Design of a Test Fixture for Rotational Core Losses in Electrical Machine Laminations

In recent years, several methods and devices have been proposed to measure rotational core losses. Technically, it is still a challenge to realize both high and uniform magnetic flux density within a wide area of the sample under test. Because of that, importance has been placed on the development of magnetizing circuits which are capable of performing 2-D rotating field tests. In this work, a novel design of a magnetizing circuit is presented, simulated, and implemented practically to obtain a uniform rotating flux density inside the test sample. This magnetizing circuit is based on an electromagnetic Halbach array. The new test fixture is suitable for the measurement of rotational core loss, and capable of measuring pulsating loss in different directions within the specimen. Measurements were carried out on five silicon steel circular samples of 20 cm diameter. Theses samples are: M15 gauge 29, M19 gauge 29, M19 gauge 24, M36 gauge 29, and M36 gauge 26. This work describes the influence of different flux patterns on core loss estimation, where three different machines, induction, BLDCM, and SRM are simulated to calculate the zones of rotating flux density in the stator cores of these machines. Experimental tests are executed, and the percentage error made by assuming all flux is pulsating is calculated. Core loss differences in electrical machine laminations have been noticed under rotating field in the case of clockwise (CW) and counterclockwise (CCW) direction excitation. Experimental results manifest that the anisotropy of the magnetic material is attributed mainly to the asymmetry in the CW and CCW rotational core losses, where the permeability varies significantly when the direction of rotating field is reversed. A study on three different types of machines at different frequencies show the importance of considering the direction of rotating field during the machine efficiency estimation. The negative power in rotational core loss which was reported in some laboratories has been addressed and interpreted according to the dynamic hysteresis loop behavior. The negative power which appears in one loss component is caused by the reversal of the field direction which can be realized from the interfered dynamic hysteresis loop, while the negative value in the total rotational loss originates in the sensors misalignment. The reason behind the unusual shape of the hysteresis loop under rotating field observed in the literature is revealed, where the harmonics under rotating field is responsible for the change in the hysteresis loop shape from the well known shape under pulsating field. The results show also that the behavior of the minor loop under rotating fields differs from its behavior under pulsating fields. The minor loop under rotating fields moves outside the major loop at saturation, while it is confined inside the major loop under pulsating fields. The effect of harmonics on core losses under a rotating field is estimated and compared with losses under pulsating fields