Inverters for Variable Frequency Drives

Inverters for Variable Frequency Drives

Prof. Thomas Jahns

May 11, 2016 UW-Madison Dynamics and Control of AC Drives

Bruce Beihoff

May 11, 2016

WEMPEC

TMJ, BCB - 2 Inverters for Variable Frequency Drives

Outline Introduction

Power Converter/Inverter Classification Structure

HVAC/ Refrigeration Inverter Applications

Future Inverter Technologies of Importance to HVAC/ Refrigeration

Conclusions

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Voltage-Source PWM Inverters

Voltage-source PWM inverters are the most widely-used class of ac drives today

Matrix Multi-Level

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Electrical Power Source

Electrical Power Converter

AC Machine

Modulation and Control

Command

Basic AC Drive Architecture

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Examples of AC Drive Power Converters

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PWM Waveform Synthesis

PWM is based on the concept of time-varying duty cycle control

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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

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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

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V/Hz Control of PWM VSI-IM Drive

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

Core of V/Hz Control

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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

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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

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Simplified Block Diagram of CRPWM VSI-IM Drive using FOC

By aligning currents appropriately with IM machine flux, torque and flux amplitudes can be individually controlled

Induction Machine

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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

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AC-AC Converter Induction Machine Drives

Cycloconverters and matrix converters eliminate the need for an intermediate dc link in the power converter

Multi-Level

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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

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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

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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

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Multi-Level

Synchronous Machine Drives

Preferred converters overlap with those of induction machines, but there are differences

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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

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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

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HVAC/Refrigeration Inverter Applications

Variable Torque – Fans – Pumps – Centrifugal and Scroll

Constant Torque

– Reciprocating Compressors

– Screw Compressors

– Positive Displacement Pumps

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HVAC/Refrigeration Inverter Applications Inverter Based VFD have found applications in every major sub-system

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Inverter Based Systems Entering Market

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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

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HVAC/ Refrigeration VFD Inverter Challenges

Motor Transient Voltages Power line Harmonics

Electromagnetic Interfence

Progress vs. Challenges (2006-2016) • 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

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High Speed VFD Compressors

Multi Piston Reciprocating Turbo Compressor ( 15,000-22,000 rpm)

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Future Technologies

Advanced Semiconductor Switches

Advanced Controls

Advanced Power System Integration

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Advanced Semiconductor Switch

Improved Silicon Based Switches

Wide Bandgap Semiconductor Switches

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Power Semiconductor Limits

Operation Frequency (Hz)

DISCRETE MOSFET

MOSFET Modules

BIPOLAR TRANSISTOR MODULES

IGBT

MODULES

100M

10M

1M

100

10

Pow

er (V

A)

100K

10K

1K

10 100 1K 10K 100K 1M

Thyristors GTOs IGCTs

• Some devices are pushing Si boundary

• SiC and GaN offer promise for advances

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Wide Band Gap Semiconductors

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Advanced Controls

Advanced Micro Controller Chip Sets

Per switch cycle Commutation and Control

Model Predictive Control / Adaptive Control

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Per Switch Cycle Direct Control

DEADBEAT-DIRECT TORQUE CONTROL OF INDUCTION MACHINES ( Example)

DIRECT INSTANTANEOUS FORCE CONTROL (DIFC) (Example)

Advantages • Precise control of speed and torque

without position sensors • Fast dynamic response with superior

stability • Very high efficiency • Intrinsic noise cancellation

possibilities…

Disadvantages • Requires more powerful processor • More complex control code

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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

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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

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Advanced Power System Integration

High Temperature Inverters

Standard Cell Power Integration

Integrated Motor Drive – pumps, fans, compressors

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High Temperature Power Electronics Lighting

Automotive Motor Drives

Solar PV

HiT Power Electronics

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>150°C Inverter Operation

SiC Diodes, MOSFETs

Stacked Ceramic

Capacitors

Packaging is key to achieving reliable operation up to 200ºC

Si or SiC are options

APEI

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High-Temperature Motor Drives

Cissoid

Performance Controls

250°C

175°C

50 kW

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Integrated Motor Drives

Danfoss

Controller in separate module mounted onto motor

Ratings up to 10 kW

Profiles of power converters are shrinking

Danfoss

Rexroth

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Future Integrated Motor Drives

Inverter

Wireless Control

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Integrated Modular Motor Drive

Pole face Coil

Back iron

SMC Pole Piece Communication

Bus Module

Controller + +

Machine Pole

Modular Pole-Drive

Unit

Power Electronics Controller

IMMD

Each modular pole-drive unit performs as an independent building block in motor drive stator configuration

CPES (UW)

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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

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200 V, 1.2 kW, 100 kHz Gen 3 IMMD Implementation

• Uses GaN switches in multi-level converter configuration

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Alternative Building DC-Based Architecture

340 VDC Bus 340 / 48 VDC

48VDC

48VDC 48 VDC Bus

Power electronics can contribute to major efficiency improvements and new electrical distribution architectures

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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

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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.

Inverters for Variable Frequency Drives

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