enercon finals assignment 5
TRANSCRIPT
1 | P a g e
Ezekiel M. Brizuela
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
2 | P a g e
Ezekiel M. Brizuela
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
3 | P a g e
Ezekiel M. Brizuela
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 kilovolt-
ampere 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.
4 | P a g e
Ezekiel M. Brizuela
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.
5 | P a g e
Ezekiel M. Brizuela
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
6 | P a g e
Ezekiel M. Brizuela
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.
7 | P a g e
Ezekiel M. Brizuela
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.
8 | P a g e
Ezekiel M. Brizuela
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.
9 | P a g e
Ezekiel M. Brizuela
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.
10 | P a g e
Ezekiel M. Brizuela
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 constant-
voltage transformers operating at one specified frequency, the open-circuit test
consists in the application of rated voltage at rated frequency usually to the low-
voltage 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.
11 | P a g e
Ezekiel M. Brizuela
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.
12 | P a g e
Ezekiel M. Brizuela
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.
13 | P a g e
Ezekiel M. Brizuela
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 low-
impedance microphone and a high-impedance amplifier input.
14 | P a g e
Ezekiel M. Brizuela
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
15 | P a g e
Ezekiel M. Brizuela
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 or115-
volt 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
16 | P a g e
Ezekiel M. Brizuela
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.
17 | P a g e
Ezekiel M. Brizuela
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.
18 | P a g e
Ezekiel M. Brizuela
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 a2 deducted by a1 multiplied
by the source voltage added up a2 of equivalent impedance multiplied by the total
current all over a1 multiplied by equivalent impedance plus the a2 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.