The needle method for induction tests: sources of error

The regional flux density in laminations of soft magnetic materials is usually determined by means of a single-winding coil inserted through drilled holes. An alternative method is to use needle contacts arranged at lamination surface points through the surface coating. This method is simple and nondestructive. However, concern exists whether the results of measurements are fully equivalent with results from the coil method. We show here that full equivalence can be expected in most cases. The test results prove to be fully equivalent if the investigated region exhibits quasi-homogeneous induction or linear induction variations. If the region encloses a local induction extremum, a systematic error arises, but it can be neglected in practice. On the other hand, strong errors may arise if the in-plane flux is combined with asymmetric off-plane flux. This is because the method takes advantage of the surface eddy-current configuration, which reflects both components of flux.