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Product category: Industrial Drives/Controls
News Release from: Lenze-ACTech | Subject: Small AC drives
Edited by the Engineeringtalk Editorial Team on 09 January 2002

Charting the rise of small AC drives

Peter Cribbin charts the rise of small AC drives and suggests that while they might not get physically much smaller, communications means their wider adoption seems assured

The history of small AC drives is not a long one; the first low-power single phase drives having been launched in the early '80s These were based mainly upon thyristor technology for the output circuitry although some pioneering companies even then were adopting the latest bi-polar transistors

All of these drives were analogue units, with simple controls, no display and relatively poor dynamic performance.

None were very compact and could certainly not be described as sub-micro, the smallest being physically around the size of one of today's 4kW 3-phase drives.

What they did offer was that they permitted the use of a standard squirrel-cage induction machine; still the most compelling reason for the use of AC drives rather than the alternatives.

Particularly they enabled the machine builder to adopt standard three phase machines for equipment being run on single-phase supplies and to abandon the use of expensive single-phase motors.

At that time the simple and slow response analogue control philosophies meant that very low power units were not a practical proposition because the fluxing difficulties of smaller motors meant that starting torque was extremely limited.

The lack of control over volts/Hz ratio, particularly on fractional kW motors, also brought the problem that most of the output current was required for generating magnetic flux within the motor and the actual load power demand was small in comparison.

The constant cost of the control circuitry and there being no significant saving in output current regardless of motor size meant that there was no cost advantage in building a fractional kW drive.

This limited the growth of small AC drives for many years and initially three phase drives made better market penetration than single-phase units.

However a number of advances, both in control technology and in the power output devices, brought down the physical size and cost and broadened the power range down to small fractions of a kW, resulting in low power drives small enough to fit in the palm of your hand.

Amongst the most significant of these multifarious developments were the shift to digital technology and the adoption of the ubiquitous microprocessor.

This plus the use of surface mount technology radically changed the technology.

Control shifted from coarse potentiometers to push button programming, assisted by the digital display of a variety of parameters.

More or less simultaneously, new, more efficient output transistors, such as the IGBT, brought about significantly improved reliability and stimulated a reduction in physical size.

This created a stronger market for AC drives and from that stimulus, the power device manufacturers started designing devices with the drives market in mind, and also led to their entering the market in their own right.

By the early 90s, there were more AC drive designs on the market than DC drives and AC drives dominated the market.

The 6 pack IGBT output power pack, complete with flywheel devices, drove down size further and led to the design of a fully encapsulated power module for AC drives, containing input bridge, output bridges and associated semiconductor devices, all in a single pack for mounting straight onto the drive heatsink.

More recently, the development of digital vector technology and the benefits of smooth flux control with high starting torque and smooth low speed performance, made the micro drive a cost effective and viable proposition and their rise into the market was as spectacular as was the reduction in their cost.

Curiously, the market started with the design and development of drives by specialist drives companies.

The multi-national control companies followed them into the market as the technology simplified and matured.

Competition was fierce and prices were driven to almost unprofitable levels, but the wheel has turned full circle.

Many of the 'monolithic' multi-national control companies, have become inured to the demands of the market, offering 'one-stop shopping' for a wide range of products but little flexibility on any one of them, looking to make their products compatible, each with the other.

Once again specialists drives companies are exploiting the opportunity to capitalise on the competitive difficulties of the global marketplace by more accurately meeting the needs of the market and by being more flexible and adaptable.

Specialist drives manufacturers like AC Tech, by offering customers, particularly OEMs, the opportunity not to put all their eggs in one basket and to shop around to meet their application demands more precisely, are winning friends for their flexible approach and thereby pulling market share from their larger competitors.

This approach has already produced dividends in the US, where AC Tech claims the highest drives manufacturing volume of any dedicated domestic manufacturer; building over 8,000 drives per month.

Although building drives up to 187kW, AC Technology's main focus is on sub 45kW drives.

Their SCF sub-micro range extends to 18.5kW and is probably the widest and most competitive range of sub-micro drives on the market today.

Their newest offering the SCL series, is designed and built to meet the needs of the European market with features like integrated EMC filter and full compliance, but sized and priced to compete.

Although different markets are imposing different demands and restrictions, such as the EMC and low voltage directives in Europe and UL, cUL and NEMA specifications in the USA, AC Tech has taken these in their stride.

They emphasise that it is concentration on their core competence, coupled with a high degree of flexibility that is keeping them ahead of more broad-based competitors.

AC Tech is adding value features like LCD displays that enable the customer to interact with the drive in plain English.

Their development of the battery powered Electronic Programming Module (EPM) is a real advance that brings valuable benefits to the user.

The drive can be programmed off line and powered down, prior to installation.

Likewise, programmes can easily be 'cloned' from one drive to another or a batch of drives programmed from a single EPM, reducing OEM manufacturing time and cost.

Should unauthorised personnel alter the drive programme, it can quickly be reset to the default settings.

Only a very few years ago high user-value features like this had not been dreamed of for sub-micro drives.

Fieldbus communication is now a commonplace offering on drives and although the variety available precludes offering all of the variants, the most popular can be offered as options, making the drive an integrated part of the overall control strategy.

AC Tech have majored on ModBus RTU but are also in the final design stage of DeviceNet for the SCF series.

The future for sub-micro AC drives still holds great promise and is reputed to be the fastest growing sector in the drives spectrum.

Sub-micro drives, now at their most competitive level, are finding their way into all manner of applications, including the domestic sector.

Fitness machines and white goods now use AC drives and in the very near future, they will find application on domestic central heating pumps for energy saving reasons.

Development proceeds apace and while it seems unlikely that drives will get physically much smaller, with limitations coming from cable and termination dimensions, small drives are becoming more interactive with the system control environment, and their wider adoption is assured.. Request a free brochure from Lenze-ACTech ...

(This was Engineeringtalk's Top Story on 8 January 2002).

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