UNIT-1
Single – Phase Transformers
1 Mark Questions
1.
2.
3.
4.
5.
6.
7.
8.
State Lenz’s law.
What are the types of magnetic losses?
Define voltage regulation of a transformer
What is the function of transformer oil in a transformer?
State Faradays First law.
Define transformation ratio in transformer
The phase angle of emf induced in the windings with respect to main flux is ---Transformer action requires
a) Constant magnetic flux
b) Increasing magnetic flux
c) Alternating magnetic flux
d) Alternating electric flux
9. The use of grain oriented laminated sheets in building a transformer core is ---10. To step 110 V ac down to 20 V ac, the turns ratio must be…..
11. If 25 W of power are applied to the primary of an ideal transformer with a turns ratio of 10,
the power delivered to the secondary load is…..
12. In a certain loaded transformer, the secondary voltage is one-fourth the primary voltage. The
secondary current is
a) one-fourth the primary current
b) four times the primary current
c) equal to the primary current
d) one-fourth the primary current and equal to the primary current
13. A transformer
a) changes ac to dc
b) changes dc to ac
c) steps up or down dc voltages
d) steps up or down ac voltages
14. Does the flux in a transformer core increases with load?
15. Why transformer is known as a static device?
16. If N1 = 10 turus ; transformation ratio is 2 ; what is N2
17. What is the angle by which no-load current will lag the ideal applied voltage?
18. What is meant by hysteric angle?
19. What are the basic requirements for transformer action.
20. What is the significance of Transformer in Powersystems.
2 Mark Questions
1. Why transformers are rated in kVA?
2. What is basic principle of Transformer?
3. Write equation for generated emf in secondary and primary windings for Transformer.
4. A 100 KVA, 1100/400V, 50Hz single phase transformer has 100 turns on the secondary
winding. The number of turns on its primary will be….
5. Draw phasor diagram for Ideal transformer on load.
6. A 400V/200V transformer has total resistance of 5Ω on its L.V. side. The resistance when
referred to H.V. side would be……….
7. Differentiate step up and step down Transformers?
8. A transformer has a 1:6 turns ratio and a secondary coil load resistance of 470 . The load
resistance as seen by the source is ------------9. Write different methods of Cooling.
10. Draw the no-load phasor diagram of a transformer.
11. Draw the regulation Vs p.f char. of a transformer.
12. Can the voltage regulation of a transformer go to negative? If so under what condition?
13. Why V1:V2≠N1:N2 in a real (practical) transformer?
14. What are the two components of no load current in transformer?
15. Draw the equivalent circuit of transformer
16. Define the turns-ratio of a transformer and relate with current ratio
17. What is the value of the magnetic flux in the side core of a shell type transformer, if the
flux in the central core is Φ?
18. Can you apply D.C supply in a single phase transformer? Give reason for your answer.
19. What do you mean by equivalent resistance of a transformer referred to primary?
20. State the condition under which transformer voltage regulation is zero.
5 Marks Questions
1. Distinguish between core type and shell type transformers.
2. Explain how eddy current and hysterisis losses of a transformer can be minimized.
3. Derive expression for Maximum Voltage regulation in Transformer.
4. A 2200/220 V, 50 Hz, single phase transformer has exciting current of 0.6 A and a core
loss of 440Watts, when its HV winding is energized at rated voltage. Calculate the two
components of the exciting current.
5. List assumptions involved in Ideal Transformer.
10 Marks Questions
1. a) Derive the equivalent circuit of a transformer from fundamentals.
b) A 40KVA, 6600/250V, 1- transformer has the following parameters.
Primary and secondary resistances of 8 Ω and 0.02 Ω respectively. Primary and
secondary leakage reactances of 15 Ω and 0.05 Ω respectively. Find full load regulation
at p.f. at 0.8 lagging.
2. a) Derive an expression for the induced e.m.f. in a transformer winding. Show that the
e.m.f /turn in primary winding is equal to e.m.f /turn in the secondary.
b) A 6300/210V, 50Hz, single phase transformer has per turn e.m.f of about 9.0 volts
and maximum flux density of 1.2 tesla. Find the number of high voltage and low
voltage winding turns and the net cross-sectional area of the core.
3. a) Derive an expression for the regulation of a transformer.
b) A 3300/1100V, single phase transformer has leakage impedance of Ze= (4+j40)
ohms referred to H.V side. When a unity power factor load of 20KW is connected on
the L.V. side, the terminal voltage is 1050 V. Calculate the voltage and power factor
as measured on its H.V terminals.
4.
a) Explain about different cooling techniques involved in Transformer designing.
b) A 20 KVA, single phase transformer has 200turns on the primary and 40turns on the
secondary. The primary is connected to 1000V, 50Hz supply. Determine i) The secondary
voltage on open circuit. ii) The maximum value of flux.
5. a) Draw the vector diagram of a transformer on under no load condition and explain
about each component?
b) The equivalent circuit parameters of 400/4000V transformer as follows. Req = 0.1
ohms, Xeq = 0.6 ohms, Xo=250 ohms, Ro=500 ohms. All are referred to LV side. If load
impedance is (400+j300) ohms; Find the secondary load voltage and primary current.
6. a) Draw the vector diagram of a transformer on under full load condition and explain
about each component?.
b) A single – phase transformer of 20KVA has 200 turns on the primary and 40 turns
on the secondary. The primary is connected to 1000V, 50Hz supply. Determine (i) the
secondary voltage on open circuit. (ii) The currents flowing through the two windings
on full load (iii) The maximum value of flux.
7. a) Define voltage regulation of a transformer. And hence find the power factor for
which zero voltage regulation.
b) The equivalent circuit parameters of 2 KVA 200/400V transformer are as follows :
Req = 0.16 
Xeq = 0.8 
Xo = 210 
Ro = 400 
All values are referred to L.V. side. If the transformer is loaded at full load
(i) 0.8 p.f lag
(ii) 0.8 p.f. lead
find its voltage regulation.
8. A 15kVA, 2200/110V transformer has R1= 1.75Ω, R2=0.0045Ω. The leakage
reactances are X1= 2.6Ω and X2=0.0075Ω. Calculate, i) Equivalent resistance referred
to primary ii) Equivalent resistance referred to secondary iii) Equivalent reactance
referred to primary (iv) Equivalent reactance referred to secondary (v) Equivalent
impedance referred to primary (vi) Equivalent impedance referred to secondary.
9. a) Why Transformers are rated in KVA? Explain.
b) A 250/125V, 5kVA single phase transformer has primary resistance of 0.2Ω and
reactance of 0.75Ω. The secondary resistance is 0.05Ω and reactance of 0.2Ω. Determine
its regulation while supplying full load on 0.8 leading p.f.
10. a) Derive Electrical equivalent circuit of Transformer referred to secondary side from
Exact Electrical equivalent circuit.
b) A 10 KVA, 2500/250 V, single phase transformer has resistances and leakage
reactances as follows: primary winding (HV side): r1 = 4.8 Ω, x1 = 11.2 Ω
Secondary winding (LV side): r2 = 0.048Ω. X2 = 0.112 Ω. Calculate Effective impedence of
Transformer referred to primary and secondary.
UNIT-II
Performance and Equivalent circuit analysis of single phase
transformers
1 Mark Questions
MID-1
1. What are the factors that effect core losses of a transformer?
2. Write the expression for efficiency of a transformer.
3. Define all day efficiency.
4. Give the expression for Maximum efficiency conditon
5. Mention the factors as which Hysterisis losses depends.
6. Mention the factors as which eddy current losses depends.
7. Did copper losses are dependent as power factor.
8. The efficiency of a transformer is always higher than that of rotating electric machines,
why?
9. What is the condition for obtaining maximum efficiency of a transformer?
10. Are there any mechanical losses in a transformer? Why?
11. What are the constant losses that occur in transformers?
MID-2
1. What is the purpose of short circuit test as transformer?
2. In O.C test in single phase transformer why do you use low power factor watt meter to
measure the power?
3. The efficiency of two identical transformers under load conditions can be determined by
which test?
2 Mark Questions
MID-1
1. If FL copper losses of a transformer are 100 W; What is the value of copper losses at ½ Full
load?
2. Full load copper loss in a transformer is 1600 watts. What will be the loss at half load ?
3. Define regulation and efficiency of a transformer.
MID-2
1.
2.
3.
4.
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6.
7.
8.
9.
Why SC test is conducted on high voltage side?
List the advantages of Auto transformers.
State two specific uses of autotransformers in the field of electrical engineering
What are the conditions for parallel operation of transformers?
What is the purpose of open circuit test on transformer?
What are the advantages of ‘Sumpners’ Test?
Compare two winding transformer and auto-transformer.
What are the advantages of auto-transformer over ordinary transformer?
Write down the volt-ampere transferred inductively and volt-ampere transferred conductively
in an auto-transformer.
10. Mention the applications of auto-transformer.
11. What are the advantages of O.C and S.C test of a transformer than the load test?
12. State why the open circuit test on a transformer is conducted at rated voltage?
5 Marks Questions
MID1
1. State the different losses take place in a single phase transformer. How do they vary
with load?
2. A 400kVA, distribution transformer has full load iron loss of 2.5kW and copper
loss of 3.5kW. During a day, its load cycle for 24 hours is,
6 hours 300kW at 0.8 p.f
10 hours 200kW at 0.7p.f
4 hours 100kW at 0.9p.f
4 hours No load
Determine its all day efficiency
MID 2
1. Derive the expression for copper saving in an auto – transformer.
2. List out the reasons that necessitate parallel operation of Transformers.
3. List out the conditions have to be met before two or more Transformers are
connected in parallel.
10 Marks Questions:
MID1
1. a) Derive expression for maximum efficiency in Transformer.
b) A 1- transformer is rated at 100 KVA, 5000/250V. The full-load copper losses
are 2000W and iron losses are 1200W. Find efficiency at
i) Full – load unity p.f. ii) Full –loaded 0.8p.f. lag
MID2
1.
2.
4.
5.
6.
a) Explain OC and SC test for a 1- transformer with relevant figures and equations.
b) In a sumpner’s test on two identical 1-  transformers rated 500KVA, 11/0.4 kV, 50
HZ, the wattmeter reading on HV side is 6000W and on LV side 15, 000W. Find the
efficiency of each transformer on half – load and 0.8 p.f. What will be its maximum
efficiency?
a) Explain sumpner’s test.
b) A 100 KVA, 6600 / 330V; 50 Hz; 1-  transformer took 10A and 436W at 100V in
a SC test, the values referring to the h.v. side. Calculate the voltage to be applied to the
h.v. side on full load at 0.8 pf lag, when the secondary terminal voltage is 330V?
a) Explain the principle of operation and advantages of an Auto transformer.
b) Two 1-  transformers with equal turns have impedances of (0.5 +j3)  and
(0.6+j10)  with respect to secondary. If they operate in parallel, determine how they
will share a total load of 100 kW at 0.8 pf lagging.
A 4KVA, 200/400V, 50Hz single phase transformer gave the following results on opencircuit and short circuit tests
open circuit test
(L.V side)
200V 0.7A 70W
short circuit test
(H.V side)
15V 10A 80W
Find the parameters of the approximate (L.V side) equivalent circuit referred to L.V side.
a) What are the core losses in a transformer? How are the core losses separated
experimentally?
b) Describe the method of calculating the regulation and efficiency of a single phase
transformer by open circuit and short circuit tests?
7.
8.
Explain with neat circuit diagram, how Sumpner’s test is performed? What are its
advantages over O.C and S.C tests? What is its limitation?
a) What is an auto transformer?
b) A 4 KVA, 1 - phase transformer rated at 50 Hz, 200V/ 400V gave the following data.
S.C. Test
:
9V,
6A,
21.6 watts
(Measured on HV side)
O.C. Test
:
(Measured on LV side)
200V, 0.7A,
60 watts
Draw the approximate equivalent circuit referred to L.V. Side. And Compute the
regulation of voltage at full load 0.9 p.f. leading and efficiency at full load u.p.f.