Lecture notes Electrical machines

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Lecture Notes Elements of Electrical Machines VEER SURENDRA SAI UNIVERSITY OF TECHNOLOGY BURLA, ODISHA, INDIA DEPARTMENT OF ELECTRICAL ENGINEERING Elements of Electrical Machines Lecture Notes Subject Code: BEE 1301 rd For 3 Semester Mechanical Engineering and Production Engineering students 1 Lecture Notes Elements of Electrical Machines Disclaimer This document does not claim any originality and cannot be used as a substitute for prescribed textbooks. The information presented here is merely a collection by the committee members for their respective teaching assignments. Various sources as mentioned at the reference of the document as well as freely available material from internet were consulted for preparing this document. The ownership of the information lies with the respective authors or institutions. Further, this document is not intended to be used for commercial purpose and the committee members are not accountable for any issues, legal or otherwise, arising out of use of this document. The committee members make no representations or warranties with respect to the accuracy or completeness of the contents of this document and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. The committee members shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. 2 Lecture Notes Elements of Electrical Machines Veer Surendra Sai University of Technology, Odisha, Burla, India Department of Electrical Engineering, (3RD SEMESTER) ELEMENTS OF ELECTRICAL MACHINES (3-1-0) (For Mechanical and Production Engineering Students) Module Chapter Lecture no. Topics D.C. Generator 1 Construction and principle of operation Module I 2 E.M.F. equation, types of generator 3 No load and load characteristics, Voltage build-up of shunt Generator 4 Voltage regulation, Application D.C. Motor 5 Back E.M.F, torque and speed equations 6 Speed control of series and shunt motors 7 Characteristics and performance curves 8 Motor starters 9 & 10 Losses and Efficiency of D.C machines Module II Single phase 11 Construction and principle of operation Transformer 12 E.M.F. equation 13 Phasor diagram 14 Actual and approximate equivalent circuits 15 Open and short circuit tests, voltage regulation 16 Losses and efficiency Three Phase 17 Construction and principle of operation Transformer 18 Connection of three single –phase units in wye, delta, open delta configurations 19 Autotransformer; conventional transformer connected as Autotransformer 20 Special Transformers – induction heating and high impedance and high frequency transformer Three- phase 21 construction and principle of operation 3 Lecture Notes Elements of Electrical Machines Module III alternators 22 E.M.F. equation, distribution and pitch factors 23 Synchronous reactance 24 performance of alternators on no-load and load; Phasor diagram; voltage regulation 25 power calculations of turbine and hydro-generators 26 synchronization of an alternator Three-Phase 27 Construction and principle operation Synchronous 28 & 29 Complete phasor diagrams, V- curves Motor 30 Starting methods of synchronous motor and applications Three-Phase 31 Construction of slip ring and squirrel cage type Module IV induction Motor induction motors 32 Starting and running torque of a three phase induction motor 33 torque-slip characteristics; maximum torque calculations 34 speed control of induction motors 35 Open and short-circuit tests 36 losses and efficiency 37 starting of induction motors Single-Phase 38 Construction and principle of operation Induction Motor 39 Analysis of single phase motor based on double revolving field theorem 40 Torque slip characteristic, capacitor- start and capacitor run motors REFERENCES 1 I. J. Nagrath & D.P. Kothari, “Theory and problems of Electrical Machines”, TMH Publication, New Delhi. 2 B.L.Theraja & A.K.Theraja, “A Text Book of Electrical Technology”, Volume II, S. Chand & Company Ltd. 3 P. S. Bhimbra, “Electrical Machinary”, Khanna Publishers, New Delhi. 4 NPTEL course materials. 5 http://electrical4u.com/, Electrical4u, online Electrical Engineering study site, accessed on January 2015. 4 Lecture Notes Elements of Electrical Machines D.C Generator An electrical Generator is a machine which converts mechanical energy (or power) into electrical energy (or power). The generator operates on the principle of the production of dynamically induced emf i.e., whenever flux is cut by the conductor, dynamically induced emf is produced in it according to the laws of electromagnetic induction, which will cause a flow of current in the conductor if the circuit is closed. Hence, the basic essential parts of an electric generator are: A magnetic field and A conductor or conductors which can so move as to cut the flux In dc generators the field is produced by the field magnets which are stationary. Permanent magnets are used for very small capacity machines and electromagnets are used for large machines to create magnetic flux. The conductors are situated on the periphery of the armature being rotated by the prime-mover. Fig. 1.1 Basics of dc generators 5 Lecture Notes Elements of Electrical Machines 1.1 Practical DC generator construction Fig. 1.2 Cut-away view of dc practical generators The actual DC generator consists of the following essential parts:  Magnetic frame or Yoke  Pole Cores and Pole Shoes  Pole Coils or Field Coils  Armature Core  Armature Windings or Conductors  Commutator  Brushes and Bearings a) Magnetic frame or Yoke Purpose of Yoke is 1. It act as a protecting cover for whole machine 2. It provides mechanical support for poles 6 Lecture Notes Elements of Electrical Machines 3. It carries the magnetic flux produced by poles b) Pole Cores and Pole Shoes The field magnets consist of pole cores and pole shoes. The Pole shoes serve two purposes: 1. They spread out the flux in the air gap 2. They support the exciting coils c) Armature When current is passed through field coils, they electro-magnetize the poles which produce the necessary flux. The Armature serves two purposes: 1. Armature houses the armature conductors or coils 2. It provides low reluctance path for flux It is drum shaped and is built up of laminations made sheet steel to reduce eddy current loss. Slots are punched on the outer periphery of the disc. The Armature windings or conductors are wound in the form of flat rectangular coils and are placed in the slots of the Armature. The Armature windings are insulated from the armature body by insulating materials. d) Commutator and brushes The function of Commutator is to facilitate collection of current from the armature conductors and converts the alternating current induced in the armature conductors into unidirectional current in the external load circuit. The commutator is made up of insulated copper segments. Two brushes are pressed to the commutator to permit current flow. The Brushes are made of carbon or Graphite. Bearings are used for smooth running of the machine. 1.2 E.M.F. equation 7 Lecture Notes Elements of Electrical Machines Let,  flux per pole in weber z total number of armature conductors=no. of slots no. of conductors/slot P no. of generator poles A no. of parallel paths in armature N armature rotation in revolutions per minute (rpm) E emf induced in any parallel path in armature Generated emf, E emf generated in any one of the parallel path i.e. E g d  n 1 Average emf generated/conductor volts, dt Now, flux cut per conductor in one revolution, dP weber N No. of revolutions per second 60 60 Time for one revolution, dt second N Hence, according to Faradays laws of Electromagnetic induction, dPN EMF generated/conductor volts dt 60 For a simplex lap-wound generator: No. of parallel paths P z No. of conductors in one path P PN zzN Hence, EMF generated/path volts 60 P 60 For a simplex wave-wound generator: No. of parallel paths 2 8 Lecture Notes Elements of Electrical Machines z No. of conductors in one path 2 PN zzNP Hence, EMF generated/path volts 60 2 120 zN P In general generated EMF, E g 60 A 1.3 Types of generator DC generators are usually classified according to the way in which their fields are excited. DC generators may be divided into, (a) separately excited dc generators, and (b) self excited dc generators. a) separately excited dc generators Separately excited generators are those whose field magnets are energized from an independent external source of dc current. b) self excited dc generators Self excited generators are those whose field magnets are energized by the current produced by the generators themselves. Due to residual magnetism, there is always present some flux in the poles. When the armature is rotated, some emf and hence some current flows which is partly or fully passed through the field coils thereby strengthening the residual pole flux. There are three types of self excited dc generators named according to the manner in which their field coils (or windings) are connected to the armature. In shunt the two windings, field and armature are in parallel while in series type the two windings are in series. In compound type the part of the field winding is in parallel while other part in series with the armature winding. 9 Lecture Notes Elements of Electrical Machines Fig.1.3 DC generators classification 10 Lecture Notes Elements of Electrical Machines D.C. Motor An electric motor is a machine which converts electrical energy into mechanical energy. 2.1 Principle of operation It is based on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force whose direction is given by Fleming's Left-hand rule and whose magnitude is given by Force, F = B I l newton 2 Where B is the magnetic field in weber/m . I is the current in amperes and l is the length of the coil in meter. Fleming’s left hand rule says that if we extend the index finger, middle finger and thumb of our left hand in such a way that the current carrying conductor is placed in a magnetic field (represented by the index finger) is perpendicular to the direction of current (represented by the middle finger), then the conductor experiences a force in the direction (represented by the thumb) mutually perpendicular to both the direction of field and the current in the conductor. Figure 2.1: Force in DC Motor Figure 2.2 : Magnetic Field in DC Motor 11 Lecture Notes Elements of Electrical Machines Figure 2.3 : Torque in DC Motor Figure 2.4 : Current Flow in DC Motor Constructionally, there is no basic difference between a dc generator and motor. In fact, the same dc machine can be used interchangeably as a generator or as a motor. The basic construction of a dc motor contains a current carrying armature which is connected to the supply end through commutator segments and brushes and placed within the north south poles of a permanent or an electro-magnet. Fig. 2.5 Flemings Left hand rule 2.2 Back E.M.F When the motor armature rotates, the conductors also rotate and hence cut the flux. In accordance with the laws of electromagnetic induction, emf is induced in them whose direction, as found by 12 Lecture Notes Elements of Electrical Machines Fleming’s Right hand Rule, is in opposition to the applied voltage. Because of its opposing direction, it is referred to as counter emf of back emf E . V has to drive I against the opposition of E . The power b a b required to overcome this opposition is . E I b a 2.3 Voltage Equation of a Motor V The voltage applied across the motor armature has to, (a) Overcome the back emf , and E b (b) Supply the armature ohmic drop I R a a Hence, V E I R b a a This is known as voltage equation of a dc motor. Now, multiplying both sides by I , we get a 2 VI E I I R a b a a a Where, VI Electrical power input to the armature a E I Electrical equivalent of mechanical power developed in the armature b a 2 I R copper loss in the armature a a 2.4 Condition for maximum efficiency 2 The gross mechanical power developed by motor is, P VI I R m a a a Differentiating both side with respect to I and equating the result to zero, we get a dP m  V 2I R 0 a a dI a Hence, I R V / 2 a a As, V E I R and I R V / 2 b a a a a Hence, E V / 2 b 13 Lecture Notes Elements of Electrical Machines Thus gross mechanical power developed by a motor is maximum when back emf is equal to half the supply voltage. This condition is, however, not realized in practice, because in that case current would be much beyond the normal current of the motor. Moreover, half the input would be wasted in the form of heat and taking other losses (mechanical and magnetic) into consideration, the motor efficiency will be well below 50 percent. 2.5 Torque The turning or twisting moment of a force about an axis is called torque. It is measured by the product of the force and the radius at which this force acts. Consider a pulley of radius r meter acted upon by a circumferential force of F newton which causes it to rotate at N rpm. Then torqueT F r newton-metre(N-m Work done by this force in one revolution =Force distance  F 2r joule Power developed F 2r N joule/second or wattF r 2N watt 2N Now, angular velocity  in radian per second and F r =torque T Hence, power developed T watt or P T watt Moreover, if N is in rpm, then  2N / 60 rad/s 2N 2 NT Hence, PT or P .NT 60 60 9.55 2.5.1 Armature torque of a motor N Let T be the torque developed by the armature of a motor running at rps. If T is in N-m, then a a power developed T 2N watt a 14 Lecture Notes Elements of Electrical Machines We also know that electrical power converted into mechanical power in the armature  E I b a watt. Comparing above equations, we get T 2N E I a b a E I b a After simplification, if N in rps, T a 2N E I b a If N is in rpm, then T 9.55 N-m a N Also,T 0.159ZIP/ AN-m a a 2.5.2 Shaft torque The whole of the armature torque, as calculated above, is not available for doing useful work, because of iron and friction losses in the motor. The torque which is available for doing useful work is known as shaft torqueT . The motor output is given by sh Output T 2N watt provided T is in N-m and N in rps. sh sh Output _ in _ watts Hence, , if N is in rps T sh 2N Output _ in _ watts Output And, if N is in rpm, then T 9.55 sh 2N / 60 N 2.6 Speed Control of DC Motor Speed control means intentional change of the drive speed to a value required for performing the specific work process. Speed control is a different concept from speed regulation where there is natural change in speed due to change in load on the shaft. Speed control is either done manually by the operator or by means of some automatic control device. One of the important features of dc motor is that its speed can be controlled with relative ease. We know that the expression of speed control dc motor is given as, 15 Lecture Notes Elements of Electrical Machines V I R V I R A  a a a a N . K rps  Z P  Where, R armature circuit resistance. a Therefore speed (N ) of 3 types of dc motor – SERIES, SHUNT AND COMPOUND can be controlled by changing the quantities on RHS of the expression. So speed can be varied by changing (i) terminal voltage of the armature V , (ii) armature circuit resistance R and (iii) flux per pole . The first two cases involve change that affects armature circuit and the third one involves change in magnetic field. Therefore speed control of dc motor is classified as 1) armature control methods and 2) field control methods. 2.7 Motor starters The starting of DC motor is somewhat different from the starting of all other types of electrical motors. This difference is credited to the fact that a dc motor unlike other types of motor has a very high starting current that has the potential of damaging the internal circuit of the armature winding of dc motor if not restricted to some limited value. This limitation to the starting current of dc motor is brought about by means of the starter. Thus the distinguishing fact about the starting methods of dc motor is that it is facilitated by means of a starter. Or rather a device containing a variable resistance connected in series to the armature winding so as to limit the starting current of dc motor to a desired optimum value taking into consideration the safety aspect of the motor. Starters can be of several types and requires a great deal of explanation and some intricate level understanding. But on a brief over-view the main types of starters used in the industry today can be illustrated as:- 1. 3 point starter. 16 Lecture Notes Elements of Electrical Machines 2. 4 point starter. Fig. 2.6 17 Lecture Notes Elements of Electrical Machines Fig. 2.7 Fig. 2.8 18 Lecture Notes Elements of Electrical Machines Single phase Transformer 3.1 Principle of operation and construction 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. Although transformers have no moving parts, they are essential to electromechanical energy conversion. They make it possible to increase or decrease the voltage so that power can be transmitted at a voltage level that results in low costs, and can be distributed and used safely. In addition, they can provide matching of impedances, and regulate the flow of power (real or reactive) in a network.  The physical basis of a transformer is mutual induction between two circuits linked by a common magnetic field.  In its simplest form, it consists of two inductive coils which are electrically separated but magnetically linked through a path of low reluctance. The two coils posses high mutual inductance.  If one coil 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 emf. Constructionally, the transformers are of two general types, distinguished from each other merely by the manner in which the primary and secondary coils are placed around the laminated core. a) Core type b) Shell type In the so-called core type transformer, the windings surround a considerable part of the core whereas in shell-type transformer, the core surrounds considerable portions of the winding. 19 Lecture Notes Elements of Electrical Machines Fig.3.1 Physical diagram of a transformer 3.2 Elementary theory of an ideal transformer An ideal transformer is one which has no losses i.e. its windings have no ohmic resistance, there is no magnetic leakage and hence which has no copper and core losses. An ideal transformer consists of two purely inductive coils wound on a loss free core. In its simplest form it consist of, two inductive coils which are electrically separated but magnetically linked through a path of low reluctance. If one coil (primary) 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 Induction. If the second coil (secondary circuit) is closed, a current flows in it and so electric energy is transferred (entirely magnetically) from the first coil to the second coil. 20

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