Letters to the Editor

A Tutorial on Reactors I would like to compliment Eric Wolf on his informative article, "Drive to Succeed" (April 2001, p. 36). As a former mechanical design engineer for a consulting engineering company, I can confess to having specified variable-frequency drives on mechanical equipment without fully understanding the workings of these devices.

07/01/2001


A Tutorial on Reactors

I would like to compliment Eric Wolf on his informative article, "Drive to Succeed" (April 2001, p. 36). As a former mechanical design engineer for a consulting engineering company, I can confess to having specified variable-frequency drives on mechanical equipment without fully understanding the workings of these devices. Your article has gone a long way in demystifying them. That brings me to my question: Your article mentions line and load reactors. Are these in fact another name for resistors? Also, when you mention 3-percent and 5-percent reactors, does this mean 3 percent or 5 percent of the full load power is lost (as heat)? Also, are the line and load reactors always in the circuit or can they be bypassed when not needed?

COLIN OAKELY, Facilities Manager, DuPage County, Ill.

Eric N. Wolf, P.E., Lockwood Greene Engineers, Indianapolis, responds:

With regard to your questions, line and load reactors are not resistors. They are inductors. Inductors are commonly referred to as "reactors" in the power industry because they are used to add reactance to an electrical system. Inductors are one of the basic components of electricity along with capacitors and resistors. Reactors are most commonly added to an electrical system to limit short-circuit current and to filter out unwanted waveforms such as harmonics. Capacitors are most commonly added to an electrical system to improve the power factor, but are also used for filtering along with reactors. Resistors are rarely used in an electrical system except for transformer grounding and controlling the speed of wound rotor motors.

The 3-percent and 5-percent ratings of reactors refer to the impedance of the reactors on a per unit (normalized) basis. The 3-percent and 5-percent values do not refer to the heating loss of the inductor. The heat loss is based on the resistance of the copper—or aluminum—wire used in the winding of the reactor. The value of heat loss from a reactor has to be obtained from the manufacturer of the reactor. However, the heat loss is greater for the 5-percent reactor, compared to the 3 percent, and the increased heat loss affects the overall efficiency of the drive. This is why you don't want to add more reactance than you need.

The line and load reactors, if used, would always be in the circuit. The only way to remove them would be to physically change it by wiring in the drive unit or adding contactors to switch around the reactors when not wanted. However, the latter option would be very expensive. It is best to determine approximately what is needed and have it built into the drive. Consulting engineers or variable-frequency drive representatives can help you to determine the size and amount of reactors needed.

Code Checking

The May 2001 issue of Consulting-Specifying Engineer contains a listing of what it calls "State Building Codes," which is a misnomer to say the least.

First of all, it is a listing of some, but not all, of the applicable state mechanical and electrical codes and standards. It does not list any of the applicable state building codes which also contain many requirements pertaining to mechanical and electrical work and list other applicable National Fire Protection Association (NFPA) and additional standards, or portions thereof, as applicable.

Second, it is a far from complete listing. For example, in Pennsylvania, where I live, the [local] department of health enforces NFPA 101, 1985 (yes, I am embarrassed, but 1985 is correct).

What you have listed is helpful, but I am concerned that some unsuspecting and inexperienced engineers may consider the list complete and therefore have problems because of that. States frequently change their standards and, in my opinion, there is only one way to determine what codes are applicable to any given project:

  • Contact all of the local and state authorities having jurisdiction over the specific project to determine exactly what codes and standards they will be enforcing for that particular project.

  • Contact the department of health for health-care projects.

  • Refer to the listings in those codes and standards.

  • Read all of the listings so obtained to determine how and where they affect the particular project.

FRANK M. LEBMAN, P.E., Ballinger, Philadelphia

Building Code Update

In reference to the listing of state building codes in the May 2001 issue, I would like to inform you that the State of Arizona does have a state plumbing code. It is the 1994 Uniform Plumbing Code with no amendments. It was just passed in 2000. It is out of date, to say the least, but it is on the books.

BILL HUG, Director of Engineering, Cavco Industries LLC, Phoenix

Do Lawyers Overbill

About the only merit of "Working with Lawyers" (April 2000, p. 21), was that the article identified some of the things which normal people despise in legal behavior. [For example], I find it extraordinarily petty to charge hapless customers as much as 0.1 of an hour billing time for a 30-second phone call. [Another disturbing note] is the untrustworthiness implied in the advice that one must always "memorialize" in writing the content of each encounter between lawyer and customer, lest someone misrepresent it later.

It's extraordinary to me that I can work on a handshake with all sorts of trades—engineers, contractors, suppliers, even accountants, dentists and trade-show managers—but I can't trust lawyers on such an agreement. I can call any of these folks, and they can call me, without billing for it. I rarely have cause to suspect that the project plumbing, heating, ventilation and air-conditioning subcontractor is magnifying the problem to fatten his fee, but I always seem to have cause to suspect the lawyer of doing so.

I don't suppose the legal "profession" can be excised from modern American business practice, but I do think that they need to be careful, lest we be even more highly motivated to find some substitute for them—maybe HAL the computer from "2001: A Space Odyssey," who never charges for "research"—and then they'd be where they deserve to be: without gainful employment.

MARTIN S. HARRIS, JR., Architect, Vergennes, Vt.

Revisiting HVAC System Deficiencies

The sidebar in, "Humidity Counts—In the Right Amounts," April 2001, p. 48, states that indoor-air quality and operational problems caused by heating, ventilation and air-conditioning (HVAC) systems were attributed primarily to: "haphazard installation and operational practices" and "poor workmanship issues or incomplete start-up, testing or debugging."

During the past decade, I have reviewed hundreds of HVAC systems. Most were wet inside, corroded and rife with mold and other fungi.

These deficiencies are not generally due to "haphazard installation" or "poor workmanship," on the part of others, as claimed. Instead, they are the result of poor design, which can and should be avoided by astute engineering design, preparation of sound and explicit specifications and follow-up inspection by a qualified designer.

System commissioning is simply not the answer. The test and measurements serve only to tell you what you have and how you can make the most out of it. It does nothing to improve the product and assure high quality.

WARREN TRENT, P.E., Trent Technologies, Tyler, Texas

Telecom Hotels, structurally speaking

The article, "Imperative Infrastructure," May 2001, p. 56, quotes a the Telecommunications Industry Association's report of a 20.9 percent increase in equipment sales for last year. This suggests that the industry is expanding at a rate critical to proper design management. As with any rapidly advancing industry, the "behind the scenes" components of the design are often overlooked, neglected or avoided altogether. The structural integrity of the system installation as a whole is one such component.

More often than not, the structural considerations of the design are ignored... In high seismic areas in particular, damage from earthquakes is a real possibility. Cable pileup can create very large loadings. Coupled with double or triple levels of cable racks, the standard superstructure design quickly becomes inadequate...

To the accidental credit of many designs, seas of column stanchions create system redundancies that, when called upon to perform, function adequately. These designs add to the illusion that all is well. Cursory design reviews can uncover simple weaknesses in system layouts that could lead to future problems. In particular are locations where heavy cable concentrations turn corners and run independent of the sea of stanchions, and are often supported by an inadequate number of stanchions to provide the desired redundancy.

While tier-to-tier cable-rack bracing is often provided in excess, the locations of these braces are critical to a properly functioning system. Fewer braces will often do the proper job if strategically placed. Many owners can save money by downsizing or placing elements like stanchions and braces more effectively.

Cable racks and systems should be provided with displayed load ratings and limits —similar to occupancy ratings—to help minimize the risk of overloads during current and future system upgrades.

Telecommunications improvements are not always routine interior improvements. With battery rooms and heavy cable concentrations, the added weight to the building structure should also be considered. Transfer of lateral loads and establishing effective load paths is critical. Attaching to any available interior partition may be convenient but not necessarily adequate. Many interior partitions are barely adequate to serve as interior partitions.

As the system demands become greater, so does the importance of structural integrity. Every system installation need not be engineered, but there are many that must be engineered or face the threat of lawsuits.

W. CAREY DUTCHER, SE, Dutcher & Associates, Fresno, Calif.





No comments