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Lect 2a Direct Current Motor Drives.pptx

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Lect 2a Direct Current Motor Drives.pptx

  1. 1. Direct Current Motor Drives:
  2. 2. DC DRIVES Vs AC DRIVES DC drives: Advantage in control unit Disadvantage in motor AC Drives: Advantage in motor Disadvantage in control unit
  3. 3. • DC DRIVES: Electric drives that use DC motors as the prime movers • Dominates variable speed applications before PE converters were introduced • DC motor: industry workhorse for decades • Will AC drive replaces DC drive ? – Predicted 30 years ago – AC will eventually replace DC – at a slow rate – DC strong presence – easy control – huge numbers DC DRIVES
  4. 4. DC Motors • Limitations: • Advantage: simple torque and speed control without sophisticated electronics • Regular Maintenance • Expensive motor • Heavy motor • Sparking
  5. 5. General Torque Equation Translational (linear) motion: dt d J T   Rotational motion: dt dv M F  F : Force (Nm) M : Mass (Kg ) v : velocity (m/s) T : Torque (Nm) J : Moment of Inertia (Kgm2 )  : angular velocity ( rad/s )
  6. 6. Torque Equation: Motor drives dt d J T T or dt d J T T L e L e       0   L e T T Acceleration 0   L e T T Deceleration 0   L e T T Constant speed Te : motor torque (Nm) TL : Load torque (Nm)
  7. 7. …continue Drive accelerates or decelerates depending on whether Te is greater or less than TL During acceleration, motor must supply not only the load torque but also dynamic torque, ( Jd/dt ). During deceleration, the dynamic torque, ( Jd/dt ), has a negative sign. Therefore, it assists the motor torque, Te.
  8. 8. Torque Equation: Graphical Te Forward running Speed Forward braking Reverse acc. Reverse running Reverse braking Forward acc.
  9. 9. Load Torque Load torque, TL, is complex, depending on applications. SPEED TORQUE TL = k2 TL = k TL = k In general:
  10. 10. 4Q OPERATION SPEED TORQUE I III II IV  Te  Te  Te  Te FM FB RM RB F: FORWARD R: REVERSE M : MOTORING B: BRAKING
  11. 11. 4Q OPERATION: LIFT SYSTEM Counterweight Cage Motor Positive speed Negative torque
  12. 12. 4Q OPERATION: LIFT SYSTEM Convention: Upward motion of the cage: Positive speed Weight of the empty cage < Counterweight Weight of the full-loaded cage > Counterweight Principle: What causes the motion? Motor : motoring P =T = +ve Load (counterweight) : braking P =T = -ve
  13. 13. You are at 10th floor, calling fully-loaded cage from gnd floor You are at gnd floor, calling empty cage from 10th floor You are at 10th floor, calling empty cage from gnd floor You are at gnd floor, calling Fully-loaded cage from 10th floor Torque Speed FM FB RM RB
  14. 14. DC MOTOR DRIVES Principle of operation Torque-speed characteristic Methods of speed control Armature voltage control Variable voltage source Phase-controlled Rectifier Switch-mode converter (Chopper) 1Q-Converter 2Q-Converter 4Q-Converter
  15. 15. Current in Current out Stator: field windings Rotor: armature windings Principle of Operation DC Motors
  16. 16. Equivalent circuit of DC motor a t i k Te   Electromagnetic torque   E a k e Armature back e.m.f. Lf Rf if a a a a t e dt di L i R v    + ea _ La Ra ia + Vt _ + Vf _ dt di L i R v f f f f  
  17. 17. Torque-speed characteristics a a a a E I R V   In steady state,      T a e T a k V T k R    2 Therefore speed is given by, Three possible methods of speed control: Armature resistance Ra Field flux F Armature voltage Va a a a a a e dt di L i R V    Armature circuit:
  18. 18. Torque-speed characteristics of DC motor Torque Speed Maximum load Torque No load speed Separately excited DC motors have good speed regulation. Full load speed
  19. 19. DC Motor Speed Control Torque Speed Maximum Torque By Changing Ra Ra increasing • Power loss in Ra • Does not maintain maximum torque capability • Poor speed regulation
  20. 20. DC Motor Speed Control Torque Speed Maximum Torque Flux Decreasing Trated • Slow transient response • Does not maintain maximum torque capability By Decreasing Flux
  21. 21. DC Motor Speed Control Torque Speed Maximum Torque By Changing Armature voltage Trated Va increasing • good speed regulation • maintain maximum torque capability
  22. 22. Speed control of DC Motors Below base speed: Armature voltage control (retain maximum torque capability) Above base speed: Field weakening (i.e. flux reduced) (Trading-off torque capability for speed) Torque speed Line of Maximum Torque Limitation Armature voltage control Field flux control base
  23. 23. Methods of Armature Voltage Control Phase-controlled rectifier (AC–DC) T Q1 Q2 Q3 Q4  3-phase Or 1-phase supply + Vt  ia
  24. 24. Methods of Armature Voltage Control 1. Ward-Leonard Scheme 2. Phase-controlled rectifier (AC–DC) 3. Switch-Mode Converter (Chopper) (DC–DC)
  25. 25. Phase-controlled rectifier: 4Q Operation Q1 Q2 Q3 Q4  T 1 or 3- phase supply 1 or 3- phase supply + Vt  Methods of Armature Voltage Control
  26. 26. Phase-controlled rectifier : 4Q Operation Q1 Q2 Q3 Q4  T F1 F2 R1 R2 + Va - 3-phase supply AN ALTERNATIVE WAY
  27. 27. Switch–mode converters: 1Q Converter Q1 Q2 Q3 Q4  T + Vt - T1 Converters For DC motor Drives
  28. 28. Switch–mode converters: 2Q Converter + Vt - T1 D1 T2 D2 Q1 Q2 Q3 Q4  T Q1  T1 and D2 Q2  D1 and T2 Converters For DC motor Drives
  29. 29. Switch–mode converters: 4Q Converter Q1 Q2 Q3 Q4  T + Vt - T1 D1 T2 D2 D3 D4 T3 T4 Converters For DC motor Drives
  30. 30. Switch–mode converters • Switching at high frequency  Reduces current ripple  Increases control bandwidth • Suitable for high performance applications Advantages of Switch mode converters

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