Name of the College : Kings College Of Engineering
Department : Electrical & Electronics Engineering
Subject : Electrical Machines
Website : kings.ac.in
Document Type : Question Bank
Download Model/Sample Question Paper : https://www.pdfquestion.in/uploads/ki…ES%20-%20I.pdf
Kings Electrical Machines Question Paper
Unit – I
Introduction :
Part-A :
1. What is reluctance?
2. What is stacking factor?
3. What is leakage flux?
4. What is fringing effect?
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5. Compare electric and magnetic circuits.
6. What is statically induced emf?
7. What is dynamically induced emf?
8. State the three basic types of rotating electrical machines.
9. State two types of induction motors.
10. What are the different types of induced emfs? Give Examples.
11. What are the types of magnetic losses?
12. Define the term self inductance.
13. Define the term mutual inductance.
14. Define coefficient of coupling.
15. State Lenz’s law.
16. Write equation for energy density for a magnetic circuit.
17. What are the factors on which hysteresis loss depends?
18. What is core loss? What is its significance in electric machines?
19. What is eddy current loss?
20. How are hysteresis and eddy current losses 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 turns 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. Give the emf equation of a transformer and define each term.
4. Does transformer draw any current when secondary is open? Why?
5. Define voltage regulation of a transformer.
6. Full load copper loss in a transformer is 1600W. what will be the loss at half load?
7. Define all day efficiency of a transformer.
8. Why transformers are rated in kVA?
9. What are the typical uses of auto transformer?
10. What are the application of step-up & step-down transformer?
11. How transformers are classified according to their construction?
12. Explain on the material used for core construction?
13. How does change in frequency affect the operation of a given Transformer?
14. What is the angle by which no-load current will lag the ideal applied voltage?
15. List the arrangement of stepped core arrangement in a transformer.
16. What is the function of transformer oil in a transformer?
17. Distinguish power transformers & distribution transformers.
18. What are properties of an ideal transformer?
19. Give the condition to be satisfied for parallel operation of transformer.
20. What are different losses occurring 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)
6. Find the all day efficiency of a 500kVA, distribution transformer whose iron loss and full load copper loss are 1.5kW and 6kW respectively. In a day it is loaded as follows : Duration(Hi) Output(Po) in kW Power factor(cosØ2)
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)
Unit – III
Electromechanical Energy Conversion :
Part-A :
1. Write the energy balance equation of a motor.
2. What is the energy conversion medium in a singly-excited magnetic field system?
3. What is the basic feature of an electro magnetic energy conversion devices?
4. Why does the energy storage in a magnetic material occurs mainly in the air gap?
5. What is multiply excited magnetic field system?
6. Give the relation between energy and co-energy for linear system.
7. Give an examples for each of single and multiple excited system
8. Write the applications of singly and doubly fed magnetic systems.
9. Why the field in rotating machines should be quasi-static in nature?
10. What are the causes for irrecoverable energy loss when the flux in the magnetic circuit undergoes a cycle
11. Why the relationship between current & coil flux linkages of electromechanical energy conversion devices are linear
12. With respect to the magnetic field ,in which direction does the force act?
Part-B :
1. Explain in detail about energy in magnetic system (16)
2. (a) With one example derive the co-energy of a multi excited magnetic field system (8)
(b) two coupled coils have self and mutual inductance of L11 = 3+0.5 x ;L22 = 2+0.5x ; L12= L21=0.3x Over a certain range of linear displacement x. The first coil is excited by a constant current of 15A and the second by a constant current of -8A.
(i) Mechanical work done if x changes from 0.6 to1m
(ii)Energy supplied by each electrical source in part 1 (8)
3. (a) With neat sketch explain the multiple excited magnetic field system in electro mechanical energy conversion systems. Also obtain the expression for field energy in the system. (8)
(b) Draw and explain the general block diagram of an electromechanical energy conversion device (8)
4. Two coupled coils have self and mutual inductance of L11=2+1/(2x); L22=1+1/(2x) : L12= L21=1/(2x). Over a certain range of linear displacement x. The first coil is excited by a constant current of 20A and the second by a constant current of -10A.
(i) Mechanical work done if x changes from 0.5to1m
(ii) Energy supplied by each electrical source in part 1
(iii) Change in field energy in part1
Hence verify that the energy supplied by the sources is equal to the increase in field energy plus the mechanical work done (16)