# Design of Electrical Machines Important Questions and Answers

1. How Materials are classified according to their degree of magnetism?

Classification of Material Based on Degree of Magnetism

1. Ferromagnetic material.
2. Paramagnetic material.
3. Diamagnetic material.

2. Name any two methods to reduce armature reaction.

Methods to Reduce Armature Reaction are

1. Compensating windings are provided to neutralize the effects of armature reaction.
2. By increasing reluctance of pole tips, the distorting effects of armature reaction can be reduced.
3. By increasing the length of air gap at pole tips.

The slot loading is the number of ampere conductors per slot. This value should not exceed 15000 A.

I Z ≤ 1500 A (Zs → No. of conductors/sot).

4. What are the factors affecting the choice of flux density of core in a transformer?

• Core area
• Cost of iron
• Length of mean turn of windings
• Cost of conductor
• Iron loss
• Temperature gradient across core
• Magnetizing current
• Harmonics.

5. How crawling can be prevented by design in an induction motor?

• Chording
• Integral slot winding
• Skewing.

6. Define dispersion coefficient of an induction motor.

It is defined as the ratio of magnetizing current to ideal short circuit current.

Dispersion coefficient = Magnetizing Current/ Ideal Short Circuit Current

7. What is run away speed of Synchronous Machine?

It is Define as the speed which the prime mover wound have if it is suddenly unloaded, when working at its rated load.

8. Give the need for damper winding in Synchronous Machine.

• Damper winding is used to start the synchronous motor as an inductor motor.
• Damper (amortisseur) winding is used to reduce the oscillations developed in rotor of alternator when it is suddenly loaded.

9. What are the major considerations in Electrical Machine Design.

Major considerations in Electrical Machine Design are

• Cost
• Durability
• Compliance with performance criteria as laid down in specifications.

10. Write down the classification of magnetic materials.

• Ferromagnetic materials
• Paramagnetic materials
• Diamagnetic materials.

11. Mention any two guiding factors for the choice of number of poles.

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.

12. Define the term; ‘Voltage Regulation’.

It is defined as change in voltage from no load voltage to full load voltage when the full load voltages is thrown off.

13. What are the methods by which heat dissipation occurs in a transformers?

• Conduction
• Convection (Natural and Artificial).

14. Write down the equation for output coefficient in a Induction Motor.

Output Coefficient Equation

C0 = 11 x kw Bav ac x 10-3

Where

• kw = winding factor
• ac = ampere conductor
• Bav = average flux density

15. What are the factors that influence the choice of specific magnetic loading in a synchronous machine?

Factors influence the choice of specific magnetic loading in Synchronous Machine

• Iron loss
• Stability
• Transient short circuit current
• Voltage
• Parallel Operation.

16. Define Short Circuit Ratio of a Synchronous Machine.

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

17.  What is meant by peripheral speed? Write the expression for peripheral speed of a rotating machine.

The peripheral speed is a translation speed that may exist at the surface of rotor while it is rotating.

Peripheral Speed = = Va = πDr n (m/s)

Where,

• Dr = Diameter of rotor (m)
• n = Speed of rotor (rps).

18. What is meant by magnetic loading?

The magnetic loading is the total flux around the armature periphery at the airgap.

Total magnetic loading = pΦ = No. of poles x flux/pole.

19. Define field form factor.

Field form factor = kf = Average gap density-pole pitch (Bav)/Maximum flux density in air gap (Bg)

20. What is meant by unbalanced magnetic pull?

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 unbalance magnetic pull, the length of rotor is kept small and ball bearings are employed.

21. Write the advantages of shell type transformer over core type transformer?

• Leakage reactance can be reduced to any desired value.
• They can produce greater withstanding forces under short circuit condition as the windings are surrounded and supported by the core.
• The cooling is better in core than in windings.

22. What is meant by stacking factor?

Stacking Factor = Sf = Area of cross section of iron in core/Cross sectional area of core including insulation area

23. What are the advantages and disadvantages of larger air gap length in induction motor?

• Better cooling
• Reduction inunbalanced magnetic pull
• Higher overload capacity
• Low noise
• More stability

24. What are the factors to be considered for selecting the number of slots in induction machine stator?

• Iron loss
• Magnetizing current
• Leakage reactance
• Tooth pulsation loss
• Cost

25. What is runaway speed?

It is defined as the speed which the prime mover would have, if it is suddenly unloaded, when working at its rated load.

26. How the value of SCR affects the design of alternator?

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.

27. Mentioned the main areas of design of electrical machines.

The main area of design of electrical machines involves the dimensioning of the:

• magnetic circuit
• electric circuit
• insulation system

28. What are the important specifications of a DC Machine as per IS 4722:1968?

The important specifications of a DC machine as per IS4722:1968 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

29. What are the factors that influence the choice of commutator diameter?

The choice of commutator diameter depends upon peripheral speed and the thickness of commutator segment.

30. Why circular coils are preferred in transformers?

The circular coils are preferred in transformers because the rectangular coil make use more length of copper for the same number turns as compared to circular core.

31. What are the factors to be considered for selecting the cooling method of a transformer?

The choice of method of cooling of transformer depends on the size, type of application and types of conditions obtaining at the site where the transformer is installed.

32. Name the different types of leakages fluxes associated with 3-phase induction motors.

The different leakage fluxes are:

• Stator and rotor slot leakage flux
• Stator and rotor zig-zag leakage flux
• Stator and rotor over change leakage flux
• Belt (or) differential leakage flux.

33. Define SCR of a synchronous machine.

It is defined as the ratio of field current required to produce rated voltage on open circuit to the field current required to circulate rated current on short circuit.

34. State the important features of turbo-alternator rotors.

The importance features of turbo alternator 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.

35. Write down the output equation of a d.c. machine.

Pa = (π2 Bav ac D2L n) x 10-3

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 code (m)

36. What are the cooling methods used for dry type of transformers?

• Air Natural (AN)
• Air Blast (AB)

37. Define Window Space Factor.