In the most basic of terms, an electric motor is a simple device that converts electrical energy into rotary motion that when transmitted to a driven load will perform mechanical work. If applied correctly, the motor will run effortlessly. If set up correctly, the motor will perform reliably at minimal cost. If maintained regularly and correctly, the motor will have a long service life. So much so that, for most motors, only 2% of their total lifetime cost will be attributed to the original purchase price, with the remainder attributed to energy costs.
Although the world is currently witnessing a renaissance of the electric motor with the advent and continued proliferation of e-motor transportation vehicles, their fundamental design and purpose remains the same. Similarly, application, setup, and maintenance will continue to influence the performance, reliability, and life cycle of the e-motor.
Application, setup, and regular maintenance are all cornerstone elements of precision maintenance that provide a simple, effective, and easy approach to ensure your motors work efficiently for a long, long, time.
APPLICATION
A new machine pretty much guarantees the motor has been sized correctly to its intended load. Application problems start to arise when the maintenance department changes out a defective or failed motor with one that’s not “like for like.” Motor application mis-matching can also occur when the production team modifies operating conditions by: changing the line or machine speed beyond its original design range; changing the raw material that’s being worked; or re-purposing the machine for a different use than that for which it was originally designed.
A motor that’s oversized for the load condition can provide adequate performance, but light loading will often be inefficient and consume excess energy under normal operation, thereby increasing the unit’s energy footprint.
Conversely, an undersized or too-light motor, at best, will stall or trip under load; may fail to operate at all; and could cause motor damage. In a worst-case scenario, the motor continues to run and overheats, causing a fire.
In special applications, when a heavy startup load is expected to reduce significantly once the equipment is running at operational speed, i.e., in a loaded conveyor drive system, there are two options: We can use reduced-horsepower motors that 1) are governed mechanically by a fluid coupling that allows the drive to come up to speed slowly without tripping the motor; or 2) are governed electrically through a use of a Variable Frequency Drive (VFD).
If a motor size is suspect, ensuring that the unit is sized correctly requires a review of the original manufacturer’s literature (design specification, Bill of Material, spare parts list, etc.). If no literature is available and you are unsure if the replacement motor is the same size as the original unit, or if you wish to change to a high-efficiency model, confer with your local motor supplier’s engineering department for assistance.
SETUP − Balancing
Purchasing a new, reputable, name-brand motor should provide some assurance that the unit’s main shaft is balanced. When it comes to new, inexpensive, offshore “no-name” motors, operations should err on the side of caution and have the balance checked and certified by a reputable motor shop. Rebuilt motors, if purchased from a reputable rebuilder, should come with a balance certificate.
Unbalanced motor shafts are noisier, vibrate more than usual, and require significantly more energy (and money) to operate over time, and often fail prematurely.
Always check a new or rebuilt motor’s balance using your vibration-analyzing equipment, or have it certified independently by a reputable local motor shop, and do so before you place the unit in operational use.
SETUP − Alignment
Arguably, the most important part of any motor setup is correct alignment between the drive and driven shaft.
Misalignment comes in two forms: 1) Angular, in which the shafts line up center-to-center, but do not in a straight line; and 2) Offset, in which the shafts do not line up center-to-center. Both conditions can occur simultaneously. And both will place tremendous stress on the driver and driven bearings and couplings, assuring rapid wear and premature failure, along with considerable increase in motor energy consumption.
Remember: Poor alignment will rapidly wear out shaft bearings, sprockets and chains, belts, and sheave pulleys
SETUP − Soft-Foot Check
A soft-foot-condition check should always be included as part of the alignment process. If aligning with a laser type alignment system, it is an easy process as virtually all laser alignment systems feature a soft foot check feature.
Soft foot is a condition that occurs when one, or more, of the motor base feet are not as flat as the others (think of a table with a short leg). Soft foot can also be encountered when the motor base grout is not flat or square. Both situations are easily remedied with the use of precision shims and correct tie-down-bolt torqueing.
If soft foot is allowed to persist, excess vibration will eventually loosen the bolts and cause the motor to vibrate more, which in turn will transfer across the drive train and its components. Again, the result will be premature wear and failure, combined with excessive energy consumption.
REGULAR MAINTENANCE − Lubrication
The reality is that most lubricated motors are grossly over lubricate. Incorrectly lubricated motors prematurely fail due to the simple act of neglecting to undo the grease drain plug, thereby forcing any excess grease to build up pressure that eventually ruptures the shaft seal. In that event, grease is free to purge into the motor winding, causing massive overheating, premature failure, and, once again, excessive energy consumption.
Some sub-fractional motors come equipped with grease nipples, even though they contain lifetime lubricated bearings and no drain port to allow excess grease entry to escape.
All motors should be assessed to understand their lubrication requirements and placed on an engineered lubrication program
REGULAR MAINTENANCE − Cleanliness
The simple act of keeping a motor dirt- and oil-free will combat the buildup of a debris/dirt based thermal blanket, allowing the unit to cool as designed and, just as important, use no more energy than designed.
REGULAR MAINTENANCE − Maintaining the Driven System
Simple maintenance of the driven systems can significantly reduce motor loads and increase energy use efficiency. Among other things:
- Ensure drive belts and chains are tensioned regularly and correctly.
- Use matched drive belts on multiple belt systems.
- Always replace sprockets at the same time worn chains are being replaced.
- Lubricate drive chains regularly.
- If the motor is coupled to a gearbox, ensure the lubricant is of the correct viscosity.
- Ensure the driven component is balanced and lubricated regularly.
- When aligning with direct coupling, use the least-expensive non-flex style that’s required, and enforce accurate alignment techniques.
- Check driven-system air filters or fluid-system filters regularly.
FINAL THOUGHT
What’s the payback from following the steps described here? Keep this in mind: With a precision-maintenance approach, a motor will virtually always outlast its driven system.TRR
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ABOUT THE AUTHOR
Ken Bannister has 40+ years of experience in the RAM industry. For the past 30, he’s been a Managing Partner and Principal Asset Management Consultant with Engtech industries Inc., where he has specialized in helping clients implement best-practice asset-management programs worldwide. A founding member and past director of the Plant Engineering and Maintenance Association of Canada, he is the author of several books, including three on lubrication, one on predictive maintenance, and one on energy reduction strategies, and is currently writing one on planning and scheduling. Contact him directly at 519-469-9173 or [email protected].
Tags: reliability, availability, maintenance, RAM, maintenance management, precision maintenance, motors and drives, machine balancing, alignment, energy efficiency, lubrication, grease, gearboxes
