Electric Motor Repair: Research Project on Soft Foot







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Electric Motor Repair:

Research Project on Soft Foot

Howard W Penrose, Ph.D., CMRP

Vice President, Engineering and Reliability


Dreisilker Electric Motors, Inc.

(c)1997 Penrose

Hello and welcome to our presentation on Electric Motor Repair. This presentation is the lead to a short series on motor repair practices and their impact on the reliability of your facility. The purpose is to provide an understanding as to the various repair practices performed on our equipment. In this way, when issues arise, we have the knowledge necessary to effectively evaluate the issues.

In this presentation, we are going to cover a research project that was independently performed on the mechanical impact of electric motor rewind. A fair amount of research had been performed on the impacts of thermal stripping on motor cores including work by General Electric, the Canadian Electrical Association and the Electrical Apparatus Association over the years. However, in each case, representatives from the motor repair industry had objected to an evaluation of the mechanical aspects of the system. Due to a number of instances identified through field service work, an independent research project was performed by Dreisilker Electric Motors Research and several partners. This presentation is a summary of the 1997 IEEE EIC conference paper on the impact of thermal stripping on electric motor reliability.

My name is Howard Penrose and I will be guiding you through this series. While most people recognize my work on electrical motor diagnostics, my career actually started as an electric motor repair journeyman and my research in electric motor repair and its impacts on reliability and testing methods started in 1985. At the time I was the motor repair supervisor on the USS Theodore Roosevelt, a brand new aircraft carrier, and my co-workers and I were trying to figure out a few oddities that we had noticed through the repair process. When I left the Navy in 1989, I went to work for Dreisilker Electric Motors in Glen Ellyn, Illinois. During my time there I was a motor rewinder, a mechanic, a machinist and finally the field service manager. The title I held was Director, Field Service and R&D as we began internally funded research in the impacts of motor repair on energy efficiency, the impact of inverters on electrical insulation and many more issues. I also worked on projects with the US Department of Energy related to the Motor Challenge program including participating in the development of MotorMaster Plus software, research, materials and technical support starting in 1992. Work on the development of motor management programs and motor system reliability started in 1993 with a focus on the impact of systems and the effectiveness of motor testing practices. These eventually led me to teach at the University of Illinois at Chicago and perform as a Senior Research Engineer at the UIC Energy Resources Center where my focus was on industrial assessment programs for energy, production, reliability and waste-stream improvements.



• Research Project performed 1996-7

• Published “Impact of Mechanical Stripping on Electric Motor Reliability,” IEEE Electrical Insulation Conference 1997

• Concerns: Possible increase in soft foot through the motor rewind process – Never previously investigated!

• Research Partners:

– Howard W Penrose, Ph.D. – University of Illinois at Chicago-Energy Resources Center, Senior Research Engineer and Adjunct Professor Industrial Engineering at the time

– Leo Dreisilker – President, Dreisilker Electric Motors, Inc.

– Test Sites

• McHenry Electric, a Dreisilker Company for burn-out studies

• Dreisilker Electric for mechanical stripping

This research project started in 1996 following a number of frustrating attempts to have the mechanical impact of motor repair practices included in other research projects. We made the decision to self-fund the project through a collaboration of Dreisilker Electric and resources at the University of Illinois at Chicago’s Energy Resources Center.

The issue began with an effort to improve alignment times for field service personnel. We were evaluating some of the new laser alignment systems in the early to mid-1990’s and had started to notice that motors rewound using higher temperatures tended to require more shim’s than those rewound using low

temperature methods. It was understood within the repair industry that there would be some level of ‘relief’ that would cause the stator frame to twist although no one had ever researched the amount or impact. The result was a project undertaken between Dreisilker Electric, McHenry Electric and my work at UIC-ERC. The intention was to drive more work in this area. However, most of the funding organizations noted that the motor repair’s trade association had been doing work on the impact of high temperature stripping processes and that was sufficient. To this date, this has been the only project of its type.



286T Rolled Steel Stator

(c)2007 SBD

The objective of the experiment was to take a sample of average-sized electric motors with different frame materials and submit samples of them to

low-temperature mechanical stripping methods and others to high low-temperature stripping methods. The stator frames selected were used complete stators (frame and core) in order to properly evaluate the impact on a typical rewind versus using ‘new motor frames’ or core-less frames. In effect, attempt to make the experiment as close to reality as possible.

All of the dimensions were to be measured by machinists and the data used in Autocad 14 as a 3-D model. Foot flatness was measured on a vertical lathe and on a flat surface with the stator positioned in the exact same spot each time.



Same Shaded 256 Colors

(c)2007 SBD

For animation purposes, the Autocad 3-D model would have a ‘skin’ applied. This would assist with the visual effect.



Diameter of Stator

(c)2007 SBD

One of the more important aspects was measurements of position relative to the feet as well as the roundness and position of any out-of-roundness that may occur.



Max Stator Facing

(c)2007 SBD

The software used to observe the changes was 3D Studio Max version 1.2, which is an animation software that will take Autocad 3D drawings in directly. This was used instead of a Finite Element Analysis as we were attempting to demonstrate the direct change in animation. It also allowed us to rotate the view in any direction in order to observe the changes.



3D Max 10 Degrees

(c)2007 SBD

It also allowed us to exaggerate the twisting effect such that we could observe the changes in the stator directly.



3D Max - 2Degrees (Realistic)

(c)2007 SBD

This particular angle of 2 degrees of ‘twisting’ allowed us to see what the changes would be on the feet and the rabbet fits. These were confirmed using the accurate measurements after the repair.



Deformed Stator

(c)2007 SBD

We confirmed the changes in Autocad by transferring the models back into Autocad and using the measurement tools in the software compare to the actual



Deformed Stator Shaded

(c)2007 SBD

This example shows one of the exaggerated deformed stators in Autocad with the components shaded such that where the changes occurred could be observed.



Deformed Stator, Right Side

(c)2007 SBD



Stators Stripped With Two

Methods, Three Temperatures

Thumm/ Mechanical - 410



Temperature Controlled Burnout - 650



Temperature Controlled Burnout - 800



Mechanical - Dreisilker Electric Motors, Inc.

Burnout - McHenry Electric and Supply, Inc.

Research and Animation – Howard W Penrose, Ph.D.

The methods being evaluated were the Thumm Mechanical Stripping system that performed excellently in the Canadian Electrical Association motor rewind study led by BC Hydro and Hydro Quebec and temperature controlled burnout. In both cases, one stator at a time was stripped. However, it should be noted that in typical high temperature burnout ovens as many stators as possible are normally placed in the oven overnight.

The Thumm method optimal core temperature for copper removal is an average of 410 degrees farenheight. The recommended temperature for coil removal in a repair shop is 650 degrees farenheight. However, recent repair trade publications have stated that temperatures as high as 750 degrees farenheight are acceptable ON ENERGY and PREMIUM EFFICIENT MOTORS WITH CARBON STEEL

CORES and ONE AT A TIME ONLY. Unfortunately, many have taken this as a license to apply 750 degrees farenheight to everything.

As a result, we determined that it would be effective to test at 410, 650 and 800 degrees.



410 F - 1.5 Hour Stripping Time

Stator A - 286T Rolled Steel

Stator B - 256T Cast Iron

Stator C - 324T Cast Iron

Stator D - 286T Cast Iron

Stator E - 215T Rolled Steel

Stator F - 324T Cast Iron

For this study, six stators were selected for the 410 degree study with four being of different size cast iron and two being of rolled steel construction. After the

publication of the study, several aluminum frames were evaluated at this temperature with the same results as rolled steel.



650 F - 8 Hour Stripping Time

Stator A - 324T Cast Aluminum

Stator B - 286T Extruded Aluminum

Stator C - 286T Extruded Aluminum

Stator D - 254T Cast Iron

Stator E - 324T Cast Aluminum

At 650 degrees, two cast aluminum, two extruded aluminum and a cast iron frame were selected.



800 F - 8 Hour Stripping Time

Stator A - 286T Cast Iron

Stator B - 324T Rolled Steel

Stator C - 365T Cast Iron

Stator D - 284T Cast Iron

Stator E - 326T Cast Iron

Stator F - 254T Cast Iron

At 800 degrees, based upon the impact of 650 degrees on the aluminum frames, only rolled steel and cast iron were evaluated.



Basic Results

410 F

Cast Iron - No Change

Rolled Steel - No Change

650 F

Aluminum - Twisting up to 8% - Soft Foot up to 60 Mil

Cast Iron - Twisting up to 2% - Soft Foot up to 12 Mil

800 F

Rolled Steel - Twisting up to 4% - Soft Foot up to 12 Mil

Cast Iron - Twisting up to 3% - Soft Foot up to 12 Mil

At 410 degrees there was no change to the rolled steel or cast iron frames.

At 650 degrees, there were significant changes to the aluminum frame with two showing soft foot increases up to 60 mil and frame twisting up to 8%. When the aluminum motors were reassembled (without new windings) it was observed that the rabbet fits were tight in two spots on each side, as the frame was oval. Two aluminum frame motors were horizontal and two vertical, which appeared to have no effect on the deformation of the stators other than the cores of the two vertical stators had slipped.

At 800 degrees, there were equal changes in both the rolled steel and cast iron frames with twisting up to 3-4% and soft foot averaging 12 mils.



Physical Observations

650 F and Greater - Stators and cores began to

oxidize after 48 hours in an air conditioned


Deformations in aluminum frame stators were


Mechanically stripped stators did not oxidize

Animation can play a roll in observing effects

In both the 650 and 800 degree tests, the temperatures removed all of the stator paint and visible varnish from the core. The test area was air conditioned, yet the stators oxidized, or rusted, within 48 hours after being removed from the ovens. The 410 degree test frames did not oxidize as the original paint and core varnish remained.

The actual deformation of the aluminum frames were severe enough to be visible.


Comparison of Stators

Stripped at 650 F 8 hours

Stripped at 410 F <1.5 hours

The picture on the left is of a stator stripped at 650 F and the picture on the right is of a stator stripped at 410 F. Note that the nameplate is in its original condition and that all of the varnish remains – note the gloss on the stator core.




More research required

Stator deformation occurs in relation to time

and temperature

Soft foot and air gap problems occur

through stator rewinds stripped 650 F and

above, significant above 650 F

Other mechanical effects, such as oxidation,

in vapor controlled ovens

The purpose of the study was to provide evidence that additional research would be required in thermal stripping impacts on the mechanical components of the electric motor. To date no additional research has been pursued, including during the motor rewind impact on efficiency study performed in the UK by EASA and AEMT without third party oversight.

The stator deformations occurred in relation to both time and temperature.

While soft foot and air gap problems occurred in stator rewinds 650 degrees and above, they were far more significant above 650 degrees. This means that motors must be stripped at no more than 650 degrees in order to maintain their mechanical condition. However, the primary reason for the repair industry to want to increase temperature has to do with reducing stripping times and reducing the amount of fuel (natural gas) necessary for a longer, lower temperature burnout.

Steps must also be taken to ensure that the stators do not become too rusty following high temperature winding removal.



Ph: 630 469-7510 http://www.dreisilker.com





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