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 eduction in 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.