AC Drives Stay Vital for the 21st Century

Physical size and weight provide the most visible evidence of the evolution of ac variable-frequency drives (VFDs) in the past 50 years. But, 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 VFD, 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," Sinner says. Insulated-gate bipolar transistors (IGBTs) remain workhorse power devices. On the control side, analog was king at first, giving way to digital control, though initially based on integrated circuits. Micro- processor (MPU)-based digital drives came somewhat later 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, Sinner comments, 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, and networked applications now amount to about 50 percent of all drives-and increases with higher-end units.

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 today's VFDs of fractional size and weight. New ac drives have added 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, Momberger adds. 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, Momberger explains. New FC-302 Automation Drive from Danfoss addresses all these features.

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. 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.

ABB regards its DTC as an advanced technology 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 drive. DTC does not use a modulator and works without motor shaft position or speed feedback. "With DTC, 100 percent torque is available at zero speed and small torque increments can be controlled at low frequencies in less than 1 millisecond," Ikonen says.

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."

IGBTs, MPUs

Rexroth released a full line of IGBT-based drives in 1988. 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, Fischbach says.

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," Thompson explains. 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, and control of synchronous (servo) and induction motors. Relatively inexpensive, add-on option modules are another popular feature, and supply extra I/O points, feedback, or communication.

DC drives led 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. 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, Kume explains. 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," he adds.

Danfoss Drives' Momberger sees growing adoption of distributed drive systems. 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.