EEE
Rony Parvejs
Lecture 3 & 4: Transformer Update: 30 April, 2015
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Rony Parvejs EEE Job Preparation Lecture 3 & 4: Transformer
TRANSFORMER
1
What is the voltage at secondary side of a transformer having a turn ratio of 1:10 if
440V dc is applied at primary?
(a) 0 V (b) 4400 V
(c) 44 V (d) 4.4 V
DPDC-14
2
What would be your comment regarding the size, of a 1 kVA. 400 Hz transformer and a
1 kVA, 50 Hz transformer?
(a) 400 Hz transformer is smaller in size
(b) 50 Hz transformer is smaller in size
(c) both the transformer are of equal size
(d) no comment can be made without seeing the transformer
(e) none of the above
MCQ BPDB-
13
3 Draw an equivalent circuit of transformer referred to secondary. DWASA-14
4 Draw the vector diagram of a single phase step down transformer. BPDB-11
5
A 400 V/100 V, 500 VA single phase transformer has an equivalent impedance of 0.5
p.u. If the primary winding impedance is 0.1 ohm ,find the secondary winding
impedance in ohm both referred to secondary.
NWPGCL-14
6 What are the no load and full load losses of transformer? PGCB-14
7 What are the load and no load losses of a transformer? What is all day efficiency of a
transformer?
BUET M.Sc.
13
8 Maximum efficiency of a transformer is 98%. It runs at 50% efficiency for 14 hours
and at no load for remaining 10 hours. Find its all day efficiency.
PGCB-14
9 A 30 kVA, 2400/240 V, 50 Hz single phase transformer, operating at no load, draws
138 W at a p.f. of 0.21 lagging. Determine
i) the exciting current of the transformer
ii) the core loss component of the exciting current and
iii) the magnetizing component of the exciting current.
BPDB-13
10 A single phase 1000/200 V takes a no load current of 3A at 0.2 power factor lagging.
Calculate the primary current and power factor when secondary current is 280 A at
power factor 0.8 lagging.
EGCB-2014
11 A single phase transformer with a ratio of 100/200 V takes a no-load current of 3A at
0.2 power factor lagging. If the secondary supplies a current of 280 A at a p.f. of 0.8
lagging, estimate the current taken by the primary.
BPDB-2015,
EGCB-12
12 The equivalent circuit impedances of a 20kVA, 8000/240V, 60Hz transformer are to be
determined. The open circuit test and the short circuit test were performed on the
primary side of the transformer, and the following data were taken:
Open circuit test (primary) Short circuit test
VOC = 8000 V VSC = 489 V
IOC = 0.214 A ISC = 2.5 A
POC = 400 W PSC = 240 W
Find the impedance of the approximate equivalent circuit referred to the primary side,
and sketch the circuit. [Example 2.2, Chapman 4th edition]
BPDB-14
(FF)
13
In short circuit test of a transformer in which side supply voltage is applied?
(a) Low voltage side
(b) High voltage side
DPDC MCQ-
14
In short circuit test of single phase transformer, the wattmeter reading roughly gives MCQ
Rony Parvejs EEE Job Preparation Lecture 3 & 4: Transformer 14 (a) Copper loss (b)Core loss (c) Both (d)None PGCB-14
15
In open circuit test of transformer power supply is given in
(a) low voltage side (b) high voltage side (c) (d)
MCQ
PGCB-14
16
Practically in open circuit test of a transformer, the wattmeter measures
(a) Copper loss (b) Core loss
(c) A slight copper loss (d) both (b) & (c)
DPDC MCQ-
14
17 Why transformer is rated in KVA? What are the instrument transformers? PGCL-11
18 Write down the conditions of parallel operation of transformers. BUET M.Sc.
Unknown
19
Show the direction of the current in primary and polarity of the load initially:
PGCB-14
20
Which of the following is not an example of transformer connection?
(a) Open delta - Y (b) (c) (d)
MCQ-
DWASA-14
21
Dy1 means-
(a) Primary is delta and secondary is star connected and secondary leads primary by 30
(b) Primary is delta and secondary is star connected and primary leads secondary by
30
MCQ
EGCB-14
22
What do you understand by DY1
(a) High voltage side is leading low voltage side by 30
(b) Low voltage side is leading high voltage side by 30
MCQ
EGCB-14
23 In a 3-phase transformer DY1 means- MCQ
EGCB-12
24
UNIT transformer at power generation is used as an-
(a) energy meter (b)step up transformer (c) auxiliary transformer
MCQ
PGCB-14
25
Why do we ground Y-neutral in D-Y transformer? (i) for flowing zero sequence current
(ii)
MCQ BPDB-
14 (FF)
26 What are the disadvantages of 3-phase Y-Y connected transformer? How this problem
can be solved?
BUET
M.Sc.12
27 A 500KVA, 13200/480 V distribution transformer has four taps 2.5% on its primary
winding. What are the voltage ratios of this transformer at each tapings?
DWASA-14
Rony Parvejs EEE Job Preparation Lecture 3 & 4: Transformer
1
What is the voltage at secondary side of a transformer having a turn ratio of 1:10 if
440V dc is applied at primary?
(a) 0 V (b) 4400 V
(c) 44 V (d) 4.4 V
DPDC-14
A transformer cannot raise or lower the voltage of a D.C. supply because Faraday's laws of electromagnetic induction
are not valid since the rate of change of flux is zero.
As transformers require an alternating magnetic flux to operate correctly, transformers cannot therefore be used to
transform or supply DC voltages or currents, since the magnetic field must be changing to induce a voltage in the
secondary winding. In other words, Transformers DO NOT Operate on DC Voltages. So, the output is zero.
Reference: Basic Electrical Engineering U.A. Bakshi
2
What would be your comment regarding the size, of a 1 kVA. 400 Hz transformer and a
1 kVA, 50 Hz transformer?
(a) 400 Hz transformer is smaller in size
(b) 50 Hz transformer is smaller in size
(c) both the transformer are of equal size
(d) no comment can be made without seeing the transformer
(e) none of the above
MCQ BPDB-
13
Due to some mechanical constraints, generally transformer size is inversely proportional to the operating
frequency. Thus 50 hz, transformer will have max. size.
The impedance of the transformer is greater for higher frequencies (as X=wL). So for constant voltage to keep
same current rating (because for constant kVA) we have to reduce the inductance of the winding. And to reduce the
inductance we have to reduce the no. of turns in the windings of the transformer. By reducing the no. of turns the size
of the transformer will be reduced for higher frequencies.
This is a short cut method based on analysis. generally power transformers use lower frequency (50hz) and we
know in electronic ckts like oscillators producing very high frqs in range of KHz we use only SMALL trfrs than pwr
trfrs.... so on analysing we can conclude tat higher frquency transformers have less volume...
NOTE: this is only shortcut method, actual reason depends on the FORMULA only for determining the size
Rony Parvejs EEE Job Preparation Lecture 3 & 4: Transformer 3 Draw an equivalent circuit of transformer referred to secondary. DWASA-14
Figure: Equivalent transformer circuit referring to the primary
Figure: Equivalent transformer circuit referring to the secondary
Reference: Electric Machinary Fundamentals - Chapman
4 Draw the vector diagram of a single phase step down transformer. BPDB-11
Figure: vector diagram of a single phase step down (VP>VS) transformer at lagging power factor.
Rony Parvejs EEE Job Preparation Lecture 3 & 4: Transformer
Figure: vector diagram of a single phase step down (VP>VS) transformer at leading power factor.
Figure: vector diagram of a single phase step down (VP>VS) transformer at unity power factor.
:
Figure: vector diagram of a single phase step down (VP>VS) transformer at no load.
Rony Parvejs EEE Job Preparation Lecture 3 & 4: Transformer
5
A 400 V/100 V, 500 VA single phase transformer has an equivalent impedance of 0.5
p.u. If the primary winding impedance is 0.1 ohm, find the secondary winding
impedance in ohm both referred to secondary.
NWPGCL-14
p.u
6 What are the no load and full load losses of transformer? PGCB-14
No Load Losses:
Iron loss: Hysteresis and eddy current loss
Very small amount of copper loss also occurs in primary
Full Load losses:
At full load, Copper loss is largest and a very small amount of eddy current loss is there.
7 What are the load and no load losses of a transformer? What is all day efficiency of a
transformer?
BUET M.Sc.
13
A transformer has mainly two types of losses, t
