If you are involved in the repair of electric motors, chances are you have heard about a core loss test (or core test, core loop test, core flux test, etc.) being performed on your motor. This article serves to provide a basic understanding of what a core loss test is and why it matters.
Core flux “Loop” test is the standard test utilized for evaluating the insulation integrity of laminated stator cores.
The test establishes a specific magnetizing level for the core by energizing the loop coil with single phase power. Calculations of the number of loop turns required for a desired core magnetizing level are made in a typical target flux range of 85,000 lines per sq. in (85 kl/in² or 1.32 Tesla).
Any defective areas of the core or tooth insulation will show up as “Hot Spots” in that they will become significantly hotter than the surrounding “normal” areas. (Using infrared thermography ensures accurate results).
Typical Acceptance Criteria
- Temperature – initial ambient core temp to final hot spot < 27 °F (15 °C) (Note: Some standards (IEEE 1068) require to be within 10° C (18 °F) like fan standard efficiency motors.)
- If core losses (Watts/lb) increase more than 20% (from before to after (stripping)), the typical acceptable range is ≤ 4 watts/lb.
Rework (core repairs and re-insulating) is needed with repeated core testing until acceptance criteria is met. In some cases, new stator laminations may be required.
Core Loss Test Prior to Burnout Process
You can see from the computerized report that the test was passed. The core loss was 1.4 watts/lb and the temperatures ranged from 80-86F
Core Loss Test after Baking and Stripping of Coils
You can see from this report that the core had losses of 1.3 watts per lb and the temperatures ranged from 82F to 94F.
Todd A. Hatfield
HECO – All Systems Go
About the author:
Todd Hatfield is a Senior Electrical Engineer at HECO. He has over 35 years' experience in generator and electric motor repair and engineering. Todd has a BS in Electrical Engineering, and his areas of expertise are: electrical and mechanical motor redesigns and engineering, root cause of failure analysis, and quality electric motor rebuilding.