Everybody agrees: motors and their control electronics were not created as equals. They live in two different environments. Electronic controls enjoy the safer, cooler, and more centralized enclosures, while motors face severe conditions of temperature, humidity, vibration, and dust in the industrial world. These conditions demand they operate in separate locations, connected by cables for power, control, and communication.

So why bring motors and their control electronics together and put them into one package?

In the past ten years technology has developed to the point where electronic components can be made to operate in the harsh environments of the factory floor. This has allowed the two disciplines-motors and controls-to merge together into one package.

Motor manufacturers are keen to sell the benefits of these new integrated motor-drive packages. By eliminating separate enclosures and long cable runs, the integrated approach promises lower system cost-as much as 20 to 40%, say some suppliers. Ownership cost of integrated motor-drives is also lower because the manufacturer can ship a pre-tested motor and drive package which means the "ultimate matching" of components, the result of designer and manufacturer collaboration on the integrated package.

Another point in their favor: the combined units greatly reduce EMC issues and eliminate long and costly cable runs between motor and drive, which are prone to reflected voltage spikes.

This emerging class of products goes under a variety of names, such as smart motor, variable-speed motor, motor with built-in drive, integral motor/drive, and integrated drive motor. Probably the clearest term in use is integrated motor drive (IMD), which is a registered product name of TB Wood's Inc. We will use the term "IMD" in this article.

In spite of their apparent advantages, OEMs and end users have not been so eager to buy the products. In fact, IMDs have fallen short of the potential envisioned for them five or more years ago. Earlier predictions of market growth and power range have been scaled back. Actually, units up to 18 kW were promised by this time, and they have not arrived. Similarly, more sophisticated controls have not fully developed because of lagging application demands and sales volumes. Market analysts estimate the European market for IMDs in 1999 was $46.4 million, while North American sales were still in the "early adoption stage," at $1.6 million.

Yet IMDs may now be poised for wider user acceptance. One positive sign for integrated motor-drive manufacturers is the growing trend toward distributed control architectures within the entire business enterprise. IMD technology can share in this move.

Tough environments

The integrated motor-controller packages have two design hurdles to overcome. First, moving the controller out to the factory floor exposes it to the hostile industrial environment, with all of its well-known complications. But more importantly, bolting the controller directly onto the motor is like putting it on an electric cooking stove and turning up the heat.

Heating effects are, in fact, the greatest enemy of IMDs. Both the motor and power-switching electronics in the controls produce heat. As a result, thermal management of these integrated packages has been improved to combat the problem. This was accomplished with optimized design of mechanical/structural components, heat sinks, cooling, and the layout of elements. The techniques have worked, up to a certain size.

Most suppliers make IMDs up to a 7.5-kW rating, or will range up to that size shortly. The 7.5-kW output represents the current plateau above which heating effects build up significantly. Commercializing larger-sized units will require more complex designs and impact product costs.

Motor speed is another factor in thermal management. At high speed, a fan driven by the motor shaft provides enough cooling, but this becomes inadequate at low speeds. This condition is exacerbated by on-going improvements in one area of IMD performance: widening of the speed range. Today, the typical speed range is 10:1, but a few manufacturers, using more sophisticated controls, offer 50:1. Extending the speed range is not a routine exercise, because more cooling is needed to offset the more severe heat generation at low-speed operation. A motor operating at 30 rpm (or below) for some periods usually requires a separate constant-speed blower.

The location of the drive electronics on the motor brings up a spatial design trade-off that also relates to heating. Most IMDs place the control unit atop the motor (or at plus or minus 90-degree radial positions), raising overall unit height but keeping the length dimension intact. A few manufacturers believe the top position exposes the electronics to extra residual heating flowing upward after the motor is turned off. For example, ABB and TB Wood's Inc. prefer axial placement of the electronics for this reason, and also to improve cooling air flow from the axial fan. These designs hold overall height and mounting dimensions identical to that of a standard motor and give up something on overall unit length. It's largely a matter of design philosophy.

Vibrations and electromagnetic interference (EMI) are some other environmental considerations for integrated motor-drives. Careful design and testing can ruggedize the electronics substantially, while special rotor balancing limits initial motor vibrations. Monitoring may be used to prevent vibrations worsening with motor usage over time. Siemens A&D mentions the special design of its CombiMaster electronics package to handle vibration and shock that can reach up to 5g. As for EMI effects, much less wiring between the integrated motor and drive inherently reduces this concern. Still, most European IMD suppliers include EMI filters in their units. For example, CombiMaster comes with Class A and B EMI filters.

Cost and market issues

Cost considerations for IMDs aren't simple either. Depending on size, a typical motor-drive package carries a premium on initial cost compared to an individual motor and drive. A rugged housing, thermal management, close-coupled wiring, and the special environmental design all add to cost. Typical price premiums today are in the 15 to 20% range.

Siemens claims lower overall cost when factors such as wiring between individual components, a control panel for the drive, panel air conditioning, and maintenance are considered. The component cost may be greater, but from a project view, savings of 20 to 30% are possible in the integrated approach. Rockwell Automation AC Drives Business cites an even larger figure for the reduction of total installed cost: up to 40%. However, total ownership cost can be hard to sell to potential users who may be more concerned with the initial cost and who hesitate about new technology.

Adding more sophisticated controls to an IMD brings further cost considerations. For example, if flux vector control is added, the cost of a feedback device (encoder) must be included in any increase of controls pricing. Recent market surveys indicate that pricing of IMDs is crucial for market expansion. In other words, to catch more buyers' attention, the initial cost of IMDs also must become more competitive with a separate motor and drive solution.

Networking will be key

Many users have concerns about the reliability of control electronics working right next to the motor. Who will be responsible for maintaining the combination package? Motors and drives are often cared for by separate maintenance departments; now someone has to look after both.

Some users see the IMD as a solution that is better suited for stand-alone applications, such as in commercial installations or HVAC.

For industrial control, the motor-drive combination may not be so practical. The motors are usually fed from central switch cabinets where the variable frequency drives are located; to move the VFD to the motor would require running an additional conduit for control wiring. And, with most controls based on PLCs or DCSs, it is relatively easy to put I/O points in the switch cabinet.

For IMDs to become more popular on the factory floor, they will have to have good communications capability. Remote control, communications, and networking features are already present, or coming soon, to this product sector.

Serial communication and bus networks incorporated into IMDs simplify control wiring. Twisted-pair wiring carries the command signals. Several motor-drive units can be daisy-chained into a network. A low-power 24V signal could energize a contactor to feed multiple IMDs in a network. Serial commands or other digital signals would then regulate individual motor-drive units in the network.

The power disconnect, fusing, or circuit breaker is still needed, but communication networks reduce the extent of control wiring. Circuit protection can be housed in a small panel or enclosure. Manual shut off of each unit is required.

Will they grow?

Power ratings of motor-drive packages have been growing slower than first envisioned. At Rockwell Automation, the present top size is 3.75 kW, the crossover point where heating effects become a substantial design problem.

No substantial size increase above 7.5 kW is seen in the near future at Baldor Electric, Lenze, or others. Although 7.5 kW is a readily available size, it represents a "practical limit" for now. Few exceptions exist.

Siemens mentions "potential" growth to 15 kW, possibly within a year. A notable exception available now is the Compact Drive product from VEM Motors GmbH. These IMDs range up to 22 kW and include field-oriented and vector control models. Drives for the smaller units come from Danfoss, while Emotron supplies the larger electronics units.

Of course, larger power ratings will come as engineers meet design and cost challenges. Above a certain point, however, physical size of hardware makes integration of motors and drives lose all meaning.

The more immediate challenge for integrated motor-drive technology is to pare down initial costs and more clearly promote product benefits to potential users. For IMDs to be a real alternative to traditional motor and drive installations in substantial numbers, some manufacturers need to take a more active stance, which is not an easy task when it means competing with alternative in-house products.