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VFD Fundamentals
- 1. © ABB Month DD, YYYY | Slide 1 ASHRAE Rocky Mountain Chapter VFD Fundamentals April 16, 2010 Jeff Miller - 2010
- 2. What is a Drive / VFD/ AFD? 0 230 460 Volts Hertz 30 60 460 V 60 Hz = 7.67 V Hz 230 V 60 Hz = 3.83 V Hz If 230 VAC Power Line: 230 V Motor 460 V M otor
- 3. What is a Drive? Motor L1 L2 L3 C L Input Converter (Diode Bridge) Output Inverter (IGBT’s) DC Bus (Filter)+ _ + _ + _ + _ + + _ _
- 4. What is a Drive?
- 5. A variable frequency drive converts incoming 60 Hz utility power into DC, then converts to a simulated variable voltage, variable frequency output VFD Fundamentals 60 Hz Power Electrical Energy ABB Zero - 120 Hz To Motor VFD RECTIFIER (AC - DC) INVERTER (DC - AC) AC DC AC VFD Zero - 120 Hz60 Hz
- 35. RECTIFIER Positive DC Bus Negative DC Bus + - INVERTER Area Under The Square-Wave Pulses Approximates The Area Under A Sine Wave Frequency Voltage
- 37. 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
- 38. Frequency = 30Hz Frequency = 60Hz
- 46. Non-Linear Loads? • Loads which draw non-sinusoidal current from the line: – Non-incandescent lighting – Computers – Uninterruptible power supplies – Telecommunications equipment – Copy machines – Battery chargers – Electronic variable speed drives – Any load with a solid state AC to DC power converter
- 47. Typical AC Drive Configuration M 460VAC 3-phase 650VDC Simulated AC (PWM) All AC Drives rectify AC to DC, then convert to simulated AC (PWM) to provide the motor Variable voltage and Frequency. The AC to DC conversion generates harmonics.
- 48. Harmonics — Definitions • Non-linear loads draw current in a non-sinusoidal or distorted manner • Harmonics or harmonic content is a mathematical concept implemented to allow quantification and simplified analysis of non-linear waveforms • Harmonics are typically present in both network currents and network voltages • Non-linear current draw creates non-linear voltage as it flows through the electrical network – Current harmonics → Voltage harmonics
- 49. Harmonic Frequencies Fundament al 5th Harmonic 7th harmonic 11th Harmonic 13th Harmonic 17th Harmonic 19th Harmonic 60 Hz 300 Hz 420 Hz 660 Hz 780 Hz 1020 Hz 1140 Hz
- 50. The Theory: Fundamental, 5th and 7th Harmonics f5 300= ω5 2π f5×= i5 t( ) 0.32 cos ω5 t× π−( )×= f7 420= ω7 2π f7×= i7 t( ) 0.09 cos ω7 t× π−( )×= 0 0.005 0.01 0.015 0.02 0.025 0.03 1 0.5 0 0.5 1 i1 t( ) i5 t( ) i7 t( ) t Fundamenta l 5th 7th iT t( ) i1 t( ) i5 t( )+ i7 t( )+= 0 0.005 0.01 0.015 0.02 0.025 0.03 1.5 1 0.5 0 0.5 1 1.5 iT t( ) t Components Summation
- 51. Harmonic Content, 6- Pulse Drive PWM Drive Harmonic Input Spectrum 5th 7th Fundamental 11th 13th
- 52. – Harmonic Current Distortion — • Added heating in transformers and cables, reduces available capacity • May stimulate a resonance condition with Power Factor Correction Capacitors –Excessive voltage –Overheating of PF correction capacitors –Tripping of PF protection equipment Voltage Distortion interfering w/ sensitive equipment. Largest Concern! Harmonics — Why worry?
- 53. – Harmonics produced by an individual load are only important to the extent that they represent a significant portion of the total connected load – Linear loads help reduce system harmonic levels – TDD equals the THD of the nonlinear load multiplied by the ratio of nonlinear load to the demand load: Harmonics — A System Issue! DL NL THDTDD NL ⋅=Where TDD = TDD of the system THDNL = THD of the nonlinear loads NL = kVA of nonlinear load DL = kVA of demand load (nonlinear + linear)
- 54. Harmonics — By the Numbers Table 10.2 Low-Voltage System Classification and Distortion Limits Special Applications General System Dedicated System Notch Depth 10% 20% 50% THD (Voltage) 3% 5% 10% Notch Area, µVs 16,400 22,800 36,500 Note: Notch area for other than 480 V systems should be multiplied by V / 480. IEEE 519 - 1992
- 55. Harmonics — By the Numbers (cont.) Table 10.3 Current Distortion Limits for General Distribution Systems ISC / IL <11 11≤h<17 17≤h<23 23≤h<35 35≤h TDD <20 4.0 2.0 1.5 0.6 0.3 5.0 20<50 7.0 3.5 2.5 1.0 0.5 8.0 50<100 10.0 4.5 4.0 1.5 0.7 12.0 100<1000 12.0 5.5 5.0 2.0 1.0 15.0 Note: All harmonic current levels are in percent with fundamental current IL as the base. IEEE 519 - 1992
- 56. Harmonics — Attenuation Options • Reactors (Chokes) • Passive Filters – Harmonic Trap – Hybrid • High Pulse Count Rectification • Active Filters – Drive Front End – Stand Alone
- 57. Reactors (Chokes) • Simplest and least expensive harmonic reduction technique • May be included in base drive package • Often meet harmonic needs provided drive load is a small portion of total connected load • May be implemented with AC line reactors or with DC link reactors – AC line reactors provide better input protection – DC link reactors provide load insensitive drive output voltage – Both types provide similar harmonic benefits • “Swinging” choke design provides enhanced light load harmonic performance
- 58. Reactors, AC Line or DC Link A C L in e R e a c t o r M D C L in k R e a c t o r M • Different design techniques • Equal harmonic reduction for same normalized % reactance • Typical full load THD (current) at drive input terminals 28% → 46%
- 59. Hybrid Filter • Installs in series with drive input • May feed multiple drives • Improves power factor (may go leading) • Typical full load THD (current) at filter input terminals 5% → 8% • Relatively unaffected by line imbalance
- 60. High Pulse Count Rectification • Typical configurations are either 12 pulse or 18 pulse • Phase shifting transformer is required • Additional drive input bridge(s) is needed • Typical full load THD (current) at transformer primary 8% → 12% (12 pulse), 4% → 6% (18 pulse) • Performance severely reduced by line imbalance (voltage or phase) • Excellent choice if step-down transformer is already required
- 61. High Pulse Count Rectification (cont.)• 6 pulse rectifier Transformer and cabling simple Current very distorted Ithd typically 45% with 3% reactor DC/AC DC/AC Transformer and cabling complicated Current distorted Ithd 8% to 12% (depending on network impedance) • 12 pulse rectifier • 18 pulse rectifier DC/AC Transformer and cabling complicated Current wave form good Ithd 4% to 6% (depending on network impedance)
- 62. Active Filter Front End with LCL Filter LCL Filter (Sine Filter) removes high frequencies >1 kHz. (Current and voltage) Full output voltage is available with 80% input voltage (400VIn = 480VOut) Full regenerative capability No transformer required Not affected by line imbalance LCL filter Line inverter (rectifier) Motor inverter Motor L L C M
- 63. Remember! Even an 80% THD nonlinear load with a will result in only 8% TDD if the nonlinear load is 10% and the linear load is 90%. (80%•(10%/(10%+90%))=8%) Harmonic Reduction Summary Effectiveness of Harmonic Mitigation Techniques (Assuming 100% Nonlinear Loading, ISC / IL = 60) Technique THD (Current) Harmonic Reduction No mitigation (reference level) 72% 3% line reactors (or equivalent DC link reactor) 39% 45.8% 5% line reactors (or equivalent DC link reactor) 33% 54.2% 5% line reactors + 5th harmonic trap filter 12% 83.3% 12 pulse input rectifier with 5% impedance transformer 10% 86.1% Hybrid filter 7% 90.3% 18 pulse input rectifier with 5% impedance transformer 5% 93.1% 12 pulse input rectifier with 5% impedance transformer + 11th harmonic trap filter 4% 94.4% Active harmonic filter 3.5% 95.1%
- 64. Summary – Practical Advice • With a main distribution transformer, 20-30% of its load on non-linear loads will typically comply with IEEE 519-1992 • Voltage distortion causes interference with sensitive equipment, not current distortion! • 5% reactors address 90+% of typical applications. They also provide protection against line transients and keep input currents low to avoid oversizing power wiring to comply with NEC. • Make VFD vendor perform a harmonic distortion calculation with the submittals.
- 66. Peak Voltage all at 50’ of cable Peak Voltage has many Contributing Factors Inverter Rated Motors Help Minimize the Issue Less dV/dT minimizes; problems with RFI/EMI Motor Insulation & Bearing Current Drive Peak Voltage 1 1040 2 1110 3 1180 4 1290 5 1350 6 2454
- 67. Recommendations • Keep cable length short as possible • Use a NEMA MG1, Part 31 motor (not “inverter duty” or “inverter ready” • Ensure that grounding is sound