Core loss testing: A good procedure gone astray?

The loop test described in IEEE 432 has long been used to evaluate the condition of stator cores before rewinding. The acceptance of this test is largely due to the rigorous peer-review process underlying all IEEE Standards. Originally developed to test stator cores, the loop test is increasingly used to perform testing of other types of electrical cores, such as: squirrel cage induction motor rotors, wound rotor cores and DC armatures. These results are often misinterpreted due to a general lack of understanding of the principles behind the test. This paper discusses the basic components of core loss (hysteresis and eddy-current losses), the effect of frequency and lamination thickness, and other contributing factors. It also explains the operating frequency of squirrel cage type rotors and armatures, and provides correction factors.