LECTURE NOTES ON ELECTRICAL TECHNOLOGY

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LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 LECTURE NOTES ON ELECTRICAL TECHNOLOGY B.Tech ECE II YEAR I SEMESTER (JNTUA-R15) Mrs.S.JAREENA ASSISTANT PROFESSOR DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING CHADALAWADA RAMANAMMA ENGINEERING COLLEGE CHADALAWADA NAGAR, RENIGUNTA ROAD, TIRUPATI (A.P) - 517506 DEPARTMENT OF EEE CREC Page 1 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY ANANTAPUR II B.Tech I-Sem (E.C.E) T Tu C 3 1 3 (15A02306) ELECTRICAL TECHNOLOGY Objective: Electrical Technology contains Single phase transformers, Induction motors, DC generators and motors which are widely used in industry are covered and their performance aspects will be studied. UNIT- I DC GENERATORS D.C. Generators – Principle of Operation – Constructional Features – E. M.F Equation– Numerical Problems – Methods of Excitation – Separately Excited and Self Excited Generators – Build-Up of E.M.F - Critical Field Resistance and Critical Speed - Load Characteristics of Shunt, Series and Compound Generators- Applications UNIT – II D.C. MOTORS D.C Motors – Principle of Operation – Back E.M.F. –Torque Equation – Characteristics and Application of Shunt, Series and Compound Motors-Speed Control of D.C. Motors: Armature Voltage and Field Flux Control Methods. Three Point Starter-Losses – Constant & Variable Losses – Calculation of Efficiency - Swinburne’s Test. UNIT-III SINGLE PHASE TRANSFORMERS Single Phase Transformers - Constructional Details- Emf Equation - Operation on No Load and on Load - Phasor Diagrams-Equivalent Circuit - Losses and Efficiency-Regulation-OC and SC Tests – Sumpner’s Test - Predetermination of Efficiency and Regulation. DEPARTMENT OF EEE CREC Page 2 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 UNIT-IV 3-PHASE INDUCTION MOTORS Polyphase Induction Motors-Construction Details of Cage and Wound Rotor Machines- - Principle of Operation – Slip- Rotor Emf and Rotor Frequency - Torque Equation- Torque Slip Characteristics. UNIT – V SYNCHRONOUS MACHINES Principle And Constructional Features of Salient Pole and Round Rotor Machines – E.M.F Equation- Voltage Regulation by Synchronous Impedance Method- Theory of Operation of Synchronous Motor. OUTCOME: After going through this course the student gets a thorough knowledge on DC Motors & Generators, Transformers and Induction motors with which he/she can able to apply the above conceptual things to real-world problems and applications. TEXT BOOKS: th 1. Electric Machines –by I.J.Nagrath & D.P.Kothari,Tata Mc Graw Hill, 7 Edition.2005 2. Basic Electrical Engineering –By T.K.Nagasarkar and M.S. Sukhija Oxford University Press. REFERENCE BOOKS: 1. Electrical and Electronic Technology, Hughes, Pearson Education. 2. Electrical Machines, P. S. Bimbhra, Khanna Publishers, 2011. nd 3. Basic Electrical Engineering, 2 Edition, V.N. Mittle and Aravind Mittal, Mc Graw hill Education, 2006. DEPARTMENT OF EEE CREC Page 3 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 UNIT-I DC GENERATORS DEPARTMENT OF EEE CREC Page 4 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 Generators There are two types of generators, one is ac generator and other is dc generator. Whatever may be the types of generators, it always converts mechanical power to electrical power. An ac generator produces alternating power. A DC generator produces direct power. Both of these generators produce electrical power, based on same fundamental principle of Faraday's law of electromagnetic induction. According to these law, when an conductor moves in a magnetic field it cuts magnetic lines force, due to which an emf is induced in the conductor. The magnitude of this induced emf depends upon the rate of change of flux (magnetic line force) linkage with the conductor. This emf will cause an current to flow if the conductor circuit is closed. Hence the most basic two essential parts of a generator are 1. a magnetic field 2. conductors which move inside that magnetic field. Constructional Features A DC generator has the following parts 1. Yoke 2. Pole of generator 3. Field winding 4. Armature of DC generator 5. Brushes of generator 6. Bearing DEPARTMENT OF EEE CREC Page 5 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 Fig. A Cut Away View Of Practical DC Generator Yoke of DC Generator Yoke of DC generator serves two purposes, 1. It holds the magnetic pole cores of the generator and acts as cover of the generator. 2. It carries the magnetic field flux. In small generator, yoke are made of cast iron. Cast iron is cheaper in cost but heavier than steel. But for large construction of DC generator, where weight of the machine is concerned, lighter cast steel or rolled steel is preferable for constructing yoke of DC generator. Normally larger yokes are formed by rounding a rectangular steel slab and the edges are welded together at the bottom. Then feet, terminal box and hangers are welded to the outer periphery of the yoke frame. Armature Core of DC Generator The purpose of armature core is to hold the armature winding and provide low reluctance path for the flux through the armature from N pole to S pole. Although a DC generator provides direct current but induced current in the armature is alternating in nature. That is why, cylindrical or drum shaped armature core is build up of circular laminated sheet. In every circular lamination, slots are either die - cut or punched on the outer periphery and the key way is located on the inner periphery as shown. Air ducts are also punched of cut on each lamination for circulation of air through the core for providing better cooling. DEPARTMENT OF EEE CREC Page 6 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 Armature Winding of DC Generator Armature winding are generally formed wound. These are first wound in the form of flat rectangular coils and are then pulled into their proper shape in a coil puller. Various conductors of the coils are insulated from each other. The conductors are placed in the armature slots, which are lined with tough insulating material. This slot insulation is folded over above the armature conductors placed in it and secured in place by special hard wooden or fiber wedges. Commutator of DC Generator The commutator plays a vital role in dc generator. It collects current from armature and sends it to the load as direct current. It actually takes alternating current from armature and converts it to direct current and then send it to external load. It is cylindrical structured and is build up of wedge - shaped segments of high conductivity, hard drawn or drop forged copper. Each segment is insulated from the shaft by means of insulated commutator segment shown below. Each commutator segment is connected with corresponding armature conductor through segment riser or lug. Brushes of DC Generator The brushes are made of carbon. These are rectangular block shaped. The only function of these carbon brushes of DC generator is to collect current from commutator segments. The brushes are housed in the rectangular box shaped brush holder. As shown in figure, the brush face is placed on the commutator segment with attached to the brush holder. Bearing of DC Generator For small machine, ball bearing is used and for heavy duty dc generator, roller bearing is used. The bearing must always be lubricated properly for smooth operation and long life of generator. Emf equation for dc generator The derivation of EMF equation for DC generator has two parts: 1. Induced EMF of one conductor 2. Induced EMF of the generator Derivation for Induced EMF of One Armature Conductor For one revolution of the conductor, Let, DEPARTMENT OF EEE CREC Page 7 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 Φ = Flux produced by each pole in weber (Wb) and P = number of poles in the DC generator. therefore, Total flux produced by all the poles = ø p And, Time taken to complete one revolution = 60/N Where, N = speed of the armature conductor in rpm. Now,according to Faraday’s law of induction, the induced emf of the armature conductor is denoted by “e” which is equal to rate of cutting the flux. Therefore, Induced emf of one conductor is Derivation for Induced EMF for DC Generator Let us suppose there are Z total numbers of conductor in a generator, and arranged in such a manner that all parallel paths are always in series. Here, Z = total numbers of conductor A = number of parallel paths Then, Z/A = number of conductors connected in series We know that induced emf in each path is same across the line Therefore, Induced emf of DC generator E = emf of one conductor × number of conductor connected in series. DEPARTMENT OF EEE CREC Page 8 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 Induced emf of DC generator is Simple wave wound generator Numbers of parallel paths are only 2 = A Therefore, Induced emf for wave type of winding generator is Simple lap-wound generator Here, number of parallel paths is equal to number of conductors in one path i.e. P = A Therefore, Induced emf for lap-wound generator is Methods Of Excitation An electric generator or electric motor consists of a rotor spinning in a magnetic field. The magnetic field may be produced by permanent magnets or by field coils. In the case of a machine with field coils, a current must flow in the coils to generate the field, otherwise no power is transferred to or from the rotor. The process of generating a magnetic field by means of an electric current is called excitation. For a machine using field coils, which is most large generators, the field current must be supplied, otherwise the generator will be useless. Thus it is important to have a reliable supply. Although the output of a generator can be used once it starts up, it is also critical to be able to start the generators reliably. In any case, it is important to be able to control the field since this will maintain the system voltage. Types of excitation (1)seperately excited generator. DEPARTMENT OF EEE CREC Page 9 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 (2)self excited generator. self generator is classified into 3 types. 1.shunt generator. 2.series generator. 3.compound generator. compoud generator is again classified into 2 types. 1.short shunt generator. 2.long shunt generator. Separately excited generators. These kind of generators has provided field exciter terminals which are external DC voltage source is supplies to produce separately magnetic field winding (shunt field) for magnetize of the generator as illustrated in figure as below. Self excited field generators. This type of generator has produced a magnetic field by itself without DC sources from an external. The electromotive force that produced by generator at armature winding is supply to a field winding (shunt field) instead of DC source from outside of the generator. Therefore, field winding is necessary connected to the armature winding. They may be further classified as a) Shunt generator. This generator, shunt field winding and armature winding are connected in parallel through commutator and carbon brush as illustrated in the figure below. DEPARTMENT OF EEE CREC Page 10 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 b) Series generator The field winding and armature winding is connected in series. There is different from shunt motor due to field winding is directly connected to the electric applications (load). Therefore, field winding conductor must be sized enough to carry the load current consumption and the basic circuit as illustrated below. Series generator c) Compound generator The compound generator has provided with magnetic field in combine with excitation of shunt and series field winding, the shunt field has many turns of fine wire and caries of a small current, while the series field winding provided with a few turns of heavy wire since it is in series with an DEPARTMENT OF EEE CREC Page 11 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 armature winding and caries the load current. There are two kinds of compound generator as illustrated in figure 5 and 6. A short-shunt compound generator Characteristic of separately excited generator The generated electromotive force (EMF) is proportional to both of a magnetic density of flux per pole and the speed of the armature rotated as expression by the relation as following. Eg = κ φ n Where K = Constant for a specific machine φ = The density of flux per pole n = Speed of the armature rotation Eg = Generator voltage DEPARTMENT OF EEE CREC Page 12 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 By holding the armature speed (n) at a constant value it can show that generator voltage (Eg) is directly proportional to the magnetic flux density. Which, flux density is proportionately to the amount of field current (If). The relation of field current and generate voltage as impressed by figure . From the figure when the field current (If) is become zero a small generate voltage is produce due to a residual magnetism. As the field current increases cause to increase generated voltage linearly up to the knee of the magnetization curve. Beyond this point by increasing the field current still further causes saturation of the magnetic structure. Generator voltage (Eg) is also directly to the armature speed. The formula and a magnetization curve can be both impressed about this relation. Where Eg = Generator voltage or the value of EMF at speed n Eg' = Generator voltage or the value of EMF at speed n’ n = Speed of the generator armature ( n’ ≠ n ) DEPARTMENT OF EEE CREC Page 13 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 Example 1: The open circuit terminal voltage versus the field current for a separately excited DC generator with provided the following test data at revolving speed 1400 rpm as show by the table1 below. Magnetic curve for example 3.1 Solution Curve (a) in figure 8 shows the characteristic at revolving speed 1400 rpm obtained by the data as show in table 1. To obtain the characteristic at 1000 rpm, is made of the relation as Eg = Kφn For instance, at a field current of 0.4 Amp the terminal voltage is 114 volts, when the speed is reached to 1400 rpm and kept its field current constant at this value, the open circuit voltage at 1000 rpm becomes. DEPARTMENT OF EEE CREC Page 14 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 Voltage Regulation When we add load on the generator, the terminal voltage will decrease due to (a) The armature winding resistance is mainly of armature resistance. It is cause directly decrease in terminal voltage as following relation. Vt = Eg - Ia Ra Where, Vt = Terminal or output voltage Ia = Armature current or load current Ra = Armature resistance (a) Load characteristic of (b) Circuit diagram a separately excited DC generator The decrease in magnetic flux due to armature reaction. The armature current establishes a magneto motive force (MMF), which it distorts to main flux, and makes result in weakened flux. We can put inter-pole between main field poles to reduce the armature reaction. To have some measure by how much the terminal voltage change from no-load condition and on load condition, which is called “voltage regulation”. DEPARTMENT OF EEE CREC Page 15 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 Where Vnl = No-load terminal voltage Vfl = Full-load terminal voltage Remark: A separately excited generator has disadvantage of requiring an external DC source. It is therefore used only where a wide range of terminal voltage required. Example 2 The separately excited generator of example 1 is driven at revolving speed 1000 rpm and the field current is adjusted to 0.6 Amp. If the armature circuit resistance is 0.28 ohm, plot the output voltage as the load current is varied from 0 to 60 Amp. Neglect armature reaction effects. If the full-load current is 60 Amp, what is the voltage regulation? Solution From example 1, Eg = 153 volts when the field current is 0.6 Amp, which is the open circuit terminal voltage. When the generator is loaded, the terminal voltage is decreased by internal voltage drop, namely. Vt = Eg - Ia Ra For a load current of, say 40 Amp. V = 153 - (40 × 0.28) = 141.80 Volts. t This calculation is for a number of load currents and the external characteristic can be plotted as show in fig. 10 at full load the terminal voltage. V = 153 - (60 × 0.28) = 136.20 Volts. t DEPARTMENT OF EEE CREC Page 16 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 Calculated load characteristic of an example 3.2 Critical Field Resistance And Critical Speed The critical field resistance is the maximum field circuit resistance for a given speed with which the shunt generator would excite. The shunt generator will build up voltage only if field circuit resistance is less than critical field resistance. It is a tangent to the open circuit characteristics of the generator at a given speed. Critical resistance DEPARTMENT OF EEE CREC Page 17 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 Suppose a shunt generator has built up voltage at a certain speed. Now if the speed of the prime mover is reduced without changing Rf, the developed voltage will be less as because the O.C.C at lower speed will come down (refer to figure). If speed is further reduced to a certain critical speed (ncr), the present field resistance line will become tangential to the O.C.C at ncr. For any speed below ncr, no voltage built up is possible in a shunt generator. Critical Speed Load characteristics Self excited DC shunt generator A shunt generator has its shunt field winding connected in parallel with the armature so that the machine provides it own excitation. For voltage to build up, there must be some residual magnetism in the field poles. There will be a small voltage (Er) generated. (a) Shunt generator circuit (b) load characteristic of shunt generator DEPARTMENT OF EEE CREC Page 18 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 If the connection of the field and armature winding are such that the weak main pole flux aids to the residual flux, the induced voltage will become larger. Thus more voltage applied to the main field pole and cause to the terminal voltage increase rapidly to a large value. When we add load on the generator, the terminal voltage will decrease due to. a) The armature winding resistance b) The armature reaction c) The weakened flux due to the connection of the generator to aids or oppose to the residual flux. DEPARTMENT OF EEE CREC Page 19 LECTURE NOTES ON ELECTRICAL TECHNOLOGY –R15 Circuit diagram for the solution of example 3 b) For a load of 20 kw when the terminal voltage is 135 volts, therefore the load current Series Generator The field winding of a series generator is connect in series with the armature winding. Since it carries the load current, the series field winding consists of only a few turns of thick wire. At no- load, the generator voltage is small due to residual field flux only. When a load is added, the flux increase, and so does the generated voltage. (a) Circuit diagram of series generator (b) load characteristics DEPARTMENT OF EEE CREC Page 20

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