Dcmachine

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Dcmachine

  1. 1. Direct-Current Machine
  2. 2. Electric Machine <ul><li>Electric machines can be used as motors and generators </li></ul><ul><li>Electric motor and generators are rotating energy-transfer electromechanical motion devices </li></ul><ul><li>Electric motors convert electrical energy to mechanical energy </li></ul><ul><li>Generators convert mechanical energy to electrical energy </li></ul>
  3. 3. Electric Machine <ul><li>Electric machines can be divided into 2 types: </li></ul><ul><ul><ul><li>AC machines </li></ul></ul></ul><ul><ul><ul><li>DC machines </li></ul></ul></ul><ul><li>Several types DC machines </li></ul><ul><ul><ul><li>Separately excited </li></ul></ul></ul><ul><ul><ul><li>Shunt connected </li></ul></ul></ul><ul><ul><ul><li>Series connected </li></ul></ul></ul><ul><ul><ul><li>Compound connected </li></ul></ul></ul><ul><ul><ul><li>Permanent magnet </li></ul></ul></ul>
  4. 4. Electric Machine <ul><li>All Electric machines have: </li></ul><ul><ul><ul><li>Stationary members (stator) </li></ul></ul></ul><ul><ul><ul><li>rotating members (rotor) </li></ul></ul></ul><ul><ul><ul><li>Air gap which is separating stator and rotor </li></ul></ul></ul><ul><li>The rotor and stator are coupled magnetically </li></ul>
  5. 5. <ul><li>Schematic representation of a DC Machine </li></ul>DC Machines Stator Rotor + V f - I f I f I f N  f 2 S
  6. 6. Electric Machine <ul><li>The armature winding is placed in the rotor slot and connected to rotating commutator which rectifies the induced voltage </li></ul><ul><li>The brushes which are connected to the armature winding, ride on commutator </li></ul>
  7. 7. DC Machines <ul><li>Elementary two-pole DC Machine </li></ul>
  8. 8. Electric Machine <ul><li>The armature winding consists of identical coils carried in slots that are uniformly distributed around the periphery of the rotor </li></ul><ul><li>Conventional DC machines are excited by direct current, in particular if a voltage-fed converter is used a dc voltage uf is supplied to the stationary field winding </li></ul><ul><li>Hence the excitation magnetic field is produced by the field coils </li></ul><ul><li>Due to the commutator, armature and field windings produce stationary magnetomotive forces that are displaced by 90 electrical degrees </li></ul>
  9. 9. <ul><li>The field winding is placed on the stator and supplied from a DC Source. </li></ul>DC Machines Rotor N  f 2 S Armature Winding x x x x x x x x
  10. 10. Magnetic Flux in DC machines  f /2 rotor stator I f S N V f + - . . . . . . x x x x x x Armature Winding I f  a
  11. 11. <ul><li>The current is induced in the Rotor Winding ( i.e. the Armature Winding ) since it is placed in the field ( Flux Lines ) of the Field Winding. </li></ul>DC Machines  f
  12. 12. <ul><li>mmf produced by the armature and mmf produced by the field winding are orthogonal. </li></ul>Orthogonality of Magnetic Fields in DC Machines B I L F Magnetic field due to field winding Magnetic field due to armature winding 90 o
  13. 13. <ul><li>The force acting on the rotor, is expressed as </li></ul>DC Machines l f f T e T e = x f l
  14. 14. <ul><li>The Field winding is placed on the stator and the current (voltage) is induced in the rotor winding which is referred also as the armature winding. </li></ul><ul><li>In DC Machines, the mmf produced by the field winding and the mmf produced by the armature winding are at right-angle with respect to each other. </li></ul><ul><li>The torque is produced from the interaction of these two fields. </li></ul>DC Machines
  15. 17. Transducer with stator and rotor windings
  16. 18. Equivalent circuit for separately excited DC motors SEPARATELY EXCITED DC MOTORS
  17. 19. Electric Machine <ul><li>Conventional separately excited DC electric machine </li></ul><ul><ul><ul><li>Stator and rotor windings excited by dc current </li></ul></ul></ul><ul><ul><ul><li>The rotor has the commutator </li></ul></ul></ul><ul><ul><ul><li>Dc voltage to the armature windings is supplied through the brushes which establish electric contact with the commutator </li></ul></ul></ul><ul><ul><ul><li>The brushes are fixed with respect to the stator and they are placed in the specified angular displacement </li></ul></ul></ul><ul><ul><ul><li>To maximize the electromagnetic torque, the stator and rotor magnetic axes are displaced by 90 electrical degrees using a commutator </li></ul></ul></ul>
  18. 20. Electric Machine <ul><li>Electric machine can be either a motor or a generator depending on whether it drives a load or it is driven by a prime mover </li></ul><ul><li>The direction of the armature current is reversed when an electric machine changes from motor to generator operation </li></ul><ul><li>However line voltage polarity, direction of rotation and field current are the same </li></ul><ul><li>( MOTOR ) If is greater than , the armature current is positive </li></ul><ul><li>( GENERATOR ) If is greater than , the armature current is negative </li></ul>
  19. 21. Electric Machine <ul><li>Conventional separately excited DC electric machine </li></ul><ul><ul><ul><li>Using kirchhoff’s second law and assuming the differential equation of a motor </li></ul></ul></ul><ul><ul><ul><li>In motor application, the output is the angular velocity </li></ul></ul></ul>
  20. 22. Equivalent circuit for separately excited DC generators SEPARATELY EXCITED DC GENERATORS
  21. 23. Electric Machine <ul><li>Conventional separately excited DC electric machine </li></ul><ul><ul><ul><li>Using kirchhoff’s second law and assuming the differential equation of a generator </li></ul></ul></ul><ul><ul><ul><li>The steady state operating condition for a generator are </li></ul></ul></ul><ul><ul><ul><li>In generator application, the output is the voltage induced </li></ul></ul></ul>
  22. 24. DC Machines Energy stored in inductor is stored in the magnetic field within the coil The mutual inductance between the armature and field windings <ul><li>The armature and field magnetic axes are displaced by 90 electrical degrees and the magnetizing reluctance is constant </li></ul>
  23. 25. DC Machines <ul><li>The torque equation </li></ul><ul><li>Electromagnetic power </li></ul><ul><li>Given that </li></ul><ul><li>Therefore </li></ul><ul><li>Electromotive force formula is given as </li></ul><ul><li>Substituting (2) into (1), yields </li></ul>using and Steady state relationship between the angular velocity end electromagnetic torque
  24. 26. <ul><li>The DC Machine Dynamic Equations for the circuit represented bellow is </li></ul>DC Machines
  25. 27. DC Machines <ul><li>The flux linkage equations are: </li></ul>Where L ff = field self-inductance L aa = armature self-inductance L af = mutual inductance between the field and rotating armature coils
  26. 28. DC Machines - Shunt Connected <ul><li>The Shunt Configuration for a DC Machine is as shown below, </li></ul>
  27. 29. DC Machines - Shunt Connected <ul><li>The Dynamic Equations (assuming r f ext = 0 ) are follows, </li></ul>Where L ff = field self-inductance L fa = mutual inductance between the field and rotating armature coils e a = induced voltage in the armature coils (also called counter or back emf )
  28. 30. DC Machines - Shunt Connected <ul><li>The torque equation for a Shunt Connected DC-Machine is </li></ul>
  29. 31. DC Machines - Shunt Connected <ul><li>For DC Machines , </li></ul>mmf field mmf armature + -

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