control dc motors by three phase convertes
- 1. power Semiconductor Drives U nit-2 1 Unit -2 Control DC Motors by Three Phase Converters
- 2. power Semiconductor Drives U nit-2 2 LECTURE PLAN Sl. No. Module as per Session Planner Lectu re No. Slide No. From-To 1 Introduction to three phase converters L 1 3-9 2 Three - phase semi controlled rectifier control of D.C. separately excited motor L2 10-20 3 Three phase fully controlled rectifier control of d.c. separately excited motor L3 21-29 4 Three - phase semi controlled rectifier control of D.C. series motor L 4 30-38 5 Speed torque Characteristics For different values of firing angle L 5 39-50 6 Three - phase Fully controlled rectifier control of D.C. series motor L 6 & L7 51-60 7 Problems L 8 61-63
- 3. power Semiconductor Drives U nit-2 3 Lecture-1 Introduction to three phase converters
- 4. power Semiconductor Drives U nit-2 4 4 3 Phase Controlled Rectifiers • Operate from 3 phase ac supply voltage. • They provide higher dc output voltage as compared to 1-Phase controlled rectifier. • Higher dc output power. • Higher output voltage ripple frequency. • Filtering requirements are simplified for smoothing out load voltage and load current.
- 5. power Semiconductor Drives U nit-2 5 5 • Extensively used in high power variable speed industrial dc drives. • In three phase drives, the armature current is mostly continuous and therefore the motor performance is better compared to that of single phase drive. Contd..
- 6. power Semiconductor Drives U nit-2 6 Three-phase Half wave Converter Drives
- 7. power Semiconductor Drives U nit-2 7 Contd… • This type of converter is employed for motors in the typical range of 10-50 HP. • This drive is impractical for most industrial applications because supply currents would contain D.C components. • With a three phase half wave converter the armature circuit gives armature voltage as
- 8. power Semiconductor Drives U nit-2 8 Contd.. • Where Vm is the peak of the phase voltage of a star connected three phase A.C supply • If then
- 9. power Semiconductor Drives U nit-2 9 Lecture-2 Three - phase semi controlled rectifier control of D.C. separately excited motor
- 10. power Semiconductor Drives U nit-2 10 Three - phase semi controlled rectifier control of D.C. separately excited motor
- 11. power Semiconductor Drives U nit-2 11 Contd… • This is One quadrant drive i. e Only 1st quadrant operation is possible . • It is limited to application in the range of 15-50 HP • The field converter should also be a single phase or three phase semi- converter
- 12. power Semiconductor Drives U nit-2 12 Voltage across Load and current through load for different firing angle
- 13. power Semiconductor Drives U nit-2 13 Contd..
- 14. power Semiconductor Drives U nit-2 14 Contd…
- 15. power Semiconductor Drives U nit-2 15 Contd.. • In 3- phase semi converter, SCRs are gated at interval of 1200 in a proper sequence • At large firing angles the motor current may be continuous or discontinuous, depending on the current demand and speed
- 16. power Semiconductor Drives U nit-2 16 Contd.. • A 3- phase semi converter has the unique feature of working as a six pulse converter for and as a three pulse converter for
- 17. power Semiconductor Drives U nit-2 17 Torque-speed characteristics
- 18. power Semiconductor Drives U nit-2 18 Contd.. • If the motor current is continuous, then the motor armature voltage is given by
- 19. power Semiconductor Drives U nit-2 19 Speed-Torque Characteristics for continuous conduction mode for different firing angle
- 20. power Semiconductor Drives U nit-2 20 Lecture-3 Three phase fully controlled rectifier control of d.c. separately excited motor
- 21. power Semiconductor Drives U nit-2 21 Three phase fully controlled rectifier control of D.C. separately excited motor
- 22. power Semiconductor Drives U nit-2 22 22 Out put Voltage and current waveform
- 23. power Semiconductor Drives U nit-2 23 23 Contd…
- 24. power Semiconductor Drives U nit-2 24 24 • The thyristors are triggered at an interval of / 3. • The frequency of output ripple voltage is 6fS. • T1 is triggered at t = (/6 + ), T6 is already conducting when T1 is turned ON. • During the interval (/6 + ) to (/2 + ), T1 and T6 conduct together & the output load voltage is equal to vab = (van – vbn) Contd…
- 25. power Semiconductor Drives U nit-2 25 25 • T2 is triggered at t = (/2 + ), T6 turns off naturally as it is reverse biased as soon as T2 is triggered. • During the interval (/2 + ) to (5/6 + ), T1 and T2 conduct together & the output load voltage vO = vac = (van – vcn) • Thyristors are numbered in the order in which they are triggered. • The thyristor triggering sequence is 12, 23, 34, 45, 56, 61, 12, 23, 34, ……… Contd…
- 26. power Semiconductor Drives U nit-2 26 Torque-speed characteristics
- 27. power Semiconductor Drives U nit-2 27 Contd.. • If the motor current is continuous, then the motor armature voltage is given by
- 28. power Semiconductor Drives U nit-2 28 TWO QUADRANT OPERATION OF A 3- PHASE FULL CONVERTER
- 29. power Semiconductor Drives U nit-2 29 Speed-Torque Characteristics for continuous conduction mode for different firing angle
- 30. power Semiconductor Drives U nit-2 30 Lecture-4 Three - phase semi controlled rectifier control of D.C. series motor
- 31. power Semiconductor Drives U nit-2 31 Three - phase semi controlled rectifier control of D.C. series motor
- 32. power Semiconductor Drives U nit-2 32 Voltage across Load and current through load for different firing angle
- 33. power Semiconductor Drives U nit-2 33 Voltage across Load and current through load for different firing angle
- 34. power Semiconductor Drives U nit-2 34 Contd..
- 35. power Semiconductor Drives U nit-2 35 Contd.. • In 3- phase semi converter, SCRs are gated at interval of 1200 in a proper sequence • At large firing angles the motor current may be continuous or discontinuous, depending on the current demand and speed
- 36. power Semiconductor Drives U nit-2 36 Contd.. • A 3- phase semi converter has the unique feature of working as a six pulse converter for and as a three pulse converter for
- 37. power Semiconductor Drives U nit-2 37 Contd.. • If the motor current is continuous, then the motor armature voltage is given by
- 38. power Semiconductor Drives U nit-2 38 Variation of Output voltage with firing angle
- 39. power Semiconductor Drives U nit-2 39 Lecture-5 Speed torque Characteristics For different values of firing angle
- 40. power Semiconductor Drives U nit-2 40 Speed torque Characteristics For different values of firing angle • In case of series motor field circuit is connected in series with the armature and motor terminal voltage is controlled by semi converter or fully controlled converter. • Series motors are particularly suited for applications that require high starting torque such as crane, hoists, elevators, vehicles etc..
- 41. power Semiconductor Drives U nit-2 41 Contd… • Inherently, series motor can provide essentially constant power output and therefore particularly suitable for traction drives • Speed control is very difficult with the series motor because any change in the load current will immediately be reflected in a speed change and hence , all speed control systems will use separately excited system.
- 42. power Semiconductor Drives U nit-2 42 Contd… • For series motor total resistance R = (Ra +Rf ) which includes both series field winding resistance and armature winding resistance. • Similarly, L = ( La + Lf) which includes both series field winding inductance and armature winding inductance . • Back emf is expressed by the equation, e = Ka Ф n ……….. (1)
- 43. power Semiconductor Drives U nit-2 43 Contd.. • The flux φ has two components , One component Фa which is produced by the armature current flowing through the series field winding. The other component Фres is due to the residual magnetism. • Фres is very small and can be assumed constant. • Therefore, Ф = Фa + Фres ……………(2)
- 44. power Semiconductor Drives U nit-2 44 Contd. • If linearity is assumed , Фa = Kf ia ……….(3) • Substituting eq (2) and eq (3) in eq (1), we ge te = Ka ( Kf ia + Фres) n = Ka Kf ia n + Ka Фres n --------------- (4) • Average back emf is given by E = Kaf Ia N + Kres N ---------- (5) • The torque developed by the motor is given by t = Ka Ф ia --------------------- (6)
- 45. power Semiconductor Drives U nit-2 45 Contd.. • If the flux Фres is neglected, then from eq(2), eq(3), and from eq(6) • t = Kaf ia 2 --------------------(7) • Torque is therefore, developed in the same direction for either direction of current .Hence, speed reversal in the series motor can be achieved by reversing either field winding or the armature terminals but not both.
- 46. power Semiconductor Drives U nit-2 46 Contd.. • The average developed torque is T = Kaf Ia 2 ------------------ (8) • The voltage equation of the armature circuit is given by va = R . ia + La . (dia / dt ) + e ------- (9) • The average armature voltage is given by Va = Ra . Ia + E ----------------- (10)
- 47. power Semiconductor Drives U nit-2 47 Contd… • Replacing the average value of back emf (E) from eq(5) Va = Ra . Ia + Kaf Ia N + Kres N ---- (11) Now the speed from above equation becomes
- 48. power Semiconductor Drives U nit-2 48 Contd… • Neglecting the ripple in the motor current T = Kaf Ia 2 ------------- (13) • Therefore , From eq (11) • Substituting the value of Ia in Eq(8)
- 49. power Semiconductor Drives U nit-2 49 Contd.. • Substituting the value of Ia in Eq(12)
- 50. power Semiconductor Drives U nit-2 50 Speed torque Characteristics For different values of firing angle
- 51. power Semiconductor Drives U nit-2 51 Lecture-6 & Lecture-7 Three - phase Fully controlled rectifier control of D.C. series motor
- 52. power Semiconductor Drives U nit-2 52 Three - phase Fully controlled rectifier control of D.C. series motor
- 53. power Semiconductor Drives U nit-2 53 53 Out put Voltage and current waveform
- 54. power Semiconductor Drives U nit-2 54 54 Contd…
- 55. power Semiconductor Drives U nit-2 55 55 • The thyristors are triggered at an interval of / 3. • The frequency of output ripple voltage is 6fS. • T1 is triggered at t = (/6 + ), T6 is already conducting when T1 is turned ON. • During the interval (/6 + ) to (/2 + ), T1 and T6 conduct together & the output load voltage is equal to vab = (van – vbn) Contd…
- 56. power Semiconductor Drives U nit-2 56 56 • T2 is triggered at t = (/2 + ), T6 turns off naturally as it is reverse biased as soon as T2 is triggered. • During the interval (/2 + ) to (5/6 + ), T1 and T2 conduct together & the output load voltage vO = vac = (van – vcn) • Thyristors are numbered in the order in which they are triggered. • The thyristor triggering sequence is 12, 23, 34, 45, 56, 61, 12, 23, 34, ……… Contd…
- 57. power Semiconductor Drives U nit-2 57 Contd.. • If the motor current is continuous, then the motor armature voltage is given by
- 58. power Semiconductor Drives U nit-2 58 TWO QUADRANT OPERATION OF A 3- PHASE FULL CONVERTER
- 59. power Semiconductor Drives U nit-2 59 Speed-Torque Characteristics We know, for series motor Replacing the value of Va for fully controlled converter
- 60. power Semiconductor Drives U nit-2 60 Speed-Torque Characteristics for continuous conduction mode for different firing angle
- 61. power Semiconductor Drives U nit-2 61 Lecture-8 Problems
- 62. power Semiconductor Drives U nit-2 62 Problems • The speed of a 150 HP, 650V, 1750 rpm, separately excited D.C motor is controlled by 3- phase, full converter. The converter is operating from a 3-phase, 460v, 50Hz supply. The rated armature current of the motor is 170A. The motor parameters are Ra= 0.099 Ohms, La= 0.73 mH, and Ka Ф = 0.33 V/ rpm. Neglect the losses in converter system. Determine (a) No load speeds at firing angle = 00 and 300 . Assume that at no-load, the armature current is 10% of the rated current and is continuous.
- 63. power Semiconductor Drives U nit-2 63 Contd.. (b) The firing angle to obtain rated speed of 1750 RPM at rated motor current. Also compute the supply power factor. (c) The speed regulation or the firing angle obtained in part (b)