Its all about electrical machine engeering course

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its about the math problems. All i need is the correct process and answer. Please make sure that not a single question is left out. Thank yo

Written Assessment Task 1
TAFE
Student Name:
Section B (Total Marks 20)
Blank spaces in the following statements represent omissions. Write the appropriate information in
the space provided on your answer sheet. Each response is worth one mark
Corrent
Transformers are connected in parallel to provide more toad
vottage
at the same
Transformers output is rated in KVA
factor
so they can be connected to a load of any power
cross
In a double wound transformer, the higher voltage winding will have a
turns than the lower voltage winding
sectional area conductor and coble
Tapped
transformers are used in the power supply network to compensate for
voltage drop in the transmission system
The conditions to be fulfilled to connect identical single phase transformers in parallel are
and
An advantage of an auto-transformer over a double wound transformer is the auto-transformer
takes less
to manufacture
transformer must always be short circuited before
The secondary of a
disconnecting its associated meter.
The Australian standard secondary current of an instrument type current transformer is
amperes.
O
transformers should always be
The primary circuit of instrument type
protected with fuses.
o
The notation Yy0 indicates that a three phase transformer is connected
with
phase shift.
.
steel is used for the laminations in a transformer core.
sheet
e voltage
The secondary voltage of a transformer is produced by se
Page 6 of 9
Last Updated: 12 March 2018
Written Assessment Task v1
TAFE
Written Assessment Task 1
Student Name:
short circuit the secondary terminals
short circuit the primary terminals
open circuit the secondary leads
(D)
open circuit the primary leads
9. If an open circuit occurs in one phase of a delta connected three phase bank of transformers,
then:
(A)
a two phase supply only is available
(B)
a three phase supply can be maintained at reduced load
the output voltage rises considerably
a normal three phase supply can be maintained at full load
10. When an auto-transformer is properly designed, one of the features compared to a double
wound transformer of the same rating is:
(A)
larger in physical size
requires less material to manufacture
has a lower efficiency
has higher losses
11. In an auto-transformer the current in the primary is 10 amperes and the current in the secondary
is 20 amperes; the current in the common part is:
30 amperes
20 amperes
(C)
15 amperes
(D)
10 amperes
Oce
WWWW
ese
12. The iron loss of a transformer can be determined by measuring the power taken by the
transformer when:
the secondary is open circuited and normal voltage is applied to the primary
winding
the secondary is open circuited and half normal voltage is applied to the primary
winding
(C)
the secondary is short circuited
(D) normal voltage is applied to the secondary
(A)
B)
13. A transformer is rated at 20 kVA. The power output at full load and 0.8 power factor is:
25 kW
(B
20 kW
16 kW
8 kW
(D)
TAFE
Written Assessment Task 1
Student Name:
14. If the iron losses in a transformer are 400 watts, the copper losses at maximum efficiency will
be:
B)
@ce
500 watts
400 watts
300 watts
200 watts
(D)
15. For two transformers to equally share a load, the regulation of one to the other should be:
Equal
(C)
(D)
Twice
Half
Quarter
16. Regulation of transformers relates to the:
(A)
change in primary current
change in secondary voltage to open circuit voltage
change in primary current to secondary current
change in secondary no load voltage to rated full load voltage
17. A Dy 11 transformer has:
a delta primary star secondary voltage lagging primary voltage by 30°
a delta primary star secondary with secondary voltage leading primary voltage by
30°
a delta secondary star primary with primary voltage leading secondary voltage by
30°
a delta secondary star primary with primary voltage lagging secondary voltage by
30°
(C)
(D)
wu
18. What is the effect of magnetic leakage?
The transformer oil needs to cleaned up regularly
(B)
The secondary voltage wont be as high as expected
The primary voltage wont be as high as expected
Other magnetic devices will be affected
(A)
19. When doing an insulation resistance test on a transformer:
Between any coil and the core should be greater than 1MS2
Between the two ends of the primary winding should be more than 1012
The earth wire resistance should be less than 222
Between primary and secondary winding should be greater than 5012
A
(D)
20. An isolation transformer is one that:
(A)
Has a common winding like an auto-transformer
Has both the primary and secondary windings earthed
Only has the secondary winding earthed
Only has the primary winding earthed
(B)
(D)
Last Updated: 12 March 2018
Written Assessment Task v1
ritten Assessment Task 1
TAFE
Student Name:
Section A (Total Marks 20)
For multiple choice questions write the correct answer in the space provided on your answer sheet.
Each question is worth one mark.
1. High grade silicon steel is used for transformer cores because it:
reduces hysteresis loss.
keeps the iron loss constant
increases convection currents in the cooling oil
is cheaper than ordinary steel
2. The no-load current of a transformer is:
(A)
magnetizing current only
(B)
iron loss current only
the sum of the magnetising and iron loss currents
(D the phasor sum of the magnetizing and iron loss currents
3. The primary and secondary currents of a single phase transformer are approximately:
(A)
in phase
(B)
60 degrees out of phase
(C)
90 degrees out of phase
D 180 degrees out of phase
4. With increasing non-inductive load the secondary terminal voltage of a single phase transformer
would:
(A) remain unchanged
(B)
rise
(С
fall slightly
become unstable
wwwww
5. The efficiency of a transformer:
(A
is constant over a wide range
(B) varies with the load
(C)
varies with the iron losses
(D)
has a maximum of 90 per cent
6. The standard secondary voltage of an instrument type potential transformer is:
(A)
415 volts
(B) 240 volts
120 volts
110 volts
7. The secondary current of a standard current transformer is:
0.6 amperes
(B) 5 amperes
10 amperes
(D)
25 amperes
8. If the instrument is to be removed from the secondary of a current transformer it is necessary to:
Page 3 of 9
Last Updated: 12 March 2018
Written Assessment Task v1
Written Assessment Task 1
TAFE
Student Name:
8. Three single phase 11kV/240V transformers are connected delta-star to a three phase 11kV
supply. If a three phase delta connected load having a phase impedance of 4012 is connected,
find:
Primary phase voltage
Secondary phase voltage
Secondary line voltage
Secondary phase current
Primary phase current
(10 Marks)
Primary line current
UGA
9. Redraw the three single phase transformer on your answer sheet and connect them in Y/D
formation.
(7Marks)
A B
С
Supply
| |
C
A1
B:
C2
Az
HV
HV
HV
ICS
LV
ma
b:
Figure 2 b 2
C1
a2
C2
di
END
Page 9 of 9
Last Updated: 12 March 2018
Written Assessment Task v1
Written Assessment Task 1
TAFE
Student Name:
Section C (Total Marks 40)
All working must be shown on your answer sheet. Total marks available for this section is 40
marks. Marks are shown for each question.
1. A transformer with a core flux of 5 milliwebers has a primary winding of 1 000 turns. Calculate
the primary voltage if the transformer operates on a 50 hertz supply.
(3 Marks).
2. Using a phasor diagram having a suitable scale, calculate the primary current and power
factor of a transformer with the following specifications:
Type of transformer single phase.
Primary turns 360
Secondary turns 45
Primary voltage 240V
No-load current 2A
No-load phase angle 75.50 lag
Secondary current 30A
Secondary p.f. 0.866 lag
(10 Marks)
3. A 50 kVA transformer has a full load copper loss of 950 W and an iron loss of 600 W. Calculate
the efficiency at 80% load if operating at unity power factor. (4 Marks)
4. The open circuit voltage of a transformer secondary winding is 255 volts and at full load the
voltage falls to 240 volts. Calculate the percentage regulation.
(3 Marks)
5. A 240 V to 120 V auto-transformer supplies a 20 ohm resistive load. Calculate:
the secondary current
the transformation ratio
the current in the common section of the winding
(3 Marks)
6. The maximum efficiency of a transformer occurs when the input equals 10 800 watts and the
load is supplied with 10 000 watts. Calculate:
copper loss
(2 Marks)
iron loss
7. A single phase transformer is rated at 25 kVA, 250 volts, 50 hertz. Determine:
power output at unity power factor
rated full load secondary current
power output at 0.8 power factor
• secondary current when supplying 20 kW at 0.8 power factor leading (8 Marks)
Page 8 of 9
Last Updated: 12 March 2018
Written Assessment Task v1

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