When a core loss test reveals localized hot spots, or visual inspection identifies physical damage, the ability to repair the damage in a cost-effective manner means the difference between repair or replacement.

The two primary reasons for performing stator core testing in the service center are (1) to verify that the stator core is acceptable for continued use and in the event of a rewind, and (2) to verify that the repair process has not adversely changed the stator core condition. The purpose of this article is to discuss how we determine, assess and compare stator core test results. It is extremely important to understand that variance in test procedures may invalidate comparison.

Las dos razones principales para realizar la prueba de núcleo del estator en el centro de servicios son (1) verificar que el núcleo del estator es apto para uso continuo y en caso de rebobinado y (2) verificar que el proceso de rebobinado no ha alterado de forma adversa la condición del núcleo del estator. El propósito de este artículo es discutir como determinar, evaluar y comparar los resultados de la prueba de núcleo. Es muy importante comprender que la variación de los procedimientos de prueba puede invalidar la comparación.

EASA’s Stator Core Test Form provides a step-by-step procedure for calculating the number of turns and cable size required for a loop test.

By this time we should all know that stator core loss testing is a required part of a quality rewind. A core loss test before and after burn-off is specified in the EASA Recommended Practice for the Repair of Rotating Electrical Apparatus (ANSI/EASA AR100) and The Effect of Repair/Rewinding on Motor Efficiency; EASA/AEMT Rewind Study and Good Practice Guide to Maintain Motor Efficiency.

Topics covered in this article include:

• Lessons learned
• Rebates add incentive
• Learning from other members
• Determining if the core is usable
• Accurate iron measurements

Prior to rewinding it is advisable to assess the condition of the core iron of stators, armatures and wound rotors. The assessment is performed by a core test, which magnetizes the core to a prescribed magnetic flux density. The core tester can do more than just probe for hot spots; it can give an indication of the overall core condition in terms of watts per pound (or watts per kilogram) of loss. A detailed explanation of stator core testing is given in Tech Note 17, so that won't be repeated here. Our objective is to explain the proper uses and interpretation of core testing using a core tester.

The procedure for core loss testing of stators is well-defined, but there is not as much information available for special cases like rotors, armatures or high-frequency motors. While the same basic principles apply as for stator testing, frequency is the variable that affects how we should interpret the results.