St.JOSEPH’S COLLEGE OF ENGINEEREING,CHENNAI-119 …

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St.JOSEPH’S COLLEGE OF ENGINEEREING,CHENNAI-119

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

EE2355- Design Of Electrical Machines

ASSIGNMENT-1

Part- A

1. Define heating time constant.

2. Define cooling time constant.

3. What is meant by Pheripheral speed? Write the expression for Pheripheral speed of a rotating

machine.

4. What are the major design considerations?

5. What are the limitations is design?

6. What is specific Electric loading?

7. What are the different types of magnetic materials according to their degree of magnetism?

8. List out the types of rating in electrical machines

9. List any three methods used for determination of motor rating

10. What is specific Magnetic loading?

11. Write down the output equation of a DC machine.

12. What is real and apparent flux densities?

13. Give main parts of a DC motor?

14. What are the factors to be considered for the selection of number of poles in DC machines?

15. What are the advantages of having large number of poles?

16. What are the losses occurring at commutator?

17. How main dimensions are calculated from D2L?

18. What is meant unbalanced magnetic pull?

19. Write down the output co-efficient equation of a DC machine.

20. Define slot pitch and pole pitch.

Part-B

1. Explain in detail about various Electrical Engineering materials.



ELECTRICAL ENGINEERING MATERIALS:

Electrical engineering materials used in the construction of all commercial

machines are classified as,

Conducting material

Insulating material

Magnetic material

Select the above materials properly to improve the efficiency of the machine,

reduce the size, weight, cost and increase the reliability of operation.

Conducting materials:

Conducting materials used in electrical engineering can be divided into two

groups,

Materials of high conductivity which are used for making conductors for all types of

windings used in electrical machines. These materials must posses least possible

electrical resistivity. High conductivity materials should meet the following requirements,

highest possible conductivity (least resistivity),

least possible temperature coefficient of resistance,

sufficient mechanical strength,

good welding and soldering of joints which ensure high reliability and low

resistance,

Property of rolling are important in the manufacture of wires.

Materials of high resistivity which are normally used for making heating devices,

thermo couples, resistance, etc.,

1

Various conducting materials are,

sliver

copper

aluminium

iron and steel

super conducting materials

8

MAGNETIC MATERIALS:

The magnetic properties of materials are characterized by their relative

permeability.

1. Ferromagnetic material: The relative permeability of these materials are much greater

than unity.

2. Paramagnetic material: The relative permeability of these materials are slightly

greater than unity.

3. Diamagnetic material: The relative permeability of these materials are slightly less

than unity.

Types of magnetic materials:

soft magnetic materials

o Solid core materialsIron

Gray cast iron

Cast steel

Soft steel

o Electrical sheet and strip

o special purpose alloys

Hard magnetic materials



9

Type Part Material used Operating flux density

Transformer winding i. Hot rolled silicon steelii. Cold rolled grain oriented

silicon steel

i. 1.1 - 1.4ii. 1.5 – 1.7

core i. Hot rolled silicon steelii. Cold rolled grain oriented

silicon steel

i. 1.0 – 1.2ii. 1.3 – 1.5

D.C Machines Yoke Cast steel 1.0 – 1.3

Pole Silicon steel, dynamograde 1.4 – 1.6

Air gap Air 0.43 – 0.76

Armature teeth Silicon steel, dynamograde 1.9 – 2.2

Armature core Silicon steel, dynamograde 1.2 – 1.4

Magnetic circuits of electrical machines:

Type Part Material used Operating flux

density

Induction Motor Stator core Silicon steel, dynamograde 1.1 – 1.4

Stator teeth Silicon steel, dynamograde 1.8 – 1.9

Air gap Air 0.35 – 0.65

Rotor teeth Silicon steel, dynamograde 1.8 – 1.9

Rotor core Silicon steel, dynamograde 1.1 – 1.4

Synchronous

Machines

Stator core Silicon steel, dynamograde 1.1 – 1.4

Stator teeth Silicon steel, dynamograde 1.8 – 1.9

Air gap Air 0.5 - -0.7

Rotor pole Silicon steel(salient), Forged

steel

(non salient)

1.2 – 1.4

Rotor core 1.1 – 1.4



11

INSULATING MATERIALS:

Insulating materials are used to provide an electrical insulation between parts

at different potentials. Design of electrical machine is limited by the restriction imposed

by the insulating materials.

Properties of insulating materials:

The proper selection of an insulating material for a particular condition needs

the knowledge of their electrical and mechanical properties.

The three fundamentals electrical qualities of insulating materials of great importance to

the designer of electrical machine or equipment are,

insulation resistance or resistivity

electric strength or dielectric strength

dielectric loss angle

In addition to the above, other properties such as mechanical strength, heat

resistance, etc.,

An ideal insulating materials must have the following properties,

high insulating resistance

high dielectric strength

low dielectric loss and low dielectric loss angle

no attraction for moisture

good heat conductivity

sufficient mechanical strength to withstand vibration and bending

solid materials should have a high melting or softening point.

liquid material should not evaporate.

2. Derive the expression for temperature rise in two dimension for a coil.

Heat flow in two dimensions:

In practical heat does not travel along parallel paths and the dissipating surface are not

homogenous. In practices the heat flow is in different directions through windings (copper) and

cores(iron) have insulation. The thermal resistivity of built up windings and cores depends upon

relative thickness of insulation to copper or iron.

Let,

l – length of coil in m

w – width of coil in m

t – thickness of coil in m

ρy – thermal resistivity along aa in Ωm

ρx – thermal resistivity along bb in Ωm

q – heat produced per unit volume in W/m3.

ABCD is the boundary of rectangular space, the wall which have thickness dx in the direction bb

and dx w/t in the direction aa.

Thermal resistance of horizontal elementary strips



3. Derive an expression for temperature rise -time curve for an electrical machine.



4. Describe any two methods used for determination of motor rating for variable load drives with

suitable diagrams.



RATING OF MACHINES:

DETERMINATION OF MOTOR RATING: Different methods to calculate the proper rating of motors for variable load drives.

method of variable losses

equivalent current method

equivalent torque method

equivalent power method

Methods of average losses:

The method consists of finding average losses Qav in the motor when it operates

according to the given load diagram. These losses are compared with Qnom. The method of

average losses presuppose that when Qav = Qnom , the motor will operate without temperature rise

going above the permissible for the particular class of insulation.

θper=Qnom/Sλ

and therefore θm=Qav/Sλ

θm= θper=Qav/Sλ=Qnom/Sλ



5. Discuss about the various duties and ratings of rotating machines and give their respective

temperature - rise curves.

TYPES OF DUTIES AND RATING OF MACHINES: Various types of duty as per IS:4722-1968 “Specification for rotating machines”.

Continuous duty(S1)

Short time duty(S2)

Intermittent periodic duty(S3)

Intermittent periodic duty with starting(S4)

Intermittent periodic duty with starting and braking(S5)

Continuous duty with intermittent periodic loading(S6)

Continuous duty with starting and braking(S7)

Continuous duty with periodic speed changes(S8)



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St.JOSEPH’S COLLEGE OF ENGINEEREING,CHENNAI-119 …

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