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DC MOTORS

DC MOTORS . SEE 3433 ELECTRICAL MACHINES. DC MOTOR. - Shunt motors - Separately excited - Starter . 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 - Starter

  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. Hoist DC MOTOR

  8. - Some applications require the control the speed DC MOTOR - 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 Shunt motor Vt = IaRa + Ea DC MOTOR 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 Ra Ra↑  TL Te Simple control Losses in external resistor

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

  14. 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

  15. Te Maximum Torque capability  base

  16. 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/

  17. 0 to base  armature voltage, above base  field flux reduction

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

  19. 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

  20. 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 ?

  21. 3-phase AC source Armature voltage control Field voltage control 0 to base  armature voltage, above base  field flux reduction Separately Excited DC motor AC to DC converter + Vdc - + Vdc - AC to DC converter

  22. Starter in DC Motor • At stand-still, w = 0  Ea = 0 Ra  Ia + Vt – eg, Vt = 100, Ra = 0.1  Ia = 1000 A !

  23. Starter in DC Motor • We can limit Ia at start-up by: 1) Controlling Vt using variable supply – e.g. using power electronics converter 2) Adding external resistor  known as starter Ra  Ia + Vt – + Ea – When Ea = 0 Rst • As speed builds up (so too Ea), Rst is gradually reduced

  24. Starter in DC Motor • As speed builds up (so too Ea), Rst is gradually reduced Ia Starter circuit 4 3 2 1 Imax Imin 1 2 3 4 t (s) speed t (s)

  25. Starter in DC Motor Practical Starter circuit

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