AC Drives Stay Vital for the 21st Century

Physical size and weight provide the most visible evidence of the remarkable evolution of ac variable-frequency drives (VFDs) in the past 50 years. However, what's under the skin is even more dramatic for the performance, efficiency, and reliability now delivered by these motor controls. Making it all happen were advances in power-switching transistors, microprocessors, other hardware, plus sof...

By Frank J. Bartos September 1, 2004

This article contains online extra material

  • AC drives

  • Variable-frequency drives

  • Multiple control types in one drive

  • V/Hz control

  • Flux-vector control

  • Sensorless vector control

Physical size and weight provide the most visible evidence of the remarkable evolution of ac variable-frequency drives (VFDs) in the past 50 years. However, what’s under the skin is even more dramatic for the performance, efficiency, and reliability now delivered by these motor controls. Making it all happen were advances in power-switching transistors, microprocessors, other hardware, plus software functions that ease users’ concerns for drive application and maintenance.

Early ac drives operated in open loop but had limited performance. A major step forward was development of field-oriented (flux vector) control for induction motors by Felix Blaschke at Siemens in 1971—followed by others—which eventually pushed VFDs to meet or exceed dc drive performance in many applications. Sensorless-vector control (eliminating a shaft encoder) and other drive algorithm advances followed. And the evolution is accelerating.

At Rockwell Automation, William L. Sinner, product line manager, notes two historic changes affecting the power and control sides of VFDs, respectively. Early ac drives (1980s) employed multiple transistors per phase due to their limited voltage and current ratings. This has changed to all-in-one packages, so that a 10-hp drive today has a structure smaller than one transistor pack of the vintage drive. ‘New generations of transistors continue to be improved as manufacturers develop smaller and more efficient power devices,’ says Sinner. Insulated-gate bipolar transistors (IGBTs) remain present-day workhorse power devices.

On the control side, analog was king at first, giving way to digital control, though initially based on integrated circuits. Microprocessor (MPU)-based digital drives came somewhat later, states Sinner, and at first offered only open-loop (V/Hz) control. Continuing advances in MPUs allowed adding multiple control types in the same drive, with only software parameter changes needed to switch control mode.

Multiple control types, connectivity

Multiple control modes mean state-of-the-art in VFDs. Low-end drives typically offer V/Hz and sensorless-vector control, while higher-end drives originally with flux-vector control later added other control modes. For instance, Rockwell’s PowerFlex 700S with built-in Logix processor has several modes, including servo control. Technology migration across a product line is another trend. Sinner cites low-end PowerFlex 70 as adding vector control and some high-end drives picking up V/Hz control.

Why open-loop control at the high end? For one, reviews Sinner, V/Hz operation enables control of multiple motors from one drive. One drive type for different applications also helps reduce spare parts inventory.

Connectivity is another core VFD feature today. All Rockwell drives come so equipped. In the company’s experience, networked applications now amount to about 50% of all drives—and increases with higher-end units. ‘The percentage of connected drive applications has doubled in the last three years,’ says Sinner.

In the view of Tom Momberger, product manager at Danfoss Drives, ‘Application of microprocessor technology to VFDs is probably the major development responsible for today’s ac drive capabilities.’ For physical changes, he contrasts a typical analog-type, 5-hp ac drive from 1968—an oil-cooled unit that required various manual adjustments to apply the drive—to present day VFDs of fractional size and weight. (See photo of Danfoss 5-hp model VLT drives over time.) Of course, new ac drives have added many other features, such as programming via operator keypad or computer. ‘The microprocessor has made all this possible,’ he states.

Flexibility, intelligence, and user friendliness are state-of the art VFD features, according to Momberger. Flexibility means satisfying numerous applications with one drive type that offers simple open-loop, closed-loop, flux vector, and even near-servo control. ‘This capability lowers the drive’s cost of ownership by reducing on-site inventory, operator training, and replacement part costs,’ he says.

MPUs and advanced diagnostic capabilities allow users to access intelligence built into a drive, thus lowering commissioning cost and downtime. Soft functions, like Automatic Motor Adaptation and software wizards, remove uncertainty in setting-up a drive/motor combination. ‘User-friendly evolution’ of the operator interface into software functions also shortens set-up to reduce potential operator error and simplify interaction with the drive, explains Momberger. New FC-302 Automation Drive from Danfoss addresses all these features (see Online Extra).

PWM, DTC, modularity

Among major ac drive milestones, ABB notes the arrival of industrial pulse-width modulated (PWM)-based drives and introduction of its Direct Torque Control (DTC) in 1995. (See CE International , March 1995, p. 5).

Following earlier R&D, ABB’s first industrial installations of PWM drives took place in the ’70s, explains Ilkka Ikonen, drives marketing communications, at ABB Oy, in Finland. Paper mills and subways set the basis for significant product advancement and robustness. ‘After technology had proved its reliability and competitiveness on these demanding applications, ac drives were accepted as leading control technology, starting to replace dc drives,’ he says.

ABB regards its DTC as an advanced technology, able to control motor torque and speed directly without need for separate control of voltage and frequency. Extremely fast torque-response time and accuracy are claimed, ‘typically 10 times faster than with PWM.’ DTC also is said to optimize motor flux, which improves combined energy efficiency of motor and drive. DTC does not use a modulator and works without motor shaft position or speed feedback. ‘With DTC, 100% torque is available at zero speed and small torque increments can be controlled at low frequencies in less than 1 millisecond,’ states Ikonen.

Modular design plays a large role in today’s ABB ac drives to meet a variety of user needs via ‘configured-to-order products.’ Almost limitless customer options must be accommodated for delivery time, quality, and cost, just like off-the-shelf products.

Bosch Rexroth notes the development of reliable switching devices and microprocessors as two major advances that shaped today’s smaller, more efficient, and robust VFD designs. Says, Peter Fischbach, manager for components, ‘Thyristor or bipolar transistor based isolated power modules and later, insulated-gate bipolar transistors, in combination with sine-modulated PWM control, revolutionized power section and cooling system design.’


Rexroth released a full line of IGBT-based drives in 1988. However, the company’s ac drive developments started much earlier in 1965. Its first high-speed, rack-style industrial VFD was in production in 1968. It allowed induction grinder motors to operate up to 180,000 rpm.

Development and continuous improvement of MPUs—the second milestone—allowed Rexroth to produce one of the earliest microprocessor-controlled industrial VFDs in 1982. This drive featured a dot-matrix LCD operator module with keypad and menu-guided setup, eliminating analog potentiometer-based setup. By 1989, newer developments led to a full line of IGBT flux-vector control drives. These VFDs featured maximum starting torque, improved low-speed velocity control, and using feedback, met and surpassed dc drive performance, says Fischbach.

Jim Thompson, drives engineer at Emerson Control Techniques (CT), regards early VFDs as limited by use of silicon-controlled rectifiers (SCRs) or ‘fairly complicated’ designs that implemented six-step control. ‘SCR power inverters were large, requiring complicated ‘commutation’ circuits, including many inductors and capacitors,’ he says. ‘Six-step output produced harmonics in the motor, causing undesirable additional heating. That scheme did not allow fast dynamic control of motor current’—needed for higher drive performance.

Besides advancing fast switching of power devices, IGBTs permit rapid adjustment of applied motor voltage. ‘This makes fairly high bandwidth magnetic field orientation (vector control) feasible, and allows fast, high-precision velocity profiling and positioning,’ explains Thompson. High cost of control electronics also limited performance of early ac VFDs. ‘Digital control was not very practical, mainly being provided by large ‘system-level’ circuitry (or computers) auxiliary to a drive package,’ he adds.

Today, Emerson CT considers fast-switching PWM output a prime VFD feature because of its minimal harmonic current production and dynamic motor torque control. Rich configuration features further characterize modern ac drives. Typical selectable features are speed or torque regulation, ability to accept various analog or digital references, speed or torque feedback, as well as control of synchronous (servo) and induction motors. Relatively inexpensive, add-on option modules are another popular feature, according to Thompson, which supply extra I/O points, feedback, or communication.

DC drives lead the way early on for variable-speed motor control. Yaskawa Electric has had a long history of involvement in dc and ac sides of electric machinery. AC drives made a big move to industry in the 1970s, via variable-voltage/frequency control using SCR and gate turn-off (GTO) power-switching devices, explains Dr. Tsuneo Kume, Yaskawa Electric director of R&D in the U.S. VFDs’ major industrial breakthrough came in applications like steel-mill processes and metal plating, according to Yaskawa (and others). Movement from analog to digital control circuits also started at that time.

Flux-vector control drives for paper making machines and machine tool spindle drives followed in the late ’70s. IGBTs became power devices of choice for general-purpose VFDs around 1990, explains Kume. Digital drives using integrated MPUs soon became standard at Yaskawa Electric, and sensorless vector drives followed by 1995.

Yet to come

Looking a decade ahead, Rockwell Automation’s Sinner sees VFDs getting still smarter. ‘An extension of assisted startup will allow set-up of smart drives with minimal intervention from the user,’ he says. Also, a chip embedded in the motor could automate motor identification upon drive startup.

Tighter integration with control systems also lies in the future of VFDs. Sinner differentiates between numerous existing ‘connections’ for drives and real integration that he says is just starting. Such real integration fully involves the drive in the programming and configuration environment of the control system. ‘This capability will work into lower-priced products, coming down from the high-end as a natural migration of drive features,’ he adds.

Danfoss Drives’ Momberger sees growing adoption of distributed drive systems in industry. Fueling the trend are lower-cost, higher-reliability drives that can be located next to [or on] the motor—decreasing installation costs without long motor/drive cable sets and associated conduit trays. ‘In addition, distributed drives have the advantage of minimizing EMC problems arising from long motor cables, reducing the need for costly filters,’ he says. Distributed systems also will grow from more integration of motion control and PLC functionality into VFDs.

Other developments include greater use of Ethernet-compatible communication to link application information of drives into plant wide networks and wireless access to drives, especially ones in difficult locations. ‘Ethernet represents the best opportunity for achieving an industry-standard communication system,’ Momberger adds.

Meanwhile, Bosch Rexroth’s Fischbach thinks that today’s optional drive features will become future necessities. He cites high starting torque, closed-loop speed and torque control, preventive maintenance, and direct data link to manufacturing control systems as prime examples. Other upcoming advances mentioned are:

  • Active drive front-end (including harmonic limitation) slowly gaining acceptance as energy costs and grid standards increase; and

  • Simple speed actuators evolving to a scalable, distributed, field-level machine/process control unit with PLC or process capability.

Integrate external functions

According to Emerson CT, future VFDs will accelerate integration of various external functions into the drive package. PLC and motion control functionality can now be implemented in a drive at a much lower total system cost. ‘For the most-sophisticated systems application, much of logic and motion control still must be handled by peripheral electronics, but we expect that to change rapidly,’ says Thompson. ‘We expect future drive systems will be composed entirely of enclosures filled with drive units, power wiring and limiting devices, serial communications wiring, and human-machine display and interface devices.’

ABB mentions rising environmental concerns and higher energy costs affecting future ac drives. They’re destined for wider usage in all industries and developing markets, raising the number of motors under variable-speed control from as low as 5% worldwide, according to ABB. Also noted is continuing shrinkage of drive size, even as more miniature features are added. Future VFDs will see new, non-traditional applications—replacing other types of control (or adding first-time automation).

Yaskawa likewise envisions future VFDs participating more in ‘green technology’ developments, especially with higher energy costs and regional electricity shortages possible. Efficiency and conservation are obvious desired benefits, but lower operating costs, higher reliability, and still more compact drive designs are other promises. Research on new control topologies continues in industry and academia, explains Dr. Kume, while newer, existing controls—such as three-level topology and matrix converter—will find expanded application ahead.

Benefits of three-level topology include lower surge voltage at the motor, lower leakage current, and improved thermal management at low speeds. Even newer matrix converter looms especially attractive, as it offers enhanced regeneration capabilities for VFDs and eliminates capacitors in the dc bus. Commercial release of matrix converter is on plan for 2005 for regenerative applications.

Kume predicts further performance improvement coming to vector control and sensorless vector control, particularly for torque control near zero motor speed. Meanwhile, VFD application will expand, fueled by ease of use and continued decrease in size and price. Smaller drives mean easier integration. ‘Drive units can more easily mount on the machine or motor,’ he says.

Still further ahead, new generation power devices will have major impact on ac drives, concludes Kume. Silicon carbide (SiC) technology carries the promise of lower losses and more possibility for miniaturization.

Online Extra

Practical industrial variable-frequency drives (VFDs) emerged in the late 1950s, with automation of synthetic-fiber processes being one early application. Adoption by heavier industry followed con-siderably later in the 1970s. Early ac drives operated in open loop and had limited performance. Developments in VFD controls in the next decade or so, coming from hardware and software, pr-pelled these ac drives to challenge the supremacy of dc adjustable-speed drives. Application experience and user awareness helped tip the scale.

Flux-vector (field-oriented) control, sensorless-vector control, and newer designs like three-level topology, matrix converter (see main article), and other approaches yet to come should continue to propel the evolution of VFDs. (By the way, “sensorless-vector control” is one of our industry’s misnomers. Encoderless-vector control is a more exact naming, as the term refers to motor control without using a shaft-mounted encoder or feedback device. However, motor parameters, such as voltage, current, and others are measured or “sensed,” as input to the drive’s dynamic controls.)

Multiple controls, one drive
Danfoss Drives ’ new FC-302 Automation Drive is one example of the trend to incorporate multiple motor-control topologies within the same compact drive package. Software “switches” provide this type of flexibility to handle a range of applications. “With FC-302, users are truly able to buy one drive to handle their industrial applications,” says Tom Momberger, product manager at Danfoss. “Using on board setup wizards makes applying FC-302 for a particular application as simple as answering setup prompts programmed into the drive.”

Momberger calls FC-302 drive “self aware,” with an ability to “proactively monitor” proper operation of its on board subsystems. “Problems are reported to the user for action, and the drive’s self-help feature provides guidance in resolving the problem,” he says.

FC-302 reportedly is easily programmed from a computer via the drive’s USB port. Drive displays can be customized for global users. Moreover, FC-302 has been designed to be multi-lingual. An OEM can program the drive in the local language and software instantly translates the full operating environment (including onboard help files) into a customer’s native language, explains Momberger.

Newer VFDs like the FC-302 have plug-and-play capability to add options in the field. However, this feature has become more sophisticated. For example, if fieldbus communication option has been added, the drive instantly recognizes that option and “automatically uncovers menus related to its functionality for the user,” adds Momberger. Models of FC-302 drive up to 10 hp became available in March 2004.

Transistors are key
Peter Fischbach, Bosch Rexroth manager of component sales, regards the development of insulated-gate bipolar transistors (IGBTs) as a major milestone for modern VFDs. “IGBTs combined the benefits of MOSFETs [metal-oxide semiconductor field-effect transistors] and bipolar transistors, but required less power and significantly reduced forward/switching losses,” he says.

Rexroth experimented with IGBTs in early 1987 and released a complete line of IGBT-based variable-frequency drives in 1988.

Fischbach views the future of VFDs as a balancing act between cost and required functionality. “It’s a case of what features are really needed to do the job,” he states. “With the availability of a new generation of switching devices and advanced control methods, the trend of further size reduction, higher efficiency, eliminating switching noise, and simplified setup/diagnostics continues.”

Another direction for VFDs noted at Bosch Rexroth is enhancement of the most commonly used V/Hz control through accurate process feedback. Besides load and speed feedback, this approach could be used to detect mechanical problems in pump, fan, extruder, and related applications before they caused production losses. However, vector control will eliminate older V/Hz methods, adds Fischbach.

Also reflecting the trend for multiple for multiple control technologies in the same drive is Bosch Rexroth’s new digital IndraDrive system. It includes several types of control from simple V/Hz to full servo control, plus scalable in performance and functions. Drive-based options like IEC 61131-3 PLC, safety functionality (per EN954-1), and various fieldbus interfaces are available.

PWM, add-on modules
Jim Thompson, drives engineer at Emerson Control Techniques (CT), believes that the high accuracy and fast dynamic control in today’s ac drives was enabled by the “advent of small, relatively inexpensive microprocessors and other high-density digital circuitry.” He likewise comments on the importance of power transistors to advancing ac drives. “IGBTs allow fairly low loss switching at rates as high as 16 kHz, permitting pulse-width modulated (PWM) drive output that results in a near-sinusoidal current in an ac motor,” he states.

Thompson explains that besides delivering dynamic motor torque control, PWM technology allows use of higher carrier frequencies for power switching to reduce audible noise in applications requiring a low noise environment. Another significant feature of today’s drives is ability to be configured to act as a bidirectional ac-to-dc power converter, which allows the flow of low-harmonic-content power either into or, by regeneration, away from a drive system.

Emerson CT also mentions the ready availability of small, “relatively inexpensive” add-on option modules. “This approach allows the basic drive to contain most of the commonly used functions at a minimum cost, while providing a wide range of special application features at low additional cost,” says Thompson.

Looking ahead, he concludes, “It is expected that drive design enhancements will continue the pursuit of coalescence between power and system control to engulf most functions needed to implement most of industrial electric motor-based motion control systems.”

Early use of IGBTs, SiC coming
Yaskawa Electric likewise considers development of IGBTs a major milestone in advancing variable-frequency drives. Its IGBT module became available in 1986. The company used IGBTs in its VS-616G2LN product line in 1987—“LN” designating a low-noise drive. In 1989, Yaskawa re-leased its G3 variable-frequency drive, “which was our first, all-IGBT product line,” says Dr. Tsuneo Kume, Yaskawa director of R&D in the U.S.

Power transistor advances helped Yaskawa Electric put its Varispeed-616T drive—a voltage-source, variable-frequency design with PWM control—into production in 1974.

As for the future, Kume mentions the importance of silicon carbide (SiC) technology as the next power-device advancement that will shape ac drives. However, he calls this is a mid-term development that we will not see for some time in an actual drive product.

Rockwell Automation ’s product line manager, William Sinner, also related to Control Engineering the longer-term potential of SiC technology for advancing VFDs unto the future.

Sinner is further keen on software tools. He sees “assisted startup” as another key drive feature. Instead of parameter programming of the past, a menu format now asks the user for specific inputs such as control mode, motor type, I/O points, etc. And looking ahead, assisted startup will become more available and capable to make ac drives easier to use. “Smart start” will handle I/O points, other drive parameters, and in vector control, interrogate the motor for startup information. Users need not be drive experts but can focus on their machines,” he says.

Michael Offik, general product manager—Variable Speed Products at Rockwell Automation Power Systems, provided additional VFD perspective relative to the company’s Reliance brand. He thinks that today’s drives benefit from three technology areas:

  • Software flexibility due to decreasing cost of processing and memory;

  • Growth of network connectivity; and

  • Much more user-friendly, robust designs.

Offik sees future ac drives being increasingly differentiated by software functions, while drive sizes continue to shrink. He predicts a rise in higher-speed drive applications as motor designs continue to improve. “Under VFD control you can run a motor at much higher speeds to obtain more output power in a smaller package,” he says.

AC drives have reportedly experienced 60% size reduction from one product generation to the next, according to Sinner. The general trend is expected to continue, though from a power-density stand-point it’s a rising design challenge.

Sensing the process, regeneration
“The drive will continue to become a more intelligent actuator in the process by adapting speed and current to a variety of sensor inputs,” Offik continues. By sensing such parameters as vibration, temperature, current signature, etc., users will be able to assess health of the motor-driven process (or the motor itself). Given that kind of information, process speed could be changed to reduce vibration and the line kept running longer to avoid unscheduled maintenance of motors, pumps, or related equipment. Another example of “more intelligent actuator” is the associated software algorithm that could adjust process flow rates to minimize energy consumption, explains Offik.

AC drives in the past have had a problem to provide line regeneration. “However, today’s high-speed processing and compact, cost-effective IGBTs allow for full regeneration to the power line with a syn-chronous front-end that also provides much-improved line harmonics,” says Offik. Regenerative drives especially will benefit applications with large motors that draw high currents and could distort the power line.

Efficient liquid cooling
Cooling and heatsinks represent another issue for efficient operation of VFDs. Offik mentions Rockwell Automation’s development of a liquid-cooled drive called LiquiFlo that uses Freon or water to improve cooling of the heatsink and allows smaller unit sizes. “In fact in many cases, the result is about 25% of the volume of many traditional air-cooled drives at the same rating,” he says.

Typically, these VFDs have high current ratings and are used on pumps, fans, and compressors. They come in two versions: LiquiFlo 1 rated 414-1,157 Amp (350-1,000 hp) and LiquiFlo 2 rated for 405-1, 215 Amp. LiquiFlo, originally designed for some Rockwell OEMs, has been available for the general-purpose market for the last year or two. Several thousand drives have been installed since LiquiFlo 1 was introduced in 1998, according to Offik.

Roleof Timmer, ABB marketing director for drives in Finland, adds that variable-frequency drives will continue to become smaller, smarter, and less costly in the next 10 years. “Also worth mentioning are increased use of fieldbuses and simplified user interfaces,” he says. And as more users without drive expertise become attracted to VFDs, factors such as ease of selection, installation, programming, and use will gain the needed attention.