Design of Electrical Apparatus Interview Questions and Answers:
Following are the below Design of Electrical Apparatus Interview Questions and Answers, Design of Electrical Apparatus Important Questions and Answers
1. What is Specific Electric Loading?
Ans. The Specific Electric Loading is defined as the number of armature ampere conductors per meter of armature periphery at the air gap.
2. List the advantage of Hydrogen Cooling.
Ans.
- Increased efficiency.
- Increasing rating.
- Increase is life.
- Smaller size of coolers.
- Eliminating of Fire Hazards.
- Less noise.
3. What are the factor on which no load current of transformer depend?
Ans. It depends on magnetising current and load current
4. Define (SCR)? What are the effects of SCR on machine performance.
Ans. It is defined as the ratio of field current required to produced rated voltage on open circuit to field current required to circulate rated current at sheet circuit.
With higher values of SCR the machine has higher stability limit and low value of regulation.
5. What is runaway speed?
Ans. It is defined as the speed which the prime mover would have, if it is suddenly unloaded, when working as its rated load.
6. What is the use of damper windings in Synchronous GenerÂators and Synchronous Speed.
Ans.
- The damper windings is used to reduce the oscillation developed in the rotor of alternator when it is suddenly loaded.
- The damper winding is used to start the synchronous motor as an induction motor.
7. Why is length of air gap in an induction motor kept at minimum possible range?
Ans.
- To reduce in armature reaction.
- Small value of regulation.
- Higher value of stability.
- A higher synchronising power which makes the machine less sensitive to load variations.
8. What are the effects of open slots and ducts on magnetic circuit of a machine?
Ans.
- Due to open slots, the flux will be crowd over the teeth.
- Reluctance of slotted armature with ducts will be increased.
9. What are the advantages of having circular coil in a transformer?
Ans. These circular coil in a transformer will be used in high KVA rating. It uses 1 and 2 strips only in parallel. Coil deformation will be avoided.
10. Give methods adopted to reduce the effect of armature reaction in a DC machine.
Ans. (a) Increase in length of air gap ate pile tips (b) Increase reluctance of pile tips (c) Compensating winding (d) Interpoles
11. List the advantages of lap winding in a DC machine.
Ans. The total number of brushes is equal.to the number of poles. It provides large current and low voltage. Hence it is used for large machines.
12. What is dispersion coefficient in an induction motor?
Ans. It is defined as the ratio of magnetizing current to ideal short circuit current.
13. Give the methods to reduce harmonic torques in an induction motor.
Ans. The methods to reduce harmonic torques are (a) chording (b) Integral slot windings (c) Skewing
14. What are the merits of computer aided design?
Ans. It has capability to sore amount of data, count integers and other data in advance. It makes it possible to select anoptimized design with a reduction in cost.
15. How materials are classified according to their degree of magnetism?
Ans. Classification of Material Based on Degree of Magnetism
- Ferromagnetic material
- Paramagnetic material
- Diamagnetic material.
16. Name any two methods to reduce armature reaction.
Ans. Methods to Reduce Armature Reaction
- Compensating windings are provided to neutralize the effects of armature reaction.
- By increasing reluctance of pole tips, the distorting effects of armature reaction can be reduced.
- By increasing the length of air gap at pole tips.
17. What is slot loading?
Ans. The slot loading is the number of ampere conductors per slot. This value should not exceed 15000 A.
18. Give the relationship between emf per turn and kVA rating in a transformer.
Ans. Relational between KVA rating and emf:
19. What are the factors affecting the choice of flux density of core in a transformer?
Ans. Factors Affecting Choice of flux Density in a Core of a Transformer:
- Core area
- Cost of iron
- Length of mean turn of windings
- Cost of conductor
- Iron loss
- Temperature gradient across core
- Magnetising current
- Harmonics.
20. How crawling can be prevented by design in an Induction motor?
Ans. Methods to Prevent Crawling:
- Chording
- Integral slot winding
- Skewing.
21. What is run away speed of Synchronous Machine?
Ans. It is defined as the speed which the prime mover would have if it is suddenly unloaded, when working at its rated load.
22. Give the need for damper winding in Synchronous Machine.
Ans. Need for Damper Winding in Synchronous Machine:
- Damper winding is used to start the synchronous motor as an induction motor.
- Damper (amortisseur) winding is used to reduce the oscillations developed in rotor of alternator when it is suddenly loaded.
23. What are the major considerations in Electrical Machine Design?
Ans. Major considerations in Electrical Machine Design are
- Cost
- Durability
- Compliance with performance criteria as laid down in specifications.
24. Write down the classification of magnetic materials.
Ans. Classification of Magnetic Materials
- Ferromagnetic materials
- Paramagnetic materials
- Diamagnetic materials.
25. Show how the specific magnetic and electric loadings are interdependent.
Ans. Interdependence of specific Magnetic and Electric Loadings
The output of a dc machine is proportional to the product of their specific magnetic loading (Bav) and specific electric loading (ac)
For a particular output, values of specific magnetic and electric loadings are interdependent. i.e., if one is chosen higher, the value of other has to be lower.
26. Mention any two guiding factors for the choice of number of poles.
Ans. Two Guiding Factors for the Number of Poles
- The frequency of flux reversals in the armature core generally lies between 25 to 50 Hz. Lower values of frequency are used for large machines.
- The value of current per parallel path is limited to about 200 A. Thus the current/brush arm should not be more than 400 A.
27. Define the term: ‘Voltage Regulation’.
Ans. It is defined as change in voltage from no load voltage to full load voltage when the full load voltage is thrown off.
where
- Vo = open circuit voltage (V),
- Vf = full load voltage (V).
28. What are the methods by which heat dissipation occurs in a transformer?
Ans. Methods by which heat dissipation occurs in a Transformer are
- Conduction
- Radiation
- Convection (Natural and Artificial).
29. Define : Stator Slot Pitch.
Ans.
Stator Slot Pitch
30. What are the factors that influence the choice of specific magnetic loading in a synchronous machine?
Ans. Factors influence the choice of specific magnetic loading in Synchronous Machine
- Iron loss
- Stability
- Transient short circuit current
- Voltage
- Parallel operation.
31. Define Short Circuit Ratio of a synchronous machine.
Ans. It is the ratio of field current required to produce rated voltage on open circuit to field current required to circulate rated current at short circuit,
where,
- Xd = Synchronous reactance.
32. What is meant by peripheral speed? Write the expression for peripheral speed of a rotating machine.
Ans. The peripheral speed is a translational speed that may exist at the surface of rotor while it is rotating.
where,
- Dr = Diameter of rotor (m)
- n = Speed of rotor (rps).
33. What is meant by magnetic loading?
Ans. The magnetic loading is the total flux around the armature periphery at the airgap.
Total magnetic loading = pΦ = No. of poles x flux/pole.
34. Define field form factor.
Ans.
35. What is meant unbalanced magnetic pull?
Ans. The unbalanced magnetic pull is the radial force acting on the rotor due to non-uniform air-gap around armature periphery. In order to overcome unbalanced magnetic pull, the length of rotor is kept small and ball bearings are employed.
36. Write the advantages of shell type transformer over core type transformer?
Ans.
- Leakage reactance can be reduced to any desired value.
- They can produce greater withstanding forces under short circuit conditions as the windings are surrounded and supported by the core.
- The cooling is better in core than in windings.
37. What is meant by stacking factor?
Ans.
38. What are the advantages and disadvantages of larger air gap length in induction motor?
Ans.
- Better cooling
- Reduction inunbalanced magnetic pull
- Higher overload capacity
- Low noise
- More stability.
39. What are the factors to be considered for selecting the number of slots in induction machine stator?
Ans.
- Iron loss
- Magnetising current
- Leakage reactance
- Tooth pulsation loss
- Cost.
40. How the value of SCR affects the design of alternator?
Ans. Length of air gap α SCR (Short Circuit Ratio):
For high stability and low regulation value of SCR should be high which requires large air gap. When length of air gap is large, mmf requirement will be high and so field system will be large. Hence the machine will be costlier.
41. Mention the main areas of design of electrical machines.
Ans. The main area of design of electrical machines involves the dimensioning of the:
- magnetic circuit
- electric circuit
- insulation system.
42. What are the important specifications of a DC machine as per IS 4722:1968?
Ans. The important specifications of a DC machine as per IS 4722:1968 are:
- Generator or motor
- Field excitation
- Type of field excitation
- Field exciting voltage and current
- Rate output power
- Type of enclosure
- Rate voltage
- Type of duty
- Rated current
- Speed.
43. What are the factors that influence the Choice of commutator diameter?
Ans. The choice of commutator diameter depends upon peripheral speed and the thickness of commutator segment.
44. Why circular coils are preferred in transformers?
Ans. The circular coils are preferred in transformers because the rectangular coil make use more length of copper for the same number of turns as compared to circular core.
45. What are the factors to be considered for selecting the cooling method of a transformer?
Ans. The choice of method of cooling of a transformer depends on the size, type of application and types of conditions obtaining at the site where the transformer is installed.
46. How can a 3-phase induction motor be designed for best power factor?
Ans.
For best power factor L/τ = 1.0 to 1.25
where,
- L = Core length
- τ = Pole pitch = πD/P.
47. Name the different types of leakages fluxes associated with 3-phase induction motors.
Aus. The different leakage fluxes are:
- Stator and rotor slot leakage flux
- Stator and rotor zig-zag leakage flux
- Stator and rotor overchange leakage flux
- Belt (or) differential leakage flux.
48. State the important features of turbo-alternator rotors.
Ans. The important features of turbo alter water rotors are:
- The diameter of the machine will be smaller than the axial length.
- The speed of the machine will be greater (i.e) in the order of 3000 rpm.
49. Write down the output equation of a d.c. machine.
Ans.
where
- Bav = Specific magnetic loading (Wb/m2)
- ac = Specific electric loading (AC/m)
- n = Speed in r.p.s.
- D = Diameter of armature (m)
- L = Length of Armature core (m).
50. What are the cooling methods used for dry type transformers?
Ans. (a) Air Natural (AN). (b) Air Blast (AB).
51. Define Window Space Factor.
Ans. Window Space FActor (kW) is defined as ratio of conductor copper area in window (AC) to total area of window (AW).
52. Write down the equation for output coefficient in an Induction Motor.
Ans.
where
- kW = Winding factor
- Bav = Specific magnetic loading (Wb/m2)
- ac = Specific electric loading (ac/m).
53. What is meant by an Ideal short circuit current?
Ans. It is defined as the current drawn by the motor at standstill if its resistance is neglected.
