VINAYAKAMISSIONSUNIVERSITY

V.M.K.V.ENGINEERINGCOLLEGE, SALEM

AARUPADAI VEEDU INSTITUTE OF TECHNOLOGY, PAIYANOOR

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

QUESTION BANK

ELECTRICAL MACHINES-I

(For III SEM EEE ONLY)

UNIT-I

Part-A

  1. State Faraday’s law.
  2. What is co-energy?
  3. What is field energy?
  4. Give any two examples for electromechanical transducer.
  5. Write the Laurentz’s force equation.
  6. Draw the I-λ curve of linear systems?
  7. State Ampere’s law of force.
  8. What are the main parts of rotating machines?
  9. What is a static machine?
  10. Give the examples for rotating machines.
  11. Define energy in magnetic system
  12. Define mechanical force.
  13. Write a note on flow of energy in electro mechanical devices
  14. Define coupling field.
  15. What is multiple excited magnetic field system?
  16. What is singly excited magnetic field system?
  17. Define electric field energy.
  18. Write about energy conversion in electromechanical system.
  19. What is an electromechanical transducer?
  20. What is a reluctance motor?
  21. Why the I-λ relationship of a magnetic circuit is almost linear.
  22. Give the relation ship between electrical and mechanical degree.
  23. Write the energy balance equation of a machine.
  24. What an expression for synchronous speed.

Part –B

  1. Describe in detail about coupling field reaction theory.
  2. Derive the expression for energy in magnetic system.
  3. Derive the expression for mechanical force.
  4. Explain about singly excited system.
  5. Derive the expression for field energy and co –energy in multiply excited system.
  6. 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’.One of the coils is excited by a constant current of 20A and the second by a constant current of -10A. Find (a) mechanical work done if ‘X’ changes from 0.5 to 1m.

(b) Energy supplied by each electrical source(c) Change in field energy.

Hence verify that the energy supplied by the sources is equal to the increase in the field energy plus mechanical work done.

  1. Explain about basic concepts in rotating machines.

UNIT-II

Part-A

  1. List the main parts of D.C. machine.
  2. Write the EMF equation of DC machine.
  3. Define the term pole pitch.
  4. What is coil span?
  5. What is armature reaction?
  6. What is commutation?
  7. List out the types of DC generators?
  8. What is the function of a starter?
  9. List the types of speed control schemes of DC motor.
  10. Define the term residual voltage.
  11. Draw the symbolic representation of a DC machine
  12. Name the three types of characteristics of a DC generator
  13. Define the term “critical resistance” of a DC shunt generator
  14. What is the cause of heating of Armature?
  15. What is the cause of heating of bearing?
  16. What is the permissible rise of temperature in a well-designed generator?
  17. How do we conclude that connections between field coils and armature are correct?
  18. When a generator loses its residual magnetism either due to lighting or short circuit, how can it be made to build up?
  19. Can a generator be reversed by reversing the connection between the armature and field coils?
  20. Will a generator build up if it becomes reversed?
  21. What are the two kinds of sparking produced in generator?
  22. What is the possible reason if sparking does not disappear in any position when brushes are rocked around the commutator?
  23. List out the type commutation.
  24. Write the function of interpole.
  25. What are the effects of armature reaction?

Part-B

1.List the important parts in a D.C machine and explain the function of each.

2.Derive the EMF equation of D.C. Generator.

3. A lap wound D.C generator has 8 poles, 120 slots with 8 conductors in each slot. Flux per pole is 0.04 Wb. Find (a) Induced emf generated at 600 RPM.(b) What should be speed at rotation if induced emf is to be 500 V.

4. Explain about the armature reaction.

5. Explain in detail the different characteristics of D.C generator.

6. Explain in detail the commutation of D.C generator.

7. Explain with neat sketches the principle of operation of a D.C motor.

8.Derive the torque equation of D.C motor.

9. Draw and explain the speed –torque characteristics of shunt and compound motor and list their application.

10. What are the various methods of speed control of a D.C motor?

11. Explain the necessity of a starter in the case D.C motor. Draw and explain the three point starter.

UNIT-III

Part A

  1. Write the advantages of indirect testing methods of Dc machines?
  2. List the disadvantages of direct testing?
  3. What is electrical braking?
  4. Write the types of electrical braking.
  5. What are the conditions for parallel operation of DC generator?
  6. What is the function of equalizer ring?
  7. Write the advantages and disadvantages of Hopkinson’s test.
  8. Swinburne’s test is not applicable for series field machine. Why?
  9. Hopkinson’s test is called as heat run test .Why?
  10. Regenerative braking is not applicable for series motor. Why?
  11. How can the direction of rotation of a D.C motor be reversed?
  12. What will happen if both armature and field currents are reversed?
  13. What will happen if the field of a D.C shunt motor is opened?

14. What willhappen ifthe direction of current at the terminals of a series motor is reversed?

15.What happen when D.C motor is connected across an A.C supply?

16. What happens if a shunt motor is directly connected to the supply line.

18.What are the likely causes if a D.C motor is found to run too slow under load?

19. Why does a D.C motor some times run too fast when under load? Give different possible causes and their remedies.

20.Under what conditions is sparking produced at the brushes of a D.C motor. How would you remedy it?

21. What are the possible causes of excessive sparking at brushes in a D.C motor?

22. What is stray loss?

23. Write the condition for maximum efficiency of D.C machine.

24. What is constant loss of D.C machine?

25. Write the efficiency equations for both machines in Hopkinson’s test having equal rating and load current as I1 and I2

Part B

1. A 500V D.C shunt motor takes a current of 4A on no load. The resistance of the armature and field circuit is 0.2 ohm and 250 ohm respectively. Find the efficiency when loaded and taking a current of 100A.

2. In a brake test, the effective load on the brake pulley was 38.1kg, the effective diameter of the pulley was 63.5cm and speed was 12 RPS. The motor took 49A at 220V. Calculate the horse power and the efficiency at this load.

3. The result of Hopkinson’s test conducted on a pair of D.C Shunt machines at full load are as follows: line voltage = 220V; line current=50A; motor armature current =195A; field currents=6A and 5A and armature resistance =0.03 Ohm. Calculate the efficiency of each machine for this particular condition of loading.

4. Two shunt generators having full load regulation of 6%and 5% are rated 250 kW and 500V.How will they share a current of 1000A while operating in parallel? Find the terminal voltage.

5. With a neat diagram, explain the brake test conducted on a D.Cshunt motor and draw the performance curves.

6. Explain Swinburne’s test for a D.C machine.

7. Explain the method used to find the stray losses of a D.C machine.

8. What are the requirements for parallel operation of D. C generator? Explain how two compound generators are put in to parallel operation.

9. Explain about the dynamic braking of all type of motors.

10. Explain about the plugging of all type of motors.

UNIT-IV

Part A

  1. Write the types of transformer.
  2. List the main parts of transformer?
  3. Write the EMF equation of transformer.
  4. Write the components of no load current of a transformer?
  5. Why the transformers are rated in KVA?
  6. Draw the approximate equivalent circuit of transformer?
  7. What is voltage regulation of a transformer?
  8. Draw the phasor diagram of transformer under no load.
  9. List the losses of transformer.
  10. Compare core and shell type transformer.
  11. Define the term phasor.
  12. What is leakage reactance?
  13. Draw the symbol for two winding transformer.
  14. How can eddy current loss be minimized?
  15. Write the voltage transformation ratio of transformer.
  16. Power factor of transformer is low under no load, why?
  17. What are the parameters that can be determined by no load test?
  18. What are up and down regulations?
  19. Write thestain Metz empirical formula for hysteresis loss.
  20. What are the parameters are determined by short circuit test?
  21. Write the condition for maximum efficiency of a transformer.
  22. What is function of transformer oil?

Part B

  1. A 40 kVA transformer has an iron loss of 450 W and a full load copper loss of 850W. If the power factor of the load is 0.8, calculate the full load efficiency and maximum efficiency.
  2. Obtain the parameters of the approximate equivalent circuit of a single phase,4kVA,200/400V,50Hz transformer for which the following are the test results:

O.C test: 200V, 0.75A, 70W on LV side

S.C test 15V, 10A, 80W on H.V side

  1. Discuss the constructional details of single phase transformer with suitable diagrams.
  2. Explain the principle of operation of transformer with suitable sketches and phasor diagram.
  3. Develop the equivalent circuit of single phase transformer from the first principle.
  4. Draw the phasor diagram of a transformer under full load condition at lagging power factor and explain.
  5. The O.C and S.C tests on a 5kVA,230/110V,50Hz transformer gave the following data:

O.C test (h.v side):230V,0.6A,80W

S.C test(l.v side):6V, 15A, 20W

Calculate the percentage efficiency and the regulation of the transformer on full load at 0.8 p.f lagging.

  1. Draw and explain the phasor diagram of a transformer on no load.
  2. Derive the EMF equation of transformer.
  3. Explain about the effect of resistance and leakage reactance of the winding.

UNIT –V

Part A

  1. List the connections of 3 phase transformer.
  2. Write the applications of auto transformer.
  3. What is all day efficiency?
  4. Write the conditions for parallel operation of transformer.
  5. Sumpner’s test is called as heat run test. Why?
  6. List the applications of Scott connection.
  7. Write the types of tap changers.
  8. Draw the circuit connection for step up auto transformer.
  9. Write the equation for savings of copper in single winding transformer.
  10. What is the function of tap changer?
  11. What is energy efficiency?
  12. Draw the circuit connection for step down auto transformer.
  13. Compare two winding transformer and auto transformer.
  14. Draw phasor diagram of an auto transformer.
  15. Which type of transformer is used for phase conversion?
  16. What are the four common ways of connecting three phase transformers?
  17. What is another name of Sumpner’s test
  18. Draw star delta connection of three phase transformer
  19. Mention the conditions to be fulfilled for the successful parallel operation of two three phase transformers.
  20. State the need for parallel operation of transformers.
  21. What is function of buckholz relay?
  22. Which type of transformer connection is used for three phase four wire system?
  23. Give the phase difference between V1 and V2 in star-delta connected transformer.
  24. Draw the circuit diagram for open delta connection.
  25. Write the application of V-V connection.

Part B

  1. A 200 kVA distribution transformer has core loss of 2000 watts and full load copper loss of 3000watts.In a day it is loaded as follows:

8hours-200kVA atUPF

4hours -150kVAat0.6pf lag

4hours – 100kVA at 0.8pf lag.

Find the all day efficiency.

  1. The primary and secondary voltages of an auto transformer are 500V and 400V respectively. Show with the aid of a diagram the current distribution in the winding when the load current is 100A and calculate the economy of copper in this particular case.
  2. Two similar 200KVA,single phase transformer gave the following results when tested by back to back method.W1 in the supply line 4KW ; W2 in the primary series circuit,when full load current circulated through the secondaries, 6KW.Calculate the full load efficiency of each transformer at 0.8 p.f lag.
  3. Explain the working principle of an auto transformer.
  4. Obtain the percentage saving of weight of copper in an auto transformer comparedtwo winding transformer of same rating.
  5. Draw and explain the various types of three phase transformer connections.
  6. Mention and explain the condition to be fulfilled for the successful parallel operation of two three phase transformer.
  7. Write the detailed notes on parallel operation of transformers.
  8. Derive an expression for load sharing of two transformers operating in parallel. Assume equal voltage ratios.
  9. Explain the sumpner’s test used for a transformer. Draw suitable circuit diagram.