EE-unit
PART-A
1. What are the two types
of transformer
cores
used?
Ø Core type
Ø Shell type.
2. What is a transformer?
Define an ideal
transformer?
Transformer is a static electrical device, which transfers electrical
energy one
circuit
to
another circuit
without
any change in
frequency . It
decreases or increases the input ac voltage.
An
Ideal transformer
is one which does
not involve any power loss and does not
have any change
in frequency .
It decreases or increases the input ac voltage.
3. What are the
test to be connected in a transformer
to
obtain its Equivalent
circuit?
Ø Open circuit test
Ø Short circuit test
4. Why transformer rating is in KVA?
Copper
loss of a transformer depends on current and iron
loss depends on voltage.
Hence
total transformer loss depends on
volt-ampere(VA)
and
not on phase angle between voltage and
current ie, it is
independent of load
power factor . That
is why rating of
transformers is
in KVA & not in KW.
5. What is the necessity of using stepped core ?
In transformers
of medium and large capacity, Only circular
coils are used which
are mechanically
stronger.
It is
more economical
to place circular
shape
coils around
Stepped
core: With
increase in
steps , the diameter
gets
reduced & ultimately reduce the
volume of the copper conductor and
also
the resistance of
the
coil.
6. What are
Instrument
transformers?
To measure high currents
in the order of several
hundred amperes and voltages
of several kilo-volts,
instrument transformers are used along with
ammeters & voltmeters of lower range.
Type: Current transformer - to
measure high
current.
Potential transformer - to measure high
voltage.
7. What
is the
purpose of constructing transformer core by silicon content steel laminations?
Steel and steel alloys offers
less reluctance to
the path of magnetic field.
Addition of 3
to 5 percent
silicon with steel reduces Hysteresis loss in
the core .
Laminating
the core reduces eddy current loss occurring
in the core.
8. Name two important electrical performances
of transformers .
The two
important electrical performances of
transformers
are
Ø Voltage regulation
Ø Efficiency.
9. Mention the difference between core and shell type transformers?
In core type, the windings surrounded the core considerably
and
in shell type the
core
surround the windings i.e
winding is placed inside
the core.
10. What is the purpose of laminating
the core in a transformer?
To reduce the eddy current loss
in
the
core of the
transformer.
11. Give the emf equation of a transformer and define each term?
E1 =4.44 fФmN1.
E2 =4.44 fФmN2.
f--->freq of AC input
Фm---->maximum value of flux in
the core
N1,N2--->Number of primary & secondary turns.
12. Does transformer draw any current when secondary is open? Why?
Yes, it(primary) will draw the current from the main supply in order to magnetize the core and to supply for iron and copper losses on no load. There will not be any current in the
secondary since secondary is
open.
13. Define voltage regulation of a transformer?
When a transformer is loaded with a constant primary voltage, the secondary voltage decreases for lagging PF load, and increases for leading PF load because of its internal resistance and
leakage reactance. The change in secondary terminal voltage from no load to full load expressed as a
percentage of no load or full load voltage is termed as regulation.
%regulation down=(V2noload-V2full load)*100/V2noload
%regulation up=(V2noload-V2 full load)*100/V2 full load
14. Full load copper loss in a transformer is
1600W. what will be the loss at half load?
If x is the ratio of actual load to full load then copper loss = x2(F.L
copper loss) here Wc = (0.5)2 – 1600=400W.
15. Define all day efficiency of a transformer?
It is computed on the basis of energy consumed during a certain period,
usually a day of 24 hrs. all
day
efficiency=output
in kWh/input in kWh tor 24 hrs.
16. Why transformers are rated in
kVA?
Copper loss
of a transformer depends on
current & iron
loss on voltage. Hence total losses depends on Volt-Ampere and not on PF. That is why the rating of transformers are in kVA
and not in kW.
17. What are the typical uses of auto transformer?
Ø To give small boost
to a distribution
cable to correct for the voltage drop.
Ø As induction motor starter.
18. What are the application of step-up & step-down transformer?
Step-up transformers are used in generating stations. Normally the generated voltage will be
either 11kV. This voltage(11kV) is stepped up to 110kV or 220kV or 400kV snd
transmitted through transmission
lines(simply called as sending end voltage). Step-down transformers are used in receiving stations. The voltage are stepped down to 11kV or 22kV
are
stepped down to 3phase 400V by means of a distribution transformer and made available.
19. Mention the difference between
core and shell type transformers?
In core type the windings surround the core
shell type the
core surround
the windings i.e winding is
placed
inside the core.
20. What
is the purpose of laminating the core in a transformer?
To reduce the eddy current
loss in the core of
the transformer.
21. How transformers are classified according to their construction?
Ø core type
Ø shell type.
In core type, the winding (primary and secondary) surround the core and in shell type, the core surround the winding.
22. Explain on the material used for core construction?
The core is constructed by sheet steel laminations assembled to provide a continuous
magnetic path with minimum of air gap included. The steel used is of high silicon content sometimes heat treated to produce a high permeability and a low hysteresis loss at the usual operating fluxdensities. The eddy current loss is minimized by laminating the core, the laminations being used from each other by light coat of core-plate vanish or by oxide layer on
the
surface. The thickness of lamination varies from 0.35mm
for a frequency
of 50Hz and 0.5mm
for a frequency of 25Hz. When will
a Bucholtz
relay operates and give an alarm.
23. How does change in frequency affect the operation of a given transformer?
With a change
in
frequency,
iron and
copper loss, regulation, efficiency & heating varies so the operation of transformer is
highly affected.
24. What is
the
angle by which no-load current will lag the ideal applied voltage?
In an ideal transformer, there are no copper & core loss i.e loss free core. The no
load current
is only magnetizing current therefore the no load current lags behind by angle
90˚. However the winding possess resistance and leakage reactance and therefore the no load
current lags the applied voltage slightly less than 90˚.
25. List the arrangement of stepped core arrangement in a transformer?
Ø To reduce the space effectively
Ø To obtain reduce length of mean turn of the winding
Ø To reduce
I2R loss. Why
are breathers
used in
transformers? Breathers are used to entrap the atmospheric moisture and thereby
not allowing it to pass on to the transformer oil. Also to permit the oil inside the tank to expand and contract as its temperature increases and decreases.
26. What is
the function of transformer oil
in a
transformer?
Ø It provides good insulation
Ø Cooling.
27. Can the voltage regulation go negative? If so under what condition?
Yes, if the load has leading PF.
28. Distinguish power transformers & distribution
transformers?
Power
transformer are very high rating. They are used in generating and receiving stations. Sophisticated controls are required. Voltage ranges will
be very high. Distribution transformers are used in receiving side. Voltage levels will be medium.
Power ranging will be small
in order of kVA. Complicated controls are not
needed.
29. What is the
purpose of
providing Taps in transformer and where these
are provided?
In order to attain the required voltage, taps are provided, normally at high voltages side(low current).
30.
What
is the function of a transformer?
Transformer
is a static device that
Ø Transfers electric power from one
circuit to another.
Ø It does so without change of frequency.
Ø It accomplishes this electro-magnetic
induction.
31. State
the different types of single phase transformer based on construction?
Ø Core type transformer
Ø Shell type transformer.
32. What
is an ideal transformer?
An
ideal transformer is one that has
Ø no winding resistance.
Ø no leakage flux.
Ø no iron losses in the core.
Ø l2R Loss (copper loss)
33. What
are turns ratio and transformation ratio of transformer?
The
transformation ratio is defined as the ratio of the secondary voltage to
primary voltage. It is denoted by K, K = E2/E1 = N2/N1
= I1/I2
34. Write
the e.m.f equation of two winding transformer.
E1=4.44fmN1 volts
E2= 4.44fmN2 volts
Where, m= Maximum flux in
the core, in wb.
N1=no of turns in primary winding
N2=no of turns in secondary winding
F=supply frequency in Hz
35. What
is the purpose of laminating the core in a transformer?
To
reduce the eddy current loss, transformer core are laminated.
36. Give
the expression of secondary resistance as referred to primary.
R’2=R2/K2
37. How
does change in frequency affect the operation of a given transformer?
Ø Iron loss increases with decrease in
frequency.
Ø Since total loss is greater at lower
frequency, the temperature is increased with decrease in frequency.
Ø Reactive drop is affected, regulation
at low power factors decrease with decrease in frequency.
38. What
load current of a transformer increases, how the input current increases to
meet with the new condition does.
Increase
the load current secondary current to increase, thus reduce flux in the core,
in order to maintain the flux the primary current increases.
39. Write
the name of material used for transformer core.
Transformer
core is made up of high grade of silicon steel.
40. What
are the losses in a transformer? How will you minimize them?
There are two losses occur transformer,
Ø Iron Losses or core losses.
Ø Copper losses.
Iron losses include Hysteresis loss and
Eddy current loss. Hysteresis loss is reduced by making transformer core high
grade of silicon steel. Eddy current loss is minimized by laminating the
transformer core.
Copper
loss is minimized by reducing the leakage flux which is linked with both
primary and secondary windings.
41. What
is the condition for maximum efficiency of a transformer?
Copper
loss = Iron Losses is the condition for maximum efficiency of transformer.
42. Define
the regulation of a transformer.
The
change in secondary terminal voltage from no load to full load is known
regulation of a transformer
43. Define
all day efficiency?
All day efficiency = Output power in
KWh / Input power in KWh (for 24 hours)
44. Mention
three phase transformer connections.
Ø star-star
Ø star-Delta
Ø Delta-Delta
Ø Delta-star
Ø open-Delta
Ø scott connections.
45. What
is mean by leakage flux?
Magnetic
flux can be confined into a desired path. The greater portion of the flux
(i.e., the mutual flux) remains confined to two core and links both the
windings, but a small portion called the leakage flux completes its path
through the air surrounding the coils.
46. What
is the condition for maximum regulation of transformer?
Tan =X/R is the condition for
maximum regulation transformer.
47. What
is step-up transformer?
If
the number of turns in secondary winding (N2) is greater than the
number of turns in primary winding (N1). i.e., when the secondary
voltage is greater than the primary voltage.
48. Write
short notes on impedance test.
This is economical method
for determine the following
Ø Equivalent impedance (Z01 (or)
Z02) Leakage reactance (L01 or L02) and total
resistance (R01 or R02) of the transformer as referred to
the winding.
Ø Cu loss of full load.
Ø Regulation of the transformer.
49. Define
a transformer.
Transformer
is a static electric device that transforms electrical power from one circuit
to another circuit through magnetic induction and electrical isolation without
change of frequency.
50. When
the load current of a transformer increases, how does the input current
increases to meet with the new condition?
Increase
in load causes secondary current to increases thus reduces flux in the core, in
order to maintain the flux, the primary current increases.
51. What
is the purpose of conducting open circuit and short circuit tests in
transformer?
O.C. Test
The
purpose of condition open circuit test is to determine no load loss or core
loss and no load current (I0) which is helpful in finding X0 and
R0.
S.C. Test
The
short circuit test is conducted to determine the following
Ø Equivalent impedance (Z01 or
Z02), leakage reactance (X01 or X02 ) and
total resistance (R01 or R02) of the transformer as
referred to the winding.
Ø Copper loss at full load.
Ø Regulation of the transformer.
52. What
is the function of transformer oil in a transformer?
Oil
provides better insulation to the air and it is a better conductor of heat then
air. It carries away the heat. Mineral oil is used for this purpose.
53. Why
are iron losses constant at all load in a transformer?
Because
Core flux remains constant for all loads, the core losses also remain constant.
54. What
are the applications of step up and step down transformer?
Step up transformer
Ø Transmission system.
Ø Current transformer.
Ø Welding transformer.
Step down transformer
Ø Distribution system
Ø Potential transforms.
PART-B
1.
Explain in details the construction
and operating principle of a transformer(DEC 2003, JUNE 2010)
Working
Principle = 2 Marks
Diagram = 4 Marks
Constructional
Features = 2 Marks
Theory = 8 Marks
2.
Derive the EMF equation for a
transformer (DEC 2003, JUNE 2010)( MAY 2011)
How EMF is
developed = 2 Marks
Derivation = 4 Marks
Final
Eqn with units = 2 Marks
3.
Derive the equivalent circuit of
transformer (DEC 2007), (DEC 2010)
Diagram(4
Steps) = 8 Marks
Equation = 4 Marks
Theory = 4 Marks
4.
Explain with help of circuit diagrams
how efficiency and regulation of single phase transformer are predetermined by
conducting open and short circuit tests?
(MAY 2011)
OC
Test:
Diagram(2) = 4 Marks
Equation = 2 Marks
Theory = 2 Marks
SC
Test:
Diagram(1) = 4 Marks
Equation = 2 Marks
Theory = 2 Marks
5.
Explain how the efficiency of a
transformer may be found from the open circuit and short circuit tests(MAY
2008)
OC
Test:
Diagram(2) = 4 Marks
Equation = 2 Marks
Theory = 2 Marks
SC
Test:
Diagram(1) = 4 Marks
Equation = 2 Marks
Theory = 2 Marks
6.
Describe the tests to determine core
loss and full load copper loss of a transformer (or) Explain the OC and SC
tests performed on a transformer (Nov/Dec 2011)
OC
Test:
Diagram(2) = 4 Marks
Equation = 2 Marks
Theory = 2 Marks
SC
Test:
Diagram(1) = 4 Marks
Equation = 2 Marks
Theory = 2 Marks
7.
Explain the working of single phase
transformer with neat sketch (Nov/Dec 2011)
Working
Principle = 2 Marks
Diagram = 4 Marks
Constructional
Features = 2 Marks
Theory = 8 Marks
8.
Define voltage regulation. Draw the
phasor diagram of lagging power factor and determine voltage regulation.
Voltage
Regulation = 2 Marks
Phasor
Diagram = 6 Marks
Theory = 6 Marks
Equation = 2 Marks
9.
Write short notes on (DEC 2006)
a.
Transformer on no- load b. Transformer on load
Diagram = 4 Marks
Theory = 8 Marks
Equation = 4 Marks
10.
Discuss about power losses in a
transformer.(DEC 2009)
Theory = 4 Marks
Equation = 4 Marks
11.
A 10KVA, 200/400V, 50Hz single phase
transformer gave the following test results.
OC
Test: 200V, 1.3A, 120W...............on L.V Side
SC
Test: 22V, 30A, 200W..................on H.V side
Calculate
(i). The magnetizing current and the component corresponding to core loss at normal
frequency and voltage (ii)The magnetizing branch impedances and (iii)% voltage regulation
when supplying full load at 0.8 pf leading.(DEC 2009).
Formula = 2 Marks
Steps = 3 Marks
Final
Answer = 2 Marks
Unit = 1 Mark
12.
A single phase 20KVA, 2200/220V, 50Hz
transformer. The O.C/S.C test results are as follows.
O.C
test: 220V, 4.2A, 148W........ (L.V.
Side) S.C test: 86V,
10.5A, 360W...... (H.V.Side)
Determine
(i).Regulation and efficiency at 0.8 p.f lagging at full load and (ii) Power
factor on short circuit (iii) obtain the approximate equivalent circuit
referred to H.V.side.
Formula = 2 Marks
Steps = 3 Marks
Final
Answer = 2 Marks
Unit = 1 Mark
13.
A 600KVA single Ф transformer has an
efficiency of 94% both at full load and half load at unity power factor.
Determine the efficiency at 75% of full load at 0.9 power factor.
Formula = 2 Marks
Steps = 3 Marks
Final
Answer = 2 Marks
Unit = 1 Mark
14.
The OC and SC tests on a 4KVA,
200/400V, and 50Hz single phase transformer gave the following results: OC test
on LV side: 200V, 1A, 100W.SC test with LV side shorted: 15V, 10A, 85W.Determine
the parameters of the equivalent circuit and draw the equivalent circuit
referred to LV side.
Formula = 2 Marks
Steps = 3 Marks
Final
Answer = 2 Marks
Unit = 1 Mark
15.
The parameters of a 2300/230V, 50-Hz
transformer is given below:
R1=0.286Ω R2’ =0.319Ω Ro=250Ω
X1=0.73Ω X2’=0.73Ω X0=1250Ω
The
secondary load impedance ZL= 0.387+j0.29. Solve the exact equivalent
circuit with normal voltage across the primary.
Formula = 2 Marks
Steps = 3 Marks
Final
Answer = 2 Marks
Unit = 1 Mark
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