AC Adjustable Speed Drives

Users want their drives easy to setup and program, and they plan to buy more of them, according to a Control Engineering survey of subscriber preferences for ac adjustable speed drives (also known as variable frequency drives or VFDs), conducted with help from Reed Research. Generally respondents prefer plain-vanilla open-loop Volts/Hz drives, more than in 2005.

By Peter Cleaveland for Control Engineering November 1, 2007

Users want their drives easy to setup and program, and they plan to buy more of them, according to a Control Engineering survey of subscriber preferences for ac adjustable speed drives (also known as variable frequency drives or VFDs), conducted with help from Reed Research. Generally respondents prefer plain-vanilla open-loop Volts/Hz drives, more than in 2005. Very important features include simple setup, programmability, and ability to download or copy parameters; 98% expect to buy as many or more in the next 12 months, while 45% plan to buy more; just 2% will buy fewer.

A total of 317 subscribers who evaluate, specify, recommend, install, and/or purchase these drives responded to the survey (more than twice as many as in our 2005 survey). Of these, 57% buy drives for in-plant requirements, while 24% buy them for original equipment manufacturer (OEM resale) requirements, and 19% buy them for both in-plant and OEM requirements.

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Blowers, pumps, motion

The application cited by more users than any other (48%) was for driving pumps, fans, and blowers; this was up from 2005, when 27% of respondents cited this use, and closer to the 2003 figure of 50%. In second place this year at 38% was assembly and/or conveyor lines. Materials handling equipment (including elevators, cranes and hoists) was at 25%, while 24% of respondents use VFDs for packaging machines and 17% for plastics manufacturing/extruder machines. No other category got more than 16%.

While VFDs are available in some sophisticated versions, by far the most popular type among those surveyed is plain-vanilla open-loop volts/Hz, with 89% of respondents using them, which is 10 percentage points more than in 2005. But sensorless vector (also called encoderless control) are popular as well, with 52% using them (up from 41% in 2005), and field-oriented vector control (closed loop) had plenty of users, with 41% using them, up slightly from 38% in 2005.

Setup simplicity, price, and convenience dominate among purchasing influences: 97% said simple controls/setup was either very influential (68%) or somewhat influential (29%) in their choice of drive. A close second was programmability in software, with 94% considering it very (51%) or somewhat (43%) influential. Not far behind was a related feature, ability to copy/download parameters; 89% consider that either very (51%) or somewhat (38% influential.

While no attribute was considered unimportant by a majority of respondents, 39% had little use for preset speeds, while 61% found them either very (17% ) or somewhat influential. Following that on the least-popular list was having a NEMA 12 (31% found it not influential) or NEMA 4 enclosure (29% found it non-influential). But a solid 97% of respondents were concerned about pricing.

When asked how influential certain performance features were when selecting an ac adjustable-speed drive, 91% of respondents considered torque control either very (44%) or somewhat (47%) important and 86% said the same for tripless operation; 83% were interested in zero-speed control, and 80% wanted sensorless vector control.

A substantial number (85%) of respondents were interested in dynamic braking, and a smaller number (77%) wanted line regeneration.

While 80% of respondents were concerned about the distance between the motor and the drive, only 29% considered it very influential and 50% found it somewhat influential. On the other side, 57% of respondents weren’t interested in medium-voltage (2.3 kV and above) operation, and 40% were not concerned about multiple motor control from one drive.

While there is some interest in networking drives, it’s not overwhelming. While a small majority (56%) of users surveyed network their drives; 88% use them standalone.

Nearly half of survey respondents use ac adjustable speed drives for pumps, fans and blower related applications.

Features, sizes, integration

We also asked users what additional features they would like in their drives. Several were dissatisfied with their drive’s autotune capabilities, one wishing for “Auto tune that really works reliably.” Other things on the wish list included different displays (sometimes with more information, sometimes with less), and several respondents wanted more choices of communications.

Size preferences in drives turned out to fairly evenly distributed. A solid majority (77%) of users surveyed use drives from 1 to 5 hp, 53% use drives 1 hp and below, and 58% use 6 to 10 hp units. Yet adjustable speed drives are making good penetration in the larger sizes as well, with 49% of respondents using drives larger than 50 hp.

In a related question, the average number of drives between 11 and 20 hp in use per location was just over 85, while there were 71 from 1 to 5 hp, and an average of about 62 in 1 hp and below. On the larger side, there were an average of more than 25 drives of 50 hp and above per location.

The idea of building the drive into the motor has been around for some years, but these integrated units don’t seem to have gained a lot of traction. Only 19% of respondents use them, while 75% don’t and don’t plan to. As one reader said, “If one fails, it costs more than if you have a standard drive and motor.” Another cites concerns about area electrical classification (Class I Div. I), but probably the most common reason was “Prefer to keep controls and motors separate for easy replacement/upgrade. Also prefer drives to be close together for install/maintenance and control wiring.” Of those who do like integrated drives, one cited “simple, inexpensive, easy maintenance,” while another said “I like to keep the heat out of the enclosure, and it frees up panel space.”

Customized or off-the-shelf?

AC adjustable speed drives are available with a broad selection of features, so it’s not surprising that standard off-the-shelf units fit most applications pretty well; 65% of users report that don’t use any customized units, and 59% don’t expect to use any within five years.

Of those who do buy customized drives (35% of respondents), the largest number (13%) go for custom in 1-20% of their purchases, and 12% buy them for 21-80% of purchases.

Looking ahead five years, 41% expect to use at least some customized drives.

Users comment about their drives

The survey asked subscribers if they were satisfied with the drives they used, and if not, why not. A large majority liked their drives just fine; some didn’t, with concerns expressed about reliability and support. “Obsolescence is bad. We need to have drives that last as long as the equipment is running. Cannot find parts or people to work on OEM drives so we are forced to upgrade and pay for no more functionality even though the drives are working well. Downtime risk is too great,” was one comment; another was “Some are 17 years old, and replacement parts are hard to locate.”

Some people had issues with difficulty in programming/setup. A case in point: “Too complicated. Too many parameters. Parameters not self-explanatory. Auto-tune doesn’t work right.”

Products: ac drives

Control Engineering subscribers, using a list provided, identified the following vendors as leading suppliers of ac adjustable speed drives: Rockwell Automation, ABB, Siemens, Yaskawa Electric, AutomationDirect, Baldor Electric, Danfoss Drives, Mitsubishi Electric, SEW-Eurodrive, Toshiba International, Emerson Control Techniques, Schneider Electric, Eaton, T.B. Woods, GE Industrial Systems, Omron, Bosch Rexroth, Danaher Motion, WEG Electric, and Alstom Power Conversion. For more manufacturers visit . For systems integrators, visit . Also visit company Websites listed with each product description.

HVAC drive now handles up to 350 hp

Allen-Bradley PowerFlex 400 HVAC drives from Rockwell Automation for commercial fan and pump applications are available with power ratings up to 350 hp. Features include connectivity via Modbus RTU, N2 and Siemens P1-FLN with optional LonWorks and BACnet; an embedded PID (proportional-integral-derivative) controller; three programmable skip frequencies and bands; selectable fan and pump curves and automatic sleep and restart based on load. Disconnect and contactor bypass packages combine operator interface, control, communications and power options in preconfigured assemblies. Contactor bypass packages come with three contactors, allowing drive test functionality and drive isolation when in bypass mode. The units have a UL508C plenum rating that allows them to be mounted inside an air handling system, such as ventilation ductwork.

Rockwell Automation

Expanded drive modules line

ABB’s ACS800-U4 drive modules line has been expanded to include R2– R6 frame sizes, which include a range of 1 to 200 hp. R7 and R8 frame sizes, already introduced, expand the total module range to 600 hp. All modules are offered in 480 V ac, and also are available in 230 V ac and 690 V ac configurations. This allows panel builders, system integrators and OEMs to buy the parts they need. The modules, built with IP20 protection and intended for installation into enclosures, are essentially ACS800-U1 standard drives minus the drive cover, conduit box, keypad, keypad mounting, and documentation. This allows the OEM and system integrator to buy only what theyneed and no more. The modules also save space; the company says that the R6, 200 hp unit is the smallest 200 hp drive on the market.

ABB, Automation Products, Low Voltage Drives

Closed loop vector drives

The Siemens Sinamics G120 drives, available in 1/2 to 125 hp, may be configured in a wide variety of designs. EN 60204 safety functions are fully integrated into the drive, including safe torque off, safe stop 1, safety limited speed and safe brake control. Built-in math and logic eliminate the need for a separate PLC. The units also offer enhanced energy savings with regenerative power modules, recovering line commutated energy for all power ratings and eliminating the need for a pulsed resistor and brake chopper. They also reduce control cabinet and cooling system requirements. Communication is via Profibus and Profinet.

Siemens Energy & Automation

Compact current vector drive

The V1000-series of ac drives from Yaskawa Electric America, available with power ratings from 1/8 to 25 hp, provide V/F control, open loop current vector control, PM open loop current vector control and simple closed loop speed control. Speed regulation ismonitor output, and RS-422/485 Modbus communications.

Yaskawa Electric America

575 V drives

The 1 to 10 hp GS2 variable frequency ac drives from AutomationDirect include 575 V ac models suitable for motor speed control applications. Offering dynamic braking, PID, and a removable keypad, the drives can be configured using either the digital keypad or an RS-485 port. The units are equipped with one programmable analog input, six programmable digital inputs, one programmable analog output, and two programmable relay outputs. The standard keypad allows setting speed, starting and stopping the drive, and monitor specific parameters during operation.


New VFDs added to line

The new models have been added to Baldor Electric’s variable speed ac drive line, including the VS1SP sensorless vector drive and VS1GV closed loop vector drive. Built on the H2 technology platform, they are aimed at difficult and demanding applications at a variety of voltage, power rating, or performance levels. Features include simple programming and easy operation. The 115 V ac models are available up to 3 hp, 230 V ac models up to 60 hp, and 460 V and 575 V are available up to 125 hp. Also new are the V*S1 microdrives, available in a variety of models from 1/2 to 10 hp, and the VS1PF drive, available from 5 to 700 hp for the pump and fan industry.

Baldor Electric Co.

Packaged drives for HVAC applications

VLT HVAC packaged drives from Danfoss Drives are available for Electronically Controlled Bypass (ECB) and Electro-Mechanical Bypass (EMB). ECB packages allow the user to program advanced bypass control features from the drive keypad, including automatic bypass, advanced firefight’s override, coordinated run-permissive in drive and bypass, and serial control while in bypass operation. The EMB packages are for users who prefer traditional selector switches and pilot devices. All packages provide improved protection and operation through a bypass 24 V dc switch mode power supply. Contactor operation is maintained in harsh conditions, and drive operation can be maintained at a reduced output with an input voltage as low as 70% of nominal, or with one input phase missing.

Danfoss Drives

Compact VFD, more torque

The E700 VFD from Mitsubishi Electric Automation, physically the same as the E500 model it replaces, provides 200% motor torque at 0.5 Hz for an open loop speed range of 120:1; the same communications options as the bigger F700 and A700 drives, as well as Modbus RTU, a dedicated USB port for set-up and diagnostics via a PC, remote operation over any supported network or locally using a keypad and display; and “bookshelf” mounting without a gap between units. DIN rail mounting also is possible.

Mitsubishi Electric Automation

Compact plug-and-play drives

Movitrac B drives from SEW-Eurodrive feature a modular design and multiple expansion options, including a plug-in keypad and communication modules for set-up and diagnostics. The drives are compatible with EtherNet/IP, Profinet, DeviceNet, Profibus, Interbus, CANopen, and Ethernet Modbus TCP/IP. Motion and logic control can be programmed with the company’s Movi-PLC software using any IEC 61331 language. A five-digit LED display shows many parameters.


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Summary: Important features

Net responses of very and somewhat influential features in ac variable speed drives follow.

97% Simple controls/setup;

97% Pricing;

94% Convenient operator interface;

94% Programmable in software;

93% Communication interface choices;

89% Ability to copy/download parameters;

89% Downloadable parameters;

87% EMI/RFI protection;

71% Enclosure type: NEMA 4;

71% CE Mark/other certification;

69% Enclosure type: NEMA 12; and

61% Preset speeds.