Important Short Questions and Answers: Design of Electrical Machines - Introduction

INTRODUCTION

1.What are the types of electrical engineering materials?

Basically there are three types of materials used in electrical machines.

^Ø                                                                        Magnetic materials

^Ø                                                                        Conducting materials

^Ø                                                                        Insulating materials

2.What is meant by critical magnetic field?

If the temperature of a material is raised above its critical temperature its superconductivity disappears. The field at which superconductivity vanishes is called critical magnetic field.

3.    What is askarel?

An askarel is a synthetic non flammable insulating liquid. The commonest askarel is hexa choloro diphynyl tricholoro benzene.

4.           Name the magnetic materials used for Yoke, Transformer Stampings and permanent magnet.

Yoke of a dc machine, transformer stamping, permanent magnet Yoke of a dc machine – cast steel

Transformer stamping – silicon steel

Permanent magnet – hard magnetic material (Al, Ni, Co)

5.           Comment on the use of CRGOS strip wound transformer core.

CRGOS means cold rolled grain oriented steel. This steel is manufactured by series of cold reductions and intermediate annealings. This could reduce the material has strong directional of highest permeability which results less hysteresis loss. This type of material is suitable for use in transformers.

6.    What is meant by heating?

The temperature of a machine rises when it runs under load condition starting from cold condition. The temperature raises is directly proportional to the power wasted. The heat dissipation may be due to conduction, convection or radiation.

7.    What is meant by cooling?

The cooling medium like air, water or gas is provided to absorb the heat energy to save the machine. The cooling medium is also called coo lent. The cooling is of two types like, direct and indirect cooling.

8.    What is meant by radiation?

If the heat energy is transferred from one place to other with air of gas it is called radiation.

9.           What are the electrical properties of insulating materials?

^Ø                                                                  high dielectric strength

^Ø                                                                  high resistivity

^Ø                                                                  low dielectric hysteresis

^Ø                                                                  good thermal conductivity

^Ø                                                                  high thermal stability

10.      Classification of insulating materials.

There are seven classes of insulating materials used in electrical machines according to their thermal stability in service.

11. What are the constructional elements of a transformer?

The constructional elements of a transformer are core, high and low voltage windings, cooling tubes or radiators and tank.

12.      How the design problems of an electrical machine can be classified?

The design problems of electrical machine can be classified as:

^Ø                                              Electromagnetic design

^Ø                                              Mechanical design

^Ø                                              Thermal design

^Ø                                              Dielectric design

13.      List the constructional elements of a d.c. machine?

The major constructional elements of a d.c.machine are stator, rotor, brushes and brush holders. The various parts of stator and rotor are listed below:

Stator -          

Yoke (or) Frame                                

Field pole

Pole shoe

Field winding

Interpole

Rotor - 

Armature core

Armature winding

Commutator

14. How is total m.m.f. calculated?

The total mmf required to establish the flux in the magnetic circuit is calculated using the knowledge of dimensions and configuration of the magnetic circuit. The magnetic circuit is split up into conventional parts, which may be connected in series or parallel. The flux density is calculated in every part and m.m.f. per unit length; ‘at’ is found by consulting ‘B-H’ curves. The summation of m.m.fs in series gives the total m.m.f.

15.      List the methods used for determining the motor rating for variable load drives.

^Ø                                                                    Method of average losses

^Ø                                                                    Equivalent current methods

^Ø                                                                    Equivalent torque method

^Ø                                                                    Equivalent power method

16.      Define rating.

Rating of a motor is the power output or the designated operating power limit based upon certain definite conditions assigned to it by the manufacturer.

17. What are the problems that arise during the calculation of m.m.f. for air gap?

The iron surfaces around the air gap are not smooth and so the calculation of m.m.f. for the airgap by ordinary methods gives wrong results. The problem is complicated by the fact that

^Ø                                                    One or both of the iron surfaces around the air gap may be slotted so that the flux tends to concentrated on the teeth rather than distributing itself uniformly over the air gap.

^Ø                                                    There are radial ventilating ducts in the machine for cooling purposes which effect in a similar manner as above.

^Ø                                                    In salient pole machine, the gap dimensions are not constant over whole of the pole pitch.

18.      Mention the methods used for calculating the mmf for tapered teeth.

^Ø                                                                    Graphical method

Ø                                                                 Three ordinate method ( Simpson’s Rule)

^Ø                                                                    B_{t 1/3} Method

19.      What is Carter’s gap co-efficient?

The Carter’s gap co-efficient ( K_cs) is the ratio of slot width to gap length. The formula which gives the value of K_cs directly is

K_cs  = 1 / [1 + (5 l_g/W_s )]

Where l_g = gap length; W_s = Width of slot

The other formula for Carter’s gap co-efficient ( K_cs) for parallel sided open slot is

K_cs  = 2/Π [tan^-1 y – 1/ Π log SQRT (1+ Π²)]

Where y = W_s / 2 l_g.

20.      Write down the expression for calculation of reluctance of air gap with slotted Armature.

Reluctance of air gap with slotted armature

S_g = l_g / µ y_s L = l_g /µ₀ L (y_s - K_cs W_s )

Where l_g = gap length; y_s = slot pitch; K_cs = Carter’s co-efficient; W_s = Width of slot.

21. Define field form factor.

22.      Define stacking factor.

Stacking factor is defined as the ratio of actual length of iron in a stack of assembled core plates to total axial length of stack.

23. What is gap contraction factor for slots?

The ratio of reluctance of air gap of slotted armature to reluctance of air gap of smooth armature is called gap contraction factor for slots.

24. What is gap contraction factor for ducts?

The ratio of reluctance of air gap with ducts to reluctance of air gap without ducts is known as gap contraction factor for ducts.

Where 

L = Length of core;

K_cd = Carter’s co-efficient for ducts ;

N_d = number of radial ducts;

W_d  = Width of each duct

25. Write the expression for mmf of air gap with smooth and slotted armatures.

M.M.F. for air gap with smooth armature is AT_g = 8,00,000 B l_g

M.M.F. for air gap with slotted armature is AT_g = 8,00,000 K_g B l_g

Where Kg is gap expansion factor; B is flux density; l_g is gap length.

26. Mention the problems encountered while calculating the mmf for teeth.

The calculation of mmf necessary to maintain the flux in the teeth is difficult during to the following problems:

^Ø                                                                    The teeth are wedge-shaped or tapered when parallel sided slots are used. This means that the area presented to the path of flux is not constant and this gives different values of flux density over the length of teeth.

^Ø                                                                    The slot provides another parallel path for flux, shunting the tooth. The teeth are normally worked in the saturation region and therefore their permeability is low and as a result an appreciable portion of the flux goes down the depth of slots.

27.      Explain why real flux density in the teeth is less than the apparent flux density.

The slot provides an alternate path for the flux to pass, although the flux entering an armature from the air gap follows paths principally. If the teeth density is high, the mmf acting across the teeth is very large and as the slots are in parallel with the teeth, this mmf acts, across the slots also. Thus at saturation density, the flux passing through the slots become large and cannot be neglected and calculation based on ‘no slot flux’ leads to wrong results. This shows that the real flux passing through the teeth is always less than the total or apparent flux. As a result, the real flux density in the teeth is always less than the apparent flux density.

28. What is meant by apparent and real flux density?

29. What is meant by rating of a machine?

The rating of machine refer to the whole of the numerical values of electrical and mechanical quantities with their duration and sequence assigned to the machines by the manufacturer and stated on the rating plate, the machine complying with the specified conditions.

30. Mention the different types of duties of a machine.

The following are the types of duty as per IS : 4722 – 1968 “ Specification for rotating electric machinery”

^Ø                                      S₁ = Continuous duty

^Ø                                      S₂ = Short time duty

^Ø                                      S₃ = Intermittent periodic duty

^Ø                                      S₄ =  Intermittent periodic duty with starting

^Ø                                      S₅ =  Intermittent periodic duty with starting and braking

^Ø                                      S₆ = Continuous duty with intermittent periodic loading

^Ø                                      S₇ = Continuous duty with starting and braking

^Ø                                      S₈ = Continuous duty with periodic speed changes

31.      Define continuous rating.

The continuous rating of a motor is defined as the load that may be carried by the machine for an indefinite time without the temperature rise of any part exceeding the maximum permissible value.

32. Define short time rating.

The short time rating of a motor may be defined as its output at which it may be operated for a certain specified time without exceeding the maximum permissible value of temperature rise.

33. Define duty factor.

The duty factor ( also called load factor or cyclic duration factor) is defined as the ratio of the heating period to the period of whole cycle.

34.            What is meant by intermittent rating?

The intermittent rating of a motor applies to an operating condition during which short time. Load periods alternate with period of rest or no load without the motor reaching the thermal equilibrium and without the maximum temperature rising above the maximum permissible value.

35.            What are the major considerations to evolve a good design of electrical machine?

The major considerations to achieve a good electrical machine are

^Ø                                                        Specific electric loading

^Ø                                                        Specific magnetic loading

^Ø                                                        Temperature rise

^Ø                                                        Efficiency

^Ø                                                        Length of air gap

^Ø                                                        Power factor

36.            List the standard specifications for transformer.

^Ø                                      IS 1180 – 1989 : Specifications for out door 3-phase distribution transformer Upto 100 KVA.

^Ø                                      IS 2026 – 1972 : Specifications of power transformers

37.            What is magnetic circuit?

The magnetic circuit is the path of magnetic flux. The mmf of the circuit creates flux in the path against the reluctance of the path. The equation which relates flux, mmf and reluctance is given by

38.            Write any two essential differences between magnetic and electric circuits.

^Ø                                      When the current flows in the electric circuit the energy is spent continuously, whereas in magnetic circuit the energy is needed only to create the flux but not to maintain it.

^Ø                                      Current actually flows in the electric circuit, whereas the flux does not flow in a magnetic circuit but it is only assumed to flow.

39.            What is magnetization curve?

The magnetization curve is a graph showing the relation between the magnetic field intensity, H and the flux density, B of a magnetic circuit. It is used to estimate the mmf required for flux path in the magnetic material and it is supplied by the manufacturers of stampings and laminations.

40. What is loss curve?

The loss curve is a graph showing the relation between iron loss and magnetic field intensity, H. It is used to estimate the iron loss of the magnetic materials and it is supplied by the manufacturers of stampings and laminations.

41.            What is the difference in permeability of magnetic and non-magnetic materials?

In magnetic materials the permeability is not constant and it depends on the saturation of the magnetic material. But in non-magnetic materials the permeability is constant.

42.            How to find total mmf in a series circuit?

The various steps in estimation of mmf of a section of magnetic circuit are:

^Ø                                      Determine the flux in the concerned section.

^Ø                                      Calculate the area of cross-section of the section.

^Ø                                      Calculate the flux density in the section

^Ø                                      From B – at curve of the magnetic material, determine the mmf per meter(at) for the calculated flux density

Ø                                   The mmf of the section is given by the product of length of the section and mmf per metre.

43.      List the different types of slots that are used in rotating machines.

The different types of slots are

^Ø                                                                    Parallel sided slots with flat bottom

^Ø                                                                    Parallel sided slots with circular bottom

^Ø                                                                    Tapered slots with flat bottom

^Ø                                                                    Tapered slots with circular bottom

^Ø                                                                    Circular slots

44.      Mention the undesirable effects of unbalanced magnetic pull.

The undesirable effects of unbalanced magnetic pull are

^Ø                                                                          Saturation of magnetic materials due to reduction in air gap.

^Ø                                                                          Excessive vibration and noise due to unbalanced radial forces

45.      Mention the importance of conductor dimensions.

The dimension of the conductors directly affects the following factors in rotating machines:

^Ø                                                                    Allowable temperature rise

^Ø                                                                    Resistivity

^Ø                                                                    Current density

^Ø                                                                    Specific electric loading

46.      Define slot space factor or slot insulation factor.

The slot space factor is defined as the ratio of conductor area to slot area.

47. What do you understand by slot pitch?

The slot pitch is defined as the distance between centers of two adjacent slots measured in linear scale.

Where D = Diameter of armature

S_s  = Number of slots in armature

48.      State the parameters governing slot utilization factor or slot space factor.

The following factors decide the slot utilization factor:

^Ø                                                          Voltage rating

^Ø                                                          Thickness of insulation

^Ø                                                          Number of conductors per slot

^Ø                                                          Area of cross-section of the conductor

^Ø                                                          Dimensions of the conductor

49.      Define specific permeance of a slot.

Specific permeance of a slot is defined as the permeance per unit length of slot or depth of field.

50. What is unbalanced magnetic pull?

The unbalanced magnetic pull is the radial force acting on the rotor due to non-uniform air-gap around armature periphery.