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DC MOTORS

DC MOTORS. SEE 3433 ELECTRICAL MACHINES. DC MOTOR. - Shunt motors - Separately excited - Series (we’ll look briefly). DC MOTORS. +. -. +. -. DC MOTOR. DC motor. Load torque opposing the motor torque. + V a -. + V f -. +. -. +. -. T m. T load.

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DC MOTORS

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  1. DC MOTORS SEE 3433 ELECTRICAL MACHINES

  2. DC MOTOR - Shunt motors - Separately excited - Series (we’ll look briefly)

  3. DC MOTORS

  4. + - + - DC MOTOR DC motor

  5. Load torque opposing the motor torque + Va - + Vf - + - + - Tm Tload Mechanical Load: fans, blowers, Compressors, DC MOTOR DC motor

  6. + V - + V - + - + - Tm Tload F = mg DC MOTOR DC motor

  7. DC MOTOR Hoist

  8. DC MOTOR - Some applications require the control the speed - Some applications require the control the torque - In order to control the torque or speed we need to know the T- characteristics of the motor and the mechanical load Intersections between the two characteristics will determine the operating point

  9. If It + Vt  Rcf Ia Ra  Rcw Te Tload Mechanical load DC MOTOR Shunt motor Vt = IaRa + Ea It = Ia + If Ea = k Te = kIa k = Vt - IaRa

  10. Three possible methods of speed control: Field flux Armature voltage Vt Armature resistance Ra

  11. Varying Vt  TL Vt↓ Te Requires variable DC supply

  12. Varying Vt TL Vt↓  Te Requires variable DC supply

  13. Varying Vt TL  Constant TL Te Requires variable DC supply

  14. Varying Vt Vt  Constant TL

  15. Varying Ra Ra↑ Introduction  TL Te Simple control Losses in external resistor

  16. Varying  ↓  TL Te Not possible for PM motor Maximum torque capability reduces

  17. Armature voltage control Field flux control Te Maximum Torque capability  base Method of speed control in DC motor drives Armature voltage control : retain maximum torque capability Field flux control (i.e. flux reduced) : reduce maximum torque capability For wide range of speed control 0 to base  armature voltage, above base  field flux reduction

  18. Te Maximum Torque capability  base

  19. Pmax Constant torque Constant power  base P Te 0 to base  armature voltage, above base  field flux reduction P= EaIa,max = kaIa,max Pmax = EaIa,max = kabaseIa,max    1/

  20. 0 to base  armature voltage, above base  field flux reduction

  21. If It + Vt  Rcf Ia Ra Rcw 0 to base  armature voltage, above base  field flux reduction BUT there are problems !

  22. If It + Vt  Rcf Ia Ra Rcw 0 to base  armature voltage, above base  field flux reduction Controlling Vt will also affect If Controlling If via Rcf caused losses  I2R

  23. DC supply for armature DC supply for field 0 to base  armature voltage, above base  field flux reduction Separately Excited DC motor What if we have an AC supply ?

  24. 3-phase AC source Armature voltage control Field voltage control Separately Excited DC motor AC to DC converter + Vdc - + Vdc - AC to DC converter

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