A new standard for large drives

Electrical Apparatus, Oct 2007 by Nailen, Richard L

The IEEE steps up with directions for matching drives with motors in the upper size range

Power versus Electronics.... Although volts, amperes, and ohms have the same significance in either field, practitioners in the two branches of technology often find themselves at odds.

Nowhere has that been more likely than in the proper matching of an adjustable-speed drive, or ASD, to an industrial motor. Many equipment operators have learned to their cost that the supplier of the rectifier/inverter/control system cannot take responsibility for suitability to the motor characteristics, and the motor manufacturer isn't going to be involved in the behavior of the power supply. Caught in the middle, the user may not enjoy a reliable, cost-effective combination.

That's particularly true for a "large" medium-voltage drive (using the NEMA motor standard definition of "large" as "over 500 hp"). Like the motor itself, such an ASD will generally be custom-designed. Where can a purchaser turn for sound guidance on specifying suitable drive performance?

The answer today is a new IEEE Standard, No. 1566-2006, titled IEEE Standard for Performance of Adjustable Speed AC Drives Rated 375 kW and Larger. It was developed by a petrochemical industry Working Group after seven years of work.

The scope reads:

"This standard applies to a-c adjustable speed drive (ASD) systems rated above 375 kWand above 750 V output voltage as used in petrochemical and similar applications. It covers the performance requirements for an ASD system including, but not limited to, input transformer or reactor [Figure 1] as required, power electronics, control interfaces, cooling system, switchgear, and motor. Requirements for power quality, engineering analysis, start-up assistance, training, and spare parts are also included [author's emphasis]. Certain items such as the motor, switchgear, or transformer may be excluded from the scope of vendor supply if specified in the data sheets; the system vendor shall specify any special requirements for the excluded equipment."

The motor involved may be either induction or synchronous.

What is an "ASD system?" The definition in IEEE 1566 reads: "An interconnected combination of equipment that provides a means of adjusting the speed of a mechanical load coupled to a motor." The standard's clear intent is to place responsibility for the entire system operation in the hands of a single vendor, so that the purchaser need not attempt the necessary coordination between various components. Figure 2 shows major elements of a typical large ASD system with reference to the corresponding pertinent sections of the standard.

A break from tradition

In dealing with rotating machinery, the IEEE has traditionally avoided issuing so-called design or performance standards. Most IEEE publications involving such matters have been either a "Recommended Practice" (expressing what consensus deems the "best way" of achieving some result) or a "Guide" (pointing the user in the direction of what's considered good practice). Examples include RP's 277 and 625 dealing with apparatus applications in cement plants; Guides 492 for operation of hydroelectric generators or 620 covering the format for motor thermal limit curves.

A notable recent exception to that approach is IEEE Standard 841, which includes design details, construction features, and performance required of severe-duty motors for the petrochemical industry (see "IEEE's new involvement in motor manufacturing," in EA February 1996). That standard has become widely used in other industries as well.

IEEE 841 governs only motors 500 hp and below. Although NEMA has issued some basic requirements for ratings above that size, those larger motors have always been viewed as custom products. Frame sizes and basic dimensions, construction features, or lubrication and cooling systems-such details have never been governed by industry standards.

Existing standards no longer sufficient

With the advent of adjustable-speed drives (ASD's) that vary motor speed by changing frequency and voltage, NEMA had to develop standards concerning motor compatibility with the inverter power source on the one hand, and with the mechanical implications of varying speed on the other. The result is Part 31 of NEMA MG 1, titled DefinitePurpose Inverter-Fed Polyphase Motors. As a unit of MG 1 section IV, these requirements are considered "Performance Standards" in that they spell out a number of temperature, current, torque, and overvoltage limits.

However, although the scope of Part 31 includes motors rated up through 5,000 horsepower and 7,200 volts, the limited provisions were clearly expected to apply primarily to much smaller, lower-voltage ratings, making up the majority of all ASD applications. And since MG 1 is a "motor and generator" standard, none of it applies to the electronic and control portions of the drive itself.

For smaller units, using "medium" or "NEMA-frame" motors, drive application may require little forethought. "Inverter duty" motors are widely available and largely standardized. The drive itself will probably use a "topology" or basic operating circuitry of a common type. Physically, the drive will be self-contained, either in a single free-standing enclosure or within a motor control center. Integral heat sinks provide adequate cooling.