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Inverters for Variable Frequency Drives Prof. Thomas Jahns Bruce Beihoff May 11, 2016 Dynamics and Control of AC Drives UW-Madison WEMPEC Outline Introduction Power Converter/Inverter Classification Structure HVAC/ Refrigeration Inverter Applications Future Inverter Technologies of Importance to HVAC/ Refrigeration Conclusions May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 2 WEMPEC Voltage-Source PWM Inverters Multi-Level Matrix Voltage-source PWM inverters are the most widely-used class of ac drives today May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 3 WEMPEC Basic AC Drive Architecture Electrical Power Source Electrical Power Converter AC Machine Modulation and Control Command May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 4 WEMPEC Examples of AC Drive Power Converters May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 5 WEMPEC PWM Waveform Synthesis - PWM is based on the concept of time-varying duty cycle control May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 6 WEMPEC Pulse Width Modulation for VSI Drives PWM makes it possible to use same inverter switches for frequency and amplitude control, plus low-harmonic reduction PWM voltage waveforms ‘saturate’ to 6-step when pulses drop May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 7 WEMPEC PWM Synthesis – A, B & C Phases • Three PWM sequences are progressively time-shifted by 120 elec. deg. • No adjustment of dc link voltage amplitude is necessary – Phase-controlled rectifier can be replaced by simpler uncontrolled rectifier May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 8 WEMPEC V/Hz Control of PWM VSI-IM Drive Core of V/Hz Control Simple open-loop V/Hz control of PWM VSI-IM drive follows same basic principles as for 6-step VSI drive – PWM modulation control is slaved to frequency command May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 9 WEMPEC PWM VSI Induction Motor Drives Advantages – No need to provide means to adjust bus voltage – Reduced low-frequency current harmonics and torque ripple Disadvantages – Much higher switching frequency reduces inverter efficiency – High-frequency current harmonics may still cause noticeable motor losses, requiring machine derating – High inverter switch dv/dt’s and di/dt’s can cause damage to winding insulation or bearings May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 10 WEMPEC Current-Regulated PWM (CRPWM) VSI Drive Instantaneous phase current waveforms can be regulated by closing feedback loops using measured phase currents Provides basis for high-performance ac machine torque control May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 11 WEMPEC Simplified Block Diagram of CRPWM VSI-IM Drive using FOC Induction Machine By aligning currents appropriately with IM machine flux, torque and flux amplitudes can be individually controlled May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 13 WEMPEC CRPWM-VSI Induction Motor Drives Advantages – Capable of achieving excellent dynamic torque control – Can achieve very low levels of torque ripple – Availability of phase currents simplifies protection Disadvantages – Control is more complicated than for basic VSI drive – High-performance control typically requires current sensors plus rotor position or speed feedback May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 14 WEMPEC AC-AC Converter Induction Machine Drives Multi-Level Cycloconverters and matrix converters eliminate the need for an intermediate dc link in the power converter May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 15 WEMPEC Matrix Converter: Direct ac↔ac • Three-phase ac input: Three line input • Three-phase output: Three line output – Requires 3 triple-throw switches – Each switch capable of turning on and off, plus blocking voltage & current in both polarities May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 16 WEMPEC Matrix Converter Implementation Using IGBTs • Each bilateral switch implemented using 2 IGBT’s and 2 diodes • Requires total of 18 IGBTs and and 18 diodes May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 17 WEMPEC Matrix Converter Motor Drives Advantages – High input and output power quality using PWM – Input and output power factor are independently controlled – Inherent 4-quadrant operating capabilities – No dc link capacitors Disadvantages – Unavailability of bilateral controlled switch devices requires high power semiconductor parts count – Maximum output voltage as fraction of input is limited – Harmonic filters on input lines can be substantial May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 18 WEMPEC Synchronous Machine Drives Multi-Level Preferred converters overlap with those of induction machines, but there are differences May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 19 WEMPEC Open-Loop VSI Synchronous Motor Drive No rotor position feedback needed for low-performance applications – Damper cage typically needed for improved stability Constant V/Hz control holds flux ~ constant, just as for IM May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 20 WEMPEC Current-Regulated VSI-SM Drive Growing popularity of permanent magnet SMs is creating growing opportunities for CRPWM VSI drives for lower power “Self-synchronous control” is key to field-oriented control of SM May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 21 WEMPEC HVAC/Refrigeration Inverter Applications Variable Torque – Fans – Pumps – Centrifugal and Scroll Constant Torque – Reciprocating Compressors – Screw Compressors – Positive Displacement Pumps May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 23 WEMPEC HVAC/Refrigeration Inverter Applications Inverter Based VFD have found applications in every major sub-system May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 24 WEMPEC Inverter Based Systems Entering Market May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 25 WEMPEC The Inverter VFD Value Proposition HVAC/Refrigeration Advantages • Energy savings • Low motor starting current • Reduction of thermal and mechanical stresses on motors and belts during starts • Simple installation • High power factor • Lower KVA May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 26 WEMPEC HVAC/ Refrigeration VFD Inverter Challenges Motor Transient Voltages Power line Harmonics Progress vs. Challenges (2006-2016) • • • Electromagnetic Interfence May 11, 2016 Intrinsic Filtering and Smart switching have reduced standard drive power line harmonics below 5% THD Motor Transient voltages on med/long cables have been reduced by 50% by better switching and filters EMI/RFI has been reduced by 50 % through smart layout , grounding, filters, and better switching Inverters for Variable Frequency Drives TMJ, BCB - 27 WEMPEC High Speed VFD Compressors Multi Piston Reciprocating May 11, 2016 Turbo Compressor ( 15,000-22,000 rpm) Inverters for Variable Frequency Drives TMJ, BCB - 28 WEMPEC Future Technologies Advanced Semiconductor Switches Advanced Controls Advanced Power System Integration May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 29 WEMPEC Advanced Semiconductor Switch Improved Silicon Based Switches Wide Bandgap Semiconductor Switches May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 30 WEMPEC Power Semiconductor Limits • Some devices are pushing Si boundary • SiC and GaN offer promise for advances 100M 10M Thyristors GTOs IGCTs Power (VA) 1M 100K IGBT MODULES BIPOLAR TRANSISTOR MODULES 10K MOSFET Modules 1K 100 DISCRETE MOSFET 10 10 100 1K 10K 100K 1M Operation Frequency (Hz) May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 31 WEMPEC Wide Band Gap Semiconductors May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 32 WEMPEC Advanced Controls Advanced Micro Controller Chip Sets Per switch cycle Commutation and Control Model Predictive Control / Adaptive Control May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 33 WEMPEC Per Switch Cycle Direct Control Advantages • Precise control of speed and torque without position sensors • Fast dynamic response with superior stability • Very high efficiency • Intrinsic noise cancellation possibilities… DIRECT INSTANTANEOUS FORCE CONTROL (DIFC) (Example) Disadvantages • Requires more powerful processor • More complex control code DEADBEAT-DIRECT TORQUE CONTROL OF INDUCTION MACHINES ( Example) May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 34 WEMPEC Advanced Micro Controllers Advantages • Cost decreasing at 12% /year • Control MIPS > 30 • Capable of processing 8 control loops at 1 us update cycles Disadvantages • May still be too expensive ????? • Control code still complex to integrate May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 35 WEMPEC Model Predictive Control Advantages • Robust under the most non-linear conditions • Combines auto-tuning and commissioning • Combines stability with optimality Disadvantages • Requires more powerful processor • Requires reasonable model of process May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 36 WEMPEC Advanced Power System Integration High Temperature Inverters Standard Cell Power Integration Integrated Motor Drive – pumps, fans, compressors May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 37 WEMPEC High Temperature Power Electronics Lighting HiT Power Electronics Solar PV Motor Drives Automotive May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 38 WEMPEC >150°C Inverter Operation SiC Diodes, MOSFETs May 11, 2016 Stacked Ceramic Capacitors Packaging is key to achieving reliable operation up to 200ºC Si or SiC are options APEI Inverters for Variable Frequency Drives TMJ, BCB - 39 WEMPEC High-Temperature Motor Drives 50 kW Cissoid 250°C 175°C Performance Controls May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 40 WEMPEC Integrated Motor Drives Controller in separate module mounted onto motor Ratings up to 10 kW Danfoss Profiles of power converters are shrinking Danfoss Rexroth May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 41 WEMPEC Future Integrated Motor Drives Inverter Wireless Control May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 42 WEMPEC Integrated Modular Motor Drive Machine Pole Power Electronics Controller SMC Pole Piece Back iron Coil + + Communication Bus Module Controller Pole face IMMD Modular Pole-Drive Unit CPES (UW) Each modular pole-drive unit performs as an independent building block in motor drive stator configuration May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 43 WEMPEC Gen 2 IMMD Implementation • Prototype 10 kW, 6-phase IMMD motor and drive built for experimental verification • Each drive unit includes independent controller for testing fault-tolerant control characteristics May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 44 WEMPEC Gen 3 IMMD Implementation 200 V, 1.2 kW, 100 kHz • Uses GaN switches in multi-level converter configuration May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 45 WEMPEC Alternative Building DC-Based Architecture 340 VDC Bus 340 / 48 VDC 48 VDC Bus 48VDC 48VD C Power electronics can contribute to major efficiency improvements and new electrical distribution architectures May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 46 WEMPEC Concluding Observations I Induction machines are the most popular choices for ASDs, but synchronous machines are widely used at both high power (>1 MW) and high-performance low-power applications (PM machines) PWM provides a powerful means of combining functions of adjusting frequency, changing amplitude, and suppressing low-frequency harmonics DC link drive architectures dominate ASDs, but AC-AC matrix converters may become more popular in future if mature bidirectional switches are developed May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 47 WEMPEC Concluding Observations III VSI (Voltage Source Inverters) have made major in roads through out the HVAC and Refrigeration applications Current-regulated VSI inverters are very popular when higher performance is required – Provides basis for field-oriented control – These see further use in advanced compressors High Speed compressors and higher frequency inverters are coming with advanced inverters Inverters based on wide bandgap semi-conductors that are higher frequency and very mechanically tough are beginning to appear Integrated Motor Drives are making their second foray into broader HVAC and Refrigeration applications. May 11, 2016 Inverters for Variable Frequency Drives TMJ, BCB - 49