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 material:
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 of silicon 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.
Conducting Material >
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 Material:
|
Sr. No.
|
Conducting material
|
Resistivity (ohm-m) at
200 C x 10-8
|
Temp. Coeff. of resistance in at 0 C200 C x 10-4
|
Density kg/m3
|
1900 to 2300Melting point 0C
|
|
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.
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