Rotor Bar Forces Experiment

A number of theories surround the forces in a rotor including those on the rotor bars and rotor core. The experiments depicted in this video determine the location of those forces.

IEEE DEIS Web Videos

The Institute of Electrical and Electronic Engineers, Inc. Dielectrics and Electrical Insulation Society Website (IEEE DEIS Web) has been placed historical educational videos from the 1960s on the website under http://ewh.ieee.org/soc/deis/education/all-education-material.html. When finished, there will be more than 50 educational videos available for viewing by IEEE DEIS members. The videos each have a 2-5 minute preview for non-members. The videos available as of July 21, 2009, are:

- “Rotating Machinery Workshop:” the first video about one hour in length that describes all of the major insulation tests from their basic principles.
- “Streamers in Liquids: Basic Phenomenon:” This short video of about 20 minutes describes semi-conductive and conductive streamers within insulating oils.
- “Fundamentals of Motor Insulation for Repair Shops Part 3:” This video of about one hour and fifteen minutes discusses proper insulation systems for repaired electric motors, in particular the forces on the coils in an operating motor and the proper securing of the insulation system as described in this week’s tech tip.
- “Turn Insulation Aging of Motors Exposed to Fast Pulses of Inverter Drives:” Describes the breakdown of motor insulation systems in inverter applications. Runs about 18 minutes.
- “Utilizing Reduced Build Concepts in the Development of Insulation Systems for Large Motors:” this 18 minute video discusses the concept of using thinner insulation systems for improved thermal transfer. It requires improved insulation materials.

Additional videos will be placed a minimum of one or two per week.
For more information, contact the IEEE DEIS Web Editor-in-Chief Dr. Penrose at hpenrose@ieee.org.

Tech Tip: Coil Movement Issues

During startup and operation of an electric motor, the forces on the coils of an electric motor move up and down and to the sides in the slots, are pushed and pulled in and out of the slots, flex as the coils leave the slots and move in virtually all directions in the ends of the coils. In all electric motors, the movement of the coils must be controlled while allowing for flexibility in order to avoiding fracturing the insulation system. In low voltage, mush wound machines, the paper insulation, varnish (which acts as glue), and tying tape are used to reduce the movement of the coil. In form-wound coils, the tapes, blocking, surge rings, varnish and tie tapes are used to reduce movement of the coils.

When machines are not properly secured, such as when some manufactures and repair shops skimp on insulation systems, or do not tie every slot, the movement of the conductors and coils eventually wear away the turn insulation. If additional stresses of contaminants, including abrasives and chemicals which may attack the insulation system, surges and partial discharge occur, the windings will short between conductors eventually. In other cases, if the windings age and become brittle, a poorly secured winding will generate small fractures in the insulation system in which moisture and other contaminants will penetrate and then generate pressures based upon the electrical and magnetic fields in those areas. Eventually, the insulation system will either fail between conductors or to ground. A 1983 study of motor failures indicates that over 85% of winding failures occur between conductors, and not to ground.

With electrical signature analysis (motor current signature analysis) coil movement can be detected and monitored during operation. When the machine is running, there should be no movement with coil movement signatures indicating a poorly secured winding. The signature is the speed (in hertz) times the number of slots in the stator plus and minus the line frequency.

The sign of coil movement does not necessarily indicate that the winding will fail in the near future, but that the winding is weak. In some cases, this may be an issue in the original design of the machine. However, if the signature did not exist before a machine repair, then it is an indicator of motor repair defects due to either a misunderstanding of the forces in the machine or lack of insulation material.

Impact of Lean Maintenance

I have observed the impact of ‘lean maintenance’ on everything from rotating machinery to the maintenance organization and the application of lean to the business organization, overall. The original purpose was to streamline processes and eliminate waste in all areas from communication to physical movements. The problem is that the term ‘lean’ has been used not only to trim the fat of organizations and processes, but also to make deep cuts into the muscle of the organization.

The ideas fostered by such programs as lean, RCM and re-engineering were to reduce the focus on unnecessary tasks and to assist with focusing resources. For instance, if you are looking at three different systems: one critical; one redundant; and, one unimportant, these programs started as methods of using set criteria to determine the level of attention for each type of system. Occasionally it would even be determined that the best practice was to let a piece of equipment to ‘run to failure.’

How we graduated from this concept to only a few pieces of equipment would be maintained and the rest run to failure has been a sight to behold! In effect, ignorant abusers of such programs have used the tools as a means to justify degrading their maintenance organization and workplace to one of reactive maintenance. The result has simply been a disservice to their organization and the bottom line.

What we have forgotten is that if a system was unnecessary it wouldn’t be there in the first place. So, technically, if something isn’t worth any notice at all, why not remove it? Well, you say, it isn’t cost effective to perform maintenance, so it was run to failure, but we still need it. Let’s say the item in question is a bathroom fan.

Is the bathroom fan a piece of critical equipment? (OK, I have heard the jokes and comments that you have just considered… ha ha). Let’s look at it from several perspectives: 1) Does it impact the four areas considered in RCM? (Safety, production, regulation or cost); 2) What is the reaction if it fails unexpectedly? Does it get repaired eventually? Immediately? Are complaints generated? 3) Why was it installed in the first place? Basically, is there a cause-effect relationship with the fan? Could it be that there is a regulation for air turnover, or just that such ventilation exists?

Now, let’s add in the headache factor and the real reasons for reduced maintenance. Often the case is related to the wrong maintenance being performed, other times it is a lack of understanding, and, in others, a lack of political will to maintain the maintenance organization. When times get tight, one of the first budgets cut is the maintenance organization. Once times and resources return, the mountain ahead is steep and the resources and effort, even the will, is lacking to move forward. As a result, the use of the maintenance organization to ride through cash-flow and budget shortfalls compound the destruction of the maintenance organization itself.

The roughshod methods referred to as ‘lean,’ various forms of ‘modified RCM,’ and concepts in which they attempt to refer to ‘re-engineering,’ outside of the original concepts, are often used as tools by consultants to make the end user feel better about gutting their maintenance organizations while paying significant fees. While a consultant, I railed against such practices after seeing the results over and over again. But, how do you explain to a prospective client that their ‘baby is ugly?’

Now, I have an organization and facilities in which we are implementing the common sense approaches that I have been preaching for decades. Seeing as we also provide condition-based PM, predictive, corrective and engineering resources for our clients, we also extend this approach and our recommendations for their success.

Howard W Penrose, Ph.D., CMRP

http://twitter.com/motordoc

Come visit the Dreisilker organization at http://www.dreisilker.com!

For questions, please respond to this email or contact me at hpenrose@dreisilker.com.

Motor Repair ‘Half Life?’ Reality or Myth?

It actually depends on the quality of the repair. A quality repair process can provide an average life of 20 to 25 years, as with a new electric motor. Motors that do not survive that long normally have some type of some issue such as contamination, operation, too many starts, electrical or mechanical anomalies, and other issues that reduce their life.

As was determined in the Canadian Electrical Association study, “Evaluation of Electric Motor Repair Procedures,” a quality motor repair process will maintain the original efficiency of the motor. In particular, rewinding using a mechanical process will maintain the losses of the stator whereas higher temperature processes, such as burnout, increase the eddy-current (stator) losses. Increased losses result in reduced efficiency (an average of an increase of 0.5 to 1% per rewind) in which the losses are seen as higher operating temperatures. The higher temperatures, or even hot spots in the stator core, reduce the insulation life at that point. To check for core damage, a before and after core loss test should be performed on all motors that are burned out, or the core is damaged, and there should be no change in the core loss with an upper limit of about 6 Watts per pound.

Additional issues include that 81% of motor repair facilities modify your windings through the repair process with 73% of those modifications for the convenience of the repair facility. Improper modifications to windings will result in changes to the efficiency of the motor and may result in increased stator I2R losses and possibly even rotor losses, depending on the result of the modification. These changes also increase operating temperature, also resulting in a reduced thermal life of the motor.

The results may be seen as slight increases in operating current under the same load conditions, hotter running motors, reduced efficiency, the motor tripping upon return from repair, reduced mean time between failures, and other anomalies, including significant changes to soft foot due to mechanical changes in high temperature ovens.

For more information on the impact of improper motor repair practices, go to http://www.motordiagnostics.com/presentations.htm. A number of papers can be found on the electrical and mechanical impacts of motor repair.

Impact of Losing the Basics

I have been working my way through converting historical training videos for the IEEE Dielectrics and Electrical Insulation Society website (IEEE DEIS Web). It is amazing how much information we have lost from the time when testing and test equipment required an engineer and not some of the modern equipment that has come along over the past decades.

The first one that we converted from tape to DVD and now to online digital format was the ‘Rotating Machinery Workshop.’ This video located at http://ewh.ieee.org/soc/deis/education/video-detail/education/rotating-machinery-workshop.html and is from some point in the 1960s. The video walks through the manual tests and explanations for insulation testing that we take for granted through modern test instruments. The result is the loss of the fundamental knowledge as to what the tests are supposed to tell us versus concentrating on the pass/fail values.

One of the recent ones that we are presently working on discusses ‘Fundamentals of Motor Insulation for Repair Shops,’ of which only Part 3 has been recovered, so far. However, in this hour and fifteen minutes, the presenter discusses, with pictures, calculations and descriptions, the electrical and mechanical dynamics of an insulation system that prematurely age the system and cause it to fail. This includes the forces on the windings through to the impact of contamination, vibration, coil movement, and other forces (vectors) that will take an insulation system from an average 20-25 year life down to a matter of a few years. It even shows the calculations in order to determine the effective age of the insulation system due to its operating environment.

In both of these, and other videos, if you are a member of DEIS, you have full access 24/7. You do not have to be an engineer and can get more information from clicking on the ‘join’ button at the top of the page at http://ewh.ieee.org/soc/deis

I suppose it is amazing how quickly we are able to lose key information that would help us interpret the problems that we run into with modern technologies. I even remember having a ‘discussion’ with an individual who was determined that windings only moved up and down in a slot because in a textbook it was dealt with as a static system. However, as identified in ‘Fundamentals of Motor Insulation for Repair Shops,’ they identify all of the different directions and forces involved during the operation of the machine. Even I had not fully grasped all of the forces and found the video to be very educational.

Over the past years I have met newly graduated engineers, from reputable schools, that do not even know how insulation resistance testers (ie: Meggers) work, tradespeople that do not understand how to use voltmeters and ammeters to troubleshoot motors, do not understand the differences between RMS, True RMS, and Averaging meters, and worse. We have drifted so far off the basics that engineers and tradespeople have difficulty explaining to lay people that an insulation to ground test cannot detect winding shorts!

While I am a strong believer in modern technology, future technologies, and have a fondness for computers, I have always found that it is important that I understand the basic principles behind what I am doing. Sure, you may want to start out just understanding how to perform the test. But a true professional is going to dig so that they understand the limits of those tests and what they really tell us.

For information on the basics:

http://www.motordiagnostics.com/presentations.htm
‘Electrical Motor Diagnostics: 2nd Edition’ at http://www.motordoc.com/detailEMD.htm available through our distributor or any online bookstore! (yeah, well I use the sales to support this newsletter).
IEEE DEIS Videos: http://www.ieee.org/go/deis and under the ‘Lifelong Learning’ button at the top of the page.

Want to know more? Watch for our MotorDoc Tech Tips in these newsletters!

Sincerely,
Howard W Penrose, Ph.D., CMRP
howard@motordoc.com