Electrical Machines LSEGG216A 9080V Content of Course • • • • • • • • • • • • • • • Transformer Construction Transformer Operation Transformer Losses, Efficiency & Cooling Transformer Voltage Regulation & % Impedance Parallel Operation & Auxiliary Equipment Auto Transformers & Instrument Transformers 3 Induction Motors Operating Principles 3 Induction Motors Construction 3 Induction Motors Characteristics 1 Induction Motors Split Phase 1 Induction Motors Capacitor & Shaded pole 1 Motors Universal Motor Protection 3 Synchronous Machines Alternators & Generators Assessment Theory Test 1 Theory Test 2 Practical Test Quizzes Theory Test 3 10 15 25 10 40 MUST PASS Transformer Construction Introduction       Describe the construction of the various types of lamination style and core construction used in single phase, three phase, auto and instrument transformers. Identify the different winding styles/types used in transformers. State the methods used to insulate low and high voltage transformers. Describe the construction of transformer tanks for distribution transformers. List the types of information stated on transformer nameplates. Perform basic insulation resistance, continuity and winding identification tests Transformer Uses Changing Isolation • • • Voltage Levels Current Levels Impedance values Transformer Operation  Primary coil is supplied with a AC voltage.  Current drawn produces a magnetic field  Magnetic field transported to a secondary coil via a magnetic circuit  Magnetic field induces a voltage in secondary coil V+ V+ Transformer Operation  Primary coil normally has a subscript of 1  Secondary coil has a subscript of 2 I1 V1 I2 V2 Core Types • • • • • • Core Construction Steel type Laminations core type Shell type Toroidal Core Type One Magnetic Circuit Shell Type Two Magnetic Circuits Toroidal Core Laminations Why not just solid steel? Eddy Currents Why do we laminate the core? I S Why do we laminate the core? Eddy currents are large & losses are great Large Number of flux lines cut High voltage generated across core I S Why do we laminate the core? Eddy currents are small & losses are reduced Small Number of flux lines cut Low voltage generated across core I S Losses due to Eddy Currents Pe  K e f Bm t 1  2 Pe = losses in W/m3 Ke = Constant F = Frequency Bm = Maximum Flux density t1 = Lamination thickness Hysteresis Curve • Bigger the area covered, the more losses associated with Iron losses Steel Types Silicon steel is used for laminations Silicon content 0 – 6.5% Why Silicon? • Small hysteresis curve area • Increases electrical resistivity Reduced eddy current size • Hardened grain structure • Reduced workability • Very low carbon levels <0.005% are called for or magnetic ageing will take place Losses will increase with age • Carbon can be removed by annealing in a hydrogen rich atmosphere Grain Orientation • • • • Optimum properties are developed in the rolling direction Magnetic density is increased by 30% in the coil rolling direction Magnetic saturation is decreased by 5% Given codes such as M-0, M1, M-2, M-3, M-4 and M-6 Non-orientated • Similar magnetic properties in all directions • less expensive • Used in applications where the direction of magnetic flux is changing (motors and generators) • Given codes from M-15 to M-47 Grain Size The larger the grain the less the hysteresis losses 2-10 W/kg @ 60 Hz and 1.5 tesla magnetic field strength are common with a 150μm grain size heat treatment increases the average crystal size Excessive bending, incorrect heat treatment, or even rough handling of core steel can adversely effect its magnetic properties Amorphous Steel losses up to 30% of conventional steels Made by pouring molten alloy steel on a rotating cooled wheel. This cools the metal so quickly that crystals do not form • high cost (about twice that of conventional silicon steel) • lower mechanical properties Lamination Coatings • Increase electrical resistance between laminations • Provide resistance to corrosion • Act as a lubricant during die cutting • • • • Can be organic or inorganic (such as Magnesium oxide) Dependant on the heat treatment of the laminations Wheather it is immersed in oil The working temperature of the finished item Magnetostriction A property of ferromagnetic materials that causes them to change their shape when subjected to a magnetic field first identified in 1842 by James Joule When a magnetic field is applied, the boundaries between the domains shift and the domains rotate, both these effects causing a change in the material's dimensions losses due to frictional heating The effect is responsible for the familiar "electric hum" Winding types • Three types? • Magnetic leakage Concentric Higher voltage closest to Iron Winding types Sandwich or Pancake Very high voltages on both windings Winding types Side by Side Very good insulation between windings Insulation of windings • Lacer • Oil • Traditionally a highly-refined (naphthenic) mineral oil • Polychlorinated Biphenyls PCBs Transformer Tanks Nameplate Details