The truth about the top five motor myths

Motors are everywhere. Industrial applications especially depend on electric motors to perform purposeful work. Engineers and maintenance staff are frequently working on motor application, selection, and maintenance.

02/15/2003


Key Concepts

 

  • It's what's inside the motor that matters

  • Insulating materials within the motor winding and the absence of voids prevent corona

  • It's good to push new grease into motor bearings regularly

  • Vibration has an impact on a system's life

    Sidebars:
    EPAct motor criteria
    Comments...


    Motors are everywhere. Industrial applications especially depend on electric motors to perform purposeful work. Engineers and maintenance staff are frequently working on motor application, selection, and maintenance. In a large plant, many motors are stocked as spares so that production can be quickly resumed when there is a motor problem. The abundance of motors and frequency of their application in a modern plant make dealing with motors a routine experience.

    NEMA and IEC standard dimensions make most motors interchangeable among brands and specification levels. Strategic sourcing and internet auctions make it easy for purchasing organizations to compare various motors within a standard set of dimensions and make decisions about standardization and effective use of inventory and buying power. Motors have become a commodity, right? Actually, that would be myth number one.


    Myth # 1: Motors are a commodity

    Truth: It's what's inside that matters Motor frames and industry standard dimensions allow selection of a variety of motors to fit any one application, but there are critical design elements, manufacturing techniques, and material selection decisions that can make a huge difference in the total life-cycle cost of a motor to its user. Looking at a motor from the outside sometimes gives an impression of the overall quality of the product, but that beauty might be only skin deep.

    Industrial motors are complex machines with decades of design and manufacturing knowhow packed into each one. The color of the motor tells you nothing about how it is designed or built. In this article, we'll explore four key attributes of motor design and manufacturing that make a real difference in motor performance, reliability, and operating costs. These four topics provide a good, practical grouping of the kinds of issues faced by today's motor user. They also respond to the "big myth," because the truth is, it's what's inside that matters (Fig. 1).

    Myth # 2: All motors are designed and manufactured to work reliably on inverter power

    Truth: The capability of a motor's insulation system to prevent corona is the difference between reliable operation and quick failure Inverters create instantaneous voltage spikes, which can destroy a motor in a matter of hours or days. The speed at which the digital pulses change velocity (dv/dt) when coupled with other factors such as lead length and their absolute peak voltage (V peak ) create a much more challenging environment for motor windings than does a smooth analog sine wave. The combination of these factors can lead to periodic instantaneous voltage peaks within the motor windings. These peaks induce the presence of corona, which is ionization of the air around the conductors. Corona ultimately allows a partial voltage discharge that physically destroys motor insulation. Destruction of motor insulation can lead to rapid winding failure.

    Today's motor insulation systems must be designed to meet this challenge. The resistance of the insulation system to this phenomenon is defined by the "corona inception voltage (CIV)," described in volts. This voltage is the highest voltage at which destructive corona does not appear in the motor winding. It can be measured for an insulation system in a lab. While each inverter drive provides different V peak and dv/dt characteristics, a motor with a higher CIV provides more insurance against the inception of corona discharge and motor destruction.

    What makes an insulation system have a higher CIV? Insulating materials within the motor winding (between the wires) and the absence of voids provide the only possible means of preventing corona. The motor insulation system consists of slot insulation, magnet wire insulation, group insulation, and the stator resin system. Assuming the proper material selections and assembly techniques, the Achilles heel of this system is the stator resin system. Full penetration of the insulating resin between the wires and an absence of air pockets within the resin create a better insulation system and higher CIV values for a given motor design. These conditions are accomplished by controlling the method of applying and curing the insulating resin. Informed users should consider these important material and quality control measures when selecting a motor. While other factors may help the motor survive a little longer in the presence of corona, it is the prevention of corona that makes a motor reliable when operated with an inverter.

    Myth #3: Motor efficiency is federally legislated

    Truth: The term "premium efficient" tells you nothing about an individual motor's efficiency Motor efficiency is an important life-cycle cost issue for users. The higher the efficiency of the motor, the lower the user's cost of energy to provide a specific function. When a failed motor is replaced, buying the highest efficiency motor should be an easy decision, because the payback for the premium price is often less than one year. That means the premium paid for the motor is completely offset by the savings the motor provides in the first year of operation. Further savings are achieved in each subsequent year of the motor's life. Efficient motors offer additional benefits since they run cooler and quieter than less efficient motors. For example, one large motor user even lowered the demand on his HVAC system and was able to downsize it by specifying more efficient motors.

    In 1992, the U.S. EPA created the EPAct standard for motor efficiency to encourage the use of energy-efficient motors. This is the only existing federal standard for motor efficiency in the U.S., and it includes a limited list of motor criteria (see sidebar "EPAct motor criteria"). EPAct created a baseline of minimum efficiency for the included motor types. It is not really necessary for an educated motor purchaser to ensure that the motor purchased from this list meets the minimum efficiency level, since this efficiency level is federally mandated. This is the good news.

    There are many motors available that greatly exceed EPAct efficiency standards but the bad news is that they are hard to identify. Manufacturers use a variety of names and advertising techniques for these motors, but their names tell you little about their efficiency. Some manufacturers refer to their EPAct compliant motors as "premium efficient," while others reserve the name for motors that greatly exceed EPAct efficiency standards. The only way to compare efficiencies between two different motors is to compare the actual nameplate efficiencies of those two motors. There are significant price and performance considerations when comparing motor efficiencies, so caveat emptor !

    One encouraging recent development on the efficiency front is the establishment of the NEMA premium standard for premium efficient motors. NEMA, in conjunction with the Consortium for Energy Efficiency (CEE), the Department of Energy (DOE), and NEMA-member companies developed this new standard to address the confusion surrounding various names and efficiency levels. While this standard is not law, users can rest assured that their motor meets a premium efficiency standard if it is identified as "NEMA Premium." More information on this standard is available through the NEMA website, nema.org .

    Myth #4: Bearing L10 life is an effective measure of expected bearing life in a motor

    Truth: Contamination is the leading cause of premature bearing failure in motors Bearings are one of the leading causes of failure in motors. Analysis indicates that few of these failures occur because the bearing has reached the end of its useful life. Most failures are the result of the bearing being contaminated by the environment. Bearing experts agree that sealing and relubrication are the two critical actions required to ensure that the bearing does not fail prematurely.

    To protect the bearing from contamination, there are a variety of sealing arrangements available in today's motors. The selection of the right sealing arrangement for your application is the first important step to better bearing life. Relatively clean environments warrant a labyrinth type seal while environments where the motor will be subjected to water spray or contact generally warrant a rubbing contact-type seal. While a rubbing contact seal generally provides better sealing, it creates additional drag and consequently reduces motor efficiency. For severe environments, a bearing isolator is an excellent solution.

    The most important factor for bearing life is to relubricate the bearings on the recommended interval. Where relubrication is not practical, sealed-for-life bearings are a good compromise. But they are just that — a compromise. Sealed bearings are limited in life by the lubrication packed into them at the factory and its one-time life. It is much better to push new grease into the bearing on a regular interval. This process not only provides fresh grease with the proper characteristics into the raceways, but also pushes old grease and the embedded contaminants that got past the seal out of the bearing. Both of these actions protect the bearing from damage and ultimate failure. There's an old adage in the bearing industry that the best seal is moving grease.

    So what should you look for in the bearing system of a motor? If you plan to relubricate the bearing (and you should), look for open bearings contained within the motor bracket by an inner cap and sealed with the appropriate level of sealing for your application. Look to ensure that the relubrication system forces the new grease into the bearing and the old grease out and that there is a pressure relief mechanism within the relubrication path that allows the bearing to purge grease easily but doesn't allow contamination to enter the bearing (Fig. 2). This final mechanism helps avoid bearing overheating through overlubrication. Only a properly relubricated bearing truly stands a chance of living to its L10 life in the majority of motor applications.

    Myth # 5: Vibration levels on motors do not matter for my application

    Truth: Lower vibration is always better Why do customers with difficult or critical applications specify lower vibration levels? It is usually because they know that they cannot afford downtime and that higher vibration levels will cause premature failure of critical components.

    Vibration causes stress on key components, such as bearings, driven machinery, or motor windings. Vibration definitely has an impact on the system's overall life. Lower system vibration levels add to the system's overall reliability. Lower noise characteristics are an added benefit.

    Vibration inherent to the motor is the result of unbalance in motor components or poor control of the centering of components. The better the balancing of the rotor, the lower the inherent vibration level. The closer the rotating components are to perfect center of the magnetic field of the stator (evenly spaced air gap), the better the motor's balance is in operation.

    Both of these attributes can be controlled through high-quality manufacturing processes. Tighter tolerances of fitted components produce a more concentric assembly and therefore less vibration. Better balanced rotating assemblies also yield lower vibration. A quality rotor starts with a good balance before machining and balancing. This is obtained through a well-controlled rotor casting process. The more uniform the casting (no voids), the better the balance of the rotor before the balancing process. The next step should be to balance the rotating assembly as perfectly as is possible.

    NEMA standard MG-1 sets a minimum vibration level for standard motors in Part 7. All NEMA motors must meet these minimum standards. IEEE 841 sets stricter standards for machines used in the petroleum and chemical industries (see sidebar "Comparison of industrial vibration standards"). Why? Because IEEE engineers have determined that setting a stricter standard greatly improves the total reliability of critical systems. If lower vibration levels improve your motor's life in a less critical application, it would make sense to consider this factor when making a motor purchase.

    When comparing motors, users and designers should consider vibration levels, which could reflect a difference in overall quality. Ask questions about how the motor manufacturer builds balance into its product. The life of your process depends on it.

    Motors are not all the same. Being informed about insulation systems, efficiency, bearing relubrication needs, and vibration levels will help users and designers make intelligent total life cycle decisions about the motors they buy.

    More Info: The author is available to answer questions about this article. He can be reached at 864-284-5510, or at macinquemani@powersystems.rockwell.com . For more information on this topic, visit plantengineering.com . Article edited by Jack Smith, Senior Editor, 630-288-8783, jsmith@reedbusiness.com .

    Comparison of industrial vibration standards

    MG-1

    841

    Organization

    NEMA

    IEEE

    Industrial Use

    General Industrial

    Petroleum and Chemical Industries

    Vibration

    0.15 in./sec

    0.08 in./sec

    Standard

    Peak velocity

    Peak velocity

    Objective

    Designed to serve as baseline for industrial motors

    Designed to ensure longer life in demanding applications by adding requirements to MG-1



    EPAct motor criteria

    The following list represents criteria that must be met for a motor to be considered premium efficient:

    General purpose

    T-Frame

    Single speed

    Squirrel cage

    Induction motor

    230/460 V

    Design A or B

    Continuous rated

    60 Hz

    From 1 to 200 hp

    2, 4, or 6 pole

    Open or enclosed

    Comments...

    Lower vibration is always better Motors guaranteed to meet IEEE 841 will commonly exhibit vibration levels significantly lower than 0.08 in./sec Definite purpose industrial motors are available to 0.02 in./sec peak velocity vibration levels



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