IEEE

Motor Core Insulation, Reliability Due to Repair

Impact of Inter-Laminar Insulation Damage on Small to Medium Voltage Machine Life

There are opportunities to negatively impact the life of electric machines through the repair process. The potential depends on the type of core steel, inter-laminar insulation, and coil removal process. Additional impact can be the result of the machine failure such as melted or missing core material due to the type of failure.

In 1984, David C. Montgomery published “The Motor Rewind Issue – A New Look”[1] identified the impacts of core loss increases of 50%, 100%, 150%, and 200% and related it to core loss increase, temperature rise, resulting insulation life, and impact on grease/bearing life. The machine example used was a 50hp, 3600RPM drip proof motor.

Core Loss Increase

Watts/lb Increase

Temp. Rise Increase

% Potential Insulation Life

Approx. Grease Life

50%

515

7C

62%

85%

100%

1030

14C

38%

69%

150%

1545

21C

24%

58%

200%

2060

29C

14%

46%

The demonstration was to open discussion related to why motor owners observed shortened machine life compared to the original machine. This was comparable to later studies including EASA/AEMT[2] study and CEA study[3] related to poor rewind techniques. General industry acceptance is the increase of more than 20%[4] between before and after stripping core loss measurements. Even to this level there is a reduction in life, energy efficiency and increase in greenhouse gas emissions.

It becomes very important as a machine owner to review the before and after core loss tests through the repair process. This includes hot-spot tests using a commercial core loss tester.

[1] Montgomery, David, “The Motor Rewind Issue – A New Look,” IEEE Transactions on Industry Applications, Vol 1A-20, No. 5, September/October 1984

[2] EASA/AEMT, The Effect of Repair/Rewinding on Motor Efficiency, 2003

[3] Demand Side Energy Consultants, Inc., Evaluation of Electric Motor Repair Procedures Guidebook, CEA 9205 U 984, Canadian Electrical Association, November, 1995

[4] IEEE Industry Application Society, IEEE Standard for the Repair and Rewinding of AC Electric Motors in the Petroleum, Chemical, and Process Industries, IEEE Std. 1068-2009

One Response to “Motor Core Insulation, Reliability Due to Repair”

  1. It is a sour truth about motor repairing. If a motor is not repaired well then it will not last long. We can see the impact of bad repairing and core losses in the above mentioned table. Thanks!

Leave a Reply