BASIC ELECTRICITY & MAGNETISM

Transcription

1 DESCRIPTION ST-05 DURATION SHORT : 01 DAY BASIC ELECTRICITY & MAGNETISM 1) Idea about EMF, current, power, resistance, inductance, capacitance, power factor, etc.and their measurement. 2) Study of ohms law and its application, series and parallel connection. 3) Importance of insulation resistance. 4) Idea about magnetism and magnetic induction. 5) Transformer working principle, rectifier working principle. 6) Fuses and lightning arrestor. 7) Measurement of loop and insulation resistance of cables and overhead lines and measurement of earth resistance. 8) A.C and D.C principles. 9) Brief introduction to various electrical equipment like charger, stabilizers, transformers, UPS, D.C - D.C converter, A.C D.C converter.

2 2 BASIC ELECTRICITY AND MAGNETISM 1. Idea about EMF, current, power, resistance, induction, capacitance, power factor and their measurements. EMF (Electromagnetic force): The full form of EMF is electromotive force. When ever an electric current flows through a conductor, a magnetic field is immediately brought into existence in the space surrounding the conductor When electrons are motion, they produces a magnetic field. When a magnetic field embracing a conductor moves relatively to the conductor, it produces a flow of electron in the conductor for which an EMF is induced in the conductor. Measurement: With the help of voltmeter we can measure the e.m.f. Current It is the rate of flow of electric charge.if a charge of Q coulombs flows across any point in a conductor for time t sec then current I = Q/t. The unit of electric current is ampere. It is denoted as A Measurement: We can measure electric current by an Ammeter. Power :The consumption of electrical power of a circuit is calculated in watt. Its symbol is P. P= V x I, V= I x R P= I x I x R P= I 2 x R or P = V x I Where, P =Electrical power, I= current in Amp, R= Resistance in ohms, The Commercial unit of electric power consumption is Kilo-watt hour (KWH) KWH= watt x hour/1000 One KWH is called a unit. It is also referred as Board of Trade unit. Resistance: The property of substance due to which it opposes the flow of electric current through it is called its resistance. Metal, acid and salt solution offer very little resistance, hence they are very good conductor of electricity. The best electrical conductor is silver.the unit of resistance is ohm ( Ώ ). A conductor is said to be a resistance of one ohm if it allows one amp current to flow through it when one volt is applied across its terminal. Measurements: With the help of ohm-meter we can measurement the value of resistance. Inductance: The property of A.C circuits which opposes any change in the amount of current is called inductance. Its symbol is L and its unit is Henry [H]. If a current changing at the rate of one ampere per second, induces an average e.m.f of one volt in a conductor, the amount of inductance in the conductor will be one Henry. Capacitance: The capability of storing electrical charge by the two conductor plate duly separated by an insulator is known as capacity. Its symbol is C and its unit is Farad..The device is called a capacitor or a condenser.

3 3 When a capacitor is connected to a battery current flows in the circuit which charges its one plate and the other plate is negative. The quantity of electric charge store in the capacitor is directly proportional to the voltage applied to the capacitor. Thus, Q α V Q/V = constant Or Q/V = C Q = C x V where Q = charge in coulombs C =capacity in Farad. V =voltage in volt The property of A.C circuit which opposes any change in the amount of voltage is known as capacity or capacitor Power factor: It may be define as 1) cosine of phase angle between voltage and current.either lead and lag. 2) The ratio R/Z=Resistance/Impedance. 3) The ratio of True power/apparent power = Watt/Volt amp = W/VA. For example,suppose a circuit draws a current of 1000A at a voltage of 20000V and has a power factor of 0.8then, Input=1000*20000/1000=20000KVA Cos ϕ = 0.8 ; sin ϕ =0.6 Hence KW =20000 * 0.8 = 16000; KVAR=20000 * 0.6=12000 Obviously = ie KVA 2 = KW 2 + KVAR 2 2. Study of ohms law and its application, series and parallel application. Ohms Law: In a closed D.C circuit the potential difference developed across a conductor (or a resistor) is directly proportional to the current flowing through it if the temperature and other physical conduction of the conductor have been kept constant. Thus, V α I Or, V/I = constant Or, V/I=R ( Hence V/I = R or, V= I X R Or, I =V/R ) Multiple and sub multiple of ohm Prefix Its meaning Symbol Equal to Mega One million M 10 6 Kilo One thousand K 10 3 Milli One thousandth m 10-3 Micro One millionth µ 10-6 Laws of resistance: The resistance (R) offered by a conductor depends upon 1) Directly on its length (L) 2) Inversely on its cross section (A) 3) On the nature of the conductor material 4) On the temperature of the conductor. Neglecting the last maintained factor we can say that R α L Here R= Resistance of conductor in ohm

4 L α 1/A R = L/A 4 L = Length of the conductor A= Cross sectional area of the conductor Where (rho) is a constant. Where (Rho) depends upon the nature of the conductor material and is known as resistivity. Its unit is ohm-cm. Resistances in series: A combination of two or more resistors which has one and only a series circuit. In this circuit each resistor has its individual voltage drop, hence total voltage V T = V 1 =V 2 = V or V T = I x R xr 2 + I x R or V T /I = R 1 + R 2 +R or V T = I ( R 1 + R 2 + R or R T = R 1 + R 2 + R Resistances in Parallel : A combination of two or more resistors in which all the resistors are getting the same voltage ( i.e they are connected same source of e.m.f.)is called parallel circuit.the current drainage of each resistor connected in parallel is different hence, Total current (I T ) = i 1 + i 2 + i I T = V/R 1 + V/R 2 + V/R I T = V ( 1/R 1 + 1/R 2 + 1/R I T /V = 1/R 1 + 1/R 2 +1/R

5 5 3. Importance of insulation resistance : Insulation resistance is a very important factor in over head and under ground cable. As insulation resistance is inversely proportional to leakage current and leakage current is as minimum as possible in any circuit. So insulation resistance in any circuit (over head or under ground)must be high value for better performance of the circuit. Normally this value is always in Mega ohm range. Insulation resistance is measured by meggar Megger is classified according to the output voltage. It may be 100V,or 500V. 100V megger is used for measuring insulation resistance of U/G cable and 500V megger is used for over head line. 4. Idea about magnetism and magnetic induction: Magnetism and magnetic induction: Magnetic effect is one out of the three principal effect of electric current. A current carrying conductor is always surrounded by a magnetic field. As in the case of a magnetic field of a permanent magnet, a magnetic neddle or compass placed at a point in the magnetic field of a current carrying conductor will rest in the direction of the field acting at that point and not in the N-S direction. the phenomenon is known as electromagnetism. Magnetic field of a straight conductor: The magnetic field of a straight current carrying conductor is composed of concentric magnetic lines of force. The center of these lines of force lies on the axis of the conductor. Magnetic field : The space around a magnet where the field effect of its magnetism can be detected is known as magnetic field A magnet niddle placed at a point in the magnetic field will rest in the direction of the field and not in the N-S direction. Properties of lines of force; 1) A line of force complete its circuit from north pole to south pole out side the magnet and from south pole to north pole inside the magnet. 2) Two lines of force never intersect with each other. 3) The tangent drawn at any point on a line of force represents the direction of magnetic field acting at that point. Magnetic induction: When a magnet is brought near to an iron bar or an iron bar is brought near to an magnet, a magnetism produce in the iron bar. The phenomenon is known as magnetic induction.actually before attracting an iron bar before it, a magnet induces an opposite polarity in the iron bar and then due to attraction due to unlike poles, magnet attracts the iron bar. The magnet need not to touch the iron bar for magnetic induction. In various electrical measuring instrument,soft iron pole pieces are used along with bar magnet in order to give the desire shape to the magnet used, such pole pieces work on the principal of magnetic induction. The intensity of magnetic field is also known as magnetic field strength. The same is called magnetizing force in connection with electromagnet. Then its unit is ampere turn/meter (A-T/m). Magneto Motive Force: The complete path of a lines of force from north pole to south pole outside the magnet and from south pole to north pole inside the magnetic called magnetic circuit.

6 6 The force responsible for producing magnetic flux in the magnetic circuit is called magneto motive force(mmf).like resistance in electrical circuit, the opposition for the production of magnetic flux in the magnetic circuit is called the Reluctance. Its symbol is R. Here R= MMF, R= Reluctance in AT/Wb MMF =Electromagnetic force in A-T = Magnetic flux in Wb. 5. Transformers working principle, rectifier working principle Transformer: A transformer is a static (or stationary) piece of apparatus by means of which electric power in one circuit is transformed into electric power of the same frequency in another circuit. It can raise or lower the voltage in a circuit but with a corresponding decrease or decrease or increase in current. The physical basis of a transformer is mutual induction between two circuits linked by a common magnetic flux. In its simple form, it consists of two inductive coils which are electrically separated but magnetically linked through a path of low reluctance. The two coils is connected to a source of alternating voltage, an alternating flux is set up in the laminated core, most of which is linked with the other coil in which it produces mutually-induced e.m.f (according to faraday s Laws of Electromagnetic Introduction e = MDI/dt). If the second coil circuit is closed, a current flows in it and so electric energy is transferred (entirely magnetically) from the first coil to the second coil.the first coil, in which electric energy is fed from the a.c. supply mains, is called primary winding and the other from which energy is drawn out, is called secondary winding. In brief, a transformer is a device that 1. Transfers electric power from one circuit to another. 2. It does so without a change of frequency 3. It accomplishes this by electromagnetic induction and 4. Where the two electric circuits are in mutual inductive influence of each other.

7 7 Rectifier: The rectifier is a circuit which employs one or more diode to convert AC voltage into pulsating DC voltage. Figure shows single phase full wave rectifier. When input ac supply is switched on, the M and N of transformer secondary becomes +ve and ve alternatively. During the +ve half cycle of the ac input, terminal M is +ve, G is at zero potential and N is at ve potential. Hence being forward biased, diode D 1 conduct( but not D 2 which is reversed biased)and current flows along MD 1 CABG.As a result +ve half cycle of the voltage appears across R L. During the ve half cycle, when terminal N becomes +ve, then D 2 conducts and current flows along ND 2 CABG.So we find that current keeps on flowing through R L in the same

8 8 direction (ie from A-B) in both half cycle of ac input. It means that both half cycle of input ac supply are utilized. Also the frequency of the rectified output voltage is twice the supply the supply frequency. Of course, this rectified output voltage consist of dc component and may ac components of diminishing amplitude. 6. Fuses and Lighting arrestors. Fuses: It is desirable that that the design of a fuse should be such that it will blow quickly and quietly whenever the current through it reaches a dangerous value. It should also be so designed that any arc which may be set up when the fuse wire melts cannot be maintained. A common type of line fuse in which the phosphor-bronze fuse wire is inserted in a small bore porcelain tube with one caps without ventilation holes. It is rated at 1.5A and blows at approximately 3A. Another type of fuse is enclosed in an asbestos tube held between two flat body pieces or pressboard.it is rated at 1.5A and is specified to carry this current continuously with out deterioration. It must blow within 30 sec of the current reaching a value of 3A. The protector device is also the part of safety device. It provides a direct path to reach for lightning or other high voltages discharges The usual type of protector used on subscriber line and junction circuit. The most common type protector consist of two moulded carbon block held together by the mounting spring. The protector has a specified discharge voltage from 500V 700V.In this type protector a thin perforated mica separator is normally used now a days in place of thin film of red anti-dust varnish. Lightning arrestor: The equipment which protect other costly equipment from lightning is called lightning arrestor. It is also called Lightning discharger (LD). It is a protective device. Most of the S&T equipments are protected from higher voltage which may arise either due to natural or artificial cause. Natural cause is lightning and artificial cause is direct contact between power line and S&T equipment. General requirement of Lightning discharger. 1) Lightning discharger shall not be operate under maximum operating voltage. 2) Speech and signaling efficiency should not be reduce on its application. 3) It should promptly operate at specified voltage or currents. 4) It should promptly isolate the apparatus and prevent further action by lightning etc. 5) Current rating should be always such that it should not produce any heat in the component of the main apparatus. 6) It should able to carry the surge current due to heavy lightning up-to the rated value without puncturing or affecting materially the operating characteristic. 7) The material employed in the design and construction shall not create fire hazards due to operation. 8) It should be suitably and compactly house when they are installed independently on poles. 9) The design of lightning discharger shall be economical and and of good engineering practice. 7. Measurement of loop and insulation resistance of cables & overhead lines and measurement of earth resistance. Measurement of loop resistance of cable and over head line.

9 9 BRIDGE MEGGER Stn.-A Instrument used : Bridge megger. Section : Block section to block section( for over head line ) Cable repeater to cable repeater.(for under ground cable) Standard Value: 5.62 Ώ (for over head A.C.S.R -6/1/1.5 conductor) 56.2 Ώ (for R.E cable) Measured value: If the measured value for 10 km is 600 Ώ in under ground cable then loop resistance /km is 600/10 = 60 Ώ Time: Mid day. Action taken : If the measured value is more than standard value 1) Bad joints to be checked 2) No of joints to be reduce by replacing new wire in case of over head line. Measurement of insulation resistance of cable and over head line MEGGER L 1 L 2 Instrument used : Megger 100V for under ground cable and 500V for over head line Section : block section to block section( for over head line ) Cable repeater to cable repeater.(for under ground cable) Standard Value: 8 M Ώ /km for over head line( dry condition ) 1.6 M Ώ /km for overhead line (wet condition)

10 M Ώ /km for under ground cable. Measured value: Time: Action taken : If the measured value for 10 km is 1MΏ in overhead line then insulation resistance /km is 1*10= 10 M Ώ Early morning. If the measured value is below than standard value 1) Broken and crack insulator to be replaced. 2) Tree branches touches with wires to be cut down Measurement of earth resistance It is necessary to earth all telecom equipment inclusive of transmitter, receivers s and associated equipments, Sheath of telecom cable for the following reason. 1) To prevent or to reduce the cross talk. 2) To complete earth return signalling circuit. 3) To avoid shock. 4) To provide direct connection to the earth for lightning protection. Measurement of earth resistance: Earth can be tested by means of a whetstone bridge or GPO detector or a megger earth tester. To test an earth two iron bars with terminals fixed on them, are driven about 6-8 meter from the earth to be tested and 6-8 meter from each as shown in the figure. A Earth to be tested. R 1 6m 6m R 3 B 6m R 2 Iron bar C The bars are used as temporary earth and driven in the ground for 1.25 meter. Now pour salted water to ensure that the bars make a good connection with the earth. In the above diagram A is the earth under test B & C are temporary earths. Measure the resistance between A&B, A&C and B&C with the help of wheatstone bridge or GPO detector or megger earth tester. Let R 1 is the resistance measured between A&B. R 2 is the resistance measured between A&C

11 R 3 is the resistance measured 11 between B&C Therefore the resistance of A can be found by following formula. R 1 + R 2 + R 3 Resistance of A = 2 8. AC & DC principles A.C Principles: Alternating current(ac ) is produced by a voltage source whose terminal polarity keeps alternating (or reversing) with time. The positive terminal at one instant becomes negativel some time later and the negative terminal at one instant becomes terminal at some other instant.as a result of constantly reversing polarity of voltage source, the direction of current flow in the circuit also keeps reversing as shown in the figure. In addition to reversing its direction, current keeps changing in value with time from zero to maximum in one direction and back to zero and then from zero to minimum in the opposite direction and again back to zero. It is obvious that an alternating voltage will cause an alternating current. D.C Principle: D.C is known as direct current which is produces by a voltage source whose terminal have fixed polarity i.e. whose poles do not change their polarity with time. Hence, they provide a current whose direction of flow does not change with time.however, this direct current may be steady. For DC voltage this main source are electric cell, battery, DC generator etc. 9. Brief introduction to Charger, stabilizer, Transformer, UPS, D.C-D.C converter,a.c-d.c converter. Battery charger (automatic) Circuit diagram of the charger is shown in the attached drawing. It consist of input terminals where mains supply is to be connected. The mains supply is connected to primary of mains transformer T through mains ON/OFF switch Sl and flues Fl. The Secondary of mains transformer is connected to a single phase full wave rectifier bridge consisting of SCRs TSI, TS2 and diodes D1 and D2. The diodes are protected by HRC fuses F4, F5 and by RC snubber circuit. The rectified output is then filtered by free whelling diode D3, filter choke, CHL. Filter condenser C6 and bleeder resistance R6. The DC output from the filter section is connected to output terminals through a shunt SH1. This output is for equipment load. The output after the shunt SH1 is also connected to the battery terminals though a battery shunt SH2, fuse F2 and a rotary switch S2. The output voltage /current is precisely regulated by controlling the phase firing angle of SCRs. The control circuit used of latest IC technology and incorporates a built-in-current and voltage limit to protect the battery from over charging and to protect the battery from over discharging and to protect the equipment load also.

12 12 Basically the charger works either as voltage regulator in float mode and current regulator in boost/initial charging mode. In automatic mode it works either flaot or boost mode depending on the charge condition of the battery. Stabilizer Ferro Resonant technology: The stringent operating condition has required the use of Ferro resonant technology. Voltage regulators manufactured with this technology are able to handle a wide variation in voltage and deliver precisely regulated output voltage. Operating on the principal of saturation and the resultant storage capacity, these regulators demonstrate a high degree of immunity to line surges. Ferro Resonant voltage regulators do not use any electronic or electro-mechanical components, resulting in good reliability and are virtually maintenance free. These regulators demonstrate a high degree of self protecting features. The equipment is self protected against overloads, over voltage, short circuits etc. and hence fuses have been eliminated. Working Principle: As soon as the input voltage is given to the regulator, the primary coil of the transformer is energized. Corresponding voltage is also magnetically induced into the secondary coil of the main transformer. Secondary winding, connected in series, with capacitors form a resonating tank at line frequency and drive the transformer into saturation. Once the transformer is saturated, the flux density of the transformer becomes constant. It is this constant flux density available at the transformer, which results in a constant output. In other word, the transformer develops immunity to the input voltage variation. Resultant harmonics, generated by the saturation, are filtered by the combination of filter winding on the main transformer, additional filter chock and capacitor. Transformer: A transformer is a static (or stationary) piece of apparatus by means of which electric power in one circuit is transformed into electric power of the same frequency in another circuit. It can raise or lower the voltage in a circuit but with a corresponding decrease or decrease or increase in current. The physical basis of a transformer is mutual Induction between two circuits linked by a common magnetic flux. In its simple form, it consists of two inductive coils which are electrically separated but magnetically linked through a path of low reluctance. The two coils is connected to a source of alternating voltage, an alternating flux is set up in the laminated core, most of which is linked with the other coil in which it produces mutuallyinduced e.m.f (according to faraday s Laws of Electromagnetic Introduction e = MDI/dt). If the second coil circuit is closed, a current flows in it and so electric energy is transferred (entirely magnetically) from the first coil to the second coil.the first coil, in which electric energy is fed from the a.c. supply mains, is called primary winding and the other from which energy is drawn out, is called secondary winding. In brief, a transformer is a device that 1. Transfers electric power from one circuit to another. 2. It does so without a change of frequency 3. It accomplishes this by electromagnetic induction and 4. Where the two electric circuits are in mutual inductive influence of each other. There are different types of transformer are.

13 13 a) Main or power transformer: Transformer working in power supply of electrical & electronics circuit and also provides various A.C voltag is called main transformer. It may voltage step-up or step-down. b) Auto transformer: It is different type of transformer, which consist on only one winding. The winding has a common tap terminal. One end of the winding and common tap together works as primary, while the other end of the winding and the common tap terminal together works as secondary. This type of transformer can be used as voltage step-up as well as voltage step-down mainly in auto stabilizer. c) Battery eliminator transformer. This type of transformer works on 230V A.C main supply. It is used in power supply unit designed to product L.T.D.C, that is why it is known as battery eliminator transformer. d) Driver transformer: It is used for coupling and impedance matching of the two amplifier is called driver transformer. e) Out put transformer: It is a voltage step-down type marching transformer used in the output stage of radio receiver. f) Push-pull amplifier :A push-pull transformer circuit requires two such driver transformer whose one winding has a center tapped terminal also.the primary winding of the out put push-pull transformer and the secondary winding of the input push-pull transformer have center tapped terminal. These are of two types A.F & R.F. A.F transformer used in receiver of P.A amplifier and R.F transformer used in transmitter. g) I.F transformer: It is specially design to transfer electrical energy at a specified frequency. It is used in radio receiver to keep I.F frequency between 450 Khz to470khz and for T.V kept at 33.4 Mhz. h) R.F transformer: All the transformer working above 20 Khz are called as R.F transformer. The oscillator and antenna coils of a radio receiver R.F transformer. Trouble shooting: Transformer are in use have so many losses like Eddy current loss, Hystresis loss, Copper loss,leakage loss etc. To minimize the losses the core is made by jointing together insulated iron lamination instate of solid bar. The lamination are painted with varnish both side. Other losses can be reduce by using mild steel lamination also. Un interrupted power supply (UPS) Computer requires un-interrupt able power supply because an interruption cause loss of data and damage of hardware resulting on a financial loss. BATTERY RECTIFIER A.C 230V INVERTER STATIC BY PASS SWITCH

14 14 SERVICE SWITCH The 230 V A.C mains supply is given to a rectifier which converts it into D.C. This D.C charges the battery and operates an inverter circuit. The inverter circuit again convert the D.C into A.C of 230V and feed into the load. There is a static bypass switch (SBS) arrangement together with a service switch in its parallel for feeding the load directly by mains line in the event of inverter failure. D.C D.C converter. In may not be always possible to obtain from power source the correct voltage in the required form for operating communication equipment. In such circumstances conversion equipment become necessary for converting the power supply voltage into a form suitable for being fed to the equipment. As long as the power sources is AC, the necessary DC voltage can be conveniently obtained by means of a transformer and a rectifier. But,where only a low voltage D.C supply is available and a higher D.C voltage is wanted, a class of circuits known as D.C D.C converter must be used. It is a device which normally step up a low D.C input to a high D.C output and is only inverter with a rectifier and a filter figure shows how it works D.C I/P D.C/D.C Converter Step-up Transformer Rectifier Filter& Regulator D.C O/P It takes the input from the low voltage D.C supply A and switches it ON and OFF continuously, at an audio frequency in switching circuit B gives an A.C voltage fed into the step up transformer C.The high voltage output form C is fed into a rectifying and smoothing circuit D. form which emerges a final high voltage D.C out.the wave shapes of the A.C voltages are as square wave which is the commonest wave form and A.C sinusoidal wave form can be derived from it. A step-down transformer can also be used to give an output lower than the input voltage.

15 15 A.C--D.C converter. It is a system in which A.C is converted required D.C voltage and is fed directly to the equipment after smoothing D.C through filter. Before filtering the will be pulsating D.C. The basic parts of A.C D.C converter is main transformer and a rectifier. The system is used where the equipment requires only D.C voltages.

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