ELECTRICAL THINGS

Windings Electrical Parts of a transformer comprise mainly of the primary and secondary windings. The windings consist of the current-carrying conductors wound around the sections of the core, and these must be properly insulated, supported and cooled to withstand operational and test conditions. Copper and Aluminum are the primary materials used as conductors in power transformer windings. These conductors may be of circular or rectangular cross-section, depending on the current and voltage ratings of the machine and are insulated using enamel, paper, or cotton. Desirable properties of a conducting material for use in transformer ( or any Electrical Machine ) are:- High Conductivity Low temperature coefficient of resistance  High Thermal Conductivity Low coefficient of thermal expansion High ductility High tensile strength Not too heavy Low corrosion by chemicals The two materials mostly commonly used for windings of transformers are copper and aluminum . Both have their relative mer

According to the type of core construction, transformers are distinguished as:- Core type transformer Shell type transformer Core type Transformer In Core type construction, the magnetic core is built of laminations to form a rectangular frame and the coils are arranged concentrically around limbs of the core. The magnetic circuit of the Transformer has reluctances connected in series as shown below:- Single-Phase, Core type Transformer Core type Transformer has the following features: Less amount of insulation required Mechanically bad More amount of Copper (Cu) required Used for high voltage application Inter leaving windings preferred Equal Cross Section Area of both the limbs Low current rating Small KVA rating Shell Type Transformer Here the two windings are wound on same limb i.e. middle limb of the core. Hence, the windings are surrounded on both sides by the core. The magnetic circuit of the transformer has reluctances connected in parallel as shown below: Single-Phase, Shell t

  Construction details of Transformer Major Construction parts of a transformer can be categorized as:- Magnetic parts Insulating parts Electrical parts Mechanical parts and accessories Magnetic parts In a Transformer the primary and secondary windings are wound around the core of a transformer. Core is a magnetic material which allows the flow of magnetic flux lines to link both primary and secondary winding. Two basic functions of the core are: To provide an easy path for the magnetic flux to flow through it, and thereby link both primary and secondary windings. To provide mechanical support for holding the coils. Both primary and secondary coils are wounded over and around the core. There are certain properties that the core material must satisfy: Core should have low reluctance and high permeability to the flow of magnetic flux. Core is generally made of Silicon steel. Features of Silicon Steel : Ferrous Magnetic Material Low reluctance and high permeability  Low hysteresis coeffic

  Magnetic Circuits A magnetic circuits can be defined as the path which is followed by magnetic flux. An example of magnetic circuit is given below:- The Core is made of iron. Iron being a good magnetic material (high value of relative permeability), flux produced by the iron core will entirely pass through the iron core and thus magnetic field intensity(H) can be assumed to remain fixed at every point within the iron core. Magnetic field intensity developed by the iron core, H c = Ni  / l c    AT/m Flux density within the iron core, B c = μ o  μ r  H c    Wb/m^2 where μ r is the relative permeability of iron material used in the iron core. The amount of flux flowing through the iron core,  Ⲫ c = B.A c or,  Ⲫ c = μ o  μ r  H c A c or,  Ⲫ c = μ o  μ r (Ni  / l c ) A c or,  Ⲫ c = Ni  /( l c / μ o  μ r  A c ) or,  Ⲫ c = Ni  / S Product of number of turns (N) in the coil and current (i) passing it i.e. ( F=Ni ) is known as the Magneto Motive Force (MMF). MMF is the source that pro

The transformer is a device that transfers electrical energy from one electrical circuit to another electrical circuit through the medium of magnetic field and without the change of frequency. The electrical circuit that receives energy from the source is called as Primary winding and other circuit that delivers electrical energy to load is called as Secondary winding. The Primary and Secondary windings of a transformer are coupled magnetically i.e. the electrical energy input to the transformer is first converted to magnetic energy and then this magnetic energy is again converted to electrical energy by the secondary winding to be delivered to load. Transformers are static electromagnetic devices in which transformation of voltage and current or both occurs without any change in frequency, but no transformation of energy takes place. Transformers work on the principle of electromagnetic induction, i.e. mutual coupling between two coils. Transformer does not have any moving parts so ou

  What is Magnetic Material ? The materials which get magnetized in the presence of magnetic field is called Magnetic materials. Non Magnetic materials get magnetized in presence of magnetic field but they exhibit weak magnetization. Classification of Magnetic Materials On the basis of magnetic behaviour, material may be classified as Diamagnetic Paramagnetic Ferromagnetic Diamagnetic materials It is a weak form of magnetism that is non-persistent and persists only when an external field is applied. Due to an applied magnetic field a change is occur in the orbital motor of electrons, due to this change a magnetic moment is induced in this materials. The magnitude of induced magnetic moment is extremely small and in a direction opposite to that of applied magnetic field. Diamagnetic materials are repelled by magnetic field. Some of the materials that exhibit diamagnetism are Cu, Au, Ge, Si, Diamond, Al 2 O 3 , NaCl etc. Paramagnetic Materials When Paramagnetic Materials placed in a magn

  Applications of DC Generators Separately-excited DC generator are rarely used in practical applications because of the additional expenses to be limit for the separate excitation supply. DC shunt generator can be used for general purpose lighting and low-voltage DC supply system. DC series generators do not find much application because of their rising voltage characteristics at higher loads, except these are sometimes used as boosters to compensate excessive voltage drops that can take place in a long DC feeder. Flat compound DC generators are most commonly used for low-voltage DC distribution systems. Flat compound generators can also be used for charging of batteries since they can give fairly constant output terminal voltage, irrespective of load current. Over-compound type DC generators can be used for lighting and general power supply applications because over-compounding can compensate for voltage drops in the distribution lines and thus voltage at the consumer end can be

Hopkinson's Method  In this method, two identical DC machines are both mechanically and electrical coupled, and are tested simultaneously. One of the machines is run as a motor, whereas the other as a generator. the connection diagram of Hopkinson's test is given below:- For performing the test, machine I is started as a DC shunt motor and brought to rated speed with switch 'S' open. Both machines run at same speed as both are mechanically coupled. The field current of the generator (machine II) is so adjusted that its output terminal voltage changes and becomes equal to that supplied to the motor (machine I) terminals.   At this time voltmeter V 2     reads zero voltage. the switch 'S' is closed at that instant. Under this condition, the generator will neither taking nor giving current to the supply. After this state is achieved, any desired load can then be put on the generator by controlling the induced EMF of the machines. I f2  > I f1 , machine II  a

  Swinburne's Method One of the easiest ways of measuring no-load losses of a machine is by Swinburne's method. Since this is a no-load test, it cannot be performed on DC Series Motors. This method is applicable for Shunt or Flat-Compound DC generators and motors.  In this test, the machine, whether it is a generator or a motor, is run as a motor with no-load at rated speed and excitation. If machine under test is generator, then the voltage across the armature = rated terminal voltage + the voltage drop in the armature circuit resistance at rated armature current. If machine under test is motor, then the voltage across the armature = rated terminal voltage - the voltage drop in the armature circuit resistance at rated armature current. Swinburne's method for measuring no-load losses of a DC Motor I ao = No-Load Armature Current I f = Field Current V t = Terminal Voltage →  Power input to the machine = V t ( I ao +   I f )  Power taken by the armature = V t I ao Power taken

  Cumulatively Compound Motor cumulatively compound motor Torque-Armature Current Characteristics The flux in a cumulatively compound machine will be function of sum of field flux and armature flux. T = K Ⲫ I a = K I a f ( N se  I a + N sh  I f )  ,         N sh = N f = number of turns in shunt field winding  N se = number of turns in series field winding    f = Frequency There can be two cases depending upon whether Series Flux dominates or Shunt Flux dominates. ( for strong field flux ) ( for weak field flux ) For Strong Shunt Field, the machine behaves as a Shunt Machine but at higher values of Armature Current the series flux dominates and characteristics become similar to Series Excited Machine. For Weak Shunt Field, series flux dominates and machine behaves as a Series Excited DC Motor. Speed-Armature Current Characteristics E a = V s  - I a ( R a + R se  ) K Ⲫ N = V s - I a (  R a  + R se  ) Speed, N = [ V s  - I a  (  R a  + R se  )] / [ K f  (  N se  I a  + N sh  I f  ) ] Ag