Variable Frequency Drives

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VFDs, Basic operation, modes of operation and parameterization

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Variable Frequency Drives

  1. 1. Variable Frequency Drives Saqib Saeed Graduate Trainee Engineer (E&I) - Electrical
  2. 2. Contents • Introduction • Block Diagram • Building blocks • Modes of operation • VFD Parameters • Some Potential Problems • Harmonics and THD • Recent Improvements in the FFCL system
  3. 3. Variable Frequency Drives – Standard motors are constant speed and when they are energized they run at a 100% speed no matter the load. – What if the speed of the driven machine (Fan, Pump) is to be changed?
  4. 4. What is a VFD? – Variable Frequency Drive (VFD) – Governing Equation of motor speed Speed= 120 x f /P • P=No. of poles • F=Line Frequency – How to change line frequency? Constant =50Hz
  5. 5. Block Diagram VFD Fundamentals 50 Hz Power Electrical Energy ABB Variable Frequency To Motor VFD RECTIFIER (AC - DC) INVERTER (DC - AC) AC DC AC VFD Variable Frequency 50 Hz
  6. 6. VFD Explored  First, the Converter (usually a diode rectifier) converts three-phase AC power to DC power.  Next, the DC Bus stores and filters the DC power in a large bank of capacitors.  Last, the Inverter (usually a set of six IGBTs) switches or inverts the DC power in a Pulse Width Modulated (PWM) AC waveform to the motor.
  7. 7. Rectifier • Basic Building blocks of rectification – Diodes (Uncontrolled)
  8. 8. Thyristors • Controlled • Output voltage can be controlled Gate Pulse Conduction after Gate Pulse
  9. 9. Three Phase Rectifiers
  10. 10. Output Voltage
  11. 11. IS it a perfect Direct Current? • Conversion of AC into DC a perfect process? – Ripples • How to eliminate the ripples? – Filters DC bus in VFD
  12. 12. Inverter Action • Switching DC voltage ON and OFF will make it AC • Filtered output from DC bus is sent to inverter in VFD
  13. 13. RECTIFIER Positive DC Bus Negative DC Bus + - INVERTER How switching can convert DC into AC?
  14. 14. RECTIFIER Positive DC Bus Negative DC Bus + - INVERTER
  15. 15. RECTIFIER Positive DC Bus Negative DC Bus + - INVERTER
  16. 16. RECTIFIER Positive DC Bus Negative DC Bus + - INVERTER
  17. 17. RECTIFIER Positive DC Bus Negative DC Bus + - INVERTER
  18. 18. Pulse width modulation • Such a waveform is not acceptable – Nowhere near Sine wave • Contains harmonics – Multiples of fundamentals • Solution – Pulse width modulation
  19. 19. RECTIFIER Positive DC Bus Negative DC Bus + - INVERTER How Often You Switch From Positive Pulses To Negative Pulses Determines The Frequency Of The Waveform Frequency Voltage
  20. 20. Area Under the Curve
  21. 21. Basic Purpose achieved • Speed of the motor now can be controlled • Is changing motor frequency alone enough?
  22. 22. V/F Control Mode • Flux = V/F 0 230 400 Volts Hertz 25 50 400 V 50 Hz = 8 V Hz 230 V 50 Hz = 4.6 V Hz If 230 VAC Power Line:
  23. 23. V/F Control Mode • Scalar mode • Drive is unaware of what is happening in the motor Example: • A 400V scalar drive is told to run a 400V, 50 Hz motor at 50% speed Following V/F pattern, Voltage applied by the drive will also be half. • Perfect when at no load. • After loading, motor will run at less than 50% speed • Drive is unaware of it Solution Vector Control
  24. 24. MotorDrive Speed setting=50% Speed at no load=50% Load increase
  25. 25. MotorDrive Speed setting=50% Speed=40% No idea what is happening
  26. 26. Vector Control Mode • Sensor less vector control mode – No feedback through speed sensor – Feedback is derived through motor terminals – Drive need to go through “Auto tuning” • Vector control with sensor – Feedback through encoder – Better speed regulations up to 0.01% – Faster response to load variations
  27. 27. VFD input Parameters • Max./Base frequency setting • Motor rated output • Motor rated voltage • Motor rated current • Carrier frequency
  28. 28. VFD input Parameters • Frequency Reference setting methods • Stop Command method • Start frequency • Stop frequency (DC Braking starts) • Torque Boost • Frequency Skip
  29. 29. VFD input Parameters • Max./Base frequency setting
  30. 30. VFD input Parameters • Motor Rated Output • Motor Rated Voltage
  31. 31. VFD input Parameters • Motor rated Current • Carrier Frequency
  32. 32. Frequency Reference Setting Methods • Potentiometer • 0-10V input voltage • 4-20(mA)
  33. 33. Frequency Reference Setting through Potentiometer Min Max Min Max Frequency
  34. 34. Centrifuge Main Control Panel Potentiometer
  35. 35. Start/Stop Frequency/Acceleration/Deceleration Frequency Start Frequency Acceleration Time DC Brake Stop Frequency Deceleration Time Time
  36. 36. Start/Stop Frequency/Acceleration/Deceleration Frequency Start Frequency Acceleration Time DC Brake Stop Frequency Deceleration Time Time
  37. 37. Stop Command methods • Coast to stop • Ramp to stop
  38. 38. Coast to stop Coast to Stop Frequency Run Command Motor speed Command is removed
  39. 39. Ramp to stop Frequency Run Command Command is removed DC Brake Ramp to Stop
  40. 40. Torque Boost Voltage Frequency Rated Frequency Rated Voltage Voltage Boost
  41. 41. Frequency Skip
  42. 42. Auto Tuning • Drive familiarizing itself with motor VFD here I am IM
  43. 43. Auto Tuning MotorDrive Start of Auto tuning Primary resistanceLeakage reactanceDC Brake Voltage Torque Boost Voltage Slip compensation
  44. 44. Auto Tuning Procedure Auto tuning procedure Preparation Turn Power ON Start VAT 300 Select the control mode Motor ratings Can motor rotate? yes NoInput 1: Simple adjustment mode Input 2: High adjustment mode
  45. 45. Input 1: Simple adjustment mode Input 2: High adjustment mode LED flickers Start Auto-Tuning Press Fwd Revor RUN LED ON Auto-tuning End 10s for V/F mode 1min. for vector mode Auto Tuning Procedure Contd.
  46. 46. DC Injection Braking • No mechanical Contact • DC is applied at the stator winding • DC causes stator to be become a magnet with constant field • A voltage is induced inside the rotor causing current to flow • According to Lens’s law, this current will cause rotor to stop
  47. 47. Dynamic Braking • Concept of Braking – Kinetic energy keeping the object moving – Energy cannot be destroyed but can be converted Kinetic Energy Heat Energy Mechanical Brakes Wear and Tear
  48. 48. Dynamic Braking • Some other form of energy – Electrical Kinetic Energy Electrical Energy DiscardUtilize Resistive Elevators Regenerative Electric railcars
  49. 49. Resistive Dynamic Braking in Elevators
  50. 50. Braking resistor for CAN Elevator VFD Braking Unit
  51. 51. Resistive Dynamic Braking • A built in dynamic braking resistor (<15Kw) • An external DB unit
  52. 52. Potential Problems • Harmonic Distortion • Bearing Damage
  53. 53. Total Harmonic Distortion
  54. 54. THD Percentage VFD BUS Non-VFD BUS
  55. 55. Harmonics Spectrum VFD BUS Non-VFD BUS
  56. 56. Current Waveform Comparison VFD BUS Non-VFD BUS
  57. 57. 519-1992 - IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems • IEEE Std. 519 (1981) – Revision (1992) • Deals with harmonics introduced by the static power converters • Overall THD < 5% • Any single harmonic < 3%
  58. 58. IEEE standard for THD
  59. 59. AREVA report on THD in MCC (VFD) at NP plant Voltage (V) R-N Y-N B-N RMS Voltage 237 237 237 Peak Voltage 362 364 364 THD (%) 4.7 5 5 Harmonics Voltage 11 12 12
  60. 60. AREVA report on THD in MCC (VFD) at NP plant Harmonic # R-N Y-N B-N 1 3 5 3.2 3.5 3.4 7 2.9 3.1 3.2 11 1.0 1.0 1.0 13 0.9 1.0 1.1
  61. 61. Line Reactors • Installed ahead of the drive • Protect the drive from sudden disturbances • Reduces the harmonics content introduced by VFD VFD
  62. 62. High peaks without reactor Reduction Up to 35% after reactor
  63. 63. Line reactors installed at NP SS 14 Line reactors are installed, 7 on each side of MCC-VFD
  64. 64. VFD and Line Reactor Line reactor VFD Main Breaker
  65. 65. THD after installation of Line Reactors
  66. 66. Bearing Damage • Pulse width modulated voltage induces bursts of shaft currents • Grounded through bearings • Eventually bearing failure
  67. 67. Bearing Damage
  68. 68. Pitting of bearing due to Shaft Currents
  69. 69. Solution for shaft currents • Shaft grounding through carbon brushes – Wear and corrode – Need maintenance • Insulated bearings are used – Partial solution – May flow through driven equipment – Insulation may become a capacitor • Shaft grounding rings – A combination of both
  70. 70. Shaft Grounding Ring
  71. 71. Questions

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