For 50 years, variable-frequency drives (VFDs) have maintained a tradition of smaller size, greater intelligence, higher efficiency, and lower cost. This trend will become even more pronounced as performance requirements increase. Over the past 50 years, we have seen significant changes in the size and weight of AC VFDs. However, an even more significant change is the improvement in the performance, efficiency, and reliability of these motor drives. These changes are due to advancements in power switching transistors, microprocessors, and other hardware and software features, which reduce the effort required for user application and maintenance. Early AC drives operated in an open-loop manner with limited functionality. A major breakthrough was the development of field-oriented (flux vector) control technology for induction motors by Felix Blaschke of Siemens in 1971. Through the efforts of others, VFDs eventually achieved performance levels comparable to or exceeding those of DC drives in many applications. Sensorless vector control (eliminating the need for shaft angle encoders) and other drive algorithms subsequently emerged. Moreover, these improvements continue to accelerate. William L. Sinner, Product Manager at Rockwell Automation, noted two major historical shifts that impacted both the power and control aspects of variable frequency drives. Early AC drives (1980s) used multiple transistors per phase due to the limited voltage and current ratings of transistors. Today, however, they are all integrated into a single package, making a 10-horsepower (1 hp = 0.745 kW) drive smaller than a single transistor unit in a past drive. “The performance of next-generation transistors still needs to be improved so that manufacturers can develop smaller, more efficient power devices,” Sinner said. Insulated-gate bipolar transistors (IGBTs) remain the dominant power devices. In terms of control, analog control initially dominated but is now giving way to digital control, although digital control was initially based on integrated circuits. Sinner added that microprocessor-based digital drives appeared later, initially offering only open-loop (V/Hz) control. The continued development of microprocessor units enabled the inclusion of multiple control modes in a single drive, switchable simply by changing software parameters. Multiple Control Types and Interconnectivity Multiple control modes represent the current development level in the variable frequency drive (VFD) field. Typical low-end drives offer V/Hz and sensorless vector control, while high-end drives initially used flux vector control and later incorporated other control modes. For example, Rockwell's PowerFlex 700S offers several control modes, including servo control, through its built-in Logix processor. Technology transfer between product lines is another trend, illustrated by Sinner's examples of vector control in the low-end PowerFlex 70 and V/Hz control in some high-end drives. Why do high-end products use open-loop control? Sinner believes it's because V/Hz conversion allows a single drive to control multiple different types of motors. Using one type of drive in multiple different applications helps reduce spare parts inventory. Interconnectivity is another important feature of current VFDs, and all Rockwell drives are equipped with this feature. According to the company's experience, networked drives now account for 50% of all drives, and the use of high-end units is increasing. “The percentage of networked drives has doubled in the last three years,” Sinner said. Tom Momberger, product manager at Danfoss Drives, believes that “the application of microprocessor technology to variable frequency drives is the main reason for the improved performance of AC drives today.” Regarding the change in physical form, he compared the size and weight of a typical 5hp analog AC drive produced in 1968 (whose oil-cooling unit required several different manual adjustments) with those of current variable frequency drives (see a photo of a past 5hp VLT drive from Danfoss). Of course, new AC drives also incorporate many other features, such as programmability via operator keyboard or computer. “Microprocessors have made all of this possible,” Momberger said. According to Momberger, flexibility, intelligence, and a user-friendly operating environment are characteristics of the development of variable frequency drive technology. Flexibility means that a single type of drive can provide open-loop, closed-loop, flux vector, and even near-servo control to meet a variety of applications. He said, “This functionality reduces costs for drive owners by minimizing on-site inventory, operator training, and spare parts costs.” The microprocessor unit and advanced diagnostic capabilities allow users to intelligently configure a drive, thus reducing commissioning costs and downtime. Software features, such as automatic motor adaptation and software wizards, eliminate uncertainty during drive/motor connection setup. “The user-friendly interface in the software shortens the setup process to reduce potential operator errors and simplifies interaction with the drive,” Momberger explained. Danfoss’s new FC-302 automatic drive incorporates all these features: PWM, DTC, and modularity. In the significant milestones of AC drive development, ABB witnessed the emergence of industrial pulse-width modulation (PWM) based drives and the introduction of direct torque control in 1995. Ilkka Ikonen, speaking at the ABB Oy Drives market conference in Finland, noted that ABB’s first PWM drive was installed in industrial applications in the 1970s. Applications in paper mills and subways laid the foundation for the advancement and robustness of this important product. He said, "After this technology proved its reliability and competitiveness in these applications, people began to accept AC drives as the dominant control technology to replace DC drives." ABB considers its Direct Torque Control (DTC) to be an advanced technology that can directly control the torque and speed of a motor without separately controlling voltage and frequency. More importantly, it boasts fast torque response time and control accuracy, "typically 10 times faster than pulse width modulation." DTC also optimizes motor flux, thus improving the overall energy efficiency of the motor and drive. DTC does not require a regulator and does not require feedback on motor shaft position and speed during control. "With DTC, torque can reach 100% at zero speed, and micro-torque increments at low frequencies can be controlled in less than 1ms," Ikonen said. Today, ABB's AC drives can meet a variety of user needs through "make-to-order" products, with its modular design playing a crucial role. It can meet almost all buyer needs, such as delivery time, quality, and cost, as if they were readily available. Reliable switching devices and microprocessors developed by Bosch Rexroth have enabled current frequency converters to be smaller, more efficient, and more robust. "The use of thyristors or field-effect transistors based on isolated power modules, and later, insulated-gate bipolar transistors, combined with sinusoidal pulse width modulation (PWM) control, revolutionized the design of power supply sections and cooling systems," said Peter Fischbach, manager of the components department. [align=center] Figure 1: The same series of variable frequency drives from Danfoss has undergone significant changes over more than 30 years. The VLT 200 included analog PWM control, while the VLT 3000 and later transitioned to digital control. [/align] IGBTs, MPUs : Rexroth released a full range of IGBT-based drives in 1988. In fact, the company's AC drive business had been developing since 1965, and in 1968 it launched its first high-speed, rack-mount industrial variable frequency drive, enabling induction grinding motors to operate at speeds up to 180,000 rpm. The development and continuous improvement of microprocessor units (a second milestone) enabled Rexroth to produce one of the earliest microprocessor-controlled industrial variable frequency drives in 1982. These variable frequency drives (VFDs) feature a dot-matrix LCD operator module with a keyboard and menu-guided setup, replacing analog potentiometer-based settings. In 1989, further developments led to a full range of IGBT and flux vector control drives. Fischbach says these VFDs are characterized by maximum starting torque, improved low-speed speed control, and feedback control, achieving and exceeding the performance of DC drives. Jim Thompson, a drive design engineer at Emerson Control Techniques (CT), believes early VFDs were limited by the use of thyristor rectifiers and rather complex six-step control. “Power inverters using thyristors are bulky and require complex commutation circuitry, including many inductors and capacitors,” he says. “Six-step outputs generate high-order harmonics in the motor, resulting in unwanted additional heat generation, and this approach cannot quickly and dynamically control the motor current”—requiring higher-performance drives to address these issues. In addition to increasing the switching speed of power devices, IGBTs also allow for rapid adjustment of the motor's operating voltage. “This makes high-bandwidth field-oriented control (vector control) feasible, enabling fast and precise velocity profiling and positioning,” Thompson explained. The high cost of electronic control also limited the performance of early AC variable frequency drives. “Digital control wasn’t very practical then, mainly because it required adding bulky system-level circuitry (or computer) auxiliary devices to the drive module,” he added. [align=center] Figure 2: Advances in variable frequency drive technology are reflected in Rockwell Automation’s new 2hp PowerFlex 4 drive, which is 70% smaller than the 1332 AC drive (left) of the same power rating produced in 1985. The PowerFlex 4 weighs only 2.2 pounds, fitting in the palm of your hand, while the 1332 weighs 16.8 pounds. [/align] Emerson CT believes that fast pulse-width modulation output is a key feature of variable frequency drives because it generates minimal harmonic current and can dynamically control motor torque. The rich configuration features make modern AC drives even more distinctive. Typical optional features include adjustable speed or torque, acceptance of multiple analog or digital reference voltages, feedback of speed or torque, and control of synchronous (servo) and induction motors. According to Thompson, relatively inexpensive, scalable optional modules are another popular feature, including the provision of additional I/O ports, feedback, or communication functions. Early development of DC drives focused on variable speed motor control. Yaskawa has a long history of research in both DC and AC motors. Dr. Tsuneo Kume, Head of R&D at Yaskawa USA, stated that AC drives made significant progress in industry in the 1970s by employing thyristors and gate-turn-off power switches for variable voltage/frequency control. According to Yaskawa (and other companies), the major industrial breakthrough for variable frequency drives was in steel mill processing and metal plating applications. The transition from analog to digital control circuits also began around that time. [align=center] Figure 3: The PWM variable frequency drive produced by Rexroth in 1982 was one of the earliest industrial variable frequency drives to utilize microprocessor control. Its digital programming module includes a D/A converter to provide analog voltage and current indication. Kume explains that flux vector control drives for paper machines and machine tool spindle drives subsequently appeared in the late 1970s. IGBTs became the choice for general-purpose variable frequency drive power devices around 1990. Digital drives with integrated microprocessor units quickly became standard at Yaskawa, and sensorless vector control drives followed in 1995. Looking ahead to the next decade, Sinner of Rockwell Automation believes variable frequency drives will become more intelligent. He says, “Extended auxiliary start functionality will allow intelligent drives to be configured with minimal user intervention.” This involves embedding chips within the motor, enabling it to automatically detect drive startup. Closer integration with control systems is also a direction for variable frequency drives. Sinner compares many existing drive connection methods with what he calls the upcoming true integration. This true integration makes the drive a complete part of the control system programming and configuration environment. “This functionality is ported through common drive feature technology, inheriting high-end products to produce low-cost products,” he adds. Momberger of Danfoss Drives anticipates an increase in the use of distributed drive systems in industry, driving a trend towards installing low-cost, high-reliability drives near (or on) motors. This eliminates the need for long motor/drive connection cables and conduit trays, reducing installation costs. "Furthermore, distributed drives minimize electromagnetic compatibility issues caused by long motor cables, reducing the cost of expensive filters," he says. Demand for distributed systems will also grow due to the increased integration of motion control and programmable logic controllers into variable frequency drives. Other developments include greater use of Ethernet-compatible communication capabilities, allowing drive application information to be connected to factory WANs and wirelessly accessed drives, especially in harsh environments. "Ethernet presents the perfect opportunity to establish industry-standard communication systems," Momberger adds. Meanwhile, Fischbach of Bosch Rexroth believes that today's optional drive features will become essential in the future, citing high starting torque, closed-loop speed and torque control, preventative maintenance, and direct data connectivity with manufacturing control systems as key examples. In addition, he mentioned some upcoming advancements: • As energy costs and grid standards increase, active drive front-ends (including harmonic suppression) are becoming increasingly accepted; • Simple speed controllers are evolving into scalable, distributed, field-level mechanism/process control units with programmable controller functionality or processing capabilities. [align=center] Figure 4: The intuitive control panel of ABB's latest ACS550 standard drive features built-in auxiliary software and a clear multilingual display for user convenience. [/align] Integrated External Functions According to Emerson CT, future variable frequency drives will integrate more external functions. Programmable controllers and motion control functions can now be implemented in a single drive at a relatively low total system cost. "For the most advanced system applications, most logic and motion control still must be handled by external electronics, but we hope this will change soon," Thompson said. "We envision future drive systems where all components are contained within the housing, including drive units, power lines and current limiting devices, serial communication lines, HMI displays, and interface devices." ABB mentioned that increasing environmental concerns and high energy consumption are impacting future AC drives. These drives are destined for wider application across all industries and developing markets, as ABB aims to increase the number of motors using variable speed control globally (currently only 5%). It also mentions that drive size will continue to shrink, even incorporating miniaturization features. Variable frequency drives will find new, non-traditional applications – replacing other types of controllers (or joining emerging automation fields). Yaskawa also anticipates that future variable frequency drives will be more involved in the development of "green technologies," particularly in addressing high energy consumption and regional power shortages. High efficiency and energy saving are obviously desirable advantages, but low operating costs, high reliability, and more compact drive designs are also expected. Research into new control topologies continues in industry and academia, Dr. Kume explains, and existing new control technologies, such as three-level topologies and matrix converters, will have broader applications. The benefits of using three-level topologies include lower inrush voltage to motors, lower leakage current, and improved thermal management at low speeds. Even newer matrix converters are showing particular appeal, such as their enhanced regenerative capacity and elimination of DC bus capacitors. The matrix converter product is planned for market launch in 2005, targeting renewable energy applications. Kume anticipates future improvements in vector control and sensorless vector control performance, particularly in torque control when motor speeds approach zero. Driven by ease of operation and continued reductions in size and price, the application areas of variable frequency drives will expand. Smaller size means easier integration. "Drive units will be easier to install on machinery or motors," he said. Kume concluded that in the more distant future, next-generation power devices will have a significant impact on AC drives, with silicon carbide technology enabling lower energy consumption and smaller size. [align=center] Figure 5: Emerson CT's Unidrive series embodies the expansion capabilities of today's AC drives. It features a Profibus interface, extended I/O, and a feedback interface for synchronizing the drive with other machine components. [/align]