Technical Report: Improving Motor Management In A Weak Economy
By Thomas H. Bishop, P.E.
When production drops due to a weak economy, plant managers and engineers often try to control costs by delaying the replacement or repair of failed electric motors. While this may save money in the s...
By Thomas H. Bishop, P.E.
When production drops due to a weak economy, plant managers and engineers often try to control costs by delaying the replacement or repair of failed electric motors. While this may save money in the short term, it could put the plant at a competitive disadvantage once the economy starts roaring back. A better approach to motor management during slack periods is to target mission-critical equipment based on a sound motor epair/replacement policy.
It’s easy to find ‘expert’ opinions on the Internet about which motors to repair or replace, so it shouldn’t be too hard to create a good repair/replace policy, right? Unfortunately, much of this free advice ignores the type and size of the motor, as well as differences in applications, hours of operation and load conditions — i.e., how, where and when the motor is used.
Often such advice also fails to consider modifications that may be needed or the time it may take to get the new motor. Worse still, some of these ‘expert’ opinions recommend replacing every motor that fails, based on the false assumption that repair inherently degrades motor efficiency, or that energy-efficient and premium-efficient motors are impossible to repair or rewind.
Getting the facts
Energy-efficient motors: A good motor repair/replace policy should be based on scientific facts, not opinions or assumptions. For starters, nothing about energy-efficient or premium-efficient motors is magical or mysterious. Manufacturers simply improved the efficiency of these models by minimizing the amount of energy that is lost to the combination of heat, friction and windage in the process of converting electricity into work. Simply put, there are no technological breakthroughs associated with these motors.
To reduce core losses, for example, some energy-efficient models have longer stator and rotor cores than standard motors. They also have more copper in the windings to decrease copper losses, and open or shielded bearings (lubricated with a specified quantity of grease) to reduce friction. To minimize the power diverted to cooling, totally enclosed, fan-cooled (TEFC) designs use the smallest fan that can handle the job.
Effect of motor repair and rewinding: When it comes to repair, a rewind study commissioned by the US-based Electrical Apparatus Service Association (EASA) and the UKbased Association of Electrical & Mechanical Trades (AEMT) in 2003 scientifically proved that the energy efficiency of high-efficiency NEMA and IEC motors is maintained by following the good practices identified in the study. The efficiency levels of the motors tested ranged from the original EPAct level to the NEMA Premium and IEC EFF1 levels.
The study, which was performed at the University of Nottingham in the UK, tested 22 motors ranging in size from 50 hp to 200 hp (37kW to 150 kW), before and after multiple winding burnouts and rewinds. An earlier study by AEMT, in 1998, also proved that the efficiency of motors with lower hp/kW ratings can be maintained during repair, dispelling the notion that, of themselves, winding burnout and removal damage the core.
According to these studies, following best practices during repair is critical to maintaining motor efficiency. To control bearing friction loss, for example, requires using the original bearing type, maintaining or restoring the proper bearing journal and housing fits, using the correct quantity of lubricant, and running in the bearings before testing for efficiency.
For a copy of the most recent study, go to www.easa.com
nd click on the Industry Info button.
To retain the original efficiency of repaired or rewound motors, consider using a service centre that follows ANSI/EASA AR100-2006, Recommended Practice for the Repair of Rotating Electrical Apparatus (also available for download on the Industry Info page at easa.com), and the Rewind Study’s Good Practice Guide.
Developing a repair/replace policy A motor repair/replace policy is useful a tool for determining the best course of action when a motor fails. But each application is unique. Although the extensive flowchart in Figure 1 lists key decision points, it doesn’t cover every possibility.
Application review: The first step is to determine if the failed motor suits the application. A motor with an open enclosure, for instance, may not be practical for a sawmill application with lots of airborne dust and debris. A better choice might be a totally-enclosed, fan-cooled (TEFC) replacement. Processes and duty cycles often change over time, so it always pays to re-examine the application when deciding whether to repair or replace a failed motor.
If the failed motor is a good fit for the application, check the condition of the stator core. Has it sustained significant damage? Prior to failure, did the motor exceed its rated temperature rise (i. e., high core losses)? Absent special features that might affect price or availability, it may cost less to buy a new motor than to repair a badly damaged stator core.
Next, consider these decision points simultaneously:
• Has catastrophic failure occurred?
• Is there evidence of a prior catastrophic failure?
• Is the rotor damaged?
• Are other mechanical parts severely damaged?
• Is it an EPAct, NEMA Premium or IEC EFF1 motor?
Catastrophic failure: If a catastrophic failure has occurred, weigh the cost of repairing the motor against that of replacing it. Such failures typically do significant damage to the stator core and windings, as well as to the rotor, shaft, bearings and end brackets. In such cases, replacement may be the most economical option — especially if you question the suitability of the motor for the application.
Rotor damage varies widely — from surface smearing due to contact with the stator, to melted bars and end rings on die-cast designs, to lifted bars or broken end rings on fabricated designs. Surface smearing can often be repaired economically. Other kinds of rotor repair may not be feasible unless the motor is very large or has special features.
The shaft, frame or other mechanical parts may also be damaged so badly that they must be replaced. Here again, the cost of buying or making a new shaft, or of purchasing a new frame, may make repair a less attractive choice than replacing the motor — unless the motor is very large or has special features.
Prior catastrophic failure: Sometimes evidence of a prior catastrophic failure is discovered only after disassembly. Examples include a bent shaft that has bent again; a damaged rotor core or damaged rotor bars or end rings; and damaged stator core laminations.
Whether you choose to repair or replace the motor, be sure to identify the contributing causes of failure to prevent a recurrence.
Energy-efficient motors: The points discussed so far have shaped motor repair-replace decisions for more than 50 years. The advent of energy-efficient motors introduced another consideration — whether to replace the failed motor with a more energy-efficient model.
Broadly speaking, energy-efficient motors are those covered by federal regulation (such as the EPAct in the US), IEC motors labelled EFF1, as well as newer, premium-efficient models (for example, NEMA Premium). Repair considerations for these motors are the same as for standard-efficiency models.
Following the good practices of the EASA/ AEMT rewind study, qualified service centres can repair any of these motors without affecting the efficiency rating.
Before repairing a standard efficiency motor, consider the return on investment for a more energy-efficient replacement, including the expected life of the motor or process, load conditions, hours of operation, the price of electricity and any modification costs. If your analysis shows that replacement is preferable to repair, the next consideration is whether you have the mon
ey in your budget. If not, you may still opt for repair as long as it costs less than a new motor.
Assuming you have the funds for a new motor, the next decision point is availability. Motors such as those that fall under EPAct rules are normally stock items. Delivery times for larger motors, or for those with special features, often range from a few weeks to several months. If the delivery time is longer than you require, a qualified service centre usually can repair the original motor in far less time. It also may be able to add the special features you need to a stock motor — for example, by converting it to a C-face mounting.
Addressing critical motor repair and replacement concerns during economic slowdowns enables you to ramp up production quickly as market conditions improve. Implementing a sound motor repair/replacement policy now also will assure wise management of plant assets well into the future.
Thomas H. Bishop, P.E., is a technical support specialist at the Electrical Apparatus Service Association (EASA), St. Louis, MO. EASA is an international trade association of more than 2,100 firms in 58 countries that sell and service electrical, electronic, and mechanical apparatus. For more information, visit www.easa.comor use the reply number below.
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