AC Drives Trend to Efficient, Modular Designs

Whether you call them ac adjustable-speed drives(ASDs)—the name preferred by most—or use other labels, such as variable-speed drivesor variable-frequency drives, or frequency converters as the international community prefers, ASDs excel at one key point: increasingly capable control of ac motors.




  • Field-configurable I/O points

  • Modular power structure

  • Easier to install, start up

  • High power density

  • Inherent energy savings

This article contains online exclusive products and an Online Extra with further information to AC Drives.

Whether you call them ac adjustable-speed drives (ASDs)—the name preferred by most—or use other labels, such as variable-speed drives or variable-frequency drives, or frequency converters as the international community prefers, ASDs excel at one key point: increasingly capable control of ac motors. Depending on the drive's sophistication, it can control torque as well as speed to match dc drive performance. And adjustable-speed operation of motors brings inherent benefits of energy savings to a growing number of applications.

To gauge trends in this sector, Control Engineering and Reed Research Group—both part of Reed Business Information —asked subscribers about their views and preferences on ac adjustable-speed drives via an e-mail/Web-based survey. Selected results presented here are based on full responses received in May 2003 from 276 subscribers who evaluate, specify, recommend, install, and/or purchase ac drives. Among these, 60% do so for in-plant requirements, 23% for OEM (resale), and 17% for both.

Open-loop (V/Hz) control of ac drives remains the overwhelming choice for 84% of respondents, (compared to 91% last year), with sensorless vector control coming in at 40% (44% in '02) and closed-loop vector control at 37% (42%). [Percentages in brackets refer to the 2002 survey throughout this article.] Various product features serve as selection criteria for ASD users. The top three general features rated as "very important" were simple controls/setup by 70% (65% in '02) of those surveyed, convenient operator interface 53% (56%), and pricing 52% (48%). Respondents also based their choices on performance features (see table).

Adjustable-speed drives are still largely applied standalone say 91% of respondents; and 44% indicate their ASDs are networked. Among networked drives, 84% use 4-20 mA, 73% use 0-10 V, and 63% like Ethernet TCP/IP. DeviceNet came in at 61%, Modbus at 42%, and Profibus-DP at 37%. Higher usage is forecast in the next 12 months—especially for Ethernet, DeviceNet, and Profibus.

Forty-two percent (a considerable gain over 24% in '02) report their ac drive purchases will increase in the coming year, while 6% (3%) foresee a decline in purchases. Average increase in purchases is expected to be 36% (26%). Survey results indicate that in the past 12 months, each respondent purchased 52 ASDs, spending $62,000 on average.

No revolution

Evolution without much revolution is how Roddy Yates, drives product manager at Baldor Electric Co., views the world of ac adjustable-speed drives. "The quest for smaller drives continues, as does the quest for more powerful application and setup tools," Yates says. He points to the trend of enabling easy field reconfiguration of the drive's internal I/O connections for more versatile applications, for example, to reduce the number of preset parameters and reassign them as alarms.

Yates also notes growth in bus communications for drive control and monitoring. "In the industrial arena, several legacy communications buses are attempting to move into Ethernet connectivity. The flak about Ethernet not being deterministic or secure enough for industrial control seems to be subsiding as Ethernet connectivity becomes available to the plant floor," he adds.

Corey Morton, product manager for Rockwell Automation, mentions the need for modular design and a high level of commonality across multiple products. High-performance drive technology can provide these attributes to combat costs and competition. Commonality across an entire ac drive family can include programming, operator interface, and communication features. "OEMs and end-users need components that can lower installation, inventory, maintenance, and training costs," states Morton.

Five key factors continually arise at ABB related to drive developments and user requests. They are: 1) simple controls/set-up, 2) programmability, 3) convenient operator interface, 4) EMI/RFI/harmonics, and 5) fieldbus connectivity, according to Mark Kenyon, ABB's director of product marketing. Control Engineering 's survey verifies the importance of these factors, with items 1 and 3 specifically showing up as the top two general features of ac drives desired by users (see above).

Software engineers are helping to simplify drive controls by judiciously limiting users' access to "what is needed versus what is available" among the slew of parameters provided by the latest microprocessors, explains Kenyon. "Commissioning tools that walk users through the start-up process and provide on-line help if necessary are becoming the new 'buzz' words."

Programming flexibility of newer drives permits users to tailor one hardware platform to fit many applications. For simpler usage, PLC-like functions integrated into the drive's firmware can eliminate separate PLC components, says ABB.

Trends noted at Siemens Energy & Automation include technology upgrade—where general-purpose (GP) drives now offer closed-loop vector control and torque control as "standard" This movement into high-performance areas includes system drives that venture into motion control capabilities as their "standard" portfolio, according to Rudy Hauser, product manager, GP drives. Physical size of drives continues to shrink, while holding the line or even raising ambient temperature tolerance. "Products offered are migrating up to 122 °F (50 °C) ratings," says Hauser.

Siemens also sees growth in features to simplify drive start-up and commissioning. Many drives come with a programming "macro" to guide users through start-up. Hauser also mentions added capability of newer drives, allowing users to "reconnect" internal signals to customize drive requirements.

AC Drive Performance Features Valued by Users
(% of respondents selecting feature as "very important" or "important")




Survey year

*-Not on 2001 survey. Respondents could check five levels of importance on 2001 survey.
Source: Control Engineering
Photo courtesy of GE Industrial Systems

Tripless operation




Torque control




Dynamic braking




Zero speed control




Distance between drive and motor




Sensorless vector control




Flux vector control (closed-loop)




Line regeneration




ASD products

For more manufacturers, visit . For system integrators go to integrators. To request free vendor information, use "enter" numbers provided, at .

Exclusive: V/Hz or sensorless drive expands power

PowerFlex 700H ac drive, now expanded up to 350 horsepower (250 kW) at 400 and 480 V, incorporates modular design and added reliability. Users can readily configure the drive for open-loop V/Hz or sensorless vector control. For added flexibility, standard I/O options include 24- or 115-V digital plus analog I/O points. Network options such as DeviceNet, ControlNet, and EtherNet/IP are included to help integrate PowerFlex 700H into the manufacturing process. Allen-Bradley DriveExecutive and DriveTools SP software eases programming, configuration, monitoring, and troubleshooting. Rockwell Automation

Drive features easy installation

Billed as having the narrowest width in the industry, ABB's new low-voltage ACS 800 drive is designed in bookshelf style. Drive widths range from 6.5 in. for low-power units up to just 13.78 in. for a freestanding 600-hp unit! Now available in NEMA 1 or NEMA 12 enclosures without derating, ACS 800 offers supply voltages of 230-690 V. Enclosures for the freestanding drives (150-600 hp) turn easily to fit into deep, narrow spaces or wide, shallow spaces. Power cables connected into a separate pedestal bolted to the floor further simplify installation. Once the cabling is complete, the drive—with built-in wheels/casters—rolls over the pedestal and is locked into place. ABB Automation Technologies

Inverter package 70% smaller

Power ratings for Series 15J (shown) and 15P inverters have recently been increased to 10 hp at 460 V and to 5 hp at 230-V input. These new inverters supply digital open-loop (V/Hz) ac motor control in a package said to be 70% smaller than Baldor's previous designs of similar power ratings. The 10-hp unit is considered especially compact. NEMA 1 enclosure allows mounting the drives to machinery without the need for additional cabinet space. Series 15J and 15P are available from stock worldwide. Baldor Electric Co.

Exclusive: 'World-first' technology cuts dv/dt effects

The "world's first" 3-level Power Architecture technology reportedly makes its debut in Yaskawa's new G7 ac drive, managing detrimental voltage switching (dv/dt) effects of IGBT power-switching devices. At 50% the dv/dt level of conventional PWM drives, this technology solves major installation and reliability concerns in industrial plants, which include long cable length between motor and drive, induced shaft bearing current, EMI/RFI radiation, common mode currents, audible noise, and surge voltages, says Yaskawa. Since the cause of problems is reduced, expensive peripheral filters and other workaround methods are eliminated. Yaskawa Electric America Inc.

Microdrive saves energy, mounting space

Recently introduced AF-300 Mini variable-frequency ac drive provides features not previously offered in other GE microdrives. Its energy saving mode uses simplified dynamic torque vector control to model excitation currents needed at all speeds to reduce energy consumption. This feature is said to be unique in a low-cost ac microdrive. AF-300 Mini allows flush side-by-side mounting of drives to save panel space, unlike competing units that require clearance on either side. Predictive maintenance alarm allows custom wiring to a pilot light or PLC to signal when the cooling fan or internal dc-link capacitor approaches its specified wear life. Also unusual for a microdrive is a built-in dynamic braking module that eliminates external wiring. GE Industrial Systems

Exclusive: Versatile enclosed drives communicate

New 58M family of enclosed Square D adjustable-speed drives provides voltage-sag immunity, the latest communications capabilities (including Ethernet connectivity), and other enhanced options. These drives offer power ratings up to 500 hp, and can configure to comply with SEMI F47 voltage-sag requirements of the semiconductor industry. Packaged in NEMA 1, NEMA 12, integrated, or "barriered" enclosures for wall or floor mounting, 58M drives can be selected with an optional enhanced door-mounted display center. Functional features include industrial-rated control operators, circuit-breaker disconnects, fully rated isolation and bypass contactors, seismic certification, line reactors, motor protection filters, and the company's PowerLogic power monitoring. Schneider Electric

Low-power inverter uses 1-phase supply

Newly introduced to North America, Siemens' Sinamics G110 is a compact, low-power ac drive frequency inverter intended for the low-cost, high-volume marketplace. Sinamics G110 offers smooth, continuous speed control of three-phase motors in the 1/6-4 hp range, using single-phase line power supplies. Rated for 200-240 V input, these drives increase the range of the company's MicroMaster MM410 line to 4 hp. Sinamics G110 comes in three frame sizes and with mechanical/functional options for more flexibility. G110 includes a space saving flat-plate style, plus setpoint control versions using either serial or analog inputs. Fast-connection terminals allow ease of installation without the need to remove covers. Siemens Energy & Automation

Online Extra Products

Get connected
Advanced flux-vector G7 drive from Toshiba is available up to 350 hp at 460 V; to 60 hp at 230 V; and to 300 hp at 600 V input. It offers one of the largest selections of communication interfaces available. G7 drive is said to easily integrate into an overall control system to enable users with wide information access and control capabilities. Options include vector feedback support and interfaces for Profibus, Modbus, Metasys, Modbus Plus, Ethernet (Modbus TCP/IP and Ethernet I/P), DeviceNet, and Tosline F10 and S20 PLC networks. All drive data are accessible, and most units are field-upgradeable. G7 drive can also be controlled by Microsoft Windows-based software—or for PDA users, Palm Pilot software can do drive programming.
Toshiba International Corp.


Compact sensorless vector drive
Designed for low-horsepower applications (1-10 hp at 480 V input; 0.5-7.5 hp at 240 V), newly introduced MVX9000 adjustable-speed drive uses a microprocessor for sensorless vector control of three-phase ac motors. Vector control allows operators to closely monitor the drive speed without encoder feedback. MVX9000 features multiple I/O configurations, two analog inputs, and PID for setpoint and feedback control. It also can be programmed to address the needs of various industries, including HVAC, machine tool, and manufacturing or commercial applications. The drive meets NEC, NEMA, and IEEE standards.

VFD with serious energy-savings focus
FR-F500 Series variable-frequency drive (VFD) consists of units rated 5 through 75 hp at 240 V ac (and through 600 hp at 480 V). An optimum excitation current control program improves motor efficiency under low-torque conditions typical to pump and fan applications by modifying the basic variable-voltage control mode. Also, second-order deceleration torque patterns bring the output voltage below the level of fixed-torque patterns. This conserves energy by matching motor output with second-order deceleration loads of fans. Among notable features are two modes for setting parameters, where the simple mode displays only essential parameters. RS-485 communication and a parameter copy function are integrated into the standard display panel. Drive maintenance is simplified with removable terminal connectors and a replaceable fan cassette.
Mitsubishi Electric Automation, Inc.

Ethernet card links drive to controller
GS-EDRV is an Ethernet interface card from AutomationDirect that enables a high-performance Ethernet link between the company’s GS2 line of ac drives drive and a control system with Modbus TCP/IP capability. The DIN-rail-mounted card connects a drive to an Ethernet hub or PC. GS-EDRV processes the drive’s input signals, formatting them to conform to the Ethernet standard, then transmits the signals to the controller. Going the other way, the interface card receives and translates output signals from the controller and sends the signals to the drive. GS-EDRV interface card is priced at $200.

High-slip braking cuts motor stopping time

3G3RV ac inverter features a new high-slip braking function for intermittent braking applications that shortens motor stopping time by 33% without using braking resistors. Also new, a static autotune function eliminates the need to rotate the motor to determine and set parameters. It complements 3G3RV’s standard rotational autotune feature.
The new inverter is sized for the 0.5-150 hp range at 230 V ac and 0.75-500 hp at 460 V ac input. Omron has built RS-485/422 Modbus communications protocol into the drive; and option cards for DeviceNet and Profibus are available. A removable terminal strip simplifies wiring for installation and maintenance. 3G3RV’s cooling fan turns off automatically when not needed to prolonging fan life. A detachable fan design streamlines changing of the fan when required. 3G3RV ac inverter is targeted for converting applications as well as for motor control in elevators, winders, conveyors, process lines, and punch presses. List price for 3G3RV starts at $950, and the drive is available from stock.
Omron IDM Controls

Online Extra supplement to August 2003 Product Focus article on‘AC Drives’

An e-mail/Web-based survey of Control Engineering‘s subscribers in May 2003 pinpointed their views about ac adjustable-speed drives (ASDs). The following additional results are based on complete responses received from 276 subscribers who evaluate, specify, recommend, install, and/or purchase ac drives.

Pumps, fans, and blowers are the most popular applications of ASDs, reported by 50% of respondents, followed by assembly/conveyor lines and material handling equipment with 45% and 28% of respondents, respectively, indicating such usage. The order of these applications is unchanged from the 2002 survey, and percentage values are very similar.
Size of ac drives employed in manufacturing plants was part of the survey questionnaire. In the distribution of sizes tabulated below, the range of 1 to 5 horsepower (0.75-3.75 kW) was most used, according to 81% of respondents. Actually, the results suggest that all size ranges of ASDs are widely used among those surveyed. Moreover, no great shift in usage per drive size has taken place over the past two years.

Sizes of AC Drives in Use
(% of respondents indicating use)

Survey year





Up to 1

55 %



1 to 5




6 to 10




11 to 2




21 to 5




Over 5




Wayne Goodreau, Reed Research Group, also contributed to this article.

Additional vendor views
At ABB Inc., Automation Technologies (, additional key factors for ac adjustable-speed drive developments and user requests include convenient operator interface and EMI/RFI/harmonics.

“The days of providing numeric only keypads or using obscure coding to operate ac drives is virtually nonexistent. Today’s drives offer full graphic displays in up to 15 foreign languages,” according to Mark Kenyon, director of product marketing at ABB. The newest operator interfaces employ cell-phone-like technology for intuitive operation, configurable to display actual signals that eliminate the need for other meters.

With built-in filters now more common, ac drives can reduce conducted and radiated EMI/RFI emissions, allowing drive installation in almost any facility without concerns for electrical interference. “Additionally, industrial users are learning from their commercial brethren and looking for today’s drives to minimize harmonic distortion without costly external chokes or filters. Use of ac line reactors and dc bus chokes—commonly specified in commercial ac drives, are becoming much more prevalent in industrial ac drives,” says Kenyon.

ABB also mentions users’ interest in “soft” goods relative to the acquisition of ac drives. “It’s not uncommon for users to ask for a free‘test and evaluation.’ If they like the drive, they keep it and pay the invoice. If they don’t like the unit, they send it back at no charge,” he adds.

Siemens Energy & Automation ( notes a broadening of overall services and products that go along with the latest ac drives. Rudy Hauser, product manager of general-purpose drives, mentions various power-related options, such as line reactors specifically engineered for the drives, along with software packages that simplify programming and monitoring.

Part of the simplicity trend for commissioning a product, Siemens factory preprograms the drive for typical operation with four-pole motors, two-wire stop/start, and speed pot setpoint control. For many applications, “This eliminates programming altogether for those who want to operate‘right out of the box,’ ” says Hauser.
As for the flexibility trend, Siemens’ recent MicroMaster 4 family of ac drives includes access to logic programming and math functions in the MM440 model with sensorless vector control. Such capability once required use of a PLC. Another model, variable-torque-rated MM430 drive (industrial pump/fan product), supplies factory installed “control functionality” for pump staging coordination and bypass control operation.

What’s in a name?
No, you have not found a typographical error.
Two designs of ac adjustable-speed drives in this Product Focus article from competing manufacturers— Toshiba International Corp. and Yaskawa Electric —carry the same name product of “G7.” In the seemingly limitless world of naming possibilities for industrial products this is unusual indeed.

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