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At times overlooked and taken for granted, reliable and smooth electric motors work each day
by Steven E. Smith
“They
are great when they work” goes the old adage. This is the common
response of any plant manager or boss commenting on the various
electric motors that run grinders, concrete crushers, conveyors and the
countless other specialized pieces of machinery. Among the questions
any manager should also be considering is, “Are they too great?” Proper
motor sizing, age of existing inservice motors and available new
technology are among the questions any manager or owner should be
investigating as the industry enters a new era of high energy costs.
Although as an industrial superpower with generations of experience
with electric motor applications across most industries, as an
established industrial country we are still striving to enhance the
efficiencies of our electric powered systems. In the recent past, a
common practice in industrial electric power applications was to
oversize the motor to compensate for the unavoidable likelihood that
there would occasionally be brief periods of increased load. This
practice was based on the engineering concept that additional motor
size would serve as a buffer. There also was a time when utility costs
were so reasonable that many industries did not even give much concern
in using oversized motors in most applications. But, the times, they
are changing.
According
to Gerardo Elias, marketing manager of WEG Electric Motors Corp.,
technological advancements in motor design and safety controls coupled
with cost saving incentives and simple economics makes looking at
electrical motors in service at any business as a worthy use of time.
“There is a misconception when replacing a motor that a cheaper but
larger motor will be an advantage,” said Elias, who quickly added that
since an appropriately sized and maintained motor can have a functional
life of 10 to 15 years, the purchase price of an electrical motor
really only equals about 3 percent of the total cost of the unit.
Efficiency, efficiency, efficiency
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| This
graph provided by WEG Motors displays the contributing factors
associated with electric motor efficiency. When evaluating motor
efficiency, it is important to review manufacturer’s recommendations as
well as industry standards for your application. |
It
all comes down to efficiency. A review of the applicable efficiency,
power, slip along with the current level is important when evaluating
the operational setting of units currently in service. Elias said most
motors are designed to operate most efficiently between 75 percent and
100 percent capacity. So if a motor is oversized for the application,
the unit will actually consume too much power because it does not run
at lower speeds when under a smaller load.
Changes in technology are also enabling some operators to improve
efficiencies as older style motors are taken out of service. The advent
of the soft starter or variable speed drive provides a mechanism for
matching the application requirements with the correct motor speed.
This start up feature reduces energy costs as well as wear and tear on
moving components within the system.
Additional consideration is being given to further improving systems so
that motor efficiencies can be obtained. A review of gears, pulleys and
other leverage points can aid as well when evaluating the electrical
motor efficiencies. In order to obtain the appropriate actions from the
system, the proper sizing of pulleys and gears will improve torque
output. And since in industrial applications such a rock crushing the
major contributor to equipment performance is torque, it is important
to evaluate the system especially during an electric motor review.
Review
During a review of the current motors in operation at a quarry,
recycling site or other industrial setting, it is important to review
information relative to the current system. By starting with the onsite
maintenance records, inservice date, repair information and other
relative issues, shaft and pulley alignment should also can be
assessed. It is important to consider that such significant repairs as
core rewound because, while this will enable the motor to be put back
into service, the efficiency is often reduced by at least 2 percent due
to degradation in the magnetic properties of the motor that result from
the varnish removing heating phase of the rebuild process.
Elias recommended that the largest motors and those that are in
operation the most be the first motors assessed during an onsite motor
review. If it is determined that a motor is oversized, an evaluation of
its present efficiency and an operating cost analysis based on current
kilowatt per hour costs will aid in the decision making process as to
whether or not to replace the motor. And dependant upon which state the
operation is located in, there are various energy efficiency
implementation incentives to consider. In New York, the New York State
Energy Research and Development Authority is a source of information on
energy auditing and incentive opportunities.
Oversizing electric motors is more common than one might think.
According to Andrew Glover, WEG Motors product manager, it is believed
that about 80 percent of pump motors could be incorrectly sized.
Conversely, it is likely there are motors in service that are
overloaded due to undersizing. According to Elias, continuous operation
in either condition will reduce the functional utility of a motor to 70
percent of normal productive life. Increased heat during overloading
damages motor bearings and also results in insulation failure, which
causes short circuiting and burn ups. It really comes down to
accurately sizing the motor and associated equipment interconnected in
the system.
Association emphasis
The increased emphasis on motor efficiency and proper application has
been some of the focal points of the National Electrical Manufacturers
Association (www.nema.org), the Power Transmission Distributors
Association (www.ptda.org) and other authorities in the field of
utility applications. These resources can be especially advantageous
when OEM specifications for electrical power units are not available. A
review of recommendations from these independent authorities can assist
in the evaluation process of current operating systems, and when
designing or modifying systems.
Once it has been determined that a motor needs to be replaced, emphasis
on a few key components in the system are essential to optimizing any
new motor. Elias stressed the importance of matching the voltage and
current so that the new motor can effectively start under the load.
Elias indicated that the service factor is an important feature
consider. The service factor is the motor’s capacity to be overloaded
without affecting the useful life of the motor. All WEG products that
are 100 hp or less feature a service factor of 1.25 while units those
larger than 100 hp feature a service factor of 1.15.
While motor sizing and features are being finalized, Elias emphasized
that the use of the sound quality, properly sized control equipment is
what protects any new system investment. Properly sized motors are the
smart choice for efficiency and having the right control equipment in
the form of overload relays and thermal switches that are sized
according to the amount of electrical current are even smarter. Elias
recommended upgrading to NEMA premium motors that meet current
efficiency standards and indicated that the year 2010 regulations
require higher efficiency standards for general purpose applications.
WEG Electric Motors Corp. based in Suwanee, Georgia produces over 10
million electric motors annually, making WEG the largest industrial
electric motor manufacturer in the Americas and one of the largest
manufacturers in the world. Since 1961, WEG has continuously strived to
advance industrial electrical technologies and today their largest unit
in service is a 30,000 horsepower product. Additional information about
advancements in motor efficiencies as well as the product available
from WEG can be found online at www.weg.net or by calling 1-800-ASK-4WEG.
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