Archive | February, 2001


4:29 pm
February 1, 2001
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Replace Eddy Currents with AC Drives to Reduce Maintenance

The versatility, operating efficiencies, and installation economies of general purpose adjustable frequency ac drives are making them increasingly popular as replacements for aging eddy current drives.

The combined simplicity of an ac drive and standard inverter duty motor provide maintenance advantages which are not available from standard eddy current drives. Mechanical complexity is reduced, as are preventive and corrective maintenance requirements. Additionally, ac drives permit use of standard inverter duty motors, which can be repaired or rebuilt at virtually any reliable motor shop.

Versatile, efficient
AC drives incorporate safeguard function indicators including motor overload, overheat, overcurrent, undercurrent, phase loss, and ground fault protection. Modern ac drives easily provide up to 150 percent starting torque without requiring the overload protection necessary in many eddy current operations. Additionally, flexible drive bypass options allow the motor to be switched to line power for drive maintenance or in pump or fan applications requiring uninterrupted operation.

AC drives reduce expensive downtime by permitting many adjustments to be made through software rather than hardware. They are simple to install and set up, and provide instant access to operating parameters through LED readouts. In contrast to less flexible eddy current drives, they offer digital inputs for simple, accurate entry of operational settings. Additionally, digital control provides zero drift for improved application consistency and repeatability throughout the speed range.

Digital repeatability facilitates accurate entry of parameter settings including jog, braking, momentary power loss override, remote speed reference inputs, overtorque detection, multi-step speed settings, and acceleration/deceleration time selection.

AC drives easily support serial communications along with features and modifications which are difficult or impossible to accomplish with an eddy current drive.

Punch press applications
High-performance adjustable frequency drive design offers significant improvements in press operation. Attributes providing increased productivity and lower production costs include:

  • Digital selection and repeatability of optimum press speeds for reduced scrap, extended die life, and reduced maintenance
  • Increased flexibility for reduced downtime on changeovers and ongoing press upgrades
  • Precise digital coordination of feed speeds to integrate presses into a mechanized line

AC drives also provide easy modifications through software vs mechanical changes, which are comparatively expensive. Operating changes can be made and presses brought back on line quickly, reducing both setup time and downtime.

Drive replacement considerations
Press drive motors are generally selected for approximately a 3:1 speed range. This is necessary because while the press load may be essentially constant torque, the motor load may remain constant as press operating speed varies.

When replacing an eddy current drive with an ac adjustable frequency  drive, consideration must be given to overload requirements of the particular  press. As an example, consider replacing an eddy current drive with  an ac adjustable frequency drive on a 350-ton press with an existing  40 hp eddy current drive, 1800 rpm, 460V-3/60 input, 50 FLA, using a  NEMA B, squirrel cage induction motor, 200 percent starting torque,  and 220 percent eddy current coupling peak torque.

A 40 hp inverter duty high efficiency squirrel cage induction motor suitable for belted output, packaged with a 50 hp drive, would be appropriate.  This selection would provide accelerating torque comparable to the eddy current drive being replaced.

As a second example, consider converting a constant speed, NEMA D motor to an adjustable frequency drive on a 400-ton press with an existing 40 hp NEMA D main press motor, 8-13 percent slip, 460V, 52 FLA, and 1800 rpm.

When converting an existing constant speed main press motor to adjustable frequency, it is important to ensure that motor insulation is appropriate for inverter duty. Insulation Class F at a Service Factor of 1.15 is minimum; Class H at SF 1.25 is optimum.

Another factor to consider is motor frame construction. For example,  U frame motors are typically capable of continuous operation at 120-125 percent rated torque. If the motor is required to operate above its 100 percent rating, it is recommended that a larger drive be selected.

Because of the high starting torque characteristics (approximately 300 percent of rated torque in this example), the time to accelerate the flywheel to rated speed will increase. This is due to the typical drive starting torque capacity of 150 percent of rated.

Switching to adjustable frequency drives offers increased equipment reliability in a number of ways including simple, accurate entry of operational settings and adjustments with software. Mechanical complexity is greatly reduced. Installation and setup are simple, and digital repeatability and control provide zero drift for improved efficiencies. And most modern general purpose ac drives are designed for highly dependable performance with standard inverter duty motors which can be repaired or rebuilt at most motor shops. MT

Information supplied by Howard Beyer, senior application engineer, MagneTek Drives, New Berlin, WI. For additional information, contact  Ken Gniot, MagneTek Drives, 16555 W. Ryerson Rd., New Berlin, WI 5315 Continue Reading →


3:03 pm
February 1, 2001
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Behind the Scenes


Robert C. Baldwin, CMRP, Editor

I continue to be amazed by what goes on behind the scenes in business, even within industries with which I am familiar. Every activity has its specialized vocabulary and processes, and I enjoy learning about them. Perhaps that is why I became an editor, to get an opportunity to peek behind the curtain to see what’s going on.

The output of many endeavors, great as they may appear from the user side, can’t be appreciated fully without some understanding of what went into them. That is again being demonstrated to me as I work backstage with members of the professional certification committee of the Society for Maintenance & Reliability Professionals (SMRP). I’m learning about the vocabulary, software, and processes of testing and what it takes to develop a program to assess and certify competency in the field of reliability and maintenance.

The committee, which is expected to become the SMRP Certifying Organization, is developing the content and infrastructure of a program for certifying maintenance and reliability managers. An important element of our last meeting was a half-day seminar by our consulting psycometrician. We were introduced to the testing community’s vocabulary: items (questions) include stem options (multiple choices) made up of one key (correct choice) and several distracters (incorrect choices).

We used specialized software to analyze the beta test given at the SMRP conference last fall in Cleveland. It tracks everything from frequency distribution of test scores to point-biserial values for individual items to flag bad questions.

The agenda included work on questions, as well as the development of infrastructure and processes that are congruent with values promulgated by National Organization for Competency Assurance.

The most important element of the process is the definition of capabilities to be tested and certified. SMRP has published a list of capabilities that the committee believes are important to successful equipment reliability, maintenance, and asset management operations. It is available through the “capabilities inventory” link near the end of the first overview article on the certification page.

The capabilities inventory details skills in five broad categories: Business and management, people, equipment reliability, manufacturing process reliability, and work management.

This list can provide the basis for taking your company leadership behind the scenes of reliability and maintenance activity. And, possibly, the basis for you to gauge what you need to develop before you invite them to peek behind the curtain. MT


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3:01 pm
February 1, 2001
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Six Sigma and Asset Dependability

In case you haven’t noticed, the Six Sigma quality revolution is gaining greater acceptance as the long-term strategy to sustainable productivity. Simply stated, Six Sigma focuses on reducing variation in our business’ internal processes using a rigorously structured, statistical approach that is tied to business results.

When senior leadership truly understands this premise, it should be demanding to measure the Six Sigma linkage to bottom-line business results. Without this kind of educated leadership and demand for business linkage, Six Sigma will go the way of Quality Circles.

First, senior leadership must understand the business value of process variation, or the cost of poor (internal) quality (COPQ).

COPQ is determined by assigning a dollar value to waste created in the process (whether a process is manufacturing or a work process, such as accounts receivable).

The Six Sigma culture promotes that any activity or process that does not perform perfectly the first time contains COPQ—for example, warranty returns, cost of inspection, unplanned equipment failures, equipment performance rate losses, product changeovers, waiting on raw materials.

Is there COPQ in the numerous processes that make up maintenance? How about daily work schedule compliance, equipment with low MTBF, spare parts quality or availability, unnecessary maintenance or PMs?

Second, senior leadership must be committed to characterizing the process variation issues that make up the gap between current performance (baseline) and ideal performance or entitlement, which can be described as zero losses in any of the three elements of productivity:

  • Availability (zero downtime, even for PM).
  • Performance Rate (zero losses in the instantaneous best capacity rate possible for the design).
  • First Pass Yield (zero defects in every process step).

The objective of Six Sigma is to economically reduce these identified issues that comprise process variation, which are assigned a calculated business value, the cost of poor quality.

The power to improve productivity is hidden in the COPQ value of this gap between baseline and entitlement. Traditionally, we think of defects in percent yield, downtime hours, and other process measures. When these process measures are converted to COPQ dollars, and priorities are examined using the Pareto Principle, it becomes perfectly clear where the trained experts in Six Sigma methods and statistical tools must focus their talents.

So where does asset dependability fit in? Nearly everywhere. However, most Six Sigma education today lacks the treatment of the influence of assets on the gap between current performance and entitlement. Six Sigma teachings almost exclusively focus upon reducing process variation, where asset dependability variation can be contributing as much as 20 to 30 percent of the overall gap in COPQ.

Being a certified Six Sigma Expert myself and having led Six Sigma training in a reliability and maintenance environment, I am discovering 10 to 30 percent of newly identified experts-in-training are faced with asset dependability variation as the keynote issue. Trouble is, traditional Six Sigma training lacks the needed tools in reliability, maintainability, and operability, which can be used to drive out variation in asset dependability.

The Six Sigma community needs to discover asset dependability variation as the new productivity frontier. We in the reliability and maintenance business have long held the tribal knowledge that asset variation is a productivity killer. These two productivity communities need to join forces to enable our industries to gain further productivity advantage. To that end, the maintenance community needs training in Six Sigma principles and statistical tools to become an effective partner in the fight to reduce process variation, while the Six Sigma community needs to recognize the added value of assigning COPQ for gaps in asset performance.

Applying the structured, business-driven statistical approach of Six Sigma to asset dependability variation can add a new element of credibility for the reliability and maintenance community. A Six Sigma approach can be leveraged to validate what we have known in our maintenance tribal knowledge for years, but have had mixed success in quantifying the business value of reducing COPQ.

Another key element in all this is that as asset dependability shows up in the gap characterization for COPQ, the clearer the message will be to leadership of the need for specific reliability and maintenance tools, which traditionally have been difficult to sell and sustain. MT

Stanley (Stan) T. Grabill is a consultant with Sigma Breakthrough Technologies, Inc., San Marcos, TX. He has led reliability and maintenance processes at the plant, business, and corporate levels since 1988. He is a certified Six Sigma Expert (Black Belt) and Certified Plant Engineer.

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