27-08-2014, 10:43 AM
COURSE CODE : ELE400 ON SEMINAR REPORT
COURSE CODE.ppt (Size: 1.03 MB / Downloads: 10)
Principles of design of Machines
Review of Magnetic and insulating materials,
Factors and limitations in design
Specific magnetic and electric loadings,
Output, real and apparent flux densities
Separation of main dimensions for D.C., induction and synchronous machines
Design of Transformers
General considerations
Output equation
Main dimensions
Leakage reactance
Winding design
Tank and cooling tubes
Calculation of magnetizing current
Losses, efficiency and regulation of Transformers
Design of Three-phase induction motors
General considerations,
Output equation
Choice of specific electric and magnetic loadings
No. of slots in stator and rotor
Elimination of harmonic torques
Design of stator and rotor windings
Leakage reactance
Equivalent resistance of squirrel cage rotor
Magnetizing current
Temperature rise and efficiency
Design of Alternator
Classification and their comparison,
Specific loadings
Output coefficient
Main dimensions
Short circuit ratio
Elimination of harmonics in generated EMF
Stator winding design
Introduction
The magnetic flux in all electrical machines (generators, motors and transformers) plays an important role in converting or transferring the energy. Field or magnetizing winding of rotating machines produces the flux while armature winding supplies either electrical power or mechanical power. In case of transformers primary wing supplies the power demand of the secondary.
The basic design of an electrical machine involves the dimensioning of the magnetic circuit, electrical circuit, insulation system etc., and is carried out by applying analytical equations.
A designer is generally confronted with a number of problems for which there may not be one solution, but many solutions. A design should ensure that the products perform in accordance with the requirements at higher efficiency, lower weight of material for the desired output, lower temperature rise and lower cost. Also they are to be reliable and durable
Factors for consideration in Electrical Machine Design
The basic components of all electromagnetic apparatus are the field and armature windings supported by dielectric or insulation, cooling system and mechanical parts. Therefore, the factors for consideration in the design are,
Magnetic circuit or the flux path: Should establish required amount of flux using minimum mmf. The core losses should be less.
Electric circuit or windings: Should ensure required emf is induced with no complexity in winding arrangement. The copper losses should be less.
Insulation: Should ensure trouble free separation of machine parts operating at different potential and confine the current in the prescribed paths.
Cooling system or ventilation: Should ensure that the machine operates at the specified temperature.
Machine parts: Should be robust
Limitations in design
The materials used for the machine and others such as cooling etc., imposes a limitation in design. The limitations stem from saturation of iron, current density in conductors, temperature, insulation, mechanical properties, efficiency, power factor etc.
Saturation: Higher flux density reduces the volume of iron but drives the iron to operate beyond knee of the magnetization curve or in the region of saturation. Saturation of iron poses a limitation on account of increased core loss and excessive excitation required to establish a desired value of flux. It also introduces harmonics.
Current density: Higher current density reduces the volume of copper but increases the losses and temperature
Materials for Electrical Machines
The main material characteristics of relevance to electrical machines are those associated with conductors for electric circuit, the insulation system necessary to isolate the circuits, and with the specialized steels and permanent magnets used for the magnetic circuit.
These are classified as :
Conducting Material
Magnetic Material
Insulating Materia
Conducting Materials
Commonly used conducting materials are copper and aluminum. Some of the desirable properties a good conductor should possess are listed below.
Low value of resistivity or high conductivity
Low value of temperature coefficient of resistance
High tensile strength
High melting point
High resistance to corrosion
Allow brazing, soldering or welding so that the joints are reliable
Highly malleable and ductile
Durable and cheap by cost
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 with stand high temperature.
Should have good thermal conductivity
Should not undergo thermal oxidation
Should not deteriorate due to higher temperature and repeated heat cycle
Should have high value of resistivity ( like 1018 Ωcm)
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
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,H (previous Y,A,E,B,F,H,C).
What is real and apparent flux density?
The real flux density is due to the actual flux through a tooth. The apparent flux density is due to total flux that has to be pass through the tooth. Since some of the fluxes passes through slot the real flux density is always less than the apparent flux density
Continuous duty
This duty denotes that, the motor is running long enough & the motor temperature reaches the steady state value. These motors are used in Paper mill drives, Compressors, Conveyors etc.
Hydrogen cooling
Cooling system should be provided in the generator or alternator to remove the heat generated in the windings (I2R loss) of the generator. Inability to remove the heat results in damage to the winding insulation of the generator and can lead to reduction in the life span of the generator. Natural air cooling and forced air cooling is provided for the small rating generators. However for the generators rated above 60MW the amount heat generated will be enormous and air cooling is insufficient to cool the generator. Therefore hydrogen cooling is employed to remove the heat generated. Hydrogen cooling is chosen because of few characteristics of the hydrogen gas. Along with hydrogen cooling water cooling is provided in the stator winding circuit for large generators
Types of ventilation
The cooling of electrical machines by means of an air stream is called the ventilation of the machines. The cooling systems can be grouped into three types viz natural cooling, self cooling and separate cooling.
Further, the ventilation of the machine can be classified into three categories, according to the scheme of ventilation incorporated in the machine, namely,1 open circuit ventilation 2 closed circuit ventilation and 3 surface ventilation. Open circuit ventilation can be divided into two types viz induced ventilation and forced ventilation.
The ventilating system can be further classified into four types in accordance with the provision of cooling ducts and how the air passes over the heated parts of the machines.