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Who's in Control Anyway?

May 1, 2002 12:00 PM, By Michael Eby, Editor-in-Chief

An interesting debate has surfaced in the world of AC drives, pitting the mechanical engineer against the electrical engineer. The debate revolves around whether variable-frequency drives (VFDs) should be placed in Division 15 or 16 of the CSI MasterFormat specifications. This may not seem like a big deal to some of you, but the topic can generate a heated conversation in certain circles. To best present both sides of this argument, I decided to tap the expertise of two industry experts. Here are their opposing arguments for where VFDs should be located within the spec.

According to Dick Orndorff, product line manager, AC drives, Square D/Schneider Electric, specifying an AC drive typically requires more electrical than mechanical engineering considerations. And since an AC drive requires coordination for nearly every electrical parameter associated with a distribution feeder circuit, including cable size, harmonics, EMI, and RFI, it's only natural for an electrical engineer to specify, evaluate, and approve project specs that involve drives. His argument is supported by the fact that today's modern pulse width modulated (PWM), adjustable-frequency drives make motor coordination much easier than in the past.

Orndorff also feels that field modifications are much easier to coordinate when AC drives are specified in Division 16 because most problems result from actions like incorrect control circuit sequencing, improper grounding, line voltage transients, nuisance ground fault tripping, and emergency generator coordination. Solving these problems requires the careful coordination of the electrical contractor, the electrical specifying engineer, and the AC drive manufacturer.

On the other hand, Michael R. Olson, manager engineered drives, ABB, Inc., argues that project coordination/start-up is more complicated when VFDs are specified in Division 16. Changes in project scope often do not get transmitted effectively from the mechanical contractor to the electrical contractor. Further coordination difficulty can arise during start-up, as the mechanical contractor may better understand how fans and pumps are supposed to operate.

Olson also notes some key technical considerations to support his position, including: 1) long motor lead lengths, which he considers almost unavoidable in MCC construction; 2) potential EMI/RFI problems due to the use of MCC common wireways vs. grounded metal conduits; and 3) sole-source responsibility if vibration or another mechanical/operational problem arises, which he states is why drives were originally moved to the mechanical section some 20 yr ago.

As an electrical professional, I have to side with Mr. Orndorff and say the specifications should be placed in Division 16, because an AC drive is an electrical device, not a mechanical device. An AC drive isn't made with mechanical devices like dampers, valves, belts, pulleys, or gears. It consists of electrical components like diodes, rectifiers, inverters, reactors, and transformers. I consider drives to be just another individual component of the overall electrical power system. And who better to specify and coordinate these devices than the electrical engineer.

However, that's not to say the two sides shouldn't work together or share information with other disciplines. The electrical engineer must act responsibly and work closely with the mechanical engineer at all stages of the project. In fact, if this approach is taken right from the start of a project, this debate would probably disappear quietly. But that's just my opinion. I'd like to know what you think. Drop me an e-mail at meby@primediabusiness.com and let me know what side of the fence you're on.

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