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All questions of Transformer for Electrical Engineering (EE) Exam
A transformer designed for operation of 60 Hz supply is working on 50 Hz supply system without changing its voltage and current ratings. When compared with full-load efficiency at 60 Hz, the transformer efficiency on full load at 50 Hz will- a)increase by a factor of 1.2
- b)decrease marginally
- c)remain unaltered
- d)increase marginally
Correct answer is option 'D'. Can you explain this answer?
A transformer designed for operation of 60 Hz supply is working on 50 Hz supply system without changing its voltage and current ratings. When compared with full-load efficiency at 60 Hz, the transformer efficiency on full load at 50 Hz will
a)
increase by a factor of 1.2
b)
decrease marginally
c)
remain unaltered
d)
increase marginally
 | Yash Patel answered |
For a core-type power transformers, both primary and secondary windings have circular coil sections, because this section- a)has the strongest mechanical shape.
- b)results in less core material and, therefore, less core loss.
- c)is easier to wind.
- d)require minimum conductor material and, therefore, less I2R loss.
Correct answer is option 'D'. Can you explain this answer?
For a core-type power transformers, both primary and secondary windings have circular coil sections, because this section
a)
has the strongest mechanical shape.
b)
results in less core material and, therefore, less core loss.
c)
is easier to wind.
d)
require minimum conductor material and, therefore, less I2R loss.
 | Lavanya Menon answered |
Primary and secondary windings of a core type transformer have circular coil section to reduce the mean length of each turn so that it require minimum conductor material and thus have less copper loss which will increase the efficiency.
The emf per turn of a single phase 10 kVA, 2200/220 V, 50 Hz transformer is 10 V. The net cross-sectional area of core for a maximum flux density of 1.5 T is- a)300 cm2
- b)200 cm2
- c)100 cm2
- d)25 cm2
Correct answer is option 'A'. Can you explain this answer?
The emf per turn of a single phase 10 kVA, 2200/220 V, 50 Hz transformer is 10 V. The net cross-sectional area of core for a maximum flux density of 1.5 T is
a)
300 cm2
b)
200 cm2
c)
100 cm2
d)
25 cm2
 | Jyoti Basak answered |
Maximum flux,
∴ New cross-sectional area of core,
∴ New cross-sectional area of core,
Which of the following is not true relating to an ideal transformer?- a)The iron loss in an ideal transformer is zero
- b)The winding resistance has a zero value
- c)The leakage reactance has a non-zero value
- d)The magnetizing current is zero
Correct answer is option 'C'. Can you explain this answer?
Which of the following is not true relating to an ideal transformer?
a)
The iron loss in an ideal transformer is zero
b)
The winding resistance has a zero value
c)
The leakage reactance has a non-zero value
d)
The magnetizing current is zero
| | Kajal Yadav answered |
In an ideal transformer there is no-loss (either in winding or core). The leakage reactance is also zero (as no-voltage drop).
Magnetizing current is zero due to infinite permeability of core.
Magnetizing current is zero due to infinite permeability of core.
A 10 kVA, 400 V/ 200 V, 1 -phase transformer with a percentage resistance of 3% and percentage reactance of 6% is supplying a current of 50 A to a resistive load. The value of the load voltage is- a)390 V
- b)194 V
- c)196 V
- d)192 V
Correct answer is option 'B'. Can you explain this answer?
A 10 kVA, 400 V/ 200 V, 1 -phase transformer with a percentage resistance of 3% and percentage reactance of 6% is supplying a current of 50 A to a resistive load. The value of the load voltage is
a)
390 V
b)
194 V
c)
196 V
d)
192 V
 | Manoj Chaudhary answered |
Hence, load voltage = 194 volt
The ohmic drop of a transformer is 1% while the reactance drop is 5% of the voltage. The regulation of the transformer at 0.8 power factor leading and 0.8 power factor lagging are respectively- a)2.2 % and -3.8%
- b) -3.4% and 4.6 %
- c)-2.2 % and 3.8%
- d) 3.4 % and -4.6 %
Correct answer is option 'C'. Can you explain this answer?
The ohmic drop of a transformer is 1% while the reactance drop is 5% of the voltage. The regulation of the transformer at 0.8 power factor leading and 0.8 power factor lagging are respectively
a)
2.2 % and -3.8%
b)
-3.4% and 4.6 %
c)
-2.2 % and 3.8%
d)
3.4 % and -4.6 %
 | Tanvi Rane answered |
A 50 MVA, 76.2/33 kV, 1-φ, 50 Hz, two-winding transformer with tap changer has percentage impedance of (0.5 + j7.0). What tapping must be used to maintain rated voltage at the secondary on full load at 0.8 lagging power factor?- a)3.8 % down
- b)3.8 % up
- c) 4.6 % up
- d)4.6 % down
Correct answer is option 'D'. Can you explain this answer?
A 50 MVA, 76.2/33 kV, 1-φ, 50 Hz, two-winding transformer with tap changer has percentage impedance of (0.5 + j7.0). What tapping must be used to maintain rated voltage at the secondary on full load at 0.8 lagging power factor?
a)
3.8 % down
b)
3.8 % up
c)
4.6 % up
d)
4.6 % down
 | Harshitha Pillai answered |
Hence, voltage to be raised, for maintaining rated voltage = 4.6%.
Thus, tap setting required on the HV side = -4.6% or 4.6 % down.
A 1 kVA, 400 Hz transformer is desired to be used at a frequency of 60 Hz. The kVA rating of the transformer at this reduced frequency is- a)82.5 VA
- b)550 VA
- c)150 VA
- d)500 VA
Correct answer is option 'C'. Can you explain this answer?
A 1 kVA, 400 Hz transformer is desired to be used at a frequency of 60 Hz. The kVA rating of the transformer at this reduced frequency is
a)
82.5 VA
b)
550 VA
c)
150 VA
d)
500 VA
 | Siddharth Khanna answered |
kVA rating of transformer,
or,
Since V ∝ f, therefore, S ∝ f
∴
or,
Since V ∝ f, therefore, S ∝ f
∴
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