BASIC CONSIDERATIONS IN DESIGN

1. BASIC CONSIDERATIONS IN DESIGN • The aim of the design is to completely obtain the dimensions of all the parts of the • machine to furnish the data to the manufacturer. The main aim of carrying out the • design is to achieve the following. • · Lower cost • · Lower weight • · Reduced size • · Better operating performance. • To design the electrical machines properly, one should be familiar with the following • aspects of electrical engineering. • · Various electrical materials and their properties. • · Properties of magnetic and electric fields. • · Laws governing electric circuits. • · Laws of electromagnetic induction. • · Calculation of magnetic circuits. • · Construction of various types of electrical machines. • · Behaviour of electrical machines under working conditions.

2. In addition to above, one needs a good knowledge of strength of material, • metallurgy, mechanics, and the laws of heat transfer. • While designing an electrical machine, much emphasis should be placed on lowering • its cost by saving the materials and reducing to a minimum labour consuming • operations in its manufacture. • The design should be satisfactory with respect to electric strength, mechanical • ruggedness, dynamic and thermal resistance of windings in the event of short circuit. • In order to meet the above requirements during the design of electrical machines, the • designer should be well conversant with the prices of the basic materials used in the • machines. • He should also be familiar with the amount of labour consumed in the production of • machine parts and assemblies. • From the above discussion it is quite obvious that the art of successful design is to • select proper materials and to resolve the conflict for space between magnetic, • conducting and insulating materials o as to produce a cheap and highly efficient • machine.

3. Effects of Material on Design • Engineering materials that are used in electrical machines may be divided into • three broad groups • · Conductors • · Magnetic materials • · Insulating materials. • Conductors possess high conductivity and are used to provide paths for electric • currents in electrical machine windings. • Magnetic materials possess high magnetic permeability and are used as magnetic • cores in electrical machines to set up a magnetic field in the surrounding space. • The conductivity of the insulating materials is very poor and as such they serve to • insulate one current-carrying part from another when ever they operate with a • difference in electrical potential relative to each other.

4. Effects of Conducting Material • Copper is most suitable as the conducting material for all types of • electrical rotating machines. This is normally used in round or rectangular • cross section. It has all the properties that a good conducting material must • possess. • However in case of transformers, aluminium can also be used as a • conductor instead of copper. The concurrent development of power • transformers with aluminium windings will bring about greater saving of • copper. • Aluminium when used as a conductor in small transformers, decrease the • over-all cost of the transformer. But in case of large transformers, the use of • aluminium will increase the over-all cost as well as the size. • The power transformers with aluminium as conductors, having a power • rating upto 6.0 MVA may serve as the counter parts of copper wound • transformers, as both of these can have similar performance characteristics. • The cost of these two transformers is nearly the same and thus this • substitution may be regarded as fairly equivalent from the view point of both • technology and economy.

5. Effects of Magnetic Materials • The cost of an electrical machine depends upon its size and on the weight • of magnetic and conducting materials. • The size and the weight of the machine depends upon the assigned values • of specific magnetic loading, which is limited by the saturation and core • losses of the magnetic material used in the machine. • However an increased value of specific magnetic loading could be • assigned for designing an electrical machine provided the magnetic material • has a comparatively higher saturation limit and lower core losses per kg of • material. • Presently magnetic materials having very high permeabilities and low • specific iron losses are available. These materials are much superior and result • into a reduced size of the machine with a lower over-all cost. • The most suitable magnetic materials for electrical machines, which give a • considerable reduction size and cost, are silicon steel of various grades. • The increase in flux density makes it possible to save magnetic material, • although cold rolled steel is more expensive than hot rolled steel. The specific • loss at a certain value of flux density for cold rolled grain oriented steel is • nearly half as compared to hot rolled steel at the same value of flux density.

6. Effects of Insulating Material • Insulating materials are very sensitive to temperature rise. • Every class of insulating materials has an upper limit of temperature upto • which these can be used safely. This limiting temperature imposed by the • insulating material used in the machine dictates the allowable losses in any • electrical machine. • A poor insulating material such as paper, cotton etc. can withstand a • maximum temperature of only 90°C, where as a good quality of insulating • material like ceramics, asbestos etc. can safely be used for temperatures • beyond 180°C. • If the insulating material selected for the machine is of better quality, • higher total losses from the aspect of temperature rise can be allowed. • The increased limit of allowable losses indicates that higher values can be • assigned to specific loading which would mean a reduced size of the machine • with the reduction in its over-all cost. • In electrical machines of small sizes, class A insulating material are • normally used, where as class B insulation system is mostly used in case of • medium and large machines.

7. The insulating materials belonging to class B have hot spot with-stand • capability of 130°C. Thus the machines with class B insulation system can • have a maximum temperature rise of 80°C over an ambient temperature of • 40°C with a margin of 10°C for difference between the hottest spot and • average temperature rise of the winding. • With the rapid progress in the material technology, new insulating • materials such as class F insulation system with much higher hot spot • Temperature capability are now available which can replace the traditional • class B materials. • With proper design, the saving in copper can be as high as 15 percent and • in steel of the order of 8 to 10 percent. • When the performance of the two designed machines with the same • specifications but with class B and class F insulation systems are compared, • normally it is observed that the efficiency of the machine with class F • insulation system is marginally lower because of higher losses. • This marginal reduction in efficiency is not at all a detrimental factor • looking to the reduced cost of capitalized losses, reliability and over all • savings of materials.

8. In addition to above, the use of class F insulation system in electrical • machines gives the following advantages. • Excellent dissipation factor. • Longer life to the machines. • Better chemical resistance. • Higher flame resistance characteristics. • RImperviousness to moisture and humidity • eductionin insulation thickness. • Better heat transfer in the coils of the winding. • Improved radiation resistance properties, thus suitable for nuclear • power station machines. • From the above discussion, it is concluded that the reliability of the • machine with class F insulation system is much more than with class B • system. • Hence the proper selection of the insulation system is very essential for • achieving best and economical design of electrical machines. • The improper selection of insulating material for the machine may • ultimately result into a machine which is disadvantageous from many • considerations.