Unit I INTRODUCTION S

Unit I
INTRODUCTION
S. No. TOPIC Page No.
1 Introduction
1.1 Basic Structure of Electromagnetic rotating machine
1.1.1 Major considerations in Electrical Machine Design
1.1.2 Limitations in design
1.1.3 Classification of design problems
1.2 Materials for Electrical Machines
1.2.1 Electrical conducting materials
1.2.2 Magnetic materials
1.2.3 Insulating materials
1.3 Space factor
1.4 Magnetic and Electric Loadings
1.4.1 Total loadings
1.4.2 Specific Loadings
1.4.3 Choice of Specific Magnetic loadings
1.5 Thermal considerations
1.5.1 Heat flow
1.5.2 Newton’s law of Cooling
1.5.3 Internal temperatures (Hot temperatures)
1.5.3.1 Calculation of internal temperature
1.5.3.2 Temperature gradients in cores
1.5.3.3 Heat flow in two dimensions
1.5.3.4 Thermal gradients in conductors placed in slots
1.5.3.5 Heating of turbo alternators
1.5.4 Thermal state in electrical machines
1.5.4.1 Theory of solid body heating
1.5.4.2 Heating
1.5.4.3 Cooling
1.6 Rating of machines
1.6.1 Motor ratings based on duty
1.7 Standard specifications
1.8 Solved problems
1.9 Two mark Q;A
1.10 University Question
1.11 Additional Problems

1. INTRODUCTION

In General, designing of any machine bring about application of science and technology. It is very much essential to produce machines with more cost-effective, highly durable, better quality and efficient without compromising its standard specifications. It is understand that electrical machines such as generators, motors and transformers are used to convert and transfer energy. The rotating machines are made of field and armature windings which are influencing magnetic flux. Amongst the two windings of rotating machines, field winding produces the flux and armature winding supplies electrical power/mechanical power. In stationary machine, primary winding supplies the power which is demanded by the secondary winding.
The designing of an electrical machine requires the dimensions of magnetic and electrical circuits, insulation system, etc. The analytical methods are used to find out the dimensions of the above mentioned parameters.
In general, the designer of a machine deals with the different type and number of problems. For all the problems solution differs and for sometimes, ends without a solution or many solutions for a single problem. However, it is important that solution provided by the designer should satisfy the requirements with appreciable efficiency. The designer should also ensure the machine is highly reliable, durable, least weight, lower temperature rise and lower cost.
A practical designer not only concentrates on the specifications of the machines but also should ensure the adaptability of the machine under stock. This situation sometimes forces the designer of the machine to compromise between the ideal design and a design which comply with manufacturing conditions.
It is stated earlier that a machine should be designed by following the national and international standards. Therefore, the electrical designer must be familiar with the standards. The list of recommended national and international standards is,
British Standard (BS), England
Indian Standard (IS), Bureau of Indian Standard (BIS), India
International Electro technical Commission (IEC)
NEMA (The National Electrical Manufacturers Association).

Basic Structure of Electromagnetic rotating machine
The main parts of the rotating machine and its structure have been depicted through the Figure 1.1.
Mechanical parts: The frame, bearings and shaft are the main mechanical parts of the machine
Constructional elements of rotating machines: The main constructional elements of the machine are stator and rotor. The construction of these elements differs for the DC and AC machines.
DC Machine
Stator: Yoke, Field Pole, Pole Shoe, Field Winding, Interpole
Rotor: Armature Core, Armature Winding, Commutator
Others: Brush and Brush holder
AC Machines
Synchronous Machine
Stator: Yoke, Field Pole, Pole Shoe, Field Winding, Interpole
Rotor: Armature Core, Armature Winding
Squirrel Cage Induction Motor
Stator: Frame, stator core and stator winding
Rotor: Rotor core, rotor bars and End rings
Slip Ring Induction Motor
Stator: Frame, stator core and stator winding
Rotor: Rotor core and rotor bars

Figure1.1 Basic Structure of Electromagnetic Rotating Machine
The machine functions four important circuits namely Magnetic, Electric, Dielectric and Thermal circuits.
Magnetic Circuit: This circuit provides the magnetic flux path which consists of air gap, stator and rotor teeth and stator and rotor cores (yoke).
Electric Circuit: The stator and rotor windings of the rotating machine are the main components of the electric circuit. The windings are used to transfer electrical energy between the two regions. It ensures the production of emf and development of magneto mechanical force (mmf) in the machine. In order to protect the electric circuit, the windings are suitably insulated.
Dielectric circuit: The insulation between the conductors, and core and windings are made through the dielectric circuit. The non-metallic materials preferably organic or inorganic, and natural or synthetic are recommended as the insulating materials.
Thermal Circuit: The thermal circuit is very much essential for heat dissipation which is produced inside the machine. The circuit is concerned with mode and media for dissipation of heat.
1.1.1 Major considerations in Electrical Machine Design
As mentioned above, the Electrical machine has different functions which are supported by dielectric or insulation, cooling system and mechanical parts. Therefore, it is advisable to put up extra care while designing the machine. The following list shows the important factors need to be considered for designing the machine,
Magnetic circuit or the flux path:
Magnetic circuit is the main source for core losses. It can be reduced by minimizing the usage of mmf without compromising the required amount of flux.
Electric circuit or windings:
Electric Circuit should ensure induction of required EMF in the machine with lesser complexity in winding arrangement. Additionally, the construction of electric circuit should produce lesser copper loss.
Dielectric circuit or insulation:
It should ensure the trouble free separation of machine parts which operating at different potential and restrict the current flow in the given paths of the circuit.
Cooling system or ventilation:
It should ensure operating temperature of the machine within the specified limit.
Machine parts:
It should ensure the robustness and accommodate the changes in size.
Other factors:
The machine design also involves optimization of manufacturing, operating and maintenance cost. The other factors requires consideration apart from the above mentioned are
Limitation in design (saturation, current density, insulation, temperature rise etc.,)
Customer’s needs
National and international standards
Convenience in production line and transportation
Maintenance and repairs
Environmental conditions etc.
1.1.2 Limitations in design
Many materials are involved in designing a machine and its supporting peripherals which imposes a restriction in design. These limitations due to the saturation of iron, current density of conductors, temperature change, insulation properties, mechanical properties, efficiency, power factor, commutation, etc.
Saturation: The volume of iron material decides the flux density of the field. The higher value of flex density drives the iron to operate beyond knee of the magnetization curve or in the region of saturation. This saturation of iron leads to the increase in core loss and performs under excitation and also produces harmonics. The excitation should be improved to meet the desired flux.
Current density: The current density can be improved by using greater volume of copper which results increase in the core losses and temperature.
Insulation (which is both mechanically and electrically weak): The most susceptible part of any machine is its insulation. The life of a machine depends upon the type of insulating materials used for its construction. It is most important to select the insulation material for the machine in order to withstand the electrical, mechanical and thermal stresses produced in it. In case of transformer, the mechanical strength of insulation material is one of the most important parameter. For instance, when a transformer is forced under short circuited secondary and primary is operating, it produces Large axial and radial forces. Therefore, while designing insulation for the transformer, it is essential to select the material to withstand large mechanical stresses. Further, the insulating material should be selected such a way to withstand higher operating temperature. The size of insulation is decided by the voltage stresses and mechanical stresses produced.
Temperature: As mentioned above insulating materials decides operating life of any machine. Notably, the life of the insulating materials in turn depends upon the withstand capacity of machine under higher temperature. The life of the insulating material gets reduced if it is operated more than its allowable temperature. This issue can be addressed by providing proper cooling and ventilation techniques. In the cooling system, coolant flows along proper paths absorbs the heat from the machine parts for dissipation to outside the machine. This helps to prevent the temperature rise of the machine and life time of the machine can be taken care.
Efficiency: The operating cost of any machine has been decided by its efficiency. The efficiency of the machine can be improved by using small magnetic and electrical loading with the support of large amount of conducting materials (both iron as well as copper/Aluminium). Though the capital cost of the machine is high but it reduces the operating cost.
Mechanical parts: It is necessary to address numerous technological requirements while constructing the electrical machine. The machine should be designed with lesser complexity without compromising on its technological concepts. The technology adapted for machine design should be unfailing with the performance requirements, reliability and durability. Most importantly, the design of mechanical parts for high speed machines is more critical. For example, while designing a turbo-alternator, the rotor slot dimensions are so selected that the mechanical stresses at the bottom of rotor teeth do not exceed the maximum allowable limit. In large machines, the size of the shaft is decided by considering the critical speed which depends of the deflection of the shaft.
Power factor: It is obvious that induction motors introduce poor power factor as the size and cost of induction motors are reduced with using a high value of flux density in the air gap. This meager power factor results in larger values of current for the same power demand and, therefore, conductor sizes are increased to the higher value. Further, the power factor also decides the value of flux density of the field. Hence, power factor is considered to be one of the limiting factors for machine design.
Commutation: Commutation of a relevant machine is important as it limits the output of the machine. For example, at present the maximum power output of a single unit d.c machine is approximately 10MW and this limitation is due to the commutation difficulties.
In addition to the above mentioned factors, Consumer, manufacturer or standard specifications may also create limitations in machine design.
1.1.3 Classification of design problems
The electrical machine design is a combination of most complex and diverse engineering problems. The engineering problems involved in designing rotating machines and Transformer is shown through the Table 1.1.
Table 1.1 Classification of design problems
Classification Rotating Machines Transformer
Electromagnetic Design Involves,
Stator and rotor core dimensions
Stator and rotor teeth dimensions
Air gap length
Stator and rotor windings Involves designing core and windings
Mechanical Design Involves design of frame, shaft and bearings Involves design of housing the core and winding assembly
Thermal Design Involves in the design of cooling ducts in core and cooling fans Involves in the design of cooling tubes and radiators
Dielectric Design Involves in the design of insulation between conductors Involves in the design of insulation between core and windings

Materials for Electrical Machines
Electrical Machine requires different varieties of materials for conducting, insulating and magnetic circuits. The selection of good conductors, insulators and permanent magnets are most essential for designing machine to bring out better performance.
1.2.1 Electrical conducting materials
The electrical conducting materials are divided into two main categories such as High conductivity materials and High resistivity materials. Amongst the two types, the later one is used for making resistances and heating devices.
All types of windings required for making electrical machines, apparatus and devices are made by high conductivity materials. The important properties of a good conductor are listed below in alphabetical order,
Allow brazing, soldering or welding so that the joints are reliable
Durable and cheap by cost
High melting point
High resistance to corrosion
High tensile strength
Highly malleable and ductile
Low value of resistivity or high conductivity
Low value of temperature coefficient of resistance
The above mentioned requirements differ with the intention. For instance, it is recommended to manufacture electrical windings with least possible resistivity for the cost of slight loss in tensile strength. Copper and Aluminum are the most commonly used high conductivity materials. The significant properties of copper and aluminum are shown in the Table 1.2.
Table 1.2 Properties of Copper and Aluminum
S.
No. Particulars Copper Aluminum
Resistivity at 200C 0.0172 ohm / m/ mm2 0.0269 ohm / m/ mm2
Conductivity at 200C 58.14 x 106 S/m 37.2 x 106 S/m
Density at 200C 8933 kg/m3 2689.9 m3
Temperature coefficient
(0-100oC) 0.393 % per 0C 0.4 % per 0C
Explanation: If the temperature increases by 1oC, the resistance increases by 0.4% in case of aluminum
Coefficient of linear expansion (0-100oC) 16.8×10-6 per oC 23.5 x10-6 per oC
Tensile strength 25 to 40 kg / mm2 10 to 18 kg / mm2
Mechanical property
highly malleable and ductile not highly malleable and
ductile
Melting point 10830C 6660C
Thermal conductivity (0-100oC) 599 W/m0C 238 W/m0C

From the Table 1.2, it is understood that for the desired resistance and length, cross-sectional area of aluminum is 61% larger than that of the copper conductor and almost 50% lighter than copper. Aluminum materials are lesser expensive and easily available compared to Copper.
1.2.2 Magnetic materials
The orientation of the crystals of the material and technical specifications of a machine or equipment decides the magnetic properties of a magnetic material. The significant properties of the good magnetic materials are listed below,
A high curie point. (Above Curie point or temperature the material losses the magnetic property or becomes paramagnetic, that is effectively non-magnetic)
High electrical resistivity (to reduce the eddy current loss)
High saturation induction (to minimize weight and volume of iron parts)
Low reluctance or high value of relative permeability ?r.
Narrow hysteresis loop or low Coercivity (to minimize hysteresis loss and improve the operation efficiency)
Should have a high value of energy product (expressed in joules / m3).
Magnetic materials are broadly classified into Diamagnetic, Paramagnetic, Ferromagnetic, Antiferromagnetic and Ferrimagnetic materials. Amongst the all types, ferromagnetic materials are most preferred and suitable for electrical machines. Ferromagnetic properties are confined almost entirely to iron, nickel and cobalt and their alloys. The relative permeability (?r) of ferromagnetic material is greater than 1.0. The dipoles of the magnetic field made by ferromagnetic materials align themselves in the direction of the applied field and get strongly magnetized.
Further the Ferromagnetic materials can be classified as Hard or Permanent Magnetic materials and Soft Magnetic materials.
Hard or permanent magnetic materials:
This type of materials has large hysteresis loop (obviously hysteresis loss is more) and step by step rising magnetization curve. The examples are carbon steel, tungsten steal, cobalt steel, alnico, hard ferrite etc.
Soft magnetic materials:
This type of materials has small size hysteresis loop and a steep magnetization curve. Soft Magnetic materials are further classified as,
Solid Core Materials
Generally used for yokes poles of dc machines, rotors of turbo alternator etc., where steady or dc flux is involved. The examples are cast iron, cast steel, rolled steel, forged steel etc., (in the solid form).
Electrical Sheet and Strip materials
Silicon steel (Iron + 0.3 to 4.5% silicon) cab ne utilized in the laminated form. The ageing issue and core loss of a machine can be reduced by proper addition of silicon. Low silicon content steel or dynamo grade steel is used in rotating electrical machines and are operated at high flux density. High content silicon steel (4 to 5% silicon) or transformer grade steel (or high resistance steel) is used in transformers. Further sheet steel may be hot or cold rolled. Cold Rolled Grain Oriented Steel (CRGOS) is costlier and superior to hot rolled. CRGO steel is generally used in transformers.
Special purpose alloys
Nickel iron alloys have high permeability and addition of molybdenum or chromium leads to improved magnetic material. Nickel with iron in different proportion leads to
High nickel permalloy (iron +molybdenum +copper or chromium), used in current transformers, magnetic amplifiers etc.,
Low nickel Permalloy (iron +silicon +chromium or manganese), used in transformers, induction coils, chokes etc.
Perminvor (iron +nickel +cobalt)
Pemendur (iron +cobalt +vanadium), used for microphones, oscilloscopes, etc.
Mumetal (Copper + iron)
iv. Amorphous alloys (often called metallic glasses)
Amorphous alloys are produced by rapid solidification of the alloy at cooling rates of about a million degrees centigrade/sec. The alloys solidify with a glass-like atomic structure which is non-crystalline frozen liquid. The rapid cooling is achieved by causing the molten alloy to flow through an orifice onto a rapidly rotating water cooled drum. This can produce sheets as thin as 10?m and a meter or more wide.
These alloys can be classified as iron rich based group and cobalt based group.
Material Maximum permeability ? x 10-3 Saturation magnetization in tesla Coercivity A/m Curie temperature oC Resistivity ?m x 108

3% Si grain oriented 90 2.0 6-7 745 48
2.5% Si grain non – oriented 8 2.0 40 74.5 44
180 Mica, quartz, ceramics, glass and asbestos with binders or resins of super thermal stability
1.3 Space factor
Space factor is defined as the ratio of copper area to the total winding area. It is used to select the size of conductor, air gap and insulation thickness. The expression for space factor for a field coil is given by,
S_f= (a_f T_f)/(h_f d_f )