Inexpensive Methods to Detect Corona and Partial Discharge Impact

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The use of visual inspections are critical when evaluating electric machine insulation systems, especially when you exceed 6kV systems, and lower voltage systems when a variable frequency drive is applied. I often use a UV light (cost about $10 USD) to inspect the insulation system in order to detect the effects of corona or partial discharge, which will show as a light area or ‘clear’ insulation.

2015-10-08 09.22.11Above: Inspection Using an UV Flashlight Showing PD/Corona

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Above: Corona Damage on a 13.8kV Stator


Repair Shop Rotor Testing

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One of the methods used to evaluate the condition of a rotor in the motor repair process is the combined use of a core loss tester, iron filings/magnetic paper and… infrared. This method is used to both detect the gross rotor faults (iron filings) and minor faults, such as cracks and high resistant bars (infrared) as well as the elusive faults that occur in the rotor and where the bar leaves the rotor core.

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Above: Infrared and Iron Filing Test on a Rotor using the GTI Predictive FLIR Attachment*

When a rotor bar fracture occurs in or around the core and current is applied, the current will flow around the fracture and through the surrounding laminations. This generates a hot spot. Things to be aware of include reflections from lights and lamps as well as thermal reflection from personnel near the rotor, especially when the rotor is reflective or light in color. You also want to be aware that dirt and rust will also generate hot spots as well as solid steel objects including bolts and balancing weights.

2015-12-14 09.51.37Above: Core Loss Tester*

2015-12-14 09.51.31Above: Rotor Set-Up for Core Loss Testing*

While it used to be that infrared test equipment was very expensive, systems that meet the requirements for rotor and electrical system testing are now available for under $600 from GTI Predictive (contact us at which also include thermal trending.

*All photos and infrared taken by MotorDoc LLC at Dreisilker Electric Motors, Inc. (