Unit I introduction

DESIGN OF ELECTRICAL MACHINES 1. INTRODUCTION

1 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

Major considerations in Electrical Machine Design Electrical Engineering Materials-Space factor - Choice of Specific Electrical and Magnetic loadings -Thermal considerations - Heat flow -Temperature rise - Rating of machines -Standard specifications.

2 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

Major considerations in Electrical Machine Design 





DESIGN may

be defined as a creative physical realization of theoretical concepts. ENGINEERING DESIGN is the application of Science, technology and inventions to produce various machines to solve specified tasks with optimum economy and efficiency. Problem of design and manufacture of electric machinery is to build, as economically as possible, a machine which fulfils a certain set of specifications and guarantees.

3 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

The major considerations to evolve a good design are: (i)

Cost .

(ii)

Durability.

(iii)

Compliance with performance criteria as laid down in specifications.

4 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

Design Factors 

Mechanical force required for movement in rotating electrical machines can be produced both by electrostatic and electromagnetic fields since both the fields store energy.



In electrostatic machines, the energy density is limited by the dielectric strength of the medium used.



In electromagnetic machines, magnetic effect is used for production of force and there is no comparable restriction in magnetic fields. 5 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0



Voltages that can be developed and used by normal means, the forces produced by electrostatic effects are very weak.



A small current can produce large mechanical forces by electromagnetic means and therefore all the modern electrical machines are electromagnetic type.

6 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

DESIGN FLOW CHART START GIVEN SPECIFICATION CHOICE OF MATERIALS: MAGNETIC, CONDUCTING, INSULATING, etc. ASSUMPTION OF BASIC QUANTITIES SUCH AS, FLUX DENSITY, AMP.COND/m, etc

DESIGN PROCESS: MAGNETIC CIRCUIT, ELECTRIC CIRCUIT, MECHANICAL & THERMAL DESIGN

MODIFY ASSUMPTIONS 7

A IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

B

A PERFORMANCE CALCULATION

B COMPARE WITH GIVEN SPECIFICATION IS SATISFACTORY?

NO

YES PRINT DESIGN SHEET STOP

8 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

BASIC STRUCTURAL PARTS OF AN ELECTROMAGNETIC ROTATING MACHINE MAGNETIC CIRCUIT: Provides the path for magnetic flux. Consists of air gap, stator& rotor teeth, stator &rotor, cores(yokes) cores (yokes). ELECTRIC CIRCUIT: Consists of stator &rotor windings. DIELECTRIC CIRCUIT : Consists of insulation THERMAL CIRCUIT: Considered with mode& media for dissipation of heat. MECHANICAL PARTS: Frame, bearing and shaft.

9 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

Limitations in design (i) (ii) (iii) (iv) (v) (vi) (vii) (viii) (ix)

Saturation. Temperature rise. Insulation. Efficiency. Mechanical parts. Commutation. Power factor. Consumer specifications Standard specifications.

10 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

SATURATION: The maximum allowable flux density is to be determined by the Saturation level of the ferromagnetic material. TEMPERATURE RISE:  Life of the machine depends on the type of insulating material used.  Life of the insulating material in turn depends upon the temperature rise of the machine .  Proper cooling and ventilation techniques are required to keep the temperature rise within safe limits.

11 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

INSULATION: 

The insulating material should withstand electrical, mechanical and thermal stresses produced in the machine.



The size of the insulation is not only decided by the maximum voltage stress but also by the mechanical stresses produced.



Eg. For the same operating voltage thicker insulation has to be used for large sized conductors than for smaller sized ones.

12 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

EFFICIENCY: 

Should be as high as possible to reduce the operating cost.



Magnetic and electric loadings used should be small and this requires large amount of material.



So the capital cost of a machine designed foe high efficiency is high while its running cost is low.

13 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

MECHANICAL PARTS:  Construction of a machine technological requirements.

should satisfy numerous



In I.M – length of air gap is small in order to have high p.f.



In large machines, size of the shaft is decided by considering the critical speed, which depends upon the deflection of the shaft.



Type of bearings to be used depends on the inertia forces due to unbalanced rotors and unbalanced magnetic pull, and the type of construction whether the machine is mounted vertically or horizontally.

14 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

COMMUTATION: 

Commutation conditions limits the maximum output of the machine.

POWER FACTOR: 

Poor P.F results in larger values of current for the same power, therefore larger conductor sizes have to be used.



Problem of P.F is particularly important in case if I.M.



Length of air gap is determined by P.F considerations.



Value of flux density depends on the power factor, hence P.F becomes a limiting factor. 15 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

CONSUMER’S SPECIFICATIONS: 



Specifications laid down in the consumer’s order has to be met. Design evolved should also satisfy the economical constraints imposed on the manufacturer.

STANDARD SPECIFICATIONS: 

Specifications are the biggest strain on the design, since both the consumer as well as the manufacturer cannot get away from them without satisfying them. 16 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

Modern machine manufacturing techniques 

Modern machines are characterised by a very wide range of power outputs.



The power range varies from a fraction of a watt to several hundreds of megawatt in a single unit.



Thus the ratio of power output of the smallest machine to that of the largest machine is 1:1010

17 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0



Range of rotational speeds of electrical machines is very wide.



One machine may have a speed of few revolutions per second while that of another may be several thousand revolutions per second.



The large varied fields of applications, and wide range of both power output and speed of operation of electrical machines has led to a variety of types of construction.

18 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

CLASSIFICATION ON THE BASIS OF MANUFACTURING 1.

Small size Machine (up to 750W).

2.

Medium size Machine (Few kW-250kW).

3.

Large size Machine (250kW-5000kW).

4.

Larger size Machine (hundreds of MW). 19 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0



Small size machines: Electrical machines having power output up to about 750 W may be called small machines.



Medium size machines: Electrical machines having power outputs ranging from a few kilowatt up to approximately 250 kW may be classified as medium size machines.



Large size machines: Electrical machines with power O/P in the range of 250 kW up to about 5000 kW are classified as larger size machines. The machines are usually designed and manufactured as a series and have a definite power output range.





Larger machines are designed on individual basis. The power o/p of the machines are hundreds to megawatt.

20 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

The action of Electromagnetic machines can be related to 3 BASIC PRINCIPLES namely, (i) Induction (Faraday’s law). (ii) Interaction (Biot Savart’s law). (iii) Alignment (Production of force due to alignment-in Reluctance motors).

21 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

BASIC PRINCIPLES:

Faraday’s law: 

This law states that e.m.f induced in a closed electric circuit is equal to the rate of change of flux linkage.



The direction of induced e.m.f is such that the current produced by it opposes the change in flux linkages.

22 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

The change in flux linkages can be caused in three ways: 

Coil is stationary w.r.t flux and the flux varies in magnitude w.r.t time- emf induced is called as transformer emf or pulsational emfused in transformers.



Flux is constant w.r.t time and is stationary and the coil moves through it- emf induced is called motional emf- used in rotating machines like d.c machines, induction and synchronous machines.



Both the changes mentioned above occur together i.e., the coil moves through a time varying field- this process involves both transfer and conversion of energy- thus used in commutator machines.

23 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

The electromagnetic force is given by: fe= Bil sinα newton Where B= flux density, Wb/m2;

l = length of conductor, m; I = current carried by conductor, A; α = angle between direction of current and magnetic field. When the conductor and magnetic field are perpendicular to each other, α = 90 degree.

So , fe= Bil newton

24 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

ELECTROMAGNETIC DESIGN STATOR & ROTOR ROTATING MACHINE Core

STATIONARY Core & winding

Teeth dimension Winding & air gap

25 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

MECHANICAL DESIGN ROTATING STATIONARY Frame Shaft tank) Bearings

Tank (Transformer

26 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

ELECTRICAL ENGINEERING MATERIALS It consists of 1. Conducting materials 2. Magnetic materials 3. Insulating materials Electrical Conducting materials consists of 1. High conductivity materials  used for making all types if windings required  2.



in electrical machines, apparatus, devices . in transmission and distribution. High resistivity materials (alloys). Used for making resistances and heating devices.

27 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

ELECTRICAL ENGINEERING MATERIALS ELECTRICAL CONDUCTING MATERIALS: High conductivity materials.  Copper, Aluminium, Iron & Steel, Alloys of copper. High resistivity materials (Alloys).  Nickel, Silver & Iron

28 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

FUNDAMENTAL REQUIREMENTS MATERIALS: 1. 2. 3. 4. 5.

6.

OF

HIGH

CONDUCTING

Highest possible conductivity. Least possible temperature coefficient of resistance. Adequate mechanical strength i.e., absence of brittleness. Rollability and Drawability. Good weldability and Solderability which ensures high reliability and low electrical resistance of the joints. Adequate resistance to corrosion.

29 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

ALLOYS OF COPPER: 1.

Bronze- copper based alloys containing tin, cadmium, beryllium and certain other metals are generally called Bronzes.  Beryllium copper

Cadmium copper Brass- contains 66% Cu, 34%Zn. Copper silver alloy- contains 99.1% Cu, 0.06 to 0.1% silver. 

2. 3.

30 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

MATERIALS OF HIGH RESISTIVITY: They can be classified into 3 categories: I- GROUP: (MATERIALS USED FOR PRECISION WORKS)  Consists of materials used in precision measuring instruments & in making standard resistances and resistance boxes.  Important material used is Manganin (composition of Cu86%, Mn 12%, Ni 2%) II- GROUP: (MATERIALS USED FOR RHEOSTATS) 



Consists of materials from which resistance elements are made for all kinds of rheostats and similar control devices. Principal alloy is constantan consisting of 60 to 65 % Cu and 40 to 35% Ni).

II- GROUP: (MATERIALS USED FOR HEATING DEVICES ) 

Consists of materials suitable for making high temperature elements for electric furnaces, heating devices and loading rheostats. 31

IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0





Alloys of Nickel, Chromium and Iron called as Nichrome. Alloys of Aluminium, Iron and Chromium.

ELECTRICAL CARBON MATERIALS: 







manufactured from graphite and other forms of carbon coal, etc.

Carbon brushes are often graphited i.e., heat treated to increase the size of crystals. This raises the conductivity of the brushes and reduces their hardness. Carbon brushes should acquire a mirror smooth surface in order that they does not cause wear of commutator when mounted on them. 32 IFETCE/EEE/M.SUJITH/III YEAR/VI SEM/EE 2355/DEM/PPT/VER 1.0

CLASSIFICATION OF MAGNETIC MATERIALS Based on the relative permeability, materials may classified as, 1. 2. 3.

Ferromagnetic materials. (µr much>1) Paramagnetic materials. (µr slightly>1) Diamagnetic materials. (µr slightly