NAME: OHIMEN WINNER OMOREBOKHAE MATRICULATION NUMBER: ENG1604188 DEPARTMENT: ELECTRICAL AND ELECTRONICS ENGINEERING COURSE CODE: EEE534 (ELECTRICAL MACHINE DESIGN) ASSIGNMENT QUESTIONS: 1. Write about magnetic materials, insulating, electrical and cooling materials 2. Explain magnetic materials with respect to retention of magnetic field, hysteresis curve and coercive force. ANSWERS: ELECTRICAL MATERIALS What are electrical materials? Electrical materials are used for the electrical installations, in case of house wiring or building wiring, electrical materials are used. We can’t do the electrical installations without using electrical materials. The electrical materials have many purposes, they can be used to control the current flow in an electric circuit. There are certain electrical materials like circuit breakers and fuses which can be used to protect the electrical appliances and devices and it can prevent fire hazards and damage. Electrical material such as insulators can prevent users from getting electric shocks while operating electrical appliances or instruments. Electrical materials such as currentcarrying wires or conductors are used to transfer the current from source to the load. Classification of Electrical Materials: Based on properties and area of applications, Electrical Engineering materials can be classified as below: 1. Conductors: Conductors are the materials which have very high conductivity. They are those materials which accept electricity to pass through them easily. The number of free electrons are very high in a conductor at room temperature, which is the basic reason of high conductivity of conductors. Examples included silver, copper, gold, aluminum etc. These devices offer very less opposition to the flow of current through them. These devices follow ohm’s law. In conductors the free electrons in valence shell are plenty in number for conduction. In conductors there is no energy gap between the valence band and conduction band, i.e. they overlap each other. As a result, all the electrons present in the valence band are part of conduction band and hence can be used to conduction which are plenty in number. So, they offer very low resistance to the flow of current. 2. Insulators: Insulators are those materials which provide good amount of opposition for the flow of the current. As a result, the amount of current flowing through them is almost negligible. In case of insulators the energy gap between the valence band and conduction band is very high. As a result, a lot of energy is required for the electrons to jump from valence band to conduction band. Hence the opposition offered by such materials is very high and is of order mega ohms. Hence these materials do not allow the flow of electricity though them. Insulators may have lot of charges in the valence band but are of no use. 3. Semiconductors: Semiconductors are materials which have the conductivity between conductors and insulators. Semiconductors are the elements of group-III, group-IV and group-IV elements. Semiconducting materials have covalent bond. At normal temperature the conductivity of semiconductors is very low. With increase in temperature the conductivity of semiconductors increases exponentially. Example: Germanium, Silicon, Gallium Arsenic etc. Semiconductors are those between conductors and insulators. The main difference between conductors and semiconductors is the conductivity. For conductor’s ,conductivity increases by making the material pure whereas it decreases in case of semiconductors. In case of semiconductors there is a bit of energy difference between the valence band and the conduction band. As a result, energy is required to excite the electrons from valence band to conduction band. Hence their conductivity is comparatively low than those of conductors and offer a bit resistance to the flow of current. However, the conductivity of these items increases with addition of impurities known as doping. COOLING MATERIALS An electrical machine generates heat in its various parts because of mechanical losses, magnetic losses and electrical losses. To reduce the electrical losses and thereby the heat in the copper windings, the area of the windings is increased. Copper is however an expensive and limited natural resource. If the machine can be cooled more efficiently, the amount of copper that is used can be decreased and therefore make the machine more cost effective and reduce the amount of material needed for the machine. The removal from electric machines of the heat liberated as a result of magnetic, electric, and other losses. The maximum permissible heating is determined by the heat resistance of the materials—insulation, solder, and lubricant—used in the machine. The most efficient method of heat removal is to cool the heated parts of the machine with a circulating intermediary substance that may be air, various gases (hydrogen, carbon dioxide, helium), or a liquid (transformer oil, water, chlorinated biphenyl). Special gases are used for cooling electric machines where the power consumption for air ventilation is very great, such as high-speed electric motors, turbo generators, and synchronous compensators. When a hydrogen cooling system is used, the possibility of the hydrogen mixing with air and forming a dangerously explosive mixture must be avoided. If such a danger exists but air cooling is nevertheless undesirable, as with high-power electric motors located in dangerously explosive places having poor ventilation, a cooling medium such as carbon dioxide or helium is used. Water is used to cool the stators of high-frequency electric machines, the bearings of high-power electric motors, and the step bearings of generators. Chlorinated biphenyl is used if there is a danger of the water freezing. The windings of heavy-duty transformers are cooled by circulating oil. INSULATING MATERIALS What are insulating materials? An Electrical Insulating Material/Insulating Material is used to obstruct the flow of current. It forms ionic bonds and the materials that have low conductivity and high resistivity are available in the form of solid, liquid, gaseous like the plastic used for plugs, insulating oil used in transformer, etc. These materials have very high resistance so the flow of electric current requires an extremely high voltage like kilo or megavolts to send a few milliamperes of current to them. The insulators are used primarily for storage and also in all domestic and commercial electrical equipment to isolate the conductor from the earth. Properties of Insulating Materials All the insulators when used should not only behave as an insulator over a wide range of electric voltage but must strong mechanically. They shouldn’t be affected by heat, atmosphere, chemical effects and should be free from deformation due to aging. Therefore, before selecting an insulating material, it is quite essential to know the various properties and their effects on insulation. The various properties of insulating materials are electrical properties, visual properties, mechanical, thermal, and chemical properties. Electrical Properties The electrical properties of insulating materials are divided into two types they are insulating resistance and dielectric strength. The insulating resistance is again classified into two types they are volume resistance and surface resistance. The factors affecting insulating resistance are temperature, aging, applied voltage and moisture and the factors affecting dielectric strength are temperature and humidity. Visual Properties The visual properties of insulating material are appearance, color, and its crystallinity. Mechanical Properties Some of the mechanical properties which are to be taken care of while selecting the insulating material are tension & compression, resistance to abrasion, tear, shear & impact, viscosity, porosity, solubility, moisture absorption, and machinability and mouldability. Thermal Properties The thermal properties of insulating material are melting point, flash, volatility, thermal conductivity, thermal expansion, and heat resistance. Chemical Properties The various chemical properties of insulating material are resistance to external chemical effects, effects on other materials, chemical changes in the material, hygroscopicity, and aging. Classification of Insulating Materials The classification of insulating material is based on the thermal classification, physical classification, structural, chemical classification, and the process of manufacture. Thermal Classification Thermally the insulators are classified into seven types or seven classes they are class-Y, classA, class-E, class-B, class-F, class-H, and class-C. Class-Y The class-Y limitation temperature is 900 C and the materials come under class-Y are cotton, paper, silk, and similar organic materials. Class-A The class-A limitation temperature is 1050 C and the materials come under class-A are impregnated paper, silk, polyamide, cotton, and resins. Class-E The class-E limitation temperature is 1200 C and the materials come under class-E are enameled wire insulation on the base of powdered plastics, polyvinyl epoxy resins, etc. Class-B The class-B limitation temperature is 1300 C and the materials come under class-B are inorganic materials impregnated with varnish. Class-F The class-F limitation temperature is 1550 C and the materials come under class-F are mica, polyester epoxide varnished in the high heat resistance. Class-H The class-H limitation temperature is 1800 C and the materials come under class-H are composite materials on mica, glass, fiber, etc. Class-C The class-C limitation temperature is >1800 C and the materials come under class-C are glass, mica, quartz, ceramics, Teflon, etc. Physical Classification of Insulating Materials The physical classification of insulating material is classified into three types they are solid, liquid, and gaseous. The physical classification of insulators is shown in the below figure. The solid insulating materials are fibrous, ceramic, mica, glass, rubber, and resinous. The liquid insulating materials are mineral oils, synthetic oils, transformer oils, and miscellaneous oils. The gaseous insulating materials are air, hydrogen, nitrogen, and Sulphur hexafluoride. Structural Classification The structural classification of insulating material is classified into two types they are cellulose and fibrous. Chemical Classification The chemical classification of insulating material is classified into two types they are organic and inorganic. Process of Manufacture The process of manufacture is classified into two types they are natural and synthetic. Some of the insulating materials are fiberglass, mineral wool, cellulose, natural fibers, polystyrene, polyisocyanurate, polyurethane, insulation facings, phenolic foam, urea-formaldehyde foam, etc. Applications of Insulating Material The applications of insulating material are 1. Cable and transmission lines 2. Electronic systems 3. Power systems 4. Domestic portable appliances 5. Electrical cable insulating tape 6. Personal protective equipment 7. Electrical rubber mats MAGNETIC MATERIALS What are magnetic materials? Magnetic materials are those materials in which a state of magnetization can be induced. Such materials create a magnetic field in the surrounding space, the response of a material when subjected to an external magnetic field is the root of magnetism. The spinning electrons in the material behave like tinymagnets. These tiny magnets are aligned in thedirection of applied magnetic field and thereby the material is magnetized. These materials play an important role for existence of various electrical machines. The magnetic materials having high permeability are used for building the core to from the low reluctance path for magnetic flux. Magnetic materials can be further divided in following categories: Classification of magnetic materials 1. Diamagnetic: It is a substance which create a magnetic field inopposite to an externally applied field. Susceptibility is negative, these materials have relative permeability slightly less thanunity. They repel the lines of force slightly. The examples are bismuth silver, copper, lead, gold,bismuth, zinc, etc. 2. Ferromagnetic: A type of material that is highly attracted tomagnets and can become permanently magnetizedis called as ferromagnetic. The relative permeability is much greater than unityand are dependent on the field strength.These have high susceptibility. The examples are iron, cobalt, nickel, gadolinium,etc. 3. Paramagnetic:It is a substance or body which very weaklyattracted by the poles of a magnet, but not retainingany permanent magnetism. These have relative permeability slightly greater thanunity and are magnetized slightly. They attract the lines of forces weakly. The examples are aluminum, chromium, manganese,lithium, magnesium, etc. Coercive Force: The primary criterion allowing for classification of magnetic materials is coercivity, which is a measure of stability of the remnant state. Soft magnetic materials are characterized by low values of coercivity (Hc<103 A m–1), while the coercivity of hard magnetic materials (usually permanent magnets) is higher than 104 A m–1. Finally, semi hard magnetic materials (mostly storage media) have coercivities in between the above two values. In addition, there exist a number of sophisticated magnetic materials with unusual properties, such as giant magnetostriction, giant magnetoresistance, and giant magnetoimpedance. All these materials are important for modern technology and will be considered here. In modern applications, magnetic properties are tailored through precision tuning of the material microstructure. Thus, a central issue in the development and application of both soft and hard magnetic materials is the connection between extrinsic magnetic properties (coercivity, remanence, hysteresis loop) and microstructure. A universal relation was established between the coercivity Hc and the anisotropy constant K1: Hysteresis curve Hysteresis occurs in a system that involves a magnetic field. Hysteresis is the common property of ferromagnetic substances. Generally, when the magnetization of ferromagnetic materials lags behind the magnetic field this effect can be described as the hysteresis effect. The relationship between magnetic field strength (H) andmagnetic flux density (B) will follow a curve up to a pointwhere further increases in magnetic field strength willresult in no further change in flux density. This condition is called magnetic saturation till point (a). The plotted relationship will follow a different curve backtowards zero field strength at which point it will be offsetfrom the original curve by an amount called the remnant flux density or retentivity as shown in graph at point (b).The 'thickness' of the middle, describes the amount ofhysteresis, related to the coercivity of the material as from(c) to (f) Retentivity: Once the magnetising force has been removed, the magnetism within the material will either remain or decay away quiet quickly depending on the magnetic material being used. This ability of a material to retain its magnetism is called Retentivity. When a ferromagnetic material is magnetized in one direction, it will not relax back to zero magnetization when the imposed magnetizing field is removed. It must be driven back to zero by a field in the opposite direction. If an alternating magnetic field is applied to the material, its magnetization will trace out a loop called a hysteresis loop. The lack of retraceability of the magnetization curve is the property called hysteresis and it is related to the existence of magnetic domains in the material. Once the magnetic domains are reoriented, it takes some energy to turn them back again. This property of ferrromagnetic materials is useful as a magnetic "memory". Some compositions of ferromagnetic materials will retain an imposed magnetization indefinitely and are useful as "permanent magnets". The magnetic memory aspects of iron and chromium oxides make them useful in audio tape recording and for the magnetic storage of data on computer disks. Applications of Magnetic Materials 1. Magnetic materials are used in meters, motors, electric generators, loud speakers, transformer cores, etc. 2. Large electromagnets are used to pick up heavy loads. 3. Large electromagnets are used to levitate modern trains. The so-called maglev trains are faster and provide a much smoother ride than the ordinary track system, due to theabsence of friction between the track and the train. 4. Magnetic tapes are used in sound and video recording equipment. 5. Magnetic recording materials are used in computer discs. 6. Superconducting magnets are used to contain the plasmas used in controlled nuclear fusion research.
