EM1 POSSIBLE QUESTIONS

D EPARTMENT OF ELECTRICAL AND
ELECTRONICS
ENGINEERING

QUESTION BANK

SUB.NAME: ELECTRICAL MACHINES -I

BRANCH : ELECTRICAL AND
ELECTRONICS ENGINEERING
YEAR : II
SEMESTER : IV

UNIT I

INTRODUCTION
PART – A
1. Mention the types of electrical
machines.
2. State Ohm’s law for magnetic
circuit.
3. Define leakage flux
4. Define magnetic reluctance
5. State stacking factor.
6. Mention some magnetic materials
7. What is magnetostriction?
8. Define statically induced emf.
9. Define dynamically induced emf.
10. State Fleming’s right hand rule.
11. State Fleming’s Left hand rule.
12. What are the losses called as core
loss?
13. Define coercivity.
14. What are factors on which
hysteresis loss?
15. What is core loss? What is its
significance in electric machines?
16. What is eddy current loss?
17. How hysteresis and eddy current
losses are minimized?

PART – B
1. (a) Explain about the magnetization
curve of Ferro –magnetic material. (8)
(b) Derive the relation between
mutual inductance and self
inductances of two magnetically
coupled coils. (8)

2. (a) Explain AC operation of
magnetic circuits. (8) (b) Explain in
detail about hysteresis and eddy
current losses. (8)

3. (a)Write in detail about magnetically
induced emf & force and derive the
relevant expression. (8) (b) An
electromagnetic relay has an exciting
coil of 800 turns. The coil has a cross
section of 5 cm x 5cm. Find
1. coil inductance if the air gap length
is 0.5 cm.
2. field energy stored for a coil current
of 1.25 A.
3. Permeance at air gap. (8)

4. Explain in detail about three basic
rotating electric machine types. (16)

5. (a) Compare magnetic and electric
circuit. (8) (b) An iron rod 1.8 cm
diameter is bent to form a ring of
mean diameter 25cm and wound with
250 turn of wire. A gap of 1mm exists
in between the end faces. Calculate
the current required to produce a flux
of 0.6mWb. Take relative permeability
of iron as 1200. (8)

6. (a) Explain the two different types
of magnetic circuits with neat
diagram. (8) (b) When two coils are
connected in series, their effective
inductance is found to be 10H .When
the connections of one coil are
reversed , the effective inductance is
6H.If the coefficient of coupling is 0.6,
calculate the self inductance of each
coil and the mutual inductance. (8)

UNIT II
TRANSFORMERS

PART-A
1. Mention the difference between
core and shell type transformers?
2. What is the purpose of laminating
the core in a transformer?
3. Does transformer draw any current
when secondary is open? Why?
4. Define voltage regulation of a
transformer?
5. Define all day efficiency of a
transformer?
6. Why transformers are rated in KVA?
7. What are the typical uses of auto
transformer?
8. What are the applications of step-
up & step-down transformer?
9. How transformers are classified
according to their construction?
10. Explain on the material used for
core construction?
11. How does change in frequency
affect the operation of a given
transformer?
12. What is the angle by which no
load current will lag the ideal applied
voltage?
13. List the arrangement of stepped
core arrangement in a transformer?
14. What is the function of
transformer oil in a transformer?
15. Can the voltage regulation go
negative if so under what condition?
16. Distinguish power transformers &
distribution transformers?
17. What is the purpose of providing
Taps in transformer and where these
are provided?
18. What are the typical uses of auto
transformer?
19. How does change in frequency
affect the operation of a given
transformer?
20. Why are breathers used in
transformers?
21. What is the function of
transformer oil in a transformer?

PART-B

1. (a) Explain the principle and
operation of auto transformer. (8)
(b)Draw and explain the no load
phasor diagram of a single phase
transformer. (8)

2. (a) Derive the emf equation of
single phase transformer. (8) (b) A
120kVA, 6000/400V, Y/Y, 3-phase,
50Hz transformer has a iron loss of
1800W.
The maximum efficiency occurs at ¾
full loads. Find the efficiency of the
transformer at
(i) Full load and 0.8 pf
(ii) The maximum efficiency at unity
pf. (8)

3. A100 kVA, 6.6kV/415V, single phase
transformer has an effective
impedance of (3+8j) _ referred to HV
side. Estimate the full load voltage
regulation at 0.8 pf lagging and 0.8
leading pf. (16)

4. (a) Explain the working of auto
transformer and prove that when
transformation ratio approaches
unity, the amount copper used
approaches smaller value. (8)
(b)The emf per turn of a single phase,
6.6kV/440V, 50 Hz transformer is
approximately 12V. Calculate the
number of turns in the HV and LV
windings and the net cross sectional
area of the core for a maximum flux
density of 1.5T. (8)

5. (a) Obtain the equivalent circuit of a
200/400V,50Hz,single phase
transformer from the following test
data: OC test: 200V,0.7A,70W on LV
side SC test: 15V, 10A, 85W on HV side.
(10) (b)With the help of circuit
diagrams, explain any two types of
three phase transformer connections.
(6)

7. Draw the circuit diagrams for
conducting OC and SC tests on a
single phase transformer. Also explain
how the efficiency and voltage
regulation can be estimated by these
tests. (16)

8. What is the sumpner’s test? Draw
the circuit diagram to conduct this test
and explain its
principle. (16)

9. (a) Derive the condition for
maximum efficiency in a transformer.
(8)
(b) A11000/230 V, 150 KVA, 1-phase,
50 Hz transformer has core loss of
1.4kW and F.L cu loss of 1.6
Kw .Determine (i) The kVA load for
maximum efficiency and the value of
maximum efficiency at unity p.f. (ii)
The efficiency at half F.L 0.8 pf leading.
(8)

10. Explain in detail about parallel
operation of single phase
transformers. (16)

11. Data of a 500KVA, 3300/400 V, 50
Hz, single phase transformer is given
below.
S.C test: 1250 W, 100 V –secondary
short circuited with full load current in
it O.C test: 1000 W –with normal
primary voltage.
Calculate the full load regulation and
efficiency at a power factor of 0.8(lag).
(16)

12. (a) Derive the equivalent circuit of
a single phase two winding
transformer. (8) (b) The maximum
efficiency of a single phase 250kVA,
2000/250 V transformer occurs at 80%
of
full load and is equal to 97.5% at 0.8
pf .determine the efficiency and
regulation on full load at 0.8pf lagging
if the impedance of the transformer is
9 percent. (8)

13. Explain in detail about tap
changing of transformers. (16)

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