P AYUB KHAN ASSISTANT PROFESSOR UNIT 5 AC

P. AYUB KHAN, ASSISTANT PROFESSOR UNIT -5 AC MACHINES AITS, EEE DEPT 1

P. AYUB KHAN, ASSISTANT PROFESSOR CONSTRUCTION AND WORKING OF TRANSFORMERS AITS, EEE DEPT 2

P. AYUB KHAN, ASSISTANT PROFESSOR CONTENT What is transformer Structure and working principle Construction of transformer Losses in transformer Ideal v/s practical transformer Uses and application of transformer AITS, EEE DEPT 3

P. AYUB KHAN, ASSISTANT PROFESSOR INTRODUCTION A transformer is a device that changes ac electric power at one voltage level to ac electric power at another voltage level through the action of a magnetic field. There are two or more stationary electric circuits that are coupled magnetically. It involves interchange of electric energy between two or more electric systems Transformers provide much needed capability of changing the voltage and current levels easily. � They are used to step-up generator voltage to an appropriate voltage level for power transfer. � Stepping down the transmission voltage at various levels for distribution and power utilization. AITS, EEE DEPT 4

P. AYUB KHAN, ASSISTANT PROFESSOR WHAT IS TRANSFORMER A transformer is a static piece of apparatus by means of which an electrical power is transferred from one alternating current circuit to another electrical circuit There is no electrical contact between them The desire change in voltage or current without any change in frequency Symbolically the transformer denoted as NOTE : Itworksontheprincipleofmutual induction AITS, EEE DEPT 5

P. AYUB KHAN, ASSISTANT PROFESSOR AITS, EEE DEPT 6

P. AYUB KHAN, ASSISTANT PROFESSOR STRUCTURE OF TRANSFORMER The transformer two inductive coils , these are electrical separated but linked through a common magnetic current circuit These two coils have a high mutual induction One of the two coils is connected of alternating voltage. this coil in which electrical energy is fed with the help of source called primary winding (P) shown in fig. The other winding is connected to a load the electrical energy is transformed to this winding drawn out to the load. this winding is called secondary winding(S) shown in fig. AITS, EEE DEPT 7

P. AYUB KHAN, ASSISTANT PROFESSOR The primary and secondary coil wound on a ferromagnetic metal core The function of the core is to transfer the changing magnetic flux from the primary coil to the secondary coil The primary has N 1 no of turns and the secondary has N 2 no of turns the of turns plays major important role in the function of transformer AITS, EEE DEPT 8

WORKING P. AYUB KHAN, ASSISTANT PROFESSOR PRINCIPLE The transformer works in the principle of mutual induction “The principle of mutual induction states that when the two coils are inductively coupled and if the current in coil change uniformly then the e. m. f. induced in the other coils. This e. m. f can drive a current when a closed path is provide to it. ” When the alternating current flows in the primary coils, a changing magnetic flux is generated around the primary coil. The changing magnetic flux is transferred to the secondary coil through the iron core The changing magnetic flux is cut by the secondary coil, hence induces an e. m. f in the secondary coil AITS, EEE DEPT 9

P. AYUB KHAN, ASSISTANT PROFESSOR Now if load is connected to a secondary winding, this e. m. f drives a current through it The magnitude of the output voltage can be controlled by the ratio of the no. of primary coil and secondary coil The frequency of mutually induced e. m. f as same that of the alternating source which supplying to the primary winding b AITS, EEE DEPT 10

P. AYUB KHAN, ASSISTANT PROFESSOR AITS, EEE DEPT 11

P. AYUB KHAN, ASSISTANT PROFESSOR CONSTRUCTIONOFTRANSFORMER These are two basic of transformer construction Magnetic core Windings or coils Magnetic core The core of transformer either square or rectangular type in size It is further divided into two parts vertical and horizontal The vertical portion on which coils are wounds called limb while horizontal portion is called yoke. these parts are Core is made of laminated core type constructions, eddy current losses get minimize. Generally high grade silicon steel laminations (0. 3 to 0. 5 mm) are used AITS, EEE DEPT 12

P. AYUB KHAN, ASSISTANT PROFESSOR WINDING Conducting material is used in the winding of the transformer The coils are used are wound on the limbs and insulated from each other The two different windings are wounds on two different limbs The leakage flux increases which affects the performance and efficiency of transformer To reduce the leakage flux it is necessary that the windings should be very close to each other to have high mutual induction AITS, EEE DEPT 13

P. AYUB KHAN, ASSISTANT PROFESSOR CORE TYPE CONSTRUCTION In this one magnetic circuit and cylindrical coils are used Normally L and T shaped laminations are used Commonly primary winding would on one limb while secondary on the other but performance will be reduce To get high performance it is necessary that other the two winding should be very close to each other AITS, EEE DEPT 14

P. AYUB KHAN, ASSISTANT PROFESSOR AITS, EEE DEPT 15

P. AYUB KHAN, ASSISTANT PROFESSOR SHELL TYPE CONSTRUCTION In this type two magnetic circuit are used The winding is wound on central limbs For the cell type each high voltage winding lie between two voltage portion sandwiching the high voltage winding Sub division of windings reduces the leakage flux Greater the number of sub division lesser the reactance This type of construction is used for high voltage AITS, EEE DEPT 16

P. AYUB KHAN, ASSISTANT PROFESSOR AITS, EEE DEPT 17

P. AYUB KHAN, ASSISTANT PROFESSOR LOSSES IN TRANSFORMER Copper losses : It is due to power wasted in the form of I 2 Rdue to resistance of primary and secondary. The magnitude of copper losses depend upon the current flowing through these coils. The iron losses depend on the supply voltage while the copper depend on the current. the losses are not dependent on the phase angle between current and voltage. hence the rating of the transformer is expressed as a product o f voltage and current called VA rating of transformer. It is not expressed in watts or kilowatts. Most of the timer, is rating is expressed in KVA. AITS, EEE DEPT 18

P. AYUB KHAN, ASSISTANT PROFESSOR Hysteresis loss : During magnetization and demagnetization , due to hysteresis effect some energy losses in the core called hysteresis loss Eddy current loss : The leakage magnetic flux generates the E. M. F in the core produces current is called of eddy current loss. AITS, EEE DEPT 19

P. AYUB KHAN, ASSISTANT PROFESSOR IDEAL V/S PRACTICAL TRANSFORMER A transformer is said to be ideal if it satisfies the following properties, but no transformer is ideal in practice. It has no losses Windings resistance are zero There is no flux leakage Small current is required to produce the magnetic field While the practical transformer has windings resistance , some leakage flux and has lit bit losses AITS, EEE DEPT 20

P. AYUB KHAN, ASSISTANT PROFESSOR APPLICATION AND USES The transformer used in television and photocopy • machines The transmission and distribution of alternating power is possible by transformer Simple camera flash uses fly back transformer Signal and audio transformer are used couple in amplifier Todays transformer is become an essential part of electrical engineering AITS, EEE DEPT 21

P. AYUB KHAN, ASSISTANT PROFESSOR REFERENCE Electrical engineering by UA Bakshi Principal of electrical machine by VK Mehta Electrical machine by RK Rajput www. allaboutcircuit. com www. iiee. com AITS, EEE DEPT 22

P. AYUB KHAN, ASSISTANT PROFESSOR AC MACHINES AITS, EEE DEPT 23

P. AYUB KHAN, ASSISTANT PROFESSOR CONTENTS • Definition and Types of Alternator • Working Principle of Alternator • Construction of Alternator • Armature Reaction in Alternator or Synchronous Generator • Armature Winding of Alternator • Application of Induction Generator AITS, EEE DEPT 24

P. AYUB KHAN, ASSISTANT PROFESSOR DEFINITION AND TYPESOF ALTERNATOR • An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. Most alternators use a rotating magnetic field with stationary armature. • It is also known as synchronous generator. According to application According to their design o Automotive type - used in modern automobile. o Diesel electric locomotive type - used in diesel electric multiple unit. o Marine type - used in marine. o Brush less type - used in electrical power generation plant as main source of power. o Radio alternators - used for low brand radio frequency transmission. o Salient pole type. o Cylindrical rotor type. AITS, EEE DEPT 25

P. AYUB KHAN, ASSISTANT PROFESSOR WORKING PRINCIPLE OF ALTERNATOR • The working principle of alternator depends upon Faraday's law of electromagnetic induction which says the current is induced in the conductor inside a magnetic field when there is a relative motion between that conductor and the magnetic field. #Working 1. For understanding working of alternator let's assume a single rectangular turn placed in between two opposite magnetic pole as shown. AITS, EEE DEPT 26

P. AYUB KHAN, ASSISTANT PROFESSOR The single turn loop ABCD starts rotating clockwise against axis a-b After 90° rotation the side AB or conductor AB of the loop comes in front of S-pole and conductor CD comes in front of N-pole. As per Fleming right hand rule the direction of this current will be from A to B. At the same time conductor CD comes under N pole and here also if we apply Fleming right hand rule we will get the direction of induced current and it will be from C to D. AITS, EEE DEPT 27

P. AYUB KHAN, ASSISTANT PROFESSOR Now after clockwise rotation of another 90° the turn ABCD comes at vertical position as shown below. At this position tangential motion of conductor AB and CD is just parallel to the magnetic flux lines, hence there will be no flux cutting that is no current in the conductor. While the turn ABCD comes from horizontal position to vertical position, angle between flux lines and direction of motion of conductor, reduces from 90° to 0° and consequently the induced current in the turn is reduced to zero from its maximum value. AITS, EEE DEPT 28

P. AYUB KHAN, ASSISTANT PROFESSOR As at this position the turn comes at horizontal position from its vertical position, the current in the conductors comes to its maximum value from zero. That means current is circulating in the close turn from point B to A, from A to D, from D to C and from C to B During every full revolution of the turn, the current in the turn gradually reaches to its maximum value then reduces to zero and then again it comes to its maximum value but in opposite direction and again it comes to zero. In this way the current completes one full sine wave form during each 360° revolution of the turn. Thus an alternating current is produced in a turn is rotated inside a magnetic field. From this, we come to the actual working principle of alternator. AITS, EEE DEPT 29

P. AYUB KHAN, ASSISTANT PROFESSOR CONSTRUCTION OF ALTERNATOR Pulley Rotor Collector-ring end Shield Rotor Drive End shield Stator AITS, EEE DEPT 30

P. AYUB KHAN, ASSISTANT PROFESSOR I. Stator : • Stator is the stationary part of the alternator and contains 3 -phase armature windings. Stator core is built up of silicon steel laminations to reduce eddy current losses. • The laminations are provided with slots on its inner periphery and are packed tightly together by cast iron frame. • The three phase windings are placed in these slots and serves as the armature windings of the alternator. • The armature windings are always connected in star and the neutral is connected to ground. AITS, EEE DEPT 31

P. AYUB KHAN, ASSISTANT PROFESSOR I. Rotor : • The rotor is rotating part of the alternator. It carries a field winding which is supplied with dc current • through two slip rings by a separate dc source. • This dc source (exciter) is generally a small dc generator mounted on the shaft of the alternator. • There are two types of rotors : i. Salient pole type ii. Cylindrical rotor type AITS, EEE DEPT 32

P. AYUB KHAN, ASSISTANT PROFESSOR Salient pole type • Salient means sticking out or projected out. A salient pole is a magnetic pole that is projected out of the rotor surface. • The salient pole alternators are slow-speed machines, speed varying from 150 to 600 rpm. These alternators are driven by hydraulic turbines. They are also called water-wheel generators or hydro-generators • Salient type rotor has non-uniform air-gap and two or four poles • Salient-pole construction can not be made strong enough to withstand the mechanical stress at higher speeds AITS, EEE DEPT 33

P. AYUB KHAN, ASSISTANT PROFESSOR Cylindrical rotor type Cylindrical rotor is non-projecting surface type • • Cylindrical rotor type rotor has small diameter and large length • Cylindrical rotor type rotor is used for high speed and has uniform air-gap • Cylindrical rotors have four or more poles • High speed alternators (1500 – 3000 rpm) are driven by steam turbines and use non-salient type rotors due to following reason : – Gives noiseless operation at high speeds – Flux is uniformly distributed along the periphery, so proper sine wave is obtained which gives better emf AITS, EEE DEPT 34

P. AYUB KHAN, ASSISTANT PROFESSOR ARMATURE REACTION IN ALTERNATOR OR SYNCHRONOUS GENERATOR • Every rotating electrical machine works based on Faraday's law. • Every electrical machine requires a magnetic field and a coil (Known as armature) with a relative motion between them. • In case of an alternator, we supply electricity to pole to produce magnetic field and output power is taken from the armature. Due to relative motion between field and armature, the conductor of armatures cut the flux of magnetic field and hence there would be changing flux linkage with these armature conductor. • According to Faraday's law of electromagnetic induction there would be an emf induced in the armature. Thus, as soon as the load is connected with armature terminals, there is an current flowing in the armature coil. • As soon as current starts flowing through the armature conductor there is one reverse effect of this current on the main field flux of the alternator (or synchronous generator). This reverse effect is referred as armature reaction in alternator or synchronous generator. • AITS, EEE DEPT 35

P. AYUB KHAN, ASSISTANT PROFESSOR • The armature reaction of alternator or synchronous generator, depends upon the phase angle between, stator armature current and induced voltage across the armature winding of alternator. • The phase difference between these two quantities, i. e. Armature current and voltage may vary from - 90° to + 90°. • If this angle is θ, then, • When θ = 0 (Unity Power Factor) • When θ = 90° (Lagging Zero Power Factor) • When θ = - 90° (Leading Power Factor) AITS, EEE DEPT 36

P. AYUB KHAN, ASSISTANT PROFESSOR A. When θ = 0 (Unity Power Factor) • At unity power factor, the angle between armature current I and induced emf E, is zero. That means, armature current and induced emf are in same phase B. When θ = 90° (Lagging Zero Power Factor) – At lagging zero electrical power factor, the armature current lags by 90° to induced emf in the armature. As the emf induced in the armature coil due to main field flux. The emf leads the main field flux by 90°. C. When θ = - 90° (Leading Power Factor) • At leading power factor condition, armature current I leads induced emf Eby an angle 90°. Again, we have shown just, field flux leads, induced emf Eby 90°. AITS, EEE DEPT 37

P. AYUB KHAN, ASSISTANT PROFESSOR ARMATURE WINDING OF ALTERNATOR • Armature winding in an alternator may be either closed type open type. Closed winding forms star connection in armature winding of alternator. • Common properties of armature winding. – First and most important property of an armature winding is, two sides of any coil should be under two adjacent poles. That means, coil span = pole pitch. – The winding can either be single layer or double layer. – Winding is so arranged in different armature slots, that it must produce sinusoidal emf. AITS, EEE DEPT 38

P. AYUB KHAN, ASSISTANT PROFESSOR • There are different types of armature winding used in alternator. The windings can be classified as – Single phase winding. – Lap winding – wave winding – Concentric winding – Full pitched coil winding – fractional pitched coil winding. AITS, EEE DEPT 39

P. AYUB KHAN, ASSISTANT PROFESSOR • Single phase and poly phase armature • Lap winding AITS, EEE DEPT 40

P. AYUB KHAN, ASSISTANT PROFESSOR • wave winding. • Concentric winding AITS, EEE DEPT 41

P. AYUB KHAN, ASSISTANT PROFESSOR RATING OF ALTERNATOR • Power rating of alternator is defined as the power which can be delivered by an alternator safely and efficiently under some specific conditions. • The power rating of an alternator is so specified, that at that maximum load, the temperature rise of different parts of the machine does not cross their specified safe limit. • The copper losses i. e. I 2 R loss varies with armature current and core losses vary with voltage. • The temperature rise or heating of alternator depends upon cumulative effect of copper losses and core losses. As there is no role of power factor upon these losses, the rating of alternator generally given in VA or KVA or MVA. • The electrical output of an alternator is product of power factor and VA and output is expressed in KW. Some times alternators are also rated by its power instead of VA rating. AITS, EEE DEPT 42

P. AYUB KHAN, ASSISTANT PROFESSOR APPLICATION OF INDUCTION GENERATOR • The conditions when the induction machine will behave as an induction generator are written below: I. Slip becomes negative due to this the rotor current and rotor emf attains negative value. II. The prime mover torque becomes opposite to electric torque. • These conditions can be achieve when an induction machine is coupled with the prime mover whose speed can be controlled. If the speed of the prime mover is increased such that the slip becomes negative. • Due to this, all the conditions that we have mentioned above will become fulfilled and machine will behave like an induction generator. • Induction generator is not a self excited machine therefore in order to develop the rotating magnetic field, it requires magnetizing current and reactive power. AITS, EEE DEPT 43

P. AYUB KHAN, ASSISTANT PROFESSOR • we can have a self excited or isolated induction generation in one case if we will use capacitor bank for reactive power supply instead of ac supply system • The function of the capacitor bank is to provide the lagging reactive power to the induction generator as well as load AITS, EEE DEPT 44

P. AYUB KHAN, ASSISTANT PROFESSOR • Externally excited generators are widely used for regenerative breaking of hoists driven by the three phase induction motors. • The efficiency of the externally excited generator is not so good. • We cannot use externally excited generator at lagging power factor which major drawback of this type of generator. • The amount of reactive power used to run these types of generator required is quite large. • Self excited generators are used in the wind mills. Thus this type of generator helps in converting the unconventional sources of energy into electrical energy AITS, EEE DEPT 45

P. AYUB KHAN, ASSISTANT PROFESSOR Thank You AITS, EEE DEPT 46