Electrical Md

7/27/2019 Electrical Md

1/10

Introduction of Electric Machine Design:

Electrical materials used in the construction of all commercial machines may be broadly classified into three

groups,

(i) Conducting

(ii) Insulating

(iii) Magnetic material.

The design of electrical machines and equipments mainly depends on the quality of these materials. Low grade

materials result in bulky and costly equipment generally.

Conducting materials:

Conducting materials used, are copper' and aluminium. Insulating materials used are cotton, silk, wood, paper

rubber mica, asbestos, polythene, pvc, bakelite etc. These materials are classed according to their temperature

withstanding capability. Magnetic materials used as silicon steel and nickel iron alloys. The following grades ofsilicon steel are mainly used in various electrical machines:

E11 , E12 , E21 , E31 , E41 , E42 , E43 , E310 , E320 , E330

In the above E represent electrical grade silicon steel. The first figure indicates the percentage of silicon in the

steel.

1. Low percentage.

2.Average percentage.

3.Above average percentage.

4. High percentage.

The second figure indicates the specific losses in the steel at 50 Hz

1. Normal.2. Average.

3. Low.

In the third figure 0 means that the steel is cold rolled grain oriented.

Comparison between Copper and Aluminium:

Sr.

No.

Characteristics Copper Aluminium (in terms of so many

times that of copper)

1. Resistivity ( ohm / mm2/

m ) at 200C

0.017241 1.624

2. Specific weight

(kg/mm2/km)

8.89 0.304

3. Thermal conductivity

(W/m3/0C)

350 0.571

4. Specific heat ( J/kg/0C) 389 2.333

Properties/Characteristics of Conducting Materials:

Sr.

No.

Conducting

material

Resistivity

(ohm-m)

at

Temp. Coeff. of

resistance in at 0C200

C x 10-4

Density

kg/m31900 to

2300Melting

point 0C


2/10

200 C x 10-8

1. Silver 1.6 40 10500 960

2. Copper

(Annealed)

1.7241 39.3 8890 1083

3. Copper

(Hard-drawn)

1.777 39.3 8890 1083

4. Aluminium(Annealed)

2.8 40.3 2703 660

5. Aluminium

(Hard-drawn)

2.8264 40.3 2703 660

6. Tungsten 5.50 50 2703 3300

7. Brass 7.0 15 to 20 18800 -

8. Nickel 10.5 40 8400 to

8700

1450

9. Tin 11.5 46 8850 232

10. Lead 21 41 7300 327

11. Carbon steel

(up to 0.4%

carbon)

10 to 14 40 to 50 11400 1350

12. Carbon steel

(high carbon)

15 to 45 2 to 40 7800 -

13. Silicon steel

(3.5% Si)

50 to 60 - - -

14. Manganin 48 0.5 7700 102

15. Constantan 52 0.25 to 0.5 8400

16. Nichrome 100 4.4 8900 1538

17. Iron (Cast) 75 to 98 - 8150 1530

18. Carbon 400 to 1200 (-) 12 to (-) 60 7800 3450

Insulating Materials:

Insulating materials are used in electrical machines to provide insulation of current carrying parts. It must provide a"galvanic separation" from the machine components at varying potentials. The fundamental requirement of a good

insulating material is:

(i) High dielectric strength,

(ii) High insulating resistance,

(iii) Low dielectric loss,(iv) Good mechanical strength,

(v) Good thermal conductivity,

(vi) High degree of thermal stability,

(vii) Good machinability to mass production.

Besides, these it must be easily available and economically viable.

Electric Machine Design (Continued):

Magnetic Circuit:

The law for magnetic circuits is

MMF = Flux x Reluctance.

Total mmf for the complete magnetic circuit is given by


3/10

F= ( B1/ 1) l1 + ( B1/ 1) l1 + ........

where l1 , l2 etc. are magnetic flux path lengths in the various parts of the magnetic circuit.

Transformers:

The transformer consists of the following elements:(i) Magnetic circuit consisting of limbs (core), yokes and clamping structures,

(ii) Electric circuit consisting of low voltage winding, high voltage winding and tertiary winding,

(iii) Dielectric circuit consisting of insulation in different forms,

(iv) Tank and accessories such as oil, cooling devices, conservators, breathers, terminal insulators and lead emf

equation is given by

E1 = 4.44 fT1 B Ai volts,

where Ai is the cross-sectional area of the core in sq-m.

The output equation is given as

Q = 2.22 f Bm Kw A i Awvolt amperes

where Kw is the space factor of the window, Aw is the net window area and is the current density.

All the conducting parts and the magnetic parts of the transformer must be well insulated from each other. The

insulation between core and LV winding is provided by an insulating cylinder of paper bakelite or press board.

Similar insulation is used between LV and HV winding. The end insulation is provided to insulate the windings

from the yokes. Partitions of press board are provided between HV windings of different legs, proper insulation is

provided between the coils, turns and layers of the winding.

Design and Constructional Features of a Transformer

(A) Design:

1. Specification:

Rating according to standard loading type, frame size, temperature rise, efficiency.

2. Performance:Transient behavior, system considerations, no load characteristics, short circuit characteristics, mechanical

forces, optimization.

3. Manufacture:Material procurement, handling and processes, skill and talent, transportation to site.

(B) Constructional features:

1. Cores:

Material, core and yoke, laminations, insulation, clamping, flux density, window, space optimization, core loss,weight.


4/10

2. Windings:

Type, current density, conductors, turns, layers, strands, tappings, insulation, transposition, resistance, l 2R loss,

leakage reactance, mechanical forces, thrust block, shielding, insulation gradient, weight.

3. Frame:

Size, stiffness, fixing, tank, heat exchanger, conservator, bushings, relays, weight.

Insulation Thickness :

Rated

KVA

Highest voltage of

the transformer

Thickness of

insulation

cylinder in mm

Thickness of

oil duct in mm

Insulation from

yoke to the

winding in mm

LV and HV Between

phases

LV and HV Between

phases

Up to

100kVA

100 to

500

kVA

500 to

1000

kVA

3.3 and 6.6 KV 3

3

3

2

2

2

6

9

11

8

8

8

22

22 to 31

31 to 52

Up to

6500

kVA

500 to

1000

kVA

11 kV 3.5

5.0

2.5

2.5

10

12

13

13

35

35 to 50

Up to

500

kVA

500 to

1000

kVA

5.0

6.0

3

3

20

20

25

25

55

70

AND CONSTRUCTIONAL FEATURES OF A ROTATING MACHINE

(A) Design:

1. Specification:

Rating according to standard, loading type, frame size, starting method, temperature rise, efficiency.


5/10

2. Performance:

Starting torque, transient behavior, system considerations, no load characteristics, short circuit characteristics,

forces under short circuit, losses, optimization.

3. Manufacture:

Material procurement, handling and processes, skill and talent, transportation to site.

(B) Constructional features:

1. Cores:

Material, stator and rotor stampings, clampings, insulation, flux density, core loss, weight.

2. Slots and teeth:

Suitable combination, size, space factor, flux density, insulation, core loss, mechanical forces.

3. Windings:Type, current density conductors, turns, layers strands, insulation, transposition, resistance, I 2 R loss, leakage

reactance, connections, temperature rise, cooling, slip-rings, commutator, weight

4. Frame:

Size, rigidity, fixing, enclosure-type, vibration, weight.

5. Shaft:

Size, stiffness, deflection, speed.

6. Bearings:Type, loss, cooling, lubrication.

DC MACHINE:

The output equation is

EIa = n (p) ( (IaZ) / a ) = n (p)(IzZ)

where,Iz = Ia / a = current per conductor is flux per pole, p is number of poles, n is speed in rps, Z is total number

of conductor and a is the number of parallel paths.

Also, D2L = (1/Co) P/n

where Co is output coefficient, D is diameter and L is the length of machine.

Specific Loading:

Machine rating,kW 10 50 100 500 1000 5000

Maximum gap flux densityBg , 0.65 0.80 0.85 0.95 1.00 1.05


6/10

( Wb/m2)

Specific Electric Loading:

Machine rating, kW 10 50 100 500 1000 50000

Ampere conductors per meter ac 17500 25000 28000 36500 41000 50000

Relation between Length and Diameter of Armature:

L = 0.7 (D /p)

Pole pitch is limited as per the following table:

No. of poles 2 4 6 more than 6

t cm 25 to 50 30 to 40 40 to 45 45 to 55

Slot design:

Knowing the value of current through the conductor Ia, its cross-sectional area ac can be calculated as

ac = I c/

where is current density in the armature winding.

The insulation is provided as per the following practice.

(a) Round conductors:

Diameter up

to (mm)

Total insulation

thickness (mm)

1.5 0.20

2.0 0.30

3.0 0.35

4.0 0.40

(b) Rectangular Conductors:

Width up to

(mm)

Total thickness

(mm)

1 0.34 0.4

4 and above 0.6


Slot insulation:

The slot is also provided with suitable insulation. A typical slot insulation is given below :


7/10

Width wise insulation mm

(i) Conductors wrapped with mica paper 2.0

(ii) Coil wrapped with glass tape 0.2

(iii) Coil varnish 0.2

(iv) Slot linear 0.4

(v) Clearance 0.2

Width wise insulation mm

(i) Conductors wrapped with mica paper 2 x 2 = 4.0

(ii) Coil wrapped with glass tope 2 x 0.2 = 0.4

(iii) Coil varnish 2 x 0.2 = 0.4

(iv) Slot linear 3 x 0.2 = 0.6

(v) Layer separator 1 x 2.0 = 2.0

(vi) Wedge 1 x 5.0 = 5.0

(vii) Clearance 1.6

Main Pole:

Axial length:

The axial length is 10 to 15 mm shorter than the core length of the armature. Width. The width can be calculated

from the relation,

Width = Cross-sectional area of pole Width / Axial length of the pole.

Height:

The height is calculated from the relation height

= [(1.1 to 1.25) ((Ia Z) / (2p) ) ] / (1000 ( ( dc k) ))

Commutator:

The diameter of the commutator is taken as (0.65 to 0.70) D for medium and large machines and (0.75 to 0.80) D

for small machines.

Standard size of commutator brushes is given below :

Machine Width (mm) Height (mm) Thickness (mm)

Small size machine 15 30 6,8,15

Medium size machine 22 37 9,12.5,15,18

Large size machine 30 43 12.5,15,18,22,25

INDUCTION MOTOR :

kVA input = Q = (11 k . Bav x 80 -3) D2 Lns = Co D2 Lns.

Main Dimensions :

Design criteria Length to Pole pitch ratio,

Good power factor 1 to 1.25


8/10

Best power factor = 18 L

Minimum cost 1.5 to 2

Good efficiency 1.5

Good overall design 1.0

Peripheral speed:

For normal design of motors a limiting value of peripheral speed as 30 m/s is used. However, standard

constructions have speeds up to 60 m/s and in very special cases up to 75 m/s.


Ventilating ducts:

The radial ventilating ducts of 8 to 10 mm are provided after each stack of 10 cm core length.

Stator windings:

Turns per phase,

Ts = Es / (4.44 f m k m)

current density of 3 to 5 A/mm2 is generally used.

For diameters up to 3 mm round SWG conductors are used with proper insulation. For larger machines bar or strip

conductors are used.

Stator slots:

For smaller machines up to 20 kW, 600 V and diameter less than 40 cm, semi-closed slots are used. For higher

ratings, open type slots with insulation wedge are used. Use of semi-closed slots gives low air gap contraction

factor, low value of magnetizing current low tooth pulsation losses and quieter operation as compared to open

slots.

For Ss = number of stator slots

Slot pitch, yss = Gap surface / Total number o stator slots = D / S s

Total number of stator conductors = 3 x 2 x T s = 6 Ts

Hence conductors per stator slot = Zss= 6 Ts / S s

Rotor design:

Air gap length depends on:

(i) Power factor,

(ii) Overload capacity,

(iii) Unbalanced magnetic pull,

(iv) Pulsation loss,

(v) Noise.


9/10

For small motors, any one of the following relation is used for air gap length :'

lg = ( 0.2 + 2 ( DL))mm

lg= (0.125 + 0.35D + L + 0.015 Va)mm

lg = (0.2 + D) mm

where,

D = inner diameter of stator in meters

L = length of stator in meters

Va = peripheral speed, m/s.

Air gap for 4 Pole Induction Motors:

D (cm) 15 20 25 30 45 55 65 80

Lg(mm) 0.35 0.50 0.60 0.70 1.31 1.8 2.5 4.0

Squirrel Cage Rotor Design:

Rules for number of rotor slots:

The number of rotor slots Sr in comparison with number of stator slots Sa, is given by

(i) Sr = (1.15 to 1.30) Ss(ii) To avoid Synchronous Cusps

Ss - Sr != p 2p or 5p

(iii) To avoid magnetic locking in 3 phase motor,

Ss - Sr != 3p

(iv) To avoid noise and vibrations,

Ss - Sr != 1,2, (p 1 ) or (p 2).

Rotor bar current for 3 phase machine is given by

Ib = 0.85 (( 6 Ib Ts) / Sr )

where Is, is stator current.

Skewing:

In order to eliminate the effect of any harmonic, the rotor bars are. skewed in such a way that the bars lie under

alternate poles of the same polarity.

To eliminate nth harmonic, the angle of skew will be


10/10

Q = 720 / (n x p) degrees mechanical.

In practice the rotor is skewed through one stator slot pitch.

Area of end rings:The area of end ring,

ar = ( Sr Ib ) / ( p c ) mm2

Slip:

The value of full load slip s is determined by rotor copper loss. Some typical values are given below:

Output

(kW)

0.75 3.75 7.5 18.750 37.50 75.00 150.00

Slip

5.00

4.2 4.0 3.70 3.50 3.20 3.00

Design of wound rotor:

No. of turns per phase on rotor,

Tr = k s/ k r. Er / Es. Ts

where,

k s = winding factor for stator ,

k r = winding factor for rotor,

Es = voltage per phase applied to stator,

Er = voltage per phase induced in rotor at stand still,

Ts = number of turns per phase on stator.

Minimum tooth width = Flux per pole / (Max. allowable flux density x Slots per pole x Net iron length)

Rotor core depth = m/ 2 - Bcr x L2

Electrical Md

Documents

Transcript of Electrical Md