are additive Polarity and Subtractive Polarity.
79. Explain how a test should be performed to determine the polarity of a transformer.
Polarity Test where-in situations where the secondary bushing identification is not available or when a transformer has been rewound, it may be necessary to determine the transformer polarity by test. The following procedure can be used.
80. Under what condition is it necessary to know the polarity of a transformer?
The condition must necessary to know whether it is additive and subtractive to know the polarity of a transformer.
81. List the conditions that must be fulfilled before two transformers can be operated successfully in parallel.
The ratio of the equivalent resistance to the equivalent reactance of all transformers should be the same. The voltage rating of both primaries and secondaries must be identical. The transformers must be properly connected with regard to polarity. The equivalent impedances should be inversely proportional to the respective kilovolt- ampere ratings.
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82. Under what condition will there be no circulating current when two transformers are operated in parallel at no load?
The condition of having no circulating current exists only if the ratios of transformation of the two transformers are exactly equal when two transformer are operated in parallel at no load.
83. Assuming that two transformers having equal ratios of transformation are connected in parallel, how is the total load divided between them?
When the transformers have equal ratios E1=E2, the equivalent current of the two transformers would then be on the assumption that the exciting current can be neglected in comparison to the load current.
84. Assuming that two transformers having unequal ratios of transformation are connected in parallel, indicate how the total load divides between them. The total load division for the primary current will be a 2 deducted by a 1 multiplied by the source voltage added up a 2 of equivalent impedance multiplied by the total current all over a 1 multiplied by equivalent impedance plus the a 2 multiplied by the second equivalent impedance. 85. When two similar transformers have equal value of Z, but different ratios of Re, Xe, will the total load be divided equally between them? Will their power factors be equal?
Yes, the total load will be divided equally between them but the power factor of the load delivered by two transformers will unequal.
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