UNIT 1 PRINCIPLES of ELECTRICAL MACHINE DESIGN Notes PDF

# UNIT 1 PRINCIPLES of ELECTRICAL MACHINE DESIGN - **Design of Machines** - Design may be defined as a creative physical realization of theoretical concepts. Engineering design is application of science, technology & invention to produce machines to perform specified tasks with optimum economy...

# UNIT 1 PRINCIPLES of ELECTRICAL MACHINE DESIGN - **Design of Machines** - Design may be defined as a creative physical realization of theoretical concepts. Engineering design is application of science, technology & invention to produce machines to perform specified tasks with optimum economy & efficiency. - Engineering is the economical application of scientific principles to practical design problems. If the items of cost of durability are omitted from a problem, the results obtained have no engineering value. The problem of design & manufacture of electric machinery is to build, as economically as possible, a machine which fulfills a certain set of specifications & guarantees. Thus a design is subordinated to the question of economic manufacture. - The major considerations to evolve a good design are: 1. Cost 2. Durability 3. Compliance with performance criteria as laid down in specifications. - **Design Factors** - The mechanical force required for movement in rotating electrical machines can be produced both by electrostatic & electromagnetic fields since both the fields store energy - In electrostatic machines, the energy density is limited by the dielectric strength of the medium used. - For example, if air is used as the dielectric medium, the max. value of electric intensity that can be used is 31V/M on account of dielectric breakdown, which corresponds to an energy density of about 40J/m³. - In electromagnetic machines, magnetic effect is used to sprod" of force of there is no comparable restriction in magnetic fields. - However, the max. value flux density that can be used is about 1.6 wb/m², because beyond this value magnetic saturation sets in the ferromagnetic materials required to complete the magnetic circuit of the machine. - This limits the energy density in the air gap to about 1MJ/m³, This energy density is approximately 25,000 as much for electric fields. - Thus, at voltages that can be developed & used by normal means, the forces produced by electrostatic effects are very weak. On the other hand, a small current can produce large mechanical forces by electromagnetic means & therefore all the modern electrical machines are electromagnetic type... - **The basic structure of an electromagnetic rotating electrical machine is shown in fig@. The machine consists of following parts** - Cores - Frames - Windings - Shaft - Bearing - Rotor - Stator - Air gap - **Magnetic Circuit** - It provides the path for magnetic flux & consists of air gap, stator & roter teeth, stator & roter cores (yokes). - **Electric Circuit** - In consists of stator & roter windings. The winding of a transformer or a rotating machine conveys electrical energy from working region & is concerned with development of emf & force. Windings are formed from suitably insulated conductors. - **Dielectric Circuit** - The dielectric circuit consists of insulation required to isolate one cond. from another & also the windings from core. The insulating materials are essentially non-metallic & may be organic or inorganic, natural or synthetic. - **Thermal Circuit** - The thermal circuit is concerned with mode & media for dissipation of heat produced inside the machine on account of losses. - **Mechanical Parts** - The important mechanical parts of a machine are it's frame, bearings & shaft. - **Limitations in design** - Apart from availability of suitable materials, facilities available for manufacture of required machine & facilities required for transportation, the following considerations impose limitation on design: 1. **Saturation**: Electromagnetic machines use ferro-magnetic materials. The max. allowable flux density to be used is determined by the saturation level of the ferro-magnetic material used. A high value of flux density results in increased excitation resulting in higher cost for the field system. 2. **Temperature Rise**: The most vulnerable part of a machine is it's insul?. The operating life of a machine depends upon type of insulating materials used in it's construction & life of insulating materials in turn depends upon temperature rise of machine. If an insulating material is operated beyond the max. allowable temperature, it's life is drastically reduced. Proper Cooling & ventilation techniques are required to keep the temp. rise within safe limits. 3. **Insulation**: The insulating materials used in a machine should be able to withstand the electrical, mechanical & thermal stresses which are produced in the machine. The mechanical strength of insul" is particularly important in the case of transformers. Large axial forces are produced when the secondary winding of a transformer is short-circuited with the primary. Therefore, while designing insul. for a transformer, due consideration must be given to the capability of the inst? to withstand, large mechanical stresses which may be either compressive or tensile) that are produced under shoot ckt conditions apart from electrical of thermal breakdown considerations... - The type of insul? is decided by the max. operating temp. of the machine parts where it is put. - The size of insul? is not only decided by max. vtg. stress but also by the mechanical stresses produced For example, for the same operating vitg, thicker insup has to be used for large sized conductors than for smaller sized ones. 4. **Efficiency**: The efficiency of a machine should be as high as possible to reduce the operating cost. In order to design a highly efficient machine, the magnetic & electric loadings used should be small & this requires the use of large amount of material. Therefore, the capital cost of a machine designed for high efficiency is high while it's running cast is low. 5. **Mechanical Parts**: The construction of an electrical machine har to satisfy numerous technological requirements. The construction should be as simple as possible & also it is technologically good if it is carried out using simple & economical means with as little labour as possible. But the technological techniques should be consistent with the requirements of performance, reliability & durability. - The design of mechanical parts is particularly important in the case of high speed machines. For example, while designing a turbo-alternator, techniques should be consistent with the requirements of performance the rotor slot dimensions are so selected that the mechnical stresses at the bottom of rotor teeth do not exceed the max, allowable limit. - In an induction motor, the length of air gap is kept as small as mechanically possible in order to have a high power factor. The length of air gap & also that of size of shaft are mainly decided by mechanical considerations. - The size of shaft should be such that it does not give rise to excessive unbalanced magnetic pull (U.M.P.) when deflected. - In large machines, the size of shaft is decided by considering critical speed which depends on deflection of shaft. - Bearings of rotating machines are subjected to action of roter weight, external loads, inertia forces due to unbalanced rotors & forces on account of unbalanced magnetic pull 6. **Commutation**: The problem of commutation is important in case of commutator machines as commutation conditions limit the max. olp that can be taken from a machine. For example, at present the max. power olp of a single unit dc. machine is approximately 10 TMW & this limitation is solely on account of commutation difficulties.. 7. **Power Factor**: Poor power factor results in larger values of current for the same power & therefore, larges cond. sizes have to be used. - This problem of p.f. is particularly important in case of I.M. The size & heyce cost of I.M. can be reduced by using a high value of flux density in the air gap but this results in a poor p.f. & consequently upon P.f. & hence the value of flux deysity used depends upon P.f. Thus, p.f. be comer a limiting facter. In fact, the length of air gap to be provided with on I.M. is primarily determined by p.f. considerations... 8. **Consumer's Specifications**: The limitations imposed by consumers specifications on the design of electric machinery are obvious. The specifications as laid down in the consumer's order have to be met & the design evolved should be such that it satisfies all the specifications & also the economic constraints imposed on the manufacturer. 9. **Standard Specifications**: These specifications are the biggest stray on the design because both the manufacturer as well as the consumer can not get away from them without satisfying them. - **Basic Principles**: - The action of electromagnetic machines can be related to 3 basic principles which are: 1. Induction 2. Interaction 3. Alignment - **Faraday's law of electro-magnetic induction**: - Whenever flux linkages with the coil are changes then emf is always induced in coil. Thus Flux linkages, ψ = NΦ - Where N= No. of turns in a coil, Φ= Flux linking with all of them. - In most cases, the flux & does not link with all the turns os alternatively all the turns do not link with the same flus. Under these circumstances, the sumonation of all products of magnetic flux with complete turns of magnetic cat gives the total value of flex linkages 4. - The total flux linkages thus are: ψ = N.Φ₁ + N₂Φ₂ + N₃Φ₃ + N₄Φ₄ = Nkfk - Where, Nk= Nor of turns which link with fluxfk. - In case there is a change in the value of the flux liubages of the coil, an induced emf is produced in it whose value is given by: - e= dy, volt - e=- Np - e=- Ndf - The -ve sign in eq above eg? indicates that the dir of the induced emf is such that current produced by it opposes change in flux linkages. - The change in flux linkages can be caused in 3 ways: 1. The coil is stationary & flux & flux varies in magnitude with time. 2. The Flux is constant wirit. time & is stationary for the coil moves through it. 3. Both the changes mentioned above occur together ie. The coil moves through a time varying field. - In the method outlined in i) where the coil is stationary & flux is time varying, an emf called transformer emf is produced. Since no motion is involved, there is no energy conversion & process really takes place is energy transference. This principle is used in transformers which employ stationary coils & time varying fluxes for transfer of energy at one level to another.. - In iⅱ) the flux cutting rule can be employed to illustrate the emf generated in a cond. moving in a const. stationary field. The emf generated in a cond. of length moving at right angles to a uniform, stationary, time invarying magnetic field is given by, e = Blv, volt Where, B= flux density, wb/m² (T) I = length of cond., m ✓ = linear velocity of cand., m/s - The generated emf in this case is called a "motional emf" because it is caused due to motion of a cend. Since motion is involved in prod" of this emf, the process involves electromechanical energy conversion. - This principle is utilized in rotating machines like d.c., induction & synchronous machines. - **Biot Savart's law**: - This law gives the value of force produced on account of interaction bet a magnetic field & a current carrying cond. The electromagnetic force is given by : fe = BIl sin θ, newton Where, B = flux density, wb/m² (T) l = bength of cond, m i = current carried by cond., Amp. θ = angle bet? dirn of current of dir. of magnetic field. - The dir? of force produced is perpendicular to both current & magnetic field. The magnetic field is radial in electrical machiyes, the magnetic field & the cond are perpendicular to each other & thus θ = 90°. Therefore, fe = Bli sin 90 fe = Bli newton - In fig®, B represents flum density of undisturbed magnetic field. The introd" of current carrying cond. introduces a new magnetic field. The original field & field due to cond. combine to produce a resultant field as shown in Fig⑥. - The resultant field is distorted in the neighbourhood of the cond, the resultart flux density being greater on one side & lesser on the othier & this results in prod? of an electromagnetic force in the dir? indicated. - In case the increase in flux density on one side is equal to red on other side, the electromagnetic force is given by egn①. - When either the dir of current or dir of the magnetic field is reversed, the dir force acting on cond. is reversed. - However, if the dirns of both the current as well as The magnetic field are reversed, the dir" of force Induced remains unaltered.. - fig shows the effect of reversing the current when the dion of field is worchanged. It is clear that under these cond the dir? of force produced is reversed. - Biot Savart's law can be applied to determine force bet" two. current carrying cond. Figs@ shows 2 111 current carrying cond. of 'I' separated by a distance 'D', & sithatet in a medium of permeability µ. - The two currents are I₁ & I₂. In fig® the two currents flow in the same dir while in fig ⑦ they flow in opposite dir". The resultant magnetic fields are also shown. - It is chear that when conductors corry currents in the same dir, there is a force of attraction bet? them, while there is a force of repulsion bet? them if they carry currents in the opposite directions. - **Alignment**: - If a magnetic field exists in an ambient piece low permeability medium like air & if a piece of high permeability material is placed in this field, the latter experiences a force which tries to align it with the dirn of field in such a way that it occupies a position of min. reluctance. The principle of prod of force due to alignment is used in reluctance Motors. # Design and Constructional features of a transformer: **Design features:** 1. **specifications:** - Rating according to standard, loading type - frame size, temp. rise, efficiency. 2. **performance:** - Transient behaviour, system considerations, - no load characterics, short ckt char, mechanical forces, optimization. 3. **Manufacture:** - Material procurement, handling and processes, - skill and talent, transportation to site. **Constructional features:** 1. **Cores:** - Material, Core and yoke Laminations, insulation, - Clamping, flux density, window space optimization, Coreloss, weight. 2. **Windings** - Type, Current density, Conductors, turns, - layers, strands, tappings, insulation, transportation, - resistance, I'R Loss, leakage reactance, mechanical Forces, - thrust block, shielding, insulation gradient, weight. 3. **Frame:** - Size, stiffness, fixing, tank, heat exchanger, - Conservator, bushings, relays, weight. # Design and Constructional features of a rotating machine: **Design Features:** 1. **specifications:** - Rating according to standard, loading type - frame size, starting method, temperature rise, efficiency. 2. **Performance:** - Starting torque, transient behaviour, - System Considerations, no load char., s.c char, forces - under short circuit, losses, optimization. 3. **Manufacture** - material procurement, handling and - Processes, skill and talent, transportation to site. **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, transportation, resistance, I²Rloss, - leakage reactance, Connections, temp. rise, Cooling, Sliprings, - Commutator, weight. 4. **Frame:** - Size, rigidity, fixing, enclose type, Vibration, - weight 5. **shaft:** - Size, stiffness, deflection, speed... 6. **Bearings:** - Type, loss, Cooling, Lubrication. # Specifications: - The Indian Standard Institution has prescribed specifications for transformer and rotating electrical machines. Standardisation helps in the economy of manufacturing cost and in developing a production type. To the user it helps to choose a quality product as per standard and easy to procure replacement and spares. For designer also it helps in identifying a Criterion for best design. **Important specifications for transformer are** 1. KVA, Voltage - ratio (primary Volts/secondary volts at no load), 2. Currents (Iry and ⑪Y), 3. No. of Phases, 4. Frequency, 5. Type (Power or distribution), 6. Losses at 75°C, 7. Connection of hiv and L.V windings, 8. percentage tappings, 9. vector group (for 3php), 10. % impedance. **Important specifications for rotating Machines** 1. **DC machine:** - Motor or Generator, output power kw, Voltage - Current, speed, Type of field excitation shunt, series, Compound - Excitation Voltage and current, Enclosure type, Duty type. 2. **AC machine:** - **Induction motor**: output kw, voltage, Current, no. of - Phases; frequency, Connection (Staror Delta), Rotor type. - (cage, súp ring), Enclosure type, Duty type. - **synchronous machine:** Motor or Generator, out put - Power Kw, voltage, Current, no. of Phases, frequency, - Connection (star or Delta), speed, Excitation Voltage - and Current. Enclosure type, Duty type. # Basic principles of Electrical Machines: - All electrical machines are basically electro magnetic devices. - Among them the rotating machines are electro mechanical energy conversion devices but transformer is a static machine where energy conversion does not take place, only the level of Voltage changes. - All electrical machines are based upon three principles. - i) Induction - ii) Interaction - iii) Alignment. ## principle of Induction: - In an electrical machine the electric and magnetic Circuits are mutually inter Linked. The magnetic Circuits provide Path of low reluctance to magnetic flux. A given flux & may not Link with all the no. of turns. therefore the total flux linkages is introduced bait which means Summation ofall products of flux with complete turns. Thus Total flux Linkages, 4 = N₁Φ₁ + N₂Φ₂ + N₃Φ₃ + N₄Φ₄ = Nkfk - Now according to Faraday's Law of Electromagnetic induction emf induced in an electric circuit is equal to the rate of change of this Linkages ie e = -dψ/dt # UNIT VI # COMPUTER AIDED DESIGN OF ELECTRICAL MACHINE ## 6.1 INTRODUCTION - Design of electrical machines mainly consists of, obtaining the dimensions of the various parts of the machine to suit given specifications, using available material economically and then to furnish these data to the manufacturer of the machine. - The design procedure starts with the assumption of certain basic design parameters such as flux density, ampere conductors per metre or current density etc. From the given specifications of the machine and the above values of the design parameters, the dimensions of various parts of the machine are calculated. Based on these performance and temperature rise are worked out, which are then compared with the given specifications. If no satisfactory result is obtained, the basic assumed quanitites are suitably modified, till the result is upto the satisfaction. - The design process is thus iterative as shown in design flow chart of Fig. 6.1 which is self explanatory. Normally, it takes many iterations to arrive at an optimal solution. The iterations require changes in values of variables, till both the cost and performance constraints are satisfied. This manual procedure of design (also known as hit and trial method) is tedious and time consuming. However, it can be done very quickly by use of digital computer. Hence, digital computer becomes a most valuable powerful tool in designing the electrical machines to satisfy the required performance of the customer. - In the conventional design, we calculate the approximate temperature rise which has sufficient tolerance. For examples, in the case of power transformers, the approximate temperature rise of the windings above ambient may come out to be 50°C, but the actual temperature rise will be about 40°C. Thus, in the conventional design, the designer is unnecessarily forced to choose an insulating material having large temperature rise and thus making the design costlier. - However, the use of computers in the design enables sophisticated calculations for the temperature rise, in the various parts of the machine. The exact hot spot temperature and its location can be determined with the use of computers. So, now for the same rating of the machine, an insulating material having lower temperature limit can be used and hence the initial cost of the machine can be reduced drastically. ## 6.2 ADVANTAGES OF COMPUTER AIDED DESIGN - The use of digital computers has completely revolutionized the field of design of electrical machines. Computer aided design eliminated the tedious and time consuming hand calculations, to achieve the desired result. The main advantages of computers in design of electrical machines are as follows: - It has the capability to store large amount of design data. - A large number of loops can be incorporated in the design programme. - Arithmetic operations during design can be performed at approximately high speeds, which results in reducing the time. - Highly accurate and reliable designs are obtained. - Logical decisions can be taken, thereby saving the man hour of the design engineers. - It is possible to optimize the design, with a reduction in overall cost and improvement in performance. ## 6.3 VARIOUS APPROACHES IN COMPUTER AIDED DESIGN - The two basic approaches commonly used for computer aided design of electrical machines are: 1. Analysis method 2. Synthesis method ### 6.3.1 Analysis Method - In this method, the use of computer is made only for the purpose of analysis, leaving all the decision making to the designer. Fig. 6.2 gives the flow chart of the analysis method. - In analysis method, the basic design parameters are chosen by the designer. These are fed to the computer as input data. The design details and performance is calculated by the computer. The performance is then examined by the designer and the he makes another choice of the input datas, if necessary, and the performance is recalculated. This process of design is repeated till the requirement are fully met with. ### 6.3.2 Synthesis Method - In synthesis method, the logical decisions are taken by the computers instead of the designer. The required performance of the machine is fed as the input to the computer. It assumes the suitable values for the design parameters, calculates the performance and then compares with the desired performance. If the performance is unsatisfactory, than the computer itself adjusts the values of the design parameters and recalculates the performance. The process is repeated till a satisfactory performance is obtained. - Flow chart illustrating this method has been given in Fig. 6.3. The main advantage of the synthesis method is that the optimization is possible. Most accurate and reliable designs can be obtained. However the programming in complex and time consuming. ## 6.4 COMPUTER AIDED DESIGN OF TRANSFORMER - The philosophy followed in the computer aided design of transformers is the same as that adopted by an experienced designer. Similar to normal design, input data, such as KVA rating, voltage ratio of LV to HV winding, frequency etc. and the performance limitations like percentage regulation, percentage load losses temperature rise etc. are fed to the machine as input data. The computer selects suitable values to the design parameters, such as maximum flux density, current density, window space factor etc. based upon the rating of the machine to be designed. Using various design equations, computer calculates the complete design data and the performance characteristics. The performance is then compared by the computer with the specified limits imposed by the customer. If the performance is not up to the desired limitation, the initial assumptions of the design parameters are revised by the computer and the whole process in repeated, till a optimum design is obtained. - Next the maximum or minimum limits of various design parameters are fed by the designer, for example - Flux density in the core - Current densities in LV and HV winding - Maximum height of the transformer - Radial distance between the HV windings on two adjacent limbs. - Window space factor. - Now based upon the rated power, line voltage and winding connection, the proper values of these design parameters are chosen. Using the design parameters and the design equations, the magnetic frame in designed. - Choosing proper values of the clearances between the windings and the thickness of the insulating cylinders, the LV and HV windings are designed using the conventional approach. If the HV winding on the two adjacent limbs overlap, then the design parameters are changed and the winding is redesigned is such a manner that now the HV windings do not overlap. Full load copper losses and iron losses are then calculated. The variation in copper losses is affected by increasing or decreasing the cross-sectional area of conductor of LV and HV winding. Iron losses are varied by changing the flux density in the core. Computer continuous the calculation with these changes till the full load copper losses and iron losses satisfy the specified limits. Percentage reactance, cooling system, the weight of core and copper are then computed and finally the detailed design data sheet is printed. ## 6.5 COMPUTER AIDED DESIGN OF ROTATING MACHINE - The computer aided design of rotating electrical machines is quite complicated, because of the following reasons. 1. There are a large number of independent variables. (ranging from 10 to 15 variables) 2. Some of the variables are discrete variables i.e. they can be varied in steps only. For example, number of stator slots, number of rotor slots etc. ## 6.6 OPTIMIZATION OF DESIGN - A feasible design is one which satisfies all the given specifications and the required performance. But, a feasible design need not be an optimal one as regards the cost of active material, weight of active material, cost of energy wasted as losses and such other considerations. The additional optimization requirement can be achieved by formulating the design problem as a programming problem, in which the cost of active material or any such other criteria forms the objective function and the performance of the system form the constraints for the problem. ### 6.6.1 Design Problem Formulation - The most general design problem can be stated as: Minimize (or maximize) the objective function F (X1, X2, ... xn) such that the variables X1, X2, ... Xn satisfies the constraints g. (X1, X2, Xn) o, i = 1, 2, ... m. - The above statement can be easily explained by applying it for power transformers. For power transformers the independent variables X1, X2, ... xn may be chosen as follows: 1. Maximum magnetic flux density x₁ (wb/m²) 2. Current density in HV winding x2 (A/mm²) 3. Current density in LV winding x3 (A/mm²) 4. Height of the winding x4 (m) 5. Voltage per turn x5 (V) - The objective function may be choosen from 1. Cost of the active material i.e. cost of copper winding and the cost of stampings. 2. Cost of energy wasted as losses. 3. Cost of the active material and cost of energy wasted as losses. - The constraint function g(X1, X2, ... xn) consists of the following: 1. Temperature rise of windings above ambient. 2. Temperature rise of top oil above ambient. 3. Percentage short circuit impedance. 4. Magnetic flux density in core. 5. Current density in winding. 6. Percentage efficiency at full load. 7. Percentage no load current. ### 6.6.2 Optimization Technique - The optimization technique can be broadly classified as follows: - Unconstrained optimization - Constrained optimization - In unconstrained optimization, there is no upper and lower limits of the independent variables. The aim is to minimize (or maximize) the objective function only. For an unconstrained optimization problem, both gradient and non-gradient methods are available. - In constrained optimization, the constraint are put on the independent variables. The aim is now to minimize (or maximize) the objective function such that the variables are in limits to the or constraint function are also satisfied. The constrained optimization problem can be solved using in interior or exterior penalty function technique or transformation of variables. ## EXERCISE 1. Explain Various Approaches in Computer Aided Design 2. Explain the design procedure with flow chart in Computer Aided Design 4. Explain the analysis and synthesis method for CAD # Materials for Electrical Machines: ### Conducting materials : - Commonly used Conducting materials are copper and aluminium. - Some of the desirable properties of good Conductors as - low value of resistivity (or) high conductivity - Low value of temp. Co-efficient of resistance - High tensile strength. - High melting point - High resistance to Corrosion. - Allow brazing, Soldering or welding so that the joints are reliable. - Highly melteable and ductaile. - Durable and Cheap by Cost. - Sai for the same resistance and length, Cross sectional area of aluminium is 61% larger than that of the Copper Conductor and almost 50% lighter than Copper. - Aluminium reduces the Cost of small capacity transformer, it increases the size and Cost of large Capacity transformers. Aluminium is being much used now a day's only because Copper is expensive and not easily available. Aluminium is almost 50% cheaper than Copper. ### Magnetic materials: - The Some of the Properties that a good magnetic material are - Low reluctance (or) should be highly permeable (or) should have a high value of relative permeability. - High saturation induction (to minimise wt and volume of iron Parts) - High electrical resistivity so that the eddy emf and hence eddy Current loss is less. - Narrow hysterisis loop or low coercivity so that hysteresis loss is less and efficiency of operation is high - A high Curie point. (Above Curie point or temp. the material loses the magnetic property or becomes paramagnetic, that is effectively non magnetic). - should have a high value of energy product (expressed in Joules/m³) magnetic materials Can broadly be classified as Diamagnetic, Paramagnetic, Ferromagnetic, Antiferromagnetic and Ferrimagnetic materials. only ferromagnetic materials have properties that are well suitable for electrical machines ### Insulating materials : - To avoid any electrical activity between parts at different Potentials, insulation is used. An ideal insulating material Should possess the following properties. - should have high dielectric strength. - should withstand high temp. - should have good thermal Conductivity. - should not undergo thermal oxidation. - should not deteriorate due to highes temp, and repeated heat Cycle. - should have high value of resistivity (like 10⁸ cm) - should not Consume any power or should have a low dielectric loss angle 𝛿 - should withstand stresses due to centrifugal forces (as in rotating machines), electro dynamic or mechanical forces (as in transformers) - should withstand vibration, abrasion, bending. - should not absorb moisture. - should be flexible and Cheap. - Liquid insulators should not evaporate or volatilize. - Insulating materials can be classified as solid, liquid and Gas and Vacuum. # 1.9.1 Standardization and Standards - Standardization and standard specifications play an important part in the choice, design manufacture and operation of any apparatus. - Every country has a standards organization to fix standard specifications for the manufacturers. - The specification are guidelines for the manufacturer to produce economic products without compromising quality. - The manufacturers who are compiling with the standards will be issued a certification for the products. - The quality of the certified products will be periodically monitored by the standard organization. ## Advantages of Standardization - **To manufacturer** - Reduction in cost as economy results when number of objects are built at the same time. - Easy to production planning. - Stream-lining the a production line. - **To user** - Standardization means interchangeability of equipments and spares. - **To designer** - It means Rigidity - Indian standards these are issued by Bureau of Indian Standards (BIS), Manak Bhavan, Bahadur Shah Zafar Road New Delhi ## The Standard Specifications Issued for Electrical Machines: - Standard ratings of machines - Types of enclosure - Standard dimensions of conductors to be used - Method of marking ratings and name plate details. - Performance specifications to be used - Types of insulation and permissible temperature rise - Permissible losses and range of efficiency - Procedure for testing of machine parts and machines - Auxiliary equipments to be provided - Cooling methods to be adopted - In India, the Bureau of Indian standards (BIS) has laid down their specification (ISI) for various products. The standards will be amended time to time, in order to include the latest developments in technology. - The name plate of the rotating machine has to bear the following details as per ISI specifications. - KW or KVA rating of machine - Rated working voltage - Operating speed - Full load current - Class of insulation - Frame size - Manufacturers name - Serial number of the product ## Classification of Insulating Materials Based on Thermal Consideration - The insulation system (also called insulation class) for wires used in generators, motors transformers and other wire-wound electrical components is divided into different classes according the temperature that they can safely withstand. - As per Indian Standard (Thermal evaluation and classification of Electrical Insulation, IS. No. 1271, 1985, first revision) and other international standard insulation is classified by letter grades A, E, B, F, Η (previous Y, A, E, B, F, H, C). | Sr. No. | Insulation Class | Maximum Operating Temperature in °C | Typical Materials | |---|---|---|---| | 1 | Y | 90 | Cotton, silk, paper, wood, cellulose, fiber etc., without impregnation or immersed | | 2 | A | 105 | The material of class Y impregnated with natural resins, cellulose esters, insulating oils etc., and also laminated wood, vamished paper etc. | | 3 | E | 120 | Synthetic resin enamels of vinyl acetate or nylon tapes, cotton and paper laminates with formaldehyde bonding etc. | | 4 | B | 130 | Mica, glass fiber, asbestos etc., with suitable bonding substances, built up mica, glass fiber and asbestos laminates. | | 5 | F | 155 | The materials of Class B with more thermal resistance bonding materials. | | 6 | H | 180 | Glass fiber and asbestos materials and built up mica with appropriate silicone resins. | | 7 | C | > 180 | Mica, ceramics, glass, quartz and asbestos with binders or resins of super thermal stability. | - The maximum operating temperature is the temperature the insulation can reach during operation and is the sum of standardized ambient temperature i.e. 40 degree centigrade, permissible temperature rise and allowance tolerance for hot spot in winding. For example, the maximum temperature of class B insulation is (ambient temperature 40 + allowable temperature rise 80 + hot spot tolerance 10) = 130°C. - Insulation is the weakest element against heat and is a critical factor in deciding the life of electrical equipment. The maximum operating temperatures prescribed for different class of insulation are for a healthy lifetime of 20,000 hours. The height temperature permitted for the machine parts is usually about 2000C at the maximum. Exceeding the maximum operating temperature will affect the life of the insulation. As a rule of thumb, the lifetime of the winding insulation will be reduced by half for every 10°C rise in temperature. The present day trend is to design the machirie using class F insulation for class B temperature rise.