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Variable Frequency Drives
- 2. Variable Frequency Drives Saqib Saeed Graduate Trainee Engineer (E&I) - Electrical
- 3. Contents • Introduction • Block Diagram • Building blocks • Modes of operation • VFD Parameters • Some Potential Problems • Harmonics and THD • Recent Improvements in the FFCL system
- 4. 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?
- 5. 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
- 6. 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
- 7. 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.
- 8. Rectifier • Basic Building blocks of rectification – Diodes (Uncontrolled)
- 9. Thyristors • Controlled • Output voltage can be controlled Gate Pulse Conduction after Gate Pulse
- 11. Output Voltage
- 12. 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
- 13. Inverter Action • Switching DC voltage ON and OFF will make it AC • Filtered output from DC bus is sent to inverter in VFD
- 14. RECTIFIER Positive DC Bus Negative DC Bus + - INVERTER How switching can convert DC into AC?
- 19. Pulse width modulation • Such a waveform is not acceptable – Nowhere near Sine wave • Contains harmonics – Multiples of fundamentals • Solution – Pulse width modulation
- 20. 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
- 21. Area Under the Curve
- 22. Basic Purpose achieved • Speed of the motor now can be controlled • Is changing motor frequency alone enough?
- 23. 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:
- 24. 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
- 25. MotorDrive Speed setting=50% Speed at no load=50% Load increase
- 26. MotorDrive Speed setting=50% Speed=40% No idea what is happening
- 27. 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
- 28. VFD input Parameters • Max./Base frequency setting • Motor rated output • Motor rated voltage • Motor rated current • Carrier frequency
- 29. VFD input Parameters • Frequency Reference setting methods • Stop Command method • Start frequency • Stop frequency (DC Braking starts) • Torque Boost • Frequency Skip
- 30. VFD input Parameters • Max./Base frequency setting
- 31. VFD input Parameters • Motor Rated Output • Motor Rated Voltage
- 32. VFD input Parameters • Motor rated Current • Carrier Frequency
- 33. Frequency Reference Setting Methods • Potentiometer • 0-10V input voltage • 4-20(mA)
- 34. Frequency Reference Setting through Potentiometer Min Max Min Max Frequency
- 35. Centrifuge Main Control Panel Potentiometer
- 36. Start/Stop Frequency/Acceleration/Deceleration Frequency Start Frequency Acceleration Time DC Brake Stop Frequency Deceleration Time Time
- 37. Start/Stop Frequency/Acceleration/Deceleration Frequency Start Frequency Acceleration Time DC Brake Stop Frequency Deceleration Time Time
- 38. Stop Command methods • Coast to stop • Ramp to stop
- 39. Coast to stop Coast to Stop Frequency Run Command Motor speed Command is removed
- 40. Ramp to stop Frequency Run Command Command is removed DC Brake Ramp to Stop
- 42. Frequency Skip
- 43. Auto Tuning • Drive familiarizing itself with motor VFD here I am IM
- 44. Auto Tuning MotorDrive Start of Auto tuning Primary resistanceLeakage reactanceDC Brake Voltage Torque Boost Voltage Slip compensation
- 45. 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
- 46. 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.
- 47. 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
- 48. 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
- 49. Dynamic Braking • Some other form of energy – Electrical Kinetic Energy Electrical Energy DiscardUtilize Resistive Elevators Regenerative Electric railcars
- 50. Resistive Dynamic Braking in Elevators
- 51. Braking resistor for CAN Elevator VFD Braking Unit
- 52. Resistive Dynamic Braking • A built in dynamic braking resistor (<15Kw) • An external DB unit
- 53. Potential Problems • Harmonic Distortion • Bearing Damage
- 55. THD Percentage VFD BUS Non-VFD BUS
- 56. Harmonics Spectrum VFD BUS Non-VFD BUS
- 57. Current Waveform Comparison VFD BUS Non-VFD BUS
- 58. 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%
- 59. IEEE standard for THD
- 60. 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
- 61. 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
- 62. Line Reactors • Installed ahead of the drive • Protect the drive from sudden disturbances • Reduces the harmonics content introduced by VFD VFD
- 63. High peaks without reactor Reduction Up to 35% after reactor
- 64. Line reactors installed at NP SS 14 Line reactors are installed, 7 on each side of MCC-VFD
- 65. VFD and Line Reactor Line reactor VFD Main Breaker
- 66. THD after installation of Line Reactors
- 67. Bearing Damage • Pulse width modulated voltage induces bursts of shaft currents • Grounded through bearings • Eventually bearing failure
- 68. Bearing Damage
- 70. Pitting of bearing due to Shaft Currents
- 71. 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
- 73. Questions