3 reasons why motor-power retrofits require systemic analysis

Stanton McGroarty says use VSDs to cut energy and maintenance costs.

By J. Stanton McGroarty, CMfgE, CMRP, senior technical editor

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When it comes to maintenance cost reduction, everyone’s favorite strategy is to stop doing unnecessary work. Discussions with some motor and blower suppliers have identified exciting new opportunities for maintenance reduction coupled with dramatic energy cost cutting. Please note that this level of strategic change suggests that we add a sharp accountant to the technical team.

Any discussion of electric motor economics ought to start with a reminder that the purchase price of a motor is typically something like 2% of its lifecycle cost. Repairs will usually add another 1 or 2%, which means that the cost of the electricity to run the motor makes up more than 95% of the cost of ownership of the motor during its service life. The folks at Baldor offer a tool they call BE$T for developing a more exact lifecycle cost profile for a specific application. For the purposes of most discussions, though, if you figure that the power used by a motor will cost something like 20 times the motor’s initial cost, you will have a useful picture of the comparative importance of purchase price and energy efficiency.

This cost relationship underscores the importance of understanding more than just the project or purchase cost for any electric motor-powered equipment. Even retrofits now require careful systemic analysis before the selection of a power source. Here are 3 reasons why.

Variable speed drives (VSDs) can cut the energy cost for some motor installations by 40% or more. The economics are particularly favorable for the VSD if it can replace a gearbox and a conventional motor. In these situations, the cost of the VSD may be offset by the simplification of the system, even for a retrofit. VSDs can often fit into the space formerly occupied by a motor and drive set, producing not only a reduction in maintenance requirements, but also providing a system that can be slowed down or sped up, saving energy during low-usage periods. This can be particularly useful for large HVAC or other air moving installations that need to keep running at reduced power when production is not in full swing. When the installation involves equipment that exhausts heated or cooled plant air, reducing the power setting on the system can also save heating or cooling costs in addition to the direct energy savings for the new motor. For these situations it is essential to use a system-wide view of energy usage to compare the value of the different technologies.

J. Stanton McGroarty, CMfgE, CMRP, is senior technical editor of Plant Services. He was formerly consulting manager for Strategic Asset Management International (SAMI), where he focused on project management and training for manufacturing, maintenance and reliability engineering. He has more than 30 years of manufacturing and maintenance experience in the automotive, defense, consumer products and process manufacturing industries. He holds a bachelor of science degree in mechanical engineering from the Detroit Institute of Technology and a master’s degree in management from Central Michigan University. He can be reached at smcgroarty@putman.net or check out his .

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Large, permanent magnet VSDs have now shrunk to the point where they can be used to replace gearbox and motor sets on installations as large as power plant cooling towers. The new motors can provide speed adjustments that balance cooling capacity to variations in plant output and changes in the weather. VSD installations in the 200 hp range are now achievable. Again, the maintenance and condition monitoring requirements for oil-filled gearboxes are replaced by infrequent applications of grease to motor bearings. Reliability of high horsepower applications is further enhanced by the natural soft-start capability of VSDs.

All this is a huge bonus for installations where drive locations are as inconvenient as they are in a huge cooling tower. The new, compact drives can also deliver a new financial benefit. Sometimes, when cooling capacity increases are needed, it may actually be possible to use energy savings to fund a capacity increase that can be achieved by increasing air-moving capability. Also on the subject of capacity, it is important to remember that reduced maintenance is reflected as increased run time.

Another technology that is catching up to motor improvements is blower design. Screw type blowers are now able to operate efficiently at 20 to 25% of their maximum output without creating worries about motor overheating. Lobe type blowers are capable of operating at about 50% of full capacity. This means that new blower designs are capable of being throttled back to match plant requirements as they change around the clock and around the seasons. With VSDs, these changes can be automated as appropriate, reducing the involvement of engineering and maintenance personnel in the changes. Newer blower designs and gearbox-free applications also offer significant reductions in noise and vibration, further reducing equipment wear and environmental impact.

This is an exciting menu of opportunities for energy and maintenance cost reduction. The next time an opportunity arises to change out a drive or air moving system, maybe it will make sense to invest a few hours of your team’s time to see if some of these technical enhancements apply to your systems. I’ll bet your oilers will think they do.

Read Stanton McGroarty's monthly column, Strategic Maintenance.

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