Your Opinion piece “Decision Time for Ultra-Supercritical” (March 2009) makes reference to Philo 6 and Eddysport 1. The latter should be Eddystone 1. I found the rest of the opinion piece to be spot on.
James B. Lewis, PE
Executive Engineer, Power Generation
To the Editor:
It is true that a large percent of all the electrical energy generated powers electric motors and their systems. What is so discouraging is the amount of errors and misconceptions in the article (“Effective Motor Management,” March 2009). The following is a list of some of these items.
There is no scope of product identified, hence the reader is left to make assumptions. I suspect the author is referring to three phase, AC, integral motors up to 200 hp. Without this simple clarification all of his other proposals may or may not make sense.
The claim that repair cost may actually be more expensive than the cost of a new motor would be very rare. The most common reason for taking a motor out of service is to replace failed bearings. The author fails to describe what types of failures or what industries he is referring to.
The claim that a repaired motor generally is less efficient than a new replaced motor is simply not true. This is an often-quoted myth that has circulated throughout the printed world. The Electrical Apparatus Service Association (EASA) offers a complete study that verifies that if motors are repaired in accordance with industry best practices, the efficiency can be maintained or even enhanced. (“The Effect of Repair/Rewinding on Motor Efficiency” is a free downloaded at www.easa.com.)
The concept of spares has been addressed without explaining the size or kind of motors where this practice may be used. For instance, not many users can afford spare motors in the 500 hp and larger range. In many applications, rather than keeping expensive spares the facility is designed with redundancy. Instead of one 10,000 hp motor they may install five 2,000 hp motors.
The claim that the “operating cost is a primary consideration” is not correct in most applications. The cost of down time and lost opportunity is much greater. The hotel that has to send all of its guests home because the air conditioning doesn’t work is not concerned about the cost to operate a motor.
The author suggests that each time a motor fails the motor efficiency should be determined. Such a request is expensive and not practical. Even the motor manufacturer may not have tested the motor for efficiency when it was new.
The claim that the NEMA Premium efficient motors are from 0.5 to 4 percent more efficient than EPAct motors is incorrect and misleading. The spread between EPAct and Premium efficient motors is in the two percentage point range.
Austin Bonnett Engineering LLC
To the Editor:
While the goal of the article (“Effective Motor Management,” March 2009) was noble, the article itself contains much information which should be corrected. Electric motor efficiency varies considerably, depending on motor horsepower (hp) rating and vintage. There are older U-frame motors, 200 hp and larger, with efficiencies comparable to today’s NEMA Premium motors.
For motor rated above 300 hp, electric motor manufacturers have built an efficient product for several decades. The article could be interpreted as promising efficiency improvements that will not be realized.
The statement that “EPAct motors generally are less expensive, however they usually are less efficient, run hotter and have higher operating costs” is simply wrong. When manufacturers sought to increase efficiency, the gains made in I2R losses (more copper in the winding reduced temperature rise) allowed the use of smaller cooling fans. This same practice was carried forward to the NEMA Premium motors. The fan size was reduced to lessen the windage losses, allowing the operating temperature to rise to the typical winding temperatures of pre-EPAct motors. They do not “run hotter” than NEMA Premium motors, they use the same size bearings and are not less reliable.
Regarding repair, the author stated that “the standard practice is to rush the repair.” It is hard to imagine anyone “rushing a repair” for which they are paid by the hour. Yes, motor users ask for rapid turn-around of failed critical motors. No, there are no magic shortcuts to accomplish the impossible.
“Continually repairing a motor degrades its efficiency” is another statement that has been proven incorrect. The EASA/AEMT Rewind Study (“The Effect of Repair/Rewinding on Motor Efficiency and Good Practice Guide”) found that properly done rewinds did not decrease efficiency. In numerous cases, the efficiency of electric motors was improved by following best practice procedures.
“NEMA Premium motors can save thousands of dollars in annual energy costs…” That depends on the size of the motor and annual operating hours. For a 10 hp motor, savings of thousands of dollars annually would be impossible. For a 250 hp motor, the payback is more likely to be several years.
The suggestion to stockpile spare motors ties up scarce financial resources in inventory. It is fiscally wise to let the distributor carry your inventory, in those cases. And for very large machines, the cost of a spare is prohibitive, not to mention the storage issues involved.
EASA Technical Support Specialist and Senior Member, IEEE
The Author Responds:
The intent of the article is to stimulate a thorough investigation of current motor management policies and motor management programs for readers in this industrial market, with a view toward better management and control of their total motor spend. Electric motors account for two-thirds of energy usage in industrial settings (Department of Energy). Industry data suggests that 95 percent of polyphase AC motors in use today (in the United States) are 200 hp and smaller.
In addition, we wanted to provide links to public domain information, resources and tools that may be useful to these readers in their investigation, planning and management processes.
Applied Industrial Technologies