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# DC motors

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DC motors

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### DC motors

1. 1. DC Motor Characteristics Pekik Argo DahonoInstitute of Technology Bandung
2. 2. DC Motor Constructionva vf Pekik Argo Dahono 2
3. 3. DC Motor Features• Depending on the connection between armature and field windings, a dc motor can be classified as separately excited and series excited dc motors.• In small dc motors, the field can be permanent magnet.• The armature is usually located on the rotor and the field is on the stator.• Due to the action of commutator and brush, the mmf generated by the armature current is perpendicular to the flux generated by field winding.• At present, most of dc motor applications are replaced by ac motors. Pekik Argo Dahono 3
4. 4. Separately Excited DC Motor ia Ra La Tl Lf va Ea J Rf Te vf if Pekik Argo Dahono 4
5. 5. General Expressions ofSeparately Excited DC Motor dia va  Ra ia  La  ea dt ea  K di f vf  Rf if  Lf dt    i f  d Te  Kia  Tl  B  J dt Pekik Argo Dahono 5
6. 6. Separately Excited DC Motor Tl (s ) _  1 I a (s )  1  (s )Va (s ) Ra  sLa Te (s ) sJ _ E a (s ) The system is nonlinear I f (s ) V f (s )  (s ) 1  i f  K R f  sL f Pekik Argo Dahono 6
7. 7. Constant-Field Block Diagram of DC Motors Tl (s ) _  1 I a (s )  1  (s )Va (s ) K Ra  sLa Te (s ) sJ _ E a (s ) K Pekik Argo Dahono 7
8. 8. Motor Characteristics• The system is linear when the field is constant.• Under this condition, the torque is proportional to armature current. Pekik Argo Dahono 8
9. 9. Transfer Function K sLa  Ra (s)  V (s)  Tl ( s ) s JLa  sJRa  K  s JLa  sJRa  K  2 2 a 2 2  Ra  K /  JLa / JLa Va ( s )  s    La  / J  Tl ( s ) 2 2  K    K  s  s Ra / La    2 s  s Ra / La    2   JLa   JL     a  2 o / JLa s    / J T ( s) Va ( s )  2 s  s  o 2 s  s  o 2 l  Ra / La Ko  JLa Pekik Argo Dahono 9
10. 10. Steady-State Expressions Va  Ra I a  K Va K Ia   Ra Ra Te  Tl  KI a Va Ra Va Ra   Ia   Te K K  K K 2 Pekik Argo Dahono 10
11. 11. Speed Control• The speed is proportional to armature voltage.• The speed can also be controlled by armature resistance. The armature resistance can also be used to limit the starting current.• The starting current is independent to the flux or field current.• The field can be reduced to increase the speed at the expense of reduced torque/armature- current. Pekik Argo Dahono 11
12. 12. Armature Resistance Control  Va Ra  V Ia  a  Ra T Ra K K K K 2 e Te Pekik Argo Dahono 12
13. 13. Armature Voltage Control  Va Te Va Ra V Ra  Ia  a  T K K K K 2 e Pekik Argo Dahono 13
14. 14. Field Control   Va Ra V Ra  Ia  a  T K K K K 2 e Te Pekik Argo Dahono 14
15. 15. Control of Separately Excited DC Motor Constant Torque Constant Power Va If Te  KI a P  Te  Pekik Argo Dahono 15
16. 16. Control Characteristics• Below base speed, the speed is controlled by the armature voltage at maximum field current (to maintain maximum torque/armature-current or constant torque capability).• Above base-speed, the speed can only be increased by reducing the field current (constant power capability). The maximum speed is limited by the mechanical capability and armature reaction. Pekik Argo Dahono 16
17. 17. Four-Quadrant Operation  Te Pekik Argo Dahono 17
18. 18. Braking of DC Motors• Dynamic braking : The kinetic energy is dissipated to the braking resistor.• Counter current braking or plugging: The kinetic energy is dissipated in the machine and current- limiting resistor. The current can be so high with braking method.• Regenerative braking. The kinetic energy is sent back to the source. This method can only be used when the source is able to receive the regenerated energy. Pekik Argo Dahono 18
19. 19. Control of Separately Excited DC Motor AC source M G M Pekik Argo Dahono 19
20. 20. Ward-Leonard Characteristics• Fully four-quadrant operation is possible.• Below base speed, the motor field current is constant at maximum value and the speed is controlled by the generator field current.• Above base speed, the motor speed is increased by reducing the motor field current.• If the ac motor is a synchronous motor, the ac power factor is controllable.• No harmonics are generated.• The system is large, expensive, less responsive, and needs a lot of maintenance. Pekik Argo Dahono 20
21. 21. Series DC Motors• This motor is commonly used in dc electric traction.• This motor can be used as ac comutator motor.• This motor can be operated as a universal motor. Pekik Argo Dahono 21
22. 22. Steady-State Characteristics • Speed can be controlled byIa  I f controlling the armature voltage.Assume   K f I a • Speed can also be controlledTe  Tl  KK f I a 2 by using the armature resistance and field diverter. Va  Ra I a Va Ra • In electric traction, series   K KK f I a KK f parallel connection is also used to control the speed. Va Ra  • A low frequency ac supply is KK f Te KK f desirable if the motor is operated as a series ac commutator motor. Pekik Argo Dahono 22
23. 23. Braking Methods• Mechanical Braking• Counter plug Te  Kia• Dynamic braking• Regenerative Braking Pekik Argo Dahono 23
24. 24. Counter Plug va  Eia  RaTe   Kia Pekik Argo Dahono 24
25. 25. Dynamic Braking Eia  Rb  RaTe   Kia Pekik Argo Dahono 25
26. 26. Regenerative Braking E  vaia  RaTe   Kia Pekik Argo Dahono 26
27. 27. The End