AC Microdrives Ply Parallel Paths

Despite the temptation, size alone is not the right descriptor for ac "microdrives"—those ultracompact variable-frequency drives (VFDs) that sprouted during the first-half of the 1990s. For quite a while, it seemed, each new model would be even more miniature than the one announced just a month or so earlier.

By Frank J. Bartos, CONTROL ENGINEERING February 1, 1998
  • Motors and motion control

  • Variable-frequency drives

  • Machine control

  • Motor starters

  • Microcontrollers

Despite the temptation, size alone is not the right descriptor for ac “microdrives”—those ultracompact variable-frequency drives (VFDs) that sprouted during the first-half of the 1990s. For quite a while, it seemed, each new model would be even more miniature than the one announced just a month or so earlier. Breakthroughs in semiconductors and electronic integration led to VFDs rated 0.75 kW (1 hp) or more capable of fitting well into a small hand. The term microdrive had a literal fit.

Today, one common defining line for microdrives is 4 kW; some folks say the range is even higher. One manufacturer includes a 37 kW unit in its “microdrive” product line! For the sake of discussion, this article places an arbitrary 4 kW limit on the drives covered. But there’s a lot more to microdrives than just size.

Initially, ac microdrives were intended for original equipment manufacturers (OEMs) and other users with special needs to control the speed of small induction motors without all the frills in tight equipment spaces. Compared to normal, so-called “full-featured drives,” these micros were smaller, had less functionality or options, and carried a lower price tag.

Today, a parallel track appears to be forming. Newer-generation micros add the latest features, while striving to maintain compact physical form. Miniaturization and integration of components are the technology drivers. Transistors with lower heat output allow placement of power, gate driver, and logic circuits inside compact integrated power modules.

A products table samples the great variety of microdrives available.

What’s the difference?

Most manufacturers differentiate a microdrive from a full-featured drive of the same power rating. At Mitsubishi Electric Automation (MEA, Vernon Hills, Ill.), lower cost and fewer functions/options are key distinctions—besides smaller size—according to Steve Racine, ac drives product manager. Mr. Racine explains that lower functionality (fewer I/O points and operating parameters, a simpler user interface, etc.) need not be a hindrance. “Microdrives can serve the vast majority of applications using full-featured drives,” he adds.

Simplicity is a further distinction noted for these products at Rockwell Automation/Allen-Bradley (Milwaukee, Wis.). “AC microdrives fulfill the need of machine builders and other users for compact, easy to use drives with ‘just right’ features. A limited feature set allows less complexity, aiding in lower costs and simpler start-ups,” says Stan Ho, marketing manager for ac drives.

With costs decreasing, microdrives have become competitive as replacements for motor starters. Mr. Ho cites this as one example of simpler applications where the drive doesn’t need communications or an integral EMC filter.

Quantity of I/O points directly affects size and cost of the drive, and how I/O devices are programmed impacts complexity. Another tradeoff is type of operator interface. An LED-type interface provides only cryptic parameter codes. Upgrading to a more useful alphanumeric display (LCD) is an incremental cost to obtain greater ease of use. An LCD-type operator interface further allows remote mounting plus ability to store and download preprogrammed drive parameters. The latter feature has drawn considerable customer interest, according to Mr. Ho. A-B’s Bulletin 1305 product (see table) includes an LCD operator interface.

David J. Grucza, marketing manager at GE Fuji Electric (Salem, Va.), notes three main differences between microdrives and ordinary VFDs: smaller size, less functionality, and lower cost. Smaller units ease the task of fitting equipment inside a control panel—a prime concern of OEMs. “Microdrives also have less functionality than normal inverters. For instance, normal drives will have more communication options than a micro. The latter are much less expensive, typically 25 to 30% less than a normal drive,” says Mr. Grucza.

The joint venture between GE and Fuji Electric develops packaged products: microdrives, Volts per Hertz (V/Hz) inverters, and flux vector drives. Custom and system drives are not included. The venture has two working units: GE Fuji Drives USA (also in Salem) handles marketing, engineering, and service functions; GE Fuji Drives America (Monterrey, Mexico) is the manufacturing entity. (Two other joint businesses, Fuji GE Asia and GE Fuji Europe, have yet to begin operations.)

Simple is key

From Control Techniques Drives’ (CTD, St. Louis, Mo., part of Emerson Electric Co.) viewpoint, the microdrives market is growing rapidly. It’s fueled by more than just size or efficiency of the product. As Jane Kienstra, standard products manager for CTD, puts it, “Microdrives are becoming more reliable, easier to use, and easier to understand—making them more acceptable to the people responsible for their operation.”

Ms. Kienstra also separates the typical microdrive from a standard VFD by the features offered. “Microdrives include key features, but don’t have a lot of them,” she remarks. The idea is to keep things simple and not intimidate the user. “With English read-out keypads and simple start-ups, most electricians can operate them.”

She illustrates this distinction using Control Techniques product examples. CTD’s Dinverter, for example, has simple V/Hz operation, while its full-featured Unidrive includes open-loop [sensorless] vector and other operating modes for more accurate control (see CE , March 1996, p. 9).

In Ms. Kienstra’s experience, other complex features typical to only full-featured drives are: digital lock, autotune (both commonly used in closed loop), high resolution analog I/O points, ratio control, etc. Some of these functions, however, will migrate to the next generation of microdrives.

Packaging differences

Microdrives, especially at the low-power end, tend to be mounted inside another enclosure. This appeals to OEMs and control panel builders where space to fit a drive on a panel or inside a machine is limited. Mark Borski, ac drives product manager at ABB Industrial Systems (New Berlin, Wis.), points out that such “chassis-type” microdrives have different packaging. “They don’t need the same ‘finger safe’ design as a normal variable-frequency drive. Wiring and terminal connections may also be different,” he adds.

Simplicity for use “right out of the box” is the focus for microdrives at ABB. Its ACS 100 product (see photo), introduced in 1997, is rated for 50 8C operation without a cooling fan. Flange mounting—one of three mounting styles offered with ACS 100—allows the heatsink to be located outside the control cabinet, possibly in a ventilation duct, for more efficient cooling.

“Clearly the term ‘microdrive’ is open to multiple interpretations,” comments Chris Bardwell-Jones, president of Bardac Corp. (Reston, Va.). In his view, microdrives reach beyond device size to include sophisticated control strategy, configurable software blocks, and greater application flexibility.

Bardac Corp. renamed from Sprint Electric Co., as of Jan. 1, 1998, avoids trademark conflicts in the U.S. (The Sprint Electric name remains unchanged in Europe.)

Bardac is a newcomer to the ac drives arena, though not in staff expertise. A long-standing proponent of dc drive technology, the company launched a new ac line called Profiler Series in July `97. Mr. Bardwell-Jones expects the line, initially rated up to 0.75 kW, to expand soon. Other versions of Profiler Series will focus on specific market segments such as OEMs, fan and pump, single-phase, and general-purpose drive applications. Support and service for both ac and dc products will be provided by the same team, he adds.

A market view

Confidence of the user community in technology advancements is behind the ongoing growth of the ac drive market, according to Automation Research Corp. (ARC, Dedham, Mass.). ARC’s recent study puts the North American ac drives business at $1.036 billion for 1997, with nearly 10% annual growth to $1.621 billion in 2002. These market figures (for all drive sizes) include hardware, software, and services—also the host computer, if it comes from the drive supplier—explains Paul Benoit, senior analyst at ARC.

Largest growth is expected in the micro and low-end drive segments during the 1997-2002 period. ARC designates these market segments as units rated up to 4 kW and 5-40 kW, respectively—more as a convenience than a hard definition. For microdrives specifically, North American shipments comprise 18.7% of the 1997 dollar value; market share is expected to grow to 21.1% of 2002 dollars, says ARC.

As for technology, virtually all units shipped are pulse-width-modulated-type drives. The above market figures are broken out by mode of operation in the following table. Volts per Hertz (V/Hz) dominates over the forecast period, followed by sensorless vector and flux vector. Currently, the top four users of microdrives are in the food and beverage (20.9%), automotive (16.7%), chemical (9.8%), and building automation (9.6%) industries. Values are given as percent of 1997 shipments.

Microdrive evolution

Mitsubishi’s Mr. Racine thinks that leading manufacturers will continue to offer separate micro and full-featured VFDs of the same power rating well into the future. This is due to basic cost and size issues of production.

Various functional elements are common across a VFD regardless of power or voltage rating. Mr. Racine includes here the control circuit, user interface, plug-in options, logic-level functions, etc. These items represent fixed cost as well as functionality. (Total cost of a drive adds on the variable cost associated with power and voltage.) To add full-featured functions means some cost increment over “basic” functions, say, $100 for the sake of discussion; but this may or may not be small relative to total drive price.

For larger drives (100s of kW), this difference is “virtually meaningless” states Mr. Racine. “It makes no sense for the manufacturer to market two separate product lines.” All customers get full-featured functionality. However, for a 0.75 kW unit, the $100 increment can be a large part of the total drive price. “At such power levels, a need exists for a ‘basic’ functionality VFD to cost-effectively meet the needs of 90% of users. Hence, the ‘micro’ drive,” he says.

Available space is another factor that affects development costs, explains Mr. Racine. More features require more physical volume, but this increment doesn’t change much with power and voltage ratings. It’s the larger drives that have space to spare. Adding more functions in low-end drives creates the real challenges for designers. And it’s here that product choice is needed. Some users will trade extra features for a package that’s not the ultimate in miniaturization; others put a premium on space savings for applications where simplicity and ease of use are the rule.

“Adding functionality without creating complexity,” is key to further evolution of microdrives, adds Allen-Bradley’s Mr. Ho. One way to obtain this is via software functions that can switch operating mode from V/Hz to vector control, he adds.

Mr. Ho sees tomorrow’s microdrives expanding in power range “well beyond” 20 kW. Next-generation transistors with lower losses, intelligent power modules, and other component integration methods will help maintain the compactness of the designs.

The view on number of features differs at ABB. “Microdrives, in many cases, provide a full-feature package, identical to their larger counterparts,” says Mr. Borski. New-generation micros will add more features. For example, ABB’s ACS 140 microdrive—to be introduced in first-quarter ’98—is said to include 90 parameters, along with enhanced intelligence and microprocessor capabilities.

Square D Co. (Raleigh, N.C.), part of Groupe Schneider, also sees expanded capabilities coming to micro drives. A good number of them are available already, according to Ajay Koul, product manager of minidrives. “Enrichment of operating characteristics and the feature set is making microdrives good candidates for a large variety of ‘high starting torque’ and ‘fine control’ applications,” he says.

Today’s microdrives offer a lot more functionality than products of five years ago. In Mr. Koul’s view, the latest microdrives already offer high torque control at low speeds (150% rated torque at 3 Hz, 100% torque at 1 Hz); autotuning of the drive to the motor; PLC-type control, operation, and monitoring functions; programmable logic I/O functions; integral PI controller for closed-loop usage; and more. Accessory items of earlier models, for example, a radio-frequency interference (RFI) filter or braking transistors are now built-in options.

Although not necessarily the smallest micro on the market, Square D’s Altivar 18 is said to offer many advanced features in a cost-effective package. In particular, this 1997 product comes with a built-in RFI filter and the low-speed torque performance cited above.

GE Fuji Electric’s Mr. Grucza verifies that new versions of micros coming on the market include functions typically found in normal inverters. For example, Micro Saver II (see photo), introduced just last month has autotune capability and sensorless vector control.

Motor starting role

Rudy Hauser, product manager at Siemens Energy & Automation, Drive Products Business Unit (Alpharetta, Ga.), likewise sees wider applications opening for microdrives because of evolution to smaller, more intelligent, and less costly form.

He cites an example in the control of large industrial overhead doors. The manufacturer was able to place a microdrive in the same small control enclosure previously occupied by a conventional motor starter. “With variable speed control, the raising and lowering of the door can now be programmed for maximum efficiency and safety,” remarks Mr. Hauser.

GE Fuji’s Mr. Grucza says, “Prices on microdrives are such that they are comparable to reversing or two-speed starters, while offering added benefits, for example, infinite speed adjustment, inherent soft-start capability, and built-in diagnostics.” Also, newer microdrives have enclosures typical of starters (e.g., NEMA 4), he adds.

Square D’s Mr. Koul likewise stresses the role of controlled motor starting for microdrives—besides steady-state control. “Highest duty for a motor and the associated drive occurs during startup,” he states

Bardac and Control Techniques are still other vendors that mention motor starter replacement as a developing trend for ac microdrives.

Communications, wider apps…

Some aspects of microdrives are already maturing—for example, power structure, heatsink design, and thermal management. New action appears to be coming from other areas.

A recurring theme heard from several drive manufacturers is more communication and programmable functions coming to micros. “We’ll see PLC-like capability so that a microdrive can be preprogrammed to make decisions. An interface to a high-speed, open communications protocol will be standard,” explains, Siemens’ Mr. Hauser.

Besides simplifying installation, Siemens’ Micro Master drives offer built-in control logic, communications (RS-485, Profibus-DP), and relay protective features. Micro Master series (lead photo) comes in two versions: 6SE92 for V/Hz control and 6SE32 for sensorless vector control (see products table).

Other near-future trends he notes include direct linking of microdrives to PLCs and HMI packages; low cost, intelligent operator interfaces; and more application-specific features, such as winder and motion control. “Overall, we need to strive for ‘fit and forget’ installations, making an advanced microdrive as easy to apply as a standard relay,” says Mr. Hauser.

As far as integration of PLC-type control, Square D’s Mr. Koul adds, “Half of the PLC is already in the drive; and within a few years traditional PLCs will not be needed. It’s a case of vertical functional integration.”

Looking ahead, Bardac’s Mr. Bardwell-Jones notes that further component miniaturization and new generation integrated power modules will shrink drive package sizes, while reducing costs. Typically, this creates a cycle of lower priced microdrives being specified for more applications, followed by sales volume growth and still lower production costs.

Will all low-power VFDs blend into one microdrive category? More blurring is sure to come with advancements in parts integration, semiconductor packaging, and continual cost reduction. But special applications needs of OEMs and related users still exist. To satisfy them, some ac microdrive manufacturers will continue to pursue parallel paths of product development for quite some time.

For more information, visit www.controleng. com/info .

AC Microdrive Shipments by Mode of Operation (Millions of dollars) *

1997 % of total 2002 % of total CAGR %
* North American Shipments Source: Automation Research Corp.
CAGR – Compound annual growth rate
Volts/Hertz 168.1 86.8 296.4 86.7 12.0
Sensorless vector 16.4 8.5 29.1 8.5 12.1
Flux vector 9.2 4.8 16.4 4.8 12.3
Total 193.7 100 341.9 100 12.0


Company Product Name Ratings kW V in Speed Range Hz (spd ratio) Smallest Unit (WxDxH) mm RSN Features
Enclosure type NEMA 1, unless noted: (x) – Chassis (y) – Chassis & NEMA 1 (z) – Protected chassis
(a) – sensorless vector control (b) – high torque at low speeds * – 1- & 3-phase ** – single-phase
(e) – smart EEPROM chip card & reader clones, dowloads setup parameters Input voltage: 115 V is 1-ph; 460 V is 3-ph
(g) – 230 V (1-ph) to 2.2 kW (h) – 230 V (1-ph) to 1.5 kW (j) – 230 V (3-ph) from 3 to 7.5 kW (n) – 230 V (3-ph) from 0.37 to 3.7 kW
(k) – 460 V from 0.75 kW (m) – 115 V to 0.75 kW (q) – 460 V from 0.37 kW (s) – 150% torque for 60 sec.
ABB ACS 100 0.37-2.2 230 V * 0-250 80 x 138 x 146 214 Nat cooling (50 C); Downld, copy parameters; (z)
Baldor Electric Series 10 0.37-1.5 115/230/460 0-400 105 x 123 x 159 215 LCD oper intface; DC inj. braking; (k); (m)
Bardac Profiler Series 0.18-0.75 200-250 0-180 57 x 103 x 90 216 50 C temp rating; Higher speed options; (y)
Control Dinverter A 0.25-0.75 230 0-400 74 x 192 x 140 217 High-speed version up to 960 Hz.
Techniques Dinverter B 0.75-3.7 230/460 90 x 200 x 292 230 V to 2.2 kW; Up to 960 Hz; RS-485 std.
Eurotherm Drives 601 Series 0.37-2.2 115/230 ** to 240 72 x 175 x 183 218 Integral RFI filter; All units one size; (e)
GE Fuji Micro Saver II 0.18-3.7 230, 380/460 0-120 104 x 79 x 150 219 NEMA 4 & 1 encl; CE mark; (a); (g).
C9 0.09-3.7 230 79 x 84 x 120 CE mark; (h)
LG Ind. Systems SV-iG 0.75-3.7 230/460 0.5-400 (40:1) 140 x 130 x 160 220 Space vector PWM control; (b)
MagneTek Drives GPD 205 0.09-2.2 115/230/460 0-400 (40:1) 108 x 130 x 128 221 (m); (q); (s); (z); 230 V (3-ph) to 1.5 kW
Motortronics CSD Series 0.37-3.7 230/460 0-400 107 x 135 x 162 222 Built-in proc timer; Keypad std w/integral pot
Mitsubishi A024/A044 0.12-3.7 230/460 0.5-400 (20:1) 104 x 66 x 150 223 Advanced functs via opt param unit; (b); (y)
Rockwell/Autom. Bulletin 160 0.37-2.2 230/460 0-240 (20:1) 72 x 140 x 140 224 App specif modules; Comm adapters; (x).
Allen-Bradley 1305 0.37-3.7 0-400 (20:1) 120 x 122 x 195 LCD oper intface; Comm modules; (b)
SEW-Eurodrive Movitrac MC 31C-0 0.56-1.5 230/380-500 0-240 105 x 189 x 188 225 Opt Profibus interface; (a)
Siemens Micro Master 6SE92 0.12-7.5 208-500 (30:1) 73 x 140 x 147 226 50 C temp rating; RS-485; Profibus; (y)
6SE32 (100:1) (a); Other features as above
Square D Altivar 18 0.37-15 230/460 0.5-320 112 x 121 x 182 227 Integral RFI filter; (a); (b); (g); (j); (k)
TB Wood’s E-trac XFC Series 0.37-3.7 115/230/460 (10:1) 104 x 122 x 160 228 Equiv kW at 230/460 V in same pack; (k); (m); (n)
PMC PowerMod Ctrl 97 x 53 x 132 Inverter module package for OEMs; (k); (m); (n)