View source for Variable-frequency drive

{{Short description|Type of adjustable-speed drive}}
{{Use mdy dates|date=May 2022}}
[[File:Small variable-frequency drive.jpg|right|thumb|300px|Small variable-frequency drive]]
[[File:HitachiJ100A.jpg|right|thumb|300px|Chassis of above VFD (cover removed)]]

A '''variable-frequency drive''' ('''VFD''', or '''adjustable-frequency drive''', '''adjustable-speed drive''', '''variable-speed drive''', '''AC drive''', '''micro drive''', '''inverter drive''', or '''drive''') is a type of [[AC motor|AC]] [[motor drive]] (system incorporating a motor) that controls [[speed]] and [[torque]] by varying the [[frequency]] of the input electricity. Depending on its [[Topology (electrical circuits)|topology]], it controls the associated [[voltage]] or [[electric current|current]] variation.<ref name=Campbell>{{cite book | first=Sylvester J. | last=Campbell | year=1987 | title=Solid-State AC Motor Controls | publisher=Marcel Dekker, Inc. | location=New York | isbn=978-0-8247-7728-9 | pages= 79–189 }}</ref><ref name="Jaeschke">{{cite book|last=Jaeschke|first=Ralph L.|title=Controlling Power Transmission Systems|publisher=Penton/IPC|year=1978|isbn=978-1114762060|location=Cleveland, OH|pages=210–215}}</ref><ref name=Siskind>{{cite book | first=Charles S. | last=Siskind | year=1963 | title=Electrical Control Systems in Industry | publisher=McGraw-Hill, Inc. | location=New York | isbn=978-0-07-057746-6 | page=[https://archive.org/details/electricalcontro0000unse/page/224 224] | url=https://archive.org/details/electricalcontro0000unse/page/224 }}</ref><ref name=NEMA-Guide>{{cite book | last=NEMA Standards Publication | title=Application Guide for AC Adjustable Speed Drive Systems | year=2007 | url=https://www.nema.org/standards/view/application-guide-for-ac-adjustable-speed-drive-systems| access-date=March 27, 2008 | publisher=National Electrical Manufacturers Association | location=Rosslyn, VA USA | page=4}}</ref><ref name=Lockley-2014>{{Cite conference|url=http://site.ieee.org/sas-pesias/files/2016/03/Whats-New-with-Medium-Voltage-Drives_Slides.pdf|last1=Lockley|first1=William|last2=Paes|first2=Richard|title=What's New In Medium Voltage Drives|conference=IEEE SAS & NCS 2014 PES/IAS Joint Chapter Technical Seminar|year=2014|access-date =January 28, 2022|quote=Adjustable Speed Drive System: An interconnected combination of equipment that provides a means of adjusting the speed of a mechanical load coupled to a motor.|page=slide 91 of 132}}, quote is per definition on p. 4 of NEMA Standards Publication ICS 7.2-2021.</ref>

VFDs are used in applications ranging from small appliances to large compressors.<ref>{{Cite web|title=Medium Voltage Integrated Drive and Motor|url=https://www.energy.gov/sites/prod/files/2017/03/f34/Medium%20Voltage%20Integrated%20Drive%20and%20Motor_0.pdf|website=energy.gov – DoE|language=en-GB}}</ref> Systems using VFDs can be more efficient than [[Hydraulics|hydraulic systems]], such as in systems with pumps and damper control for fans.<ref name="ABB-energy">{{cite web|title=Energy efficiency makes a difference|url=http://www02.abb.com/db/db0003/db002698.nsf/0/a3881ed5c3dbc647c12575080036dafd/$file/energy_efficiency_makes_difference_191108.pdf|access-date=May 1, 2017|archive-url=https://web.archive.org/web/20170908185247/http://www02.abb.com/db/db0003/db002698.nsf/0/a3881ed5c3dbc647c12575080036dafd/$file/Energy_efficiency_makes_difference_191108.pdf|archive-date=September 8, 2017|url-status=dead}}</ref>
	
Since the 1980s, [[power electronics]] technology has reduced VFD cost and size and has improved performance through advances in semiconductor switching devices, drive topologies, simulation and control techniques, and control hardware and software.

VFDs include low- and medium-voltage [[AC-to-AC converter|AC-AC]] and [[Power inverter|DC-AC]] topologies.

== History ==
Pulse Width Modulating (PWM) variable frequency drive projects started in the 1960s at Strömberg in Finland. Martti Harmoinen is regarded as the inventor of this technology.<ref>{{Cite web |date=2023-06-20 |title=Muistokirjoitus {{!}} Martti Harmoinen 1934–2023 |url=https://www.hs.fi/muistot/art-2000009649557.html |access-date=2023-06-22 |website=Helsingin Sanomat |language=fi}}</ref><ref>{{Cite web |title=Taajuusmuuttajasta tuli Suomen suuri innovaatio - IN0619 |url=https://insinoori-lehti.fi/digilehti/in0619/taajuusmuuttajasta-tuli-suomen-suuri-innovaatio |access-date=2023-06-22 |website=insinoori-lehti.fi |language=fi}}</ref><ref>{{Cite web |title=Suomi100-innovaatiot, sija 5: taajuusmuuttaja |url=https://etn.fi/index.php/13-news/7240-suomi100-innovaatiot-sija-5-taajuusmuuttaja |access-date=2023-06-22 |website=etn.fi}}</ref> Strömberg managed to sell the idea of PWM drive to [[Helsinki Metro]] in 1973 and in 1982 the first PWM drive SAMI10 were operational.<ref>{{Cite web |date=2021-06-14 |title=Edelläkävijät osaavat lukea signaaleja ympärillään |url=https://trafiikki.fi/edellakavijat-osaavat-lukea-signaaleja-ymparillaan/ |access-date=2023-06-22 |website=Trafiikki |language=fi}}</ref><ref>{{Cite web |title=Tekniikan Maailma |url=https://tekniikanmaailma.fi/sahkomoottorin-kaasupolkimen-suomalainen-isa-martti-harmoinen-on-poissa-helsingin-metrosta-tekniikka-levisi-teollisuuteen-sahkoautoihin-ja-hisseihin/ |access-date=2023-06-22 |website=tekniikanmaailma.fi}}</ref><ref>{{Cite web |last=museo |first=Tekniikan |date=2021-07-15 |title=Esinekummi tekee näkyväksi tekniikan ja teollisuuden tarinaa |url=https://tekniikanmuseo.wordpress.com/2021/07/15/esinekummi-tekee-nakyvaksi-tekniikan-ja-teollisuuden-tarinaa/ |access-date=2023-06-22 |website=Tekniikan museo |language=fi}}</ref>

==System description and operation==
[[File:Variable-frequency Drive System.png|right|thumb|320px|VFD system]]
A variable-frequency drive is a device used in a drive system consisting of the following three main sub-systems:  AC motor, main drive [[controller (control theory)|controller]] assembly, and drive/operator interface.<ref name=Jaeschke/>{{rp|210–211}}<ref name=NEMA-Guide/>

===AC motor===
The AC electric motor used in a VFD system is usually a [[Three-phase electric power|three-phase]] [[induction motor]]. Some types of [[Single-phase electric power|single-phase]] motors or [[synchronous motor]]s can be advantageous in some situations, but generally three-phase induction motors are preferred as the most economical. Motors that are designed for fixed-speed operation are often used. Elevated-voltage stresses imposed on induction motors that are supplied by VFDs require that such motors be designed for definite-purpose inverter-fed duty in accordance with such requirements as Part 31 of [[National Electrical Manufacturers Association|NEMA]] Standard MG-1.<ref>NEMA Guide, p. 13</ref><ref>{{Cite web |title=Products |url=https://servosupplier.com/collections/all |access-date=2023-07-27 |website=Servo Supplier |language=en}}</ref>

===Controller===
{{Further|FAM control of induction motor}}
The VFD controller is a [[solid state (electronics)|solid-state]] power electronics conversion system consisting of three distinct sub-systems: a [[rectifier]] bridge converter, a [[direct current]] (DC) link, and an inverter. [[Voltage source|Voltage-source]] inverter (VSI) drives (see 'Generic topologies' sub-section below) are by far the most common type of drives. Most drives are [[AC/AC Converter|AC-AC]] drives in that they convert AC line input to AC inverter output. However, in some applications such as common DC bus or [[Solar energy|solar]] applications, drives are configured as DC-AC drives. The most basic rectifier converter for the VSI drive is configured as a three-phase, six-pulse, [[Full-wave rectifier|full-wave]] [[diode bridge]]. In a VSI drive, the DC link consists of a [[capacitor]] which smooths out the converter's DC output [[ripple (electrical)|ripple]] and provides a stiff input to the inverter. This filtered DC voltage is converted to quasi-[[sine wave|sinusoidal]] AC voltage output using the inverter's active switching elements. VSI drives provide higher [[power factor]] and lower [[harmonic distortion]] than [[Phase fired controllers|phase-controlled]] [[current source|current-source]] inverter (CSI) and load-commutated inverter (LCI) drives (see 'Generic topologies' sub-section below). The drive controller can also be configured as a [[phase converter]] having single-phase converter input and three-phase inverter output.<ref>Campbell, pp. 79–83</ref>

Controller advances have exploited dramatic increases in the voltage and current ratings and switching frequency of solid-state power devices over the past six decades. Introduced in 1983,<ref name="Bose (2006)">{{cite book|last=Bose|first=Bimal K.|title=Power Electronics and Motor Drives : Advances and Trends|url=https://archive.org/details/powerelectronics00bose_657|url-access=limited|year=2006|publisher=Academic|location=Amsterdam|isbn=978-0-12-088405-6|page=[https://archive.org/details/powerelectronics00bose_657/page/n39 22]}}</ref> the [[insulated-gate bipolar transistor]] (IGBT) has in the past two decades come to dominate VFDs as an inverter switching device.<ref name="Bartos21st">{{cite journal|last=Bartos|first=Frank J.|date=September 1, 2004|title=AC Drives Stay Vital for the 21st Century|url=https://www.controleng.com/articles/ac-drives-stay-vital-for-the-21st-century/|journal=Control Engineering}}</ref><ref>{{cite conference  | first =Robert E. | last = Eisenbrown | chapter= AC Drives, Historical and Future Perspective of Innovation and Growth | title = Keynote Presentation for the 25th Anniversary of The Wisconsin Electric Machines and Power Electronics Consortium (WEMPEC) | pages =6–10 | publisher =WEMPEC | date =May 18, 2008 | location = University of Wisconsin, Madison, WI, USA }}</ref><ref>{{cite journal  | last =Jahn   | first =Thomas M. |author2=Owen, Edward L.  | title =AC Adjustable-Speed Drives at the Millennium: How Did We Get Here? | journal =IEEE Transactions on Power Electronics | volume = 16 | issue = 1 | pages =17–25 | date =January 2001  | doi =10.1109/63.903985 | bibcode =2001ITPE...16...17J | url =http://digital.library.wisc.edu/1793/11120 }}</ref>

In variable-[[torque]] applications suited for Volts-per-Hertz (V/Hz) drive control, AC motor characteristics require that the voltage magnitude of the inverter's output to the motor be adjusted to match the required load torque in a [[linearity|linear]] V/Hz relationship. For example, for 460&nbsp;V, 60&nbsp;Hz motors, this linear V/Hz relationship is 460/60 = 7.67&nbsp;V/Hz. While suitable in wide-ranging applications, V/Hz control is sub-optimal in high-performance applications involving low speed or demanding, dynamic speed regulation, positioning, and reversing load requirements. Some V/Hz control drives can also operate in [[quadratic function|quadratic]] V/Hz mode or can even be programmed to suit special multi-point V/Hz paths.<ref name=BasicsACDrive>{{cite web|title=Basics of AC drives|url=http://www3.sea.siemens.com/step/flash/STEPACDrives/index.htm|access-date=April 18, 2012|page=Hardware-Part 2: slide 2 of 9|archive-url=https://web.archive.org/web/20120419103737/http://www3.sea.siemens.com/step/flash/STEPACDrives/index.htm|archive-date=April 19, 2012|url-status=dead}}</ref><ref name="Bose (1980)">{{cite book | first=Bimal K. | last=Bose | year=1980 | title=Adjustable Speed AC Drive Systems | publisher=IEEE Press | location=New York | isbn=978-0-87942-146-5| bibcode=1981asad.book.....B }}</ref>

The two other drive control platforms, [[vector control (motor)|vector control]] and [[direct torque control]] (DTC), adjust the motor voltage magnitude, angle from reference, and frequency<ref name="Yano">{{cite web|last=Yano|first=Masao|title=History of Power Electronics for Motor Drives in Japan|url=http://www.ieeeghn.org/wiki/images/4/49/Yano2.pdf|access-date=April 18, 2012|page=13|display-authors=etal}}</ref> so as to precisely control the motor's magnetic flux and mechanical torque.

Although [[space vector modulation|space vector]] [[pulse-width modulation]] (SVPWM) is becoming increasingly popular,<ref name="Bose (2011)">{{cite conference |date=June 8, 2012|url=https://ieeexplore.ieee.org/document/6214597|last=Bose |first=Bimal K. |title=Global Energy Scenario and Impact on Power Electronics In the 21st Century |conference=PEIA 2011 – Workshop on Power Electronics for Industrial Applications and Renewable Energy Conversion |page=12 |location=Doha, Qatar|access-date=February 8, 2012|publisher=IEEE}}</ref> sinusoidal PWM (SPWM) is the most straightforward method used to vary drives' motor voltage (or current) and frequency. With SPWM control (see Fig. 1), quasi-sinusoidal, variable-pulse-width output is constructed from intersections of a saw-toothed [[carrier signal]] with a modulating sinusoidal signal which is variable in operating frequency as well as in voltage (or current).<ref name=Bartos21st/><ref name="Bose 2006, p. 183">Bose (2006), p. 183</ref><ref>Campbell, pp. 82–85</ref>

Operation of the motors above rated nameplate speed (base speed) is possible, but is limited to conditions that do not require more power than the nameplate rating of the motor. This is sometimes called "field weakening" and, for AC&nbsp;motors, means operating at less than rated V/Hz and above rated nameplate speed. [[Permanent magnet]] synchronous motors have quite limited field-weakening speed range due to the constant magnet [[flux linkage]]. Wound-rotor synchronous motors and induction motors have much wider speed range. For example, a 100&nbsp;HP, 460&nbsp;V, 60&nbsp;Hz, 1775&nbsp;[[revolutions per minute|RPM]] (4-pole) induction motor supplied with 460&nbsp;V, 75&nbsp;Hz (6.134&nbsp;V/Hz), would be limited to 60/75 = 80% torque at 125% speed (2218.75&nbsp;RPM) = 100% power.<ref>Bose (1980), p. 3</ref> At higher speeds, the induction motor torque has to be limited further due to the lowering of the breakaway torque{{efn|NEMA Guide defines a motor's breakaway torque as 'The torque that a motor produces at zero speed when operating on a control', and a motor's breakdown torque as 'The maximum torque that it will develop with rated voltage applied at rated frequency on sinewave power, without an abrupt drop in speed.'}} of the motor. Thus, rated power can be typically produced only up to 130–150% of the rated nameplate speed. Wound-rotor synchronous motors can be run at even higher speeds. In rolling mill drives, often 200–300% of the base speed is used. The mechanical strength of the rotor limits the maximum speed of the motor.

[[File:Pwm.png|right|thumb|240px|Fig. 1: SPWM carrier-sine input & 2-level PWM output]]

An [[embedded system|embedded]] [[microprocessor]] governs the overall operation of the VFD controller. Basic [[Computer programming|programming]] of the microprocessor is provided as user-inaccessible [[firmware]]. User programming of [[display device|display]], variable, and function block parameters is provided to control, protect, and monitor the VFD, motor, and driven equipment.<ref name=Bartos21st/><ref>Basics of AC Drives, p. Programming: slide 3 of 7</ref>

The basic drive controller can be configured to selectively include such optional power components and accessories as follows: 
* Connected upstream of converter – [[circuit breaker]] or [[fuse (electrical)|fuse]]s, isolation [[contactor]], [[Electromagnetic compatibility|EMC]] filter, line [[inductor|reactor]], passive filter
* Connected to DC link – [[braking chopper]], braking [[resistor]]
* Connected downstream of inverter—output reactor, sine wave filter, dV/dt filter.{{efn|The mathematical symbol dV/dt, defined as the [[derivative]] of voltage&nbsp;V with respect to time&nbsp;t, provides a measure of rate of voltage rise, the maximum admissible value of which expresses the capability of capacitors, motors, and other affected circuit elements to withstand high current or voltage spikes due to fast voltage changes; dV/dt is usually expressed in V/microsecond.<ref name=EPCOS>{{cite web|title=Film capacitors – Short Definition of Terms|url=http://www.epcos.com/web/generator/Web/Sections/ProductCatalog/Capacitors/FilmCapacitors/PDF/PDF__DefinitionOfTerms,property=Data__en.pdf;/PDF_DefinitionOfTerms.pdf|access-date=May 22, 2012|page=2|archive-date=September 5, 2012|archive-url=https://web.archive.org/web/20120905113706/http://www.epcos.com/web/generator/Web/Sections/ProductCatalog/Capacitors/FilmCapacitors/PDF/PDF__DefinitionOfTerms,property=Data__en.pdf;/PDF_DefinitionOfTerms.pdf|url-status=dead}}</ref>}}<ref>Basics of AC Drives, p. Hardware – Part 2: slide 7 of 9</ref>

===Operator interface===
The operator interface provides a means for an operator to start and stop the motor and adjust the operating speed. The VFD may also be controlled by a [[programmable logic controller]] through [[Modbus]] or another similar interface. Additional operator control functions might include reversing, and switching between manual speed adjustment and automatic control from an external [[process control]] signal. The operator interface often includes an [[alphanumeric]] display or indication lights and meters to provide information about the operation of the drive. An operator interface keypad and display unit is often provided on the front of the VFD controller as shown in the photograph above. The keypad display can often be cable-connected and mounted a short distance from the VFD controller. Most are also provided with [[input/output|input and output]] (I/O) terminals for connecting push buttons, switches, and other operator interface devices or control signals. A [[serial communications]] [[Computer port (hardware)|port]] is also often available to allow the VFD to be configured, adjusted, monitored, and controlled using a computer.<ref name=Bartos21st/><ref name=Cleaveland>{{cite journal | last =Cleaveland | first =Peter | title = AC Adjustable Speed Drives | journal =Control Engineering | date =November 1, 2007 }}</ref><ref>Campbell, pp. 107–129</ref>

===Speed control===
There are two main ways to control the speed of a VFD; networked or hardwired. Networked involves transmitting the intended speed over a communication protocol such as [[Modbus]], [[Modbus]]/[[Transmission Control Protocol|TCP]], [[EtherNet/IP]], or via a keypad using [[Display Serial Interface]] while hardwired involves a pure electrical means of communication. Typical means of hardwired communication are: [[4-20mA]], 0-10VDC, or using the internal 24VDC power supply with a [[potentiometer]]. Speed can also be controlled remotely and locally. Remote control instructs the VFD to ignore speed commands from the keypad while local control instructs the VFD to ignore external control and only abide by the keypad.

==Programming a VFD==
Depending on the model a VFD's operating parameters can be programmed via: dedicated programming software, internal keypad, external keypad, or SD card. VFDs will often block out most programming changes while running. Typical parameters that need to be set include: motor nameplate information, speed reference source, on/off control source and braking control. It is also common for VFDs to provide debugging information such as fault codes and the states of the input signals.

==Starting and software behavior==
Most VFDs allow auto-starting to be enabled. Which will drive the output to a designated frequency after a power cycle, or after a fault has been cleared, or after the emergency stop signal has been restored (generally emergency stops are active low logic). One popular way to control a VFD is to enable auto-start and place L1, L2, and L3 into a contactor. Powering on the contactor thus turns on the drive and has it output to a designated speed. Depending on the sophistication of the drive multiple auto-starting behavior can be developed e.g. the drive auto-starts on power up but does not auto-start from clearing an emergency stop until a reset has been cycled.

===Drive operation===
[[File:Four quadrant motion control of a motor.jpg|right|thumb|300px|Electric motor speed-torque chart]]
Referring to the accompanying chart, drive applications can be categorized as single-quadrant, two-quadrant, or four-quadrant; the chart's four quadrants are defined as follows:<ref name=TGN8>{{cite web|title=Technical guide No. 8 – Electrical Braking|url=https://library.e.abb.com/public/20be376000f34dd6b9c513580cf56423/Technical_guide_No_8_3AFE64362534_RevC.pdf|access-date=April 20, 2012}}</ref><ref name=EnergyRegen>{{cite web|title=Energy Regeneration|url=http://www.freescale.com/files/training_pdf/WBNR_ENERGY_REGENERATION.pdf|access-date=April 20, 2012}}</ref><ref name="ReferenceA">Basics of AC Drives, pp. Hardware – Part 1: slides 9–10 of 11</ref> 
* Quadrant I – Driving or motoring,<ref>Energy Regeneration, slide 3</ref> forward [[acceleration|accelerating]] quadrant with positive speed and torque 
* Quadrant II – Generating or braking, forward braking-[[deceleration|decelerating]] quadrant with positive speed and negative torque
* Quadrant III – Driving or motoring, reverse accelerating quadrant with negative speed and torque
* Quadrant IV – Generating or braking, reverse braking-decelerating quadrant with negative speed and positive torque.

Most applications involve single-quadrant loads operating in quadrant I, such as in variable-torque (e.g. centrifugal pumps or fans) and certain constant-torque (e.g. extruders) loads.

Certain applications involve two-quadrant loads operating in quadrant I and II where the speed is positive but the torque changes [[electrical polarity|polarity]] as in case of a fan decelerating faster than natural mechanical losses. Some sources define two-quadrant drives as loads operating in quadrants I and III where the speed and torque is same (positive or negative) polarity in both directions.

Certain high-performance applications involve four-quadrant loads (Quadrants I to IV) where the speed and torque can be in any direction such as in hoists, elevators, and hilly conveyors. Regeneration can occur only in the drive's DC link bus when inverter voltage is smaller in magnitude than the motor back-[[Electromotive force|EMF]] and inverter voltage and back-EMF are the same polarity.<ref>Energy Regeneration, slide 6</ref>

In starting a motor, a VFD initially applies a low frequency and voltage, thus avoiding high inrush current associated with [[Motor controller|direct-on-line starting]]. After the start of the VFD, the applied frequency and voltage are increased at a controlled rate or ramped up to accelerate the load. This starting method typically allows a motor to develop 150% of its rated torque while the VFD is drawing less than 50% of its rated current from the mains in the low-speed range. A VFD can be adjusted to produce a steady 150% starting torque from standstill right up to full speed.<ref>Campbell, pp. 95–102</ref> However, motor cooling deteriorates and can result in overheating as speed decreases such that prolonged low-speed operation with significant torque is not usually possible without separately motorized fan ventilation.

With a VFD, the stopping sequence is just the opposite as the starting sequence. The frequency and voltage applied to the motor are ramped down at a controlled rate. When the frequency approaches zero, the motor is shut off. A small amount of braking torque is available to help decelerate the load a little faster than it would stop if the motor were simply switched off and allowed to coast. Additional braking torque can be obtained by adding a braking circuit (resistor controlled by a transistor) to dissipate the braking energy. With a four-quadrant rectifier (active front-end), the VFD is able to brake the load by applying a reverse torque and injecting the energy back to the AC line.

==Benefits==

===Energy savings===
Many fixed-speed motor load applications that are supplied direct from AC line power can save energy when they are operated at variable speed by means of VFD. Such energy cost savings are especially pronounced in variable-torque centrifugal fan and pump applications, where the load's torque and power vary with the square and [[cube (algebra)|cube]], respectively, of the speed. This change gives a large power reduction compared to fixed-speed operation for a relatively small reduction in speed. For example, at 63% speed a motor load consumes only 25% of its full-speed power. This reduction is in accordance with [[affinity laws]] that define the relationship between various centrifugal load variables.

In the United States, an estimated 60–65% of electrical energy is used to supply motors, 75% of which are variable-torque fan, pump, and compressor loads.<ref name="Bose (2009b)">{{cite journal|last=Bose|first=Bimal K.|title=The Past, Present, and Future of Power Electronics|journal=IEEE Industrial Electronics Magazine|date=June 2009|volume=3|issue=2|page=9|doi=10.1109/MIE.2009.932709}}</ref> Eighteen percent of the energy used in the 40 million motors in the U.S. could be saved by efficient energy improvement technologies such as VFDs.<ref name="Spear (2005)">{{cite web|last=Spear|first=Mike|title=Adjustable Speed Drives: Drive Up Energy Efficiency|url=http://www.chemicalprocessing.com/articles/2005/489.html?page=1|publisher=ChemicalProcessing.com|access-date=January 27, 2012}}</ref><ref name="Bose (2009a)">{{cite journal|last=Bose|first=B. K.|title=Power Electronics and Motor Drives Recent Progress and Perspective|journal=IEEE Transactions on Industrial Electronics|date=February 2009|volume=56|issue=2|pages=581–588|doi=10.1109/tie.2008.2002726|s2cid=6139738}}</ref>

Only about 3% of the total installed base of AC motors are provided with AC drives.<ref name="TG4">{{cite web|url=https://library.e.abb.com/public/d3c711ec2acddb18c125788f002cf5da/ABB_Technical_guide_No_4_REVC.pdf|title=Guide to Variable Speed Drives – Technical Guide No. 4|access-date=January 27, 2012}}</ref> However, it is estimated that drive technology is adopted in as many as 30–40% of all newly installed motors.<ref name="Motoring Ahead 1/11">{{cite web|last=Lendenmann|first=Heinz|title=Motoring Ahead|url=http://www.lead-central.com/AssetManager/02427e68-6f15-4f3a-9749-d37abf613741/Documents/APW2012/Low%20Voltage%20Drives%20Motors/ABB-136_WPO_Motoring%20ahead.pdf|access-date=April 18, 2012|display-authors=etal}}</ref>

An energy consumption breakdown of the global population of AC motor installations is as shown in the following table:

{| class="wikitable"
|+ Global population of motors, 2009<ref name="Waide (2011)">{{cite web|last=Waide|first=Paul|author2=Brunner, Conrad U.|title=Energy-Efficiency Policy Opportunities for Electric Motor-Driven Systems|url=http://www.iea.org/publications/freepublications/publication/EE_for_ElectricSystems.pdf|publisher=International Energy Agency|year=2011|access-date=January 27, 2012|archive-date=August 10, 2017|archive-url=https://web.archive.org/web/20170810122020/http://www.iea.org/publications/freepublications/publication/EE_for_ElectricSystems.pdf|url-status=dead}}</ref>
!
! Small
! General Purpose – Medium-Size
! Large
|-
! Power
| 10 W – 750 W
| 0.75&nbsp;kW – 375&nbsp;kW
| 375&nbsp;kW – 10000&nbsp;kW
|-
! Phase, voltage
| 1-ph., <240 V
| 3-ph., 200 V to 1 kV
| 3-ph., 1 kV to 20 kV
|-
! % total motor energy
| 9%
| 68%
| 23%
|-
! Total stock
| 2&nbsp;billion
| 230 million
| 0.6&nbsp;million
|-
|}

===Control performance===
AC drives are used to bring about process and quality improvements in industrial and commercial applications' acceleration, flow, monitoring, pressure, speed, temperature, tension, and torque.<ref>Basics of AC drives, p. Overview: slide 5 of 6</ref>

Fixed-speed loads subject the motor to a high starting torque and to current surges that are up to eight times the full-load current. AC drives instead gradually ramp the motor up to operating speed to lessen mechanical and electrical stress, reducing maintenance and repair costs, and extending the life of the motor and the driven equipment.

Variable-speed drives can also run a motor in specialized patterns to further minimize mechanical and electrical stress. For example, an [[logistic function|S-curve pattern]] can be applied to a conveyor application for smoother deceleration and acceleration control, which reduces the backlash that can occur when a conveyor is accelerating or decelerating.

Performance factors tending to favor the use of DC drives over AC drives include such requirements as continuous operation at low speed, four-quadrant operation with regeneration, frequent acceleration and deceleration routines, and need for the motor to be protected for a hazardous area.<ref name="ABB (DC vs. AC)">{{cite web|title=DC or AC Drives? A Guide for Users of Variable-Speed drives (VSDs)|url=http://www05.abb.com/global/scot/scot239.nsf/veritydisplay/046e0b4cdc2fe50ac1257466003a8443/$file/3adw000059.pdf|access-date=March 22, 2012|page=11}}</ref> The following table compares AC and DC drives according to certain key parameters:<ref name="Rockell (AC vs. DC)">{{cite web|title=AC and DC Variable Speed Drives Application Considerations|url=http://www.reliance.com/prodserv/standriv/appnotes/d7725.pdf|access-date=March 22, 2012|page=2}}</ref><ref name="Drury (2009)">{{cite book|last=Drury|first=Bill|title=The Control Techniques Drives and Controls Handbook|url=https://archive.org/details/controltechnique00drur|url-access=limited|year=2009|publisher=Institution of Engineering and Technology|location=Stevenage, Herts, UK|isbn=978-1-84919-101-2|page=[https://archive.org/details/controltechnique00drur/page/n514 474]|edition=2nd}}</ref><ref name="Kang (2009)">{{cite web|last=Kang|first=Jun|title=General Purpose Permanent Magnet Motor Drive without Speed and Position Sensor|url=http://www.emsdrives.com/site/dmdrive.nsf/link2/NKOE-7U9HJG/$file/WP.AFD.05.pdf|access-date=September 7, 2012|archive-url=https://web.archive.org/web/20130718000207/http://www.emsdrives.com/site/dmdrive.nsf/link2/NKOE-7U9HJG/$file/WP.AFD.05.pdf|archive-date=July 18, 2013|url-status=dead}}</ref> 
{| class="wikitable"
|-
! Drive type  !! DC !! AC VFD !! AC VFD || AC VFD || AC VFD
|-
! Control platform !! Brush type DC !! V/Hz control !! Vector control  !! Vector control || Vector control
|-
! Control criteria !! Closed-loop !! Open-loop !! Open-loop !! Closed-loop || Open-loop w. HFI^
|-
! Motor !! [[direct current|DC]] !! [[induction motor|IM]] !! [[induction motor|IM]] !! [[induction motor|IM]] || Interior [[permanent magnet|PM]]
|-
| Typical speed regulation (%) || 0.01 ||  1 || 0.5 || 0.01 || 0.02
|-
| Typical speed range at constant torque (%) || 0–100 || 10–100 || 3–100 || 0–100 || 0–100
|-
| Min. speed at 100% torque (% of base) || Standstill || 8% || 2% || Standstill || Standstill (200%)
|-
| Multiple-motor operation recommended ||No || Yes || No || No || No
|-
| Fault protection (Fused only or inherent to drive) || Fused only || Inherent || Inherent || Inherent || Inherent
|-
| Maintenance || (Brushes) || Low || Low || Low || Low
|-
| Feedback device || [[Tachometer]] or [[rotary encoder|encoder]] || N/A || N/A || Encoder || N/A
|}

^ High-frequency injection

==VFD types and ratings==

===Generic topologies===
[[File:VSI Topology.jpg|right|thumb|300px|Topology of VSI drive]]
[[File:CS Inverter Topology.jpg|right|thumb|300px|Topology of CSI drive]]
[[File:6SWS.jpg|right|thumb|200px|Six-step drive waveforms]]
[[File:MC Topology.jpg|right|thumb|200px|Topology of direct matrix converter]]
AC drives can be classified according to the following generic topologies:{{efn|A topology is defined in power electronics parlance as the relationship between AC drives' various elements.}}<ref name=Morris>{{cite web|first=Ewan|last=Morris|author2=Armitage, David|title=A Guide to Standard Medium Voltage Variable Speed Drives, Part 2|url=https://library.e.abb.com/public/4fb66e46af347939c1256ed800338956/Fact%20Packs%20Part2.pdf|pages= 7–13|access-date=March 16, 2012}}</ref><ref name="Paes (2011)">{{cite journal|last=Paes|first=Richard|title=An Overview of Medium Voltage AC Adjustable Speed Drives and IEEE Std. 1566 – Standard for Performance of Adjustable Speed AC Drives Rated 375 kW and Larger|journal=Joint Power Engineering Society-Industrial Applications Society Technical Seminar|publisher=IEEE Southern Alberta Chapter|date=June 2011|pages= 1–78}}</ref> 
* '''Voltage-source inverter (VSI) drive topologies''' (see image):  In a VSI drive, the DC output of the [[diode]]-bridge converter stores energy in the capacitor bus to supply stiff voltage input to the inverter. The vast majority of drives are VSI type with PWM voltage output.{{efn|The term PWM is often used to mean VSI-PWM, which is misleading as not only VSI drives are with PWM output.}}
* '''Current-source inverter (CSI) drive topologies''' (see image): In a CSI drive, the DC output of the [[silicon controlled rectifier|SCR]]-bridge converter stores energy in series-[[Inductor]] connection to supply stiff current input to the inverter. CSI drives can be operated with either PWM or six-step waveform output.
* '''Six-step inverter drive topologies''' (see image):{{efn|The term six-step refers strictly speaking to an inverter waveform output alternative to PWM, some drives being configured as combined six-step and PWM options.}}<ref name=McMurray>{{cite journal|last=McMurray|first=William|title=Power Electronic Circuit Topology|journal=Proceedings of the IEEE|date=April 1988|volume=76|issue=4|pages=428–437|doi=10.1109/5.4428}}</ref> Now largely obsolete, six-step drives can be either VSI or CSI type and are also referred to as variable-voltage inverter drives, [[pulse-amplitude modulation]] (PAM) drives,<ref name="Carrow (2000)">{{cite book|last=Carrow|first=Robert S.|title=Electrician's Technical Reference: Variable Frequency Drives|year=2000|publisher=Delmar Thomson Learning|location=Albany, NY|isbn=978-0-7668-1923-8|pages=51}}</ref> [[power inverter|square-wave]] drives or [[chopper (electronic)|D.C. chopper]] inverter drives.<ref>Drury, p. 6</ref> In a six-step drive, the DC output of the SCR-bridge converter is smoothed via capacitor bus and series-reactor connection to supply via [[Darlington Pair]] or [[Insulated gate bipolar transistor|IGBT]] inverter quasi-sinusoidal, six-step voltage or current input to an induction motor.<ref name="Williams (2003)">{{cite journal|last=Sandy, Williams|author2=Baillie, Alastair |author3=Shipp, David |title=Understanding VSDs with ESPs – A Practical Checklist|publisher=Society of Petroleum Engineers|year=2003}}</ref> 
* '''Load commutated inverter (LCI) drive topologies''':  In an LCI drive (a special CSI case), the DC output of the SCR-bridge converter stores energy via DC link inductor circuit to supply stiff quasi-sinusoidal six-step current output of a second SCR-bridge's inverter and an over-excited synchronous machine.Low-cost SCR-thyristor-based LCI fed synchronous motor drives are often used in high-power low-dynamic-performance fan, pump and compressor applications rated up to 100 MW.<ref name=Wu-2017>{{cite book |last1=Wu |first1=B. |last2=Narimani |first2=Mehdi |title=High-power converters and AC drives |date=2017 |publisher=Wiley-IEEE Press |location=Hoboken, N.J. |isbn=9781119156031 |page=318 |edition=2nd|url=https://ieeexplore.ieee.org/servlet/opac?bknumber=7823162}} 480 pages, copyright 2017.</ref>
* '''Cycloconverter or matrix converter (MC) topologies''' (see image):  [[Cycloconverter]]s and MCs are [[AC/AC converter|AC-AC converter]]s that have no intermediate DC link for energy storage. A cycloconverter operates as a three-phase current source via three anti-parallel-connected SCR-bridges in six-pulse configuration, each cycloconverter phase acting selectively to convert fixed line frequency AC voltage to an alternating voltage at a variable load frequency. MC drives are IGBT-based. 
* '''Doubly fed slip recovery system topologies''':  A [[doubly fed electric machine|doubly fed]] [[Induction motor#Slip|slip]] recovery system feeds rectified slip power to a smoothing reactor to supply power to the AC [[supply network]] via an inverter, the speed of the motor being controlled by adjusting the DC current.

===Control platforms===
{{see also|Dqo transformation|Alpha–beta transformation}}
 
Most drives use one or more of the following control platforms:<ref name=Morris/><ref>Drury, pp. 6–9</ref>
* [[Scalar control]]
* [[Vector control (motor)|Vector control]] (VC)
** [[Field-oriented control]] (FOC) 
* [[Direct torque control]] (DTC).

===Load torque and power characteristics===
Variable-frequency drives are also categorized by the following load torque and power characteristics:
* Variable torque, such as in centrifugal fan, pump, and blower applications
* Constant torque, such as in conveyor and positive-displacement pump applications
* Constant power, such as in machine tool and traction applications.

===Available power ratings===
VFDs are available with voltage and current ratings covering a wide range of single-phase and multi-phase AC motors. Low-voltage (LV) drives are designed to operate at output voltages equal to or less than 690&nbsp;V. While motor-application LV drives are available in ratings of up to the order of 5&nbsp;or&nbsp;6 MW,<ref name=ACS800Catalog>{{cite journal | title = ACS800 Catalog – Single Drives 0.55 to 5600 kW| date =July 19, 2009}}</ref> economic considerations typically favor medium-voltage (MV) drives with much lower power ratings.  Different MV drive topologies (see Table 2) are configured in accordance with the voltage/current-combination ratings used in different drive controllers' switching devices<ref name="Wu05">{{cite web|last=Wu|first=Bin|title=High-Power Converters and AC Drives|url=http://www.ee.ryerson.ca/~bwu/seminars/pesc05_seminar.pdf|publisher=IEEE PES|year=2005|access-date=February 3, 2012|page=slide 22}}</ref> such that any given voltage rating is greater than or equal to one to the following standard nominal motor voltage ratings: generally either {{frac|2|3|4}}.16&nbsp;kV (60&nbsp;Hz) or {{frac|3|3|6}}.6&nbsp;kV (50&nbsp;Hz), with one thyristor manufacturer rated for up to 12&nbsp;kV switching. In some applications a step-up [[transformer]] is placed between a LV drive and a MV motor load. MV drives are typically rated for motor applications greater than between about {{convert|375| and |750|kW|hp|abbr=on}}. MV drives have historically required considerably more application design effort than required for LV drive applications.<ref name=BartosMV>{{cite journal|last=Bartos | first =Frank J. | title =Medium-Voltage AC Drives Shed Custom Image  | journal =Control Engineering | date =February 1, 2000 }}</ref><ref name="Lockley (2008)">{{cite journal|last=Lockley|first=Bill |author2=Wood, Barry |author3=Paes, Richard |author4=DeWinter, Frank|title=Standard 1566 for (Un)Familiar Hands|journal=IEEE Industry Applications Magazine|date=Jan–Feb 2008|volume=14|issue=1|pages=21–28|doi=10.1109/MIA.2007.909800|s2cid=16455550 }}</ref> The power rating of MV drives can reach {{convert|100|MW|hp|abbr=on}}, a range of different drive topologies being involved for different rating, performance, power quality, and reliability requirements.<ref>Wu, slide 159</ref><ref name="Klug (2005)"/><ref name="MVTopology"/>

===Drives by machines and detailed topologies===
It is lastly useful to relate VFDs in terms of the following two classifications:
* In terms of various AC machines as shown in Table 1 below<ref>Bose (2006) pp. Chapter 6–8, especially pp. 328, 397, 481</ref><ref name=USDOEVSPGuide>{{cite web|title=Variable Speed Pumping, A Guide to Successful Applications, Executive Summary|url=http://www1.eere.energy.gov/industry/bestpractices/pdfs/variable_speed_pumping.pdf|publisher=USDOE – Europump – Hydraulic Institute|access-date=January 29, 2012|page=9, Fig. ES–7|date=May 2004|archive-url=https://web.archive.org/web/20111027042737/http://www1.eere.energy.gov/industry/bestpractices/pdfs/variable_speed_pumping.pdf|archive-date=October 27, 2011|url-status=dead}}</ref>
* In terms of various detailed [[AC/AC converter|AC-AC converter]] topologies shown in Tables 2 and 3 below.<ref name=Morris/><ref name="Paes (2011)"/><ref name="Klug (2005)">{{cite book |doi=10.1109/EPE.2005.219669|chapter=High power medium voltage drives - innovations, portfolio, trends|title=2005 European Conference on Power Electronics and Applications|pages=10 pp.–P.10|year=2005|last1=Klug|first1=R.-D.|last2=Klaassen|first2=N.|isbn=90-75815-09-3|s2cid=15001359}}</ref><ref name="MVTopology">{{cite web|title=TMEiC Medium Voltage Drive Evolution|url=https://www.tmeic.com/sites/default/files/assets/files/library/MV%20Drive%20Evolution%20Brochure-2019web.pdf|access-date=February 3, 2012}}</ref><ref>Wu, Slide 159</ref><ref name=Rashid06>{{cite book|last=Rashid|first=Muhammad H., (Ed.)|title=Power Electronics Handbook: Devices, Circuits, and Applications|url=https://archive.org/details/powerelectronics00rash_987|url-access=limited|year=2006|publisher=Academic|location=Burlington, MA|isbn=978-0-12-088479-7|page=[https://archive.org/details/powerelectronics00rash_987/page/n887 903]|edition=2nd}}</ref><ref>{{cite journal |author1=J. Rodriguez |author2=Jih-Sheng Lai |author3=Fang Zheng Peng |title=Multilevel Inverters: A Survey of Topologies, Controls, and Applications|journal=IEEE Transactions on Industrial Electronics|volume=49|issue=4|pages=724–738|doi=10.1109/TIE.2002.801052|year=2002|hdl=10533/173647|hdl-access=free}}</ref><ref name="Ikonen (2005)">{{cite web|last=Ikonen|first=Mika|title=Two-Level and Three-Level Converter Comparison in Wind Power Application|year=2005|url=http://www.elkraft.ntnu.no/smola2005/Topics/15.pdf |archive-url=https://web.archive.org/web/20090419113306/http://www.elkraft.ntnu.no/smola2005/Topics/15.pdf |archive-date=April 19, 2009 |website=Institutt for elkraftteknikk |display-authors=etal}}</ref><ref name="Kouro (2012)">{{cite journal|last=Kouro|first=Samir|author2=Rodriguez, Jose |author3=Wu, Bin |author4=Bernet, Steffen |author5= Perez, Marcelo |title=Powering the Future of Industry: High-Power Adjustable Speed Drive Topologies|journal=IEEE Industry Applications Magazine|date=Jul–Aug 2012|volume=18|issue=4|pages=26–39|doi=10.1109/MIAS.2012.2192231|s2cid=5825955}}</ref>

{{hidden begin|border=1px #aaa solid|title={{center|Table 1: Drives by machines}}}}
{{clade
 |label1=Machines
 |1={{clade
  |label1=[[Induction motor|Induction]]^^^
  |1={{clade
   |label1=Cage rotor
   |1={{clade
    |1=CSI or VSI (six-step or [[pulse-width modulation|PWM]]), cycloconverter, matrix
   }}
   |label2=WRIM
   |2={{clade
    |1=Electro-mechanical
   }}
   |label3=[[Doubly fed electric machine|Doubly fed WRIM]]
   |3={{clade
    |1=Slip energy recovery (Kramer/Scherbius)
   }}
  }}
  |label2=[[Synchronous motor|Synchronous]]^^^
  |2={{clade
   |label1=WFSM
   |1={{clade
    |1=CSI (LCI), cycloconverter, VSI
   }}
   |label2=PM
   |2={{clade
    |1=Axial or disk
    |label2=Radial
    |2={{clade
     |1=Interior
     |label2=Surface
     |2={{clade
      |label1=[[Brushless DC electric motor|Trapezoidal BLDM]], Sinusoidal PMSM
      |1={{clade
       |1=VSI
      }}
     }}
    }}
   }}
   |label3=SyRM
   |3={{clade
    |1=VSI
   }}
  }}
  |label3=VRM^^^
  |3={{clade
   |label1=[[Switched reluctance motor]], [[Stepper motor]]
   |1={{clade
    |1=VSI
   }}
  }}
 }}
}}
{{hidden end}}
{{hidden begin|border=1px #aaa solid|title={{center|Table 2: Drives by detailed [[AC/AC converter|AC-AC converter]] topologies }}}}
{{clade
 |1=Topologies
 |label2=Indirect [[AC/AC converter|AC-AC]]
 |2={{clade
  |label1=LV
  |1={{clade
   |label1=[[Insulated gate bipolar transistor|IGBT]]^
   |1={{clade
    |label1=2-level VSI [[Pulse-width modulation|PWM]]
    |1={{clade
     |1= Great majority of LV drive makes
    }}
    |label2=3-level VSI [[Pulse-width modulation|PWM]]
    |2={{clade
     |1= Primarily Yaskawa
    }}
   }}
   |label2=([[Silicon controlled rectifier|SCR]]/CSI or diode/VSI)+[[Insulated gate bipolar transistor|IGBT]]
   |2={{clade
    |label1=Six-step or [[Pulse-amplitude modulation|PAM]]
    |1={{clade
     |1= For example Baker-Hughes
    }}
   }}
  }}
  |label2=MV
  |2={{clade
   |label1=VSI
   |1={{clade
    |label1=GCT^ 
    |1={{clade
     |label1=3-level NPC inverter
     |1={{clade
      |1= Primarily ABB, GE
     }}
    }}
|label2=[[Insulated gate bipolar transistor|IGBT]]^
    |2={{clade
     |label1=2-level inverter
     |1={{clade
      |1=Primarily GE-Converteam
     }}
     |label2=3-level NPC inverter 
     |2={{clade
      |1=Primarily Eaton, GE-Toshiba, Siemens
     }}
     |label3=Multilevel CHB inverter
     |3={{clade
      |1=Primarily GE, Siemens (Robicon), Toshiba
     }}
     |label4=Flying-capacitor inverter drive
     |4={{clade
      |1=Primarily GE-Converteam
     }}
     |label5=NPC/H-bridge inverter drive
     |5={{clade
      |1=Primarily Toshiba
     }}
    }}
   }}
  |label2=CSI
   |2={{clade
    |label1= Primarily ABB, GE-Converteam, Siemens
    |1={{clade
     |label1=[[Silicon controlled rectifier|SCR]]^^
     |1={{clade
      |1=LCI
     }}
    }}
    |label2=Primarily A-B
    |2={{clade
     |label1=SGCT^^ with AFE
     |1={{clade
      |1= CSR [[Pulse-width modulation|PWM]] / CSI [[Pulse-width modulation|PWM]] inverter with CME
     }}
     |label2=[[Silicon controlled rectifier|SCR]] + SGCT
     |2={{clade
      |1= 18-pulse CSI [[Pulse-width modulation|PWM]] drive
     }}
     |label3=GTO^^
     |3={{clade
      |1= Capacitor assisted CSI [[Pulse-width modulation|PWM]] drive (legacy)
     }}
    }}
   }}
  }}
 }}
 |label3=Direct [[AC/AC converter|AC-AC]]
 |3={{clade
  |label1=[[Silicon controlled rectifier|SCR]]
  |1={{clade
   |label1=[[Cycloconverter]]
   |1={{clade
    |1=Primarily ABB, Siemens, GE Converteam
   }}
  }}
  |label2=[[Insulated gate bipolar transistor|IGBT]]
  |2={{clade
   |label1=Matrix
   |1={{clade
    |1=Primarily Yaskawa
   }}
  }}
 }}
}}
{{hidden end}}
{{hidden begin|border=1px #aaa solid|title={{center|Table 3: Topology diagrams}}}}
<gallery widths=210 heights=325>
File:2LI Topology.svg|Simplified 2-Level Inverter Topology
File:3LI Topology.jpg|Simplified Neutral Point Clamped 3-Level Inverter Topology
File:CHBI Topology.jpg|Simplified Cascaded H-bridge Inverter Topology
File:FCI Topology.jpg|Simplified Flying Capacitor Inverter 4-Level Topology
File:NPC-HBI Topology.jpg|Simplified Neutral Point Clamped H-bridge Inverter Topology
</gallery>
{{hidden end}}
{{hidden begin|border=1px #aaa solid|title={{center|Legend for Tables 1 to 3}}}}
{| class="wikitable"|style=font-size:50%;line-height:50%
|-
| ^  || Inverter switching device (with std. diode rectifier)
|-
| ^^ || Inverter and rectifier switching device
|-
| ^^^|| Rotating or linear
|-
| AFE || Active front end
|-
| BLDM || PM trapezoid machine ([[Brushless DC electric motor]])
|-
| CME || Common mode elimination
|-
| CHB || Cascaded H-bridge
|-
| CSI || [[Current source inverter]]
|-
| CSR || Current source rectifier
|-
| GCT || Gate-controlled thyristor
|-
| GTO || [[Gate turn-off thyristor]]
|-
| IGBT || [[Insulated gate bipolar transistor]]
|-
| LCI || Load commutated inverter
|-
| LV || Low voltage
|-
| MV || Medium voltage
|-
| NPC || Neutral point clamped
|-
| PAM || [[Pulse-amplitude modulation]]
|-
| PM || Permanent magnet
|-
| PMSM || [[Permanent magnet synchronous generator]]
|-
| PWM || [[Pulse-width modulation]]
|-
| SCR || [[Silicon controlled rectifier]]
|-
| SGCT || Symmetrical gate controlled thyristor
|-
| SRM || [[Switched reluctance motor]]
|-
| SyRM || Synchronous reluctance machine
|-
| VRM || Variable-reluctance machine
|-
| VSI || Voltage source inverter
|-
| VVI || Variable-voltage inverter
|-
| WFSM || Wound-field synchronous machine
|-
| WRIM || Wound-rotor induction motor
|-
|}
{{hidden end}}

==Application considerations==

===AC line harmonics===
Note of clarification:.{{efn|The harmonics treatment that follows is limited for simplication reasons to LV VSI-PWM drives.}}

While [[harmonics (electrical power)|harmonics]] in the PWM output can easily be filtered by carrier-frequency-related filter inductance to supply near-sinusoidal currents to the motor load,<ref name="Bose 2006, p. 183"/> the VFD's diode-bridge rectifier converts AC line voltage to DC voltage output by super-imposing [[nonlinear system|non-linear]] half-phase current pulses thus creating harmonic current distortion, and hence voltage distortion, of the AC line input. When the VFD loads are relatively small in comparison to the large, stiff power system available from the [[electric power company]], the effects of VFD harmonic distortion of the [[Electrical grid|AC grid]] can often be within acceptable limits. Furthermore, in low-voltage networks, harmonics caused by single-phase equipment such as computers and TVs are partially cancelled by three-phase diode bridge harmonics because their 5th and 7th harmonics are in counterphase.<ref name="Hansen (2000)">{{cite journal|last=Janssen|first=Hansen|author2=Neilsen, Peter |author3=Blaabjerg, Frede |title=Harmonic Cancellation by Mixing Non-Linear Single-Phase and Three-Phase Loads|journal=IEEE Transactions on Industry Applications|date=Jan–Dec 2000|volume=36|issue=1}}</ref> However, when the proportion of VFD and other non-linear load compared to total load or of non-linear load compared to the stiffness at the AC power supply, or both, is relatively large enough, the load can have a negative impact on the AC power waveform available to other power company customers in the same grid.

When the power company's voltage becomes distorted due to harmonics, losses in other loads such as normal fixed-speed AC motors are increased. This condition may lead to overheating and shorter operating life. Also, [[Electrical substation|substation]] transformers and compensation capacitors are affected negatively. In particular, capacitors can cause resonance conditions that can unacceptably magnify harmonic levels. To limit the voltage distortion, owners of VFD load may be required to install filtering equipment to reduce harmonic distortion below acceptable limits. Alternatively, the utility may adopt a solution by installing filtering equipment of its own at substations affected by the large amount of VFD equipment being used. In high-power installations, harmonic distortion can be reduced by supplying multi-pulse rectifier-bridge VFDs from transformers with multiple phase-shifted windings.<ref name="Technical Guide No. 6">{{cite web | title = Guide to Harmonics with AC Drives – Technical Guide No. 6 |website=ABB | date =May 17, 2002 | url =http://www05.abb.com/global/scot/scot201.nsf/veritydisplay/6dff3290e5f2d651c125788d00393a58/$file/ABB_Technical%20guide%20No.6_REVC.pdf |archive-url=https://web.archive.org/web/20110626081829/http://www05.abb.com/global/scot/scot201.nsf/veritydisplay/6dff3290e5f2d651c125788d00393a58/$file/ABB_Technical%20guide%20No.6_REVC.pdf |archive-date=June 26, 2011 | access-date =July 29, 2009}}</ref>

It is also possible to replace the standard diode-bridge rectifier with a bi-directional IGBT switching device bridge mirroring the standard inverter which uses IGBT switching device output to the motor. Such rectifiers are referred to by various designations including active [[Infeed|infeed converter]] (AIC), [[active rectification|active rectifier]], IGBT supply unit (ISU), active front end (AFE), or four-quadrant operation. With PWM control and a suitable input reactor, an AFE's AC line current waveform can be nearly sinusoidal. AFE inherently regenerates energy in four-quadrant mode from the DC side to the AC grid. Thus, no braking resistor is needed, and the efficiency of the drive is improved if the drive is frequently required to brake the motor.

Two other harmonics mitigation techniques exploit use of passive or active filters connected to a common bus with at least one VFD branch load on the bus. Passive filters involve the design of one or more [[low-pass filter|low-pass]] LC filter traps, each trap being tuned as required to a harmonic frequency (5th, 7th, 11th, 13th, . . . kq+/-1, where k=integer, q=pulse number of converter).<ref name="IEEE 519">{{cite book |doi=10.1109/IEEESTD.1993.114370|publisher=IEEE|isbn=978-0-7381-0915-2|title=IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems }}</ref>

It is very common practice for power companies or their customers to impose harmonic distortion limits based on [[International Electrotechnical Commission|IEC]] or [[Institute of Electrical and Electronics Engineers|IEEE]] standards. For example, IEEE Standard 519 limits at the customer's connection point call for the maximum individual frequency voltage harmonic to be no more than 3% of the fundamental and call for the voltage [[total harmonic distortion]] (THD) to be no more than 5% for a general AC power supply system.<ref>IEEE 519, pp. 69–70</ref>

===Switching frequency foldback===
One drive uses a default switching frequency setting of 4&nbsp;kHz. Reducing the drive's switching frequency (the carrier-frequency) reduces the heat generated by the [[IGBT]]s.<ref>[[ABB]] [https://library.e.abb.com/public/583ed5f1aa2c4723b325c1473dbaabcb/ACH550-PHPB01U-EN_REVD.pdf ACH550 Switching Frequency Foldback]</ref>

A carrier frequency of at least ten times the desired output frequency is used to establish the PWM switching intervals. A carrier frequency in the range of 2,000 to 16,000&nbsp;Hz is common for LV [low voltage, under 600 Volts AC] VFDs. A higher carrier frequency produces a better sine wave approximation but incurs higher {{ill|switching loss|lt=switching losses|de|Schaltverluste}} in the IGBT, decreasing the overall power conversion efficiency.<ref>{{cite conference|last1=D'Aversa |first1=A. |last2=Hughes |first2=B. |last3=Patel |first3=S. |title=Challenges and solutions of protecting variable speed drive motors |conference=2013 66th Annual Conference for Protective Relay Engineers |date=April 8–11, 2013|page=2 |doi=10.1109/CPRE.2013.6822040 |url=https://cms-cdn.selinc.com/assets/Literature/Publications/Technical%20Papers/6596_ChallengesSolutions_BH_20130211_Web.pdf?v=20190111-001336| publisher =IEEE}}, pp. 250-256.</ref>

===Noise smoothing===
Some drives have a noise smoothing feature that can be turned on to introduce a random variation to the switching frequency. This distributes the acoustic noise over a range of frequencies to lower the peak noise intensity.

===Long-lead effects===
The carrier-frequency pulsed output voltage of a PWM VFD causes rapid rise times in these pulses, the transmission line effects of which must be considered. Since the transmission-line [[impedance (electrical)|impedance]] of the cable and motor are different, pulses tend to reflect back from the motor terminals into the cable. The resulting reflections can produce [[overvoltage]]s equal to twice the DC bus voltage or up to 3.1&nbsp;times the rated line voltage for long cable runs, putting high stress on the cable and motor windings, and eventual insulation failure.  Insulation standards for three-phase motors rated 230&nbsp;V or less adequately protect against such long-lead overvoltages.  On 460&nbsp;V or 575&nbsp;V systems and inverters with 3rd-generation 0.1-microsecond-rise-time IGBTs, the maximum recommended cable distance between VFD and motor is about 50&nbsp;m or 150&nbsp;feet. For emerging SiC MOSFET powered drives, significant overvoltages have been observed at cable lengths as short as 3 meters.<ref>{{Cite news|url=https://dvdtfilter.com/dv-dt-simulation-scenario-1-sic-drive/|title=dv/dt Simulation Scenario 1: SiC Drive – dv/dt Simulation and Mitigation|date=August 16, 2018|work=dv/dt Simulation and Mitigation|access-date=October 18, 2018|language=en-US}}</ref> Solutions to overvoltages caused by long lead lengths include minimizing cable length, lowering carrier frequency, installing dV/dt filters, using inverter-duty-rated motors (that are rated 600&nbsp;V to withstand pulse trains with rise time less than or equal to 0.1&nbsp;microsecond, of 1,600&nbsp;V peak magnitude), and installing LCR low-pass sine wave filters.<ref>Skibinski, p. 274</ref><ref name="Novak (2009)">{{cite web|last=Novak|first=Peter|title=The Basics of Variable-Frequency Drives|url=http://ecmweb.com/power_quality/variable-frequency-drives-20090501/|publisher=EC&M|access-date=April 18, 2012|date=May 1, 2009}}</ref><ref name=Finlayson>{{cite journal|last=Finlayson|first=P.T.|title=Output filters for PWM drives with induction motors|date=Jan–Feb 1998|volume=4|issue=1|pages=46–52|journal=IEEE Industry Applications Magazine|doi=10.1109/2943.644886|s2cid=25469484}}</ref><ref>{{Cite news|url=https://dvdtfilter.com/motor-voltage-waveform-simulation/|title=Motor Voltage Waveform Simulation - dv/dt Simulation and Mitigation|work=dv/dt Simulation and Mitigation|access-date=October 18, 2018|language=en-US}}</ref> Selection of optimum PWM carrier frequency for AC&nbsp;drives involves balancing noise, heat, motor insulation stress, common-mode voltage-induced motor bearing current damage, smooth motor operation, and other factors. Further harmonics attenuation can be obtained by using an LCR low-pass sine wave filter or dV/dt&nbsp;filter.<ref name="Skibinski (2004)">{{cite book|doi=10.1109/PCICON.2004.1352810|publisher=IEEE|isbn=978-0-7803-8698-3|chapter = Line and load friendly drive solutions for long length cable applications in electrical submersible pump applications|title = IEEE Geoscience and Remote Sensing Letters|pages = 269–278|year = 2004|last1 = Skibinski|first1 = G.|last2 = Breit|first2 = S.|s2cid=8945509}}</ref><ref name="Long lead (2006)">{{cite web|title=Application Report Long Drive/Motor Leads|url=https://www.yaskawa.com/pycprd/download/search/detail?n=AR.AFD.05|access-date=February 14, 2012}}</ref><ref name="Malfait (1994)">{{cite journal|last=Malfait|first=A.|author2=Reekman, R. |author3=Belmans, R. |title=Audible Noise and Losses in Variable Speed Induction Motor Drives: Influence of the Squirrel Cage Design and the Switching Frequency|journal=29th Annual Meeting Proceedings of IEEE Industry Applications Society|year=1994|pages=693–700|doi=10.1109/IAS.1994.345435 |s2cid=110917173 }}</ref><ref name="CarrierFAQs">{{cite web|title=Who Cares About Carrier Frequency?|url=http://www.reliance.com/pdf/drives/data_sheets/D7171D.pdf|access-date=February 15, 2012}}</ref>

===Motor bearing currents===
{{Main|Shaft voltage}}

Carrier frequencies above 5&nbsp;kHz are likely to cause bearing damage unless protective measures are taken.<ref>{{Cite web|url=http://www.nrel.gov/docs/fy13osti/56011.pdf|title=Minimize Adverse Motor and Adjustable Speed Drive Interactions &#124; Advanced Manufacturing Office Energy Efficiency and Renewable Energy &#124; U.S. Department of Energy}}</ref>

PWM drives are inherently associated with high-frequency common-mode voltages and currents which may cause trouble with motor bearings.<ref>{{cite journal|url=http://www.plantservices.com/articles/2007/022.html|pages=1–2| title=Bearings and Electricity Don't Match|year=2007|journal=PlantServices.com [Plant Services]|publisher=PtmanMedia|last=Yung|first=Chuck|location=Itasca, IL}}</ref> When these high-frequency voltages find a path to earth through a bearing, transfer of metal or [[electrical discharge machining]] (EDM) sparking occurs between the bearing's ball and the bearing's race. Over time, EDM-based sparking causes erosion in the bearing race that can be seen as a fluting pattern. In large motors, the [[stray capacitance]] of the windings provides paths for high-frequency currents that pass through the motor shaft ends, leading to a circulating type of bearing current.  Poor [[Ground (electricity)|grounding]] of motor stators can lead to shaft-to-ground bearing currents. Small motors with poorly grounded driven equipment are susceptible to high-frequency bearing currents.<ref name="Technical Guide No. 5">{{cite web | title = Bearing Currents in Modern AC Drive Systems – Technical Guide No. 5 |website=ABB | date =December 1, 1999 | url =http://www05.abb.com/global/scot/scot201.nsf/veritydisplay/8c253c2417ed0238c125788f003cca8e/$file/abb_technical_guide_no5_revc.pdf |archive-url=https://web.archive.org/web/20110720013656/http://www05.abb.com/global/scot/scot201.nsf/veritydisplay/8c253c2417ed0238c125788f003cca8e/$file/abb_technical_guide_no5_revc.pdf |archive-date=July 20, 2011 | access-date =June 14, 2011}}</ref>

Prevention of high-frequency bearing current damage uses three approaches: good cabling and grounding practices, interruption of bearing currents, and filtering or damping of common-mode currents through common mode chokes. Good cabling and grounding practices can include use of shielded, symmetrical-geometry power cable to supply the motor, installation of shaft grounding brushes, and conductive bearing grease.  Bearing currents can be interrupted by installation of insulated bearings and specially designed electrostatic-shielded induction motors.  Filtering and damping high-frequency bearing can be done though inserting soft magnetic cores over the three phases giving a high frequency impedance against the common mode or motor bearing currents.  Another approach is to use instead of standard 2-level inverter drives, using either 3-level inverter drives or matrix converters.<ref name="Technical Guide No. 5"/><ref name=Swamy>{{cite book |pages=XLV–LVI, Fig. 16|publisher=IEEE|doi=10.1109/OPTIM.2008.4602333|chapter=Present state and a futuristic vision of motor drive technology|title=2008 11th International Conference on Optimization of Electrical and Electronic Equipment|year=2008|last1=Mahesh Swamy|last2=Tsuneo Kume|isbn=978-1-4244-1544-1|s2cid=39690131}}</ref>

===Dynamic braking===
{{see also|Dynamic braking|Regenerative braking}}
Torque generated by the drive causes the induction motor to run at [[Synchronization (alternating current)|synchronous]] speed less the slip. If the load drives the motor faster than synchronous speed, the motor acts as a [[Electric generator|generator]], converting mechanical power back to electrical power. This power is returned to the drive's DC link element (capacitor or reactor). A DC-link-connected electronic power switch or [[braking chopper|braking DC chopper]] controls dissipation of this power as heat in a set of resistors. Cooling fans may be used to prevent resistor overheating.<ref name="ReferenceA"/>

Dynamic braking wastes braking energy by transforming it to heat. By contrast, regenerative drives recover braking energy by injecting this energy into the AC line. The capital cost of regenerative drives is, however, relatively high.<ref>{{Cite web|url=https://library.e.abb.com/public/20be376000f34dd6b9c513580cf56423/Technical_guide_No_8_3AFE64362534_RevC.pdf|title=Technical Guide No. 8, pp. 26–30}}</ref>

===Regenerative drives===
[[File:KIMO TRA U2 mL Gruppe.jpg|right|thumb|300px|Line regenerative variable frequency drives, showing capacitors (top cylinders) and inductors attached, which filter the regenerated power]]
[[File:2010 Prius Regeneration.jpg|right|thumb|300px|Simplified Drive Schematic for a Popular EHV<ref name="Olszewski (2011)">{{cite web|last=Olszewski|first=Mitch|title=Evaluation of 2010 Toyota Prius Hybrid Electric Drive System|url=http://ecee.colorado.edu/~ecen5017/notes/OakRidge_2010Prius.pdf|publisher=Oak Ridge National Laboratory|access-date=September 26, 2012|year=2011|display-authors=etal|archive-date=December 24, 2012|archive-url=https://web.archive.org/web/20121224134754/http://ecee.colorado.edu/~ecen5017/notes/OakRidge_2010Prius.pdf|url-status=dead}}</ref>]]

Regenerative AC drives have the capacity to recover the braking energy of a load moving faster than the designated motor speed (an ''overhauling'' load) and return it to the power system.<ref>{{Cite web |title=Reliance Electric FlexPak 3000 20FR4042 {{!}} Automation Industrial |url=https://20fr4042.com/blog |access-date=2023-12-09 |website=20fr4042.com |language=en}}</ref>

Cycloconverter, Scherbius, matrix, CSI, and LCI drives inherently allow return of energy from the load to the line, while voltage-source inverters require an additional converter to return energy to the supply.<ref name="Dubey">{{cite book|last=Dubey|first=Gopal K.|title=Fundamentals of Electrical Drives|year=2001|publisher=Alpha Science Int.|location=Pangbourne|isbn=978-1-84265-083-7|edition=2}}</ref><ref>Rashid, p. 902, Table 33.13</ref>

Regeneration is useful in VFDs only where the value of the recovered energy is large compared to the extra cost of a regenerative system,<ref name="Dubey"/> and if the system requires frequent braking and starting. Regenerative VFDs are widely used where speed control of overhauling loads is required.<ref name=Jaeschke/><ref name=Siskind/>
<ref>Campbell, pp. 70–190</ref>

Some examples:
* Conveyor belt drives for manufacturing, which stop every few minutes. While stopped, parts are assembled correctly; once that is done, the belt moves on. 
* A crane, where the hoist motor stops and reverses frequently, and braking is required to slow the load during lowering.
* Plug-in and hybrid electric vehicles of all types (see image and [[Hybrid Synergy Drive]]).

==See also==
{{div col|colwidth=22em}}
* [[Adjustable-speed drive]]
* [[AC/AC Converter]]
* [[Frequency changer]]
* [[Pump]]
* [[Sub-Micro AC Drive]]
* [[Variable-speed air compressor]]
{{div col end}}

==Notes==
{{notelist}}

==References==
{{reflist}}

{{Commons category|Variable frequency drives}}

{{Electric motor}}

{{good article}}

[[Category:Electric motor control]]
[[Category:Electric power systems components]]
[[Category:Frequency changers]]
[[Category:Mechanical power transmission]]
[[Category:Mechanical power control]]
[[Category:Electrical power control]]