Electric motor technology may not progress in leaps and bounds, but there have been plenty of evolutionary changes over the past few years. In an ideal world, that evolution would trigger changes in the standards governing motor design and performance. "When technology moves on, standards should be updated," says Dan Jones, a consultant to the motor industry, president of Incremotion Associates and a board member of the Motion Control Association.
In reality, though, standards don’t necessarily move in lockstep with motor technology. And that divergence between standards and actual motors can provide some insight about the practical problems confronting engineers in the field. How so? Well, consider the most recent standards revisions from the National Electrical Manufacturers Association (NEMA) and Underwriter’s Laboratories (UL).
NEMA’s well-known motor standards specify frame sizes, mountings and various performance characteristics. "These standards are supposed to have a five-year shelf life," says Greg Winchester, program manger for NEMA’s programmable motion control committee.
Yet NEMA’s ICS 16 standard has been around since 2002, governing motion control motors along with related feedback devices and controls. From a technical standpoint, the standard sorely needs an update. As both Winchester and Jones point out, servo and stepper motor makers have managed to squeeze more power into smaller packages over the past five years. Yet ICS 16 while explicitly addresses only NEMA frame sizes 17, 23 and 34. "People talk about NEMA 14 and 8 motors, but they’re not actually included in this standard," says Winchester. He and Jones both describe such motors as having "quasi-NEMA" frame sizes.
You might think the standard should get an update to reflect the full range of frame sizes actually in use today. However, when NEMA last year tried to marshall its motor-making members to rework the standard to include smaller frame sizes, there was little interest. "The feedback wasn’t exactly resounding," says Winchester. "We simply didn’t get the critical mass we needed for an update."
Winchester believes the reason has to do with the lack of installation problems in the field. "As long as users can mix and match smaller motors from different suppliers, there isn’t a strong motivation for anybody to come to the standards table," he says. So far, he’s gotten very little indication of widespread confusion about sizes or difficulties between motors whose frame sizes are not spelled out by ICS 16.
Until that confusion or widespread difficulties emerge, it looks like ICS 16 will stay as it is.
UL, by contrast, has made sweeping changes to its motor safety standard UL1004. According to Frank Ladonne, a UL principal engineer, the standard revisions came about in part because UL has recently started listing motors–as opposed to treating them as recognized components. A desire to keep pace with technology advances also played a role. "I began to see that our existing standards didn’t adequately address new technology," Ladonne explains.
As a key example, he points to the clearances that UL requires to ensure safe motor operation. A 120-volt electric motor formerly required 1/16-inch spacing between its live parts and accessible metal components. "Twenty or twenty-five years ago, that was no problem," he says. But with today’s downsized motors, which often have better insulation systems than in decades past, that clearance may not be realistic. "That same motor today might measure a half inch in its largest dimension, so a 1/16-inch could be a problem," he says.
So Ladonne has rewritten UL1004 to permit functional testing as a way to demonstrate safe clearances and creepage distances. "Manufacturers can either meet the requirement we spell out or demonstrate the safety of their design through dielectric or impulse testing," he says.
In all, the revamped UL1004 will include nine different sections covering just about every type of motor and related electrical machines. The completed sections address rotating electrical machines (UL1004-1), impedance-protected motors (UL1004-2), thermally protected motors (UL1004-3), electric generators (UL1004-4) and fire-pump motors (UL1004-5) are complete. By the end of this month, Ladonne plans to submit four more sections–for inverter-driven motors, step and servo motors, brushless DC motors and refurbished motors.
The specific safety issues related to each type of motor vary, but the standard changes all share a common underpinning in that they provide a greater role for functional testing. "As always, we’re cautious to call out only those aspects of the motor construction that impact safety because we don’t want to be in the position of stifling innovation," Ladonne says. "The key is that manufacturers now have more alternatives to demonstrate the safety of their designs."